Øystein Ole Gahr Langangen

• Klöcker, Antonia; Vihtakari, Mikko Juhani; Arostegui, Martin Christopher; Schlindwein, Axel; Ferter, Keno & Bjelland, Otte [Vis alle 11 forfattere av denne artikkelen] (2026). Wide-ranging, year-round breaching behaviour of basking sharks revealed by long-term biologging. Biology Letters. ISSN 1744-9561. 22(4). doi: 10.1098/rsbl.2025.0696. Fulltekst i vitenarkiv Vis sammendrag

  Marine megafauna, including various species of cetaceans and fishes, exhibit energetically costly breaching behaviour during which individuals rapidly propel themselves out of the water. The function of breaching remains enigmatic, largely due to short-duration observations of limited spatial extent. Here, we used animal-borne biologgers to record breaching events of plankton-feeding basking sharks (Cetorhinus maximus) over full annual cycles at high temporal resolution (5 s) across broad spatial scales (35° latitude). Recording 577 breaches across four individuals over 366 days (mean 144 yr⁻¹ shark⁻¹), breaching appears to be a routine behaviour in both sexes, peaking in autumn, but occurring across all seasons and habitats, including deep oceanic areas. Notably, we identified thermal constraints on breaching rates in colder water, with a critical minimum observed at 5°C. Beyond demonstrating that breaching is more common than previously recognized, our study suggests it serves several functions, some potentially independent of spatial and temporal contexts (e.g. seasonal courtship), such as ectoparasite removal. The ubiquity of breaching across space and time highlights the need for comprehensive ‘behaviour maps’ for ocean giants to elucidate the physiological constraints, ecological significance and management implications of breaching in a changing ocean.

• Opdal, Anders Martin Frugård; Alix, Maud; Kjesbu, Olav Sigurd; Jørgensen, Christian; Langangen, Øystein Ole Gahr & Karlsen, Ørjan [Vis alle 11 forfattere av denne artikkelen] (2026). Despite temperature effects on gonad development, timing of spawning is remarkably flexible in Atlantic cod. ICES Journal of Marine Science. ISSN 1054-3139. 83(2), s. 1–12. doi: 10.1093/icesjms/fsag020. Fulltekst i vitenarkiv Vis sammendrag

  At high latitudes, early life stage survival of fish is often associated with how spawning time relates to the timing of the spring bloom. With ocean warming, basic physiological rates of ectotherms, like fish, will speed up—including gonadal development rates, which dictate spawning time. Since warmer water is thought to influence the spring bloom timing differently than that of fish spawning time, the two may fall out of synchrony in the future. The precise mechanisms between temperature and gonadal development and spawning time have, however, been difficult to disentangle. Here, we take advantage of a series of independent laboratory experiments measuring individual oocyte development up to or near spawning for 153 Atlantic cod (Gadus morhua) kept between 3 and 12°C. From these data we derive a predictive, mechanistic equation for daily oocyte growth rate as a function of temperature and oocyte developmental status (diameter). The vitellogenic oocyte growth follows an accelerating pattern, and the model predicts that spawning can advance by up to 7 days per 1°C increase. Within-treatment variation is, however, of comparable magnitude to between-treatment temperature effects. The model was also tested in the field by back-calculating oocyte development of 82 fish (2018–2021) sampled at two locations along the Norwegian coast, using daily ambient temperatures from telemetry tags during vitellogenesis as model input. We find that Atlantic cod are able to initiate vitellogenesis over a period of several months in late summer and autumn, as well as regulate the oocyte development rate across a wide range of temperatures—both leading to significant phenotypic plasticity in spawning phenology.

• Skjæraasen, Jon Egil; Arechavala-Lopez, P.; Karlsen, Ørjan; Olsen, Esben Moland; Meager, Justin James & Nyqvist, Daniel [Vis alle 12 forfattere av denne artikkelen] (2026). Male Agonistic Behavior on Atlantic Cod Spawning Grounds. Ecology and Evolution. 16(4). doi: 10.1002/ece3.73316. Fulltekst i vitenarkiv Vis sammendrag

  The mating system of the iconic Atlantic cod (Gadus morhua) has been described as a lek, a mating system where male–male agonistic interactions are expected. However, observing such behaviors directly in marine environments is challenging. So far, the evidence supporting the importance of agonistic interactions between males is mainly derived from laboratory studies with fish kept in close confinement, possibly elevating the observed levels and their perceived importance. In natural aquatic systems, agonistic interactions may be inferred from acoustic telemetry transmitters fitted with accelerometers, given that they are associated with high activity levels. A large telemetry dataset on Atlantic cod was collected during (i) a field study at two Western Norway spawning grounds from 2019 to 2024 and (ii) a close confinement net—pen study in 2019–2020. A total of 154 sexed mature Atlantic cod were tagged with acoustic transmitters fitted with accelerometers and depth sensors, yielding ~1.5 million accelerometer detections for subsequent activity analyses. In the net-pens, male cod showed much higher activity levels than females. Overall activity levels were reduced in the field, but male activity levels remained higher than those of females during the spawning period. A prominent feature of coastal cod spawning along the Norwegian coast is that males tend to dwell deeper than females. We therefore explored whether depth influenced activity patterns of males and females. This was indeed the case with males being more active than females at fish depths > 20 m and no sex-difference in activity levels closer to the surface (< 20 m depth). No sex difference in activity levels was found outside the spawning period. Our data suggest that male–male agonistic interactions, inferred from activity levels, are present on natural spawning grounds for Atlantic cod, albeit at reduced levels compared to levels exhibited in net–pen confinement. We hypothesize that the heightened activity levels of males compared to females when occupying deeper waters are related to their overall deeper distribution and territorial and agonistic behavior at these depths, agreeing with the lek description of the cod mating system.

• Durant, Joël M.; Langangen, Øystein Ole Gahr; Dolgov, Andrey V.; Lindstrøm, Ulf Ove; Krasnov, Yuri & Nikolaeva, Natalia G. [Vis alle 7 forfattere av denne artikkelen] (2026). Importance of capelin in the diet of Barents Sea predators relative to alternative prey. Marine Ecology Progress Series. ISSN 0171-8630. 781. doi: 10.3354/meps15078. Fulltekst i vitenarkiv Vis sammendrag

  The Arcto-boreal Barents Sea is significantly impacted by temperature warming, with potential effects on trophic interactions and population dynamics. Capelin, a small pelagic fish species, dominates the Barents Sea’s relatively simple food web. Capelin are an essential link between lower trophic levels and top predators, like in many Arcto-boreal systems. This study investigates the factors controlling capelin consumption by Barents Sea predators. We analysed changes in capelin presence in the diet of predators and related it to capelin and krill abundances, using 8 unique diet data sets from major Barents Sea predators (3 fish, 2 seabird, and 2 marine mammal species) spanning 10-36 yr. Only 2 fish species showed a positive relationship between the percentage of capelin in the stomach and capelin abundance, while all fish species exhibited a negative correlation with krill abundance. This finding suggests that these species replace capelin with krill in their diet based on prey availability. Our results confirm the central role of capelin in the Barents Sea ecosystem, with most predators’ diets being influenced by that species’ abundance. Krill plays a more significant role in the Southern Barents Sea, particularly for central place foragers. Although diet data can have limitations, our study shows that diet analysis remains a valuable method to study predator-prey relationships. This information is essential for understanding predator-prey dynamics of key species in the Barents Sea, supporting ecosystem-based and multi-species fisheries management efforts under current and future climate conditions.

• Langangen, Øystein Ole Gahr; Kaartvedt, Stein; Hylland, Ketil & Durant, Joël (2026). The collapse of Atlantic cod in the Oslofjord. Frontiers in Marine Science. 13. doi: 10.3389/fmars.2026.1746235. Fulltekst i vitenarkiv Vis sammendrag

  Coastal fish populations in temperate regions increasingly experience environmental pressures and dramatic decreases in stock size. Currently, there are serious concerns about the status of the Atlantic cod ( Gadus morhua ) in the Oslofjord region, a 120-km inlet in southeastern Norway. Given the recent political debate, the large public interest, and the recent decision to close large areas in the Oslofjord to all fishing, the scarcity of quantitative estimates on cod biomass in the region presents an acute challenge. Typically, the state of the cod in the Oslofjord has been assessed by the analysis of juvenile fish datasets. However, scientific surveys targeting adult life stages are limited, and abundance and biomass estimates are based on relatively short time series and with limited spatial coverage. Here, we demonstrate how commercial landings data can be used to describe a recent decline in catch per unit effort in the region, a proxy related to biomass. These results are highly topical and can serve as a baseline for further investigations on the reasons for the collapse of the cod population. While our results demonstrate the value of the large commercial dataset, the introduction of conservation actions calls for alternative and more comprehensive monitoring in the area for documenting a potential future recovery. Our results highlight research and data needs associated with current efforts to improve the status of fish stocks in general, and the Atlantic cod in the Oslofjord in particular.

• Olsen, Tora; Stige, Leif Christian; Dupont, Nicolas; Durant, Joël M & Langangen, Øystein Ole Gahr (2025). Reply to Bogstad et al. (2025) ‘Comment on “Repeated large declines in the Barents Sea capelin populations are associated with different ecosystem conditions” by Olsen et al. (2024)’. ICES Journal of Marine Science. ISSN 1054-3139. 82(4). doi: 10.1093/icesjms/fsaf054. Fulltekst i vitenarkiv Vis sammendrag

  Herring predates on young capelin in the Barents Sea (BS), which affects the recruitment of BS capelin. However, our modelling study does not support the hypothesis that Bogstad et al. advocated that herring predation is necessary for the observed large declines (collapses) of the BS capelin population. What Bogstad et al. do not consider is the magnitude of the herring predation effect compared to other influences on the population dynamics of capelin. Our results suggest that while herring significantly reduced age-0 abundance of capelin at the start of the collapses, these reductions were probably not the main cause of the subsequent large population declines. All models have limitations, but we do not agree with Bogstad et al. that model limitations were the reason for our findings on the importance of herring predation for capelin biomass collapses.

• Eikefjord, Mathilde Bryn; Johannesen, Edda; Reecht, Yves; Fall, Johanna; Staby, Arved & Langangen, Øystein Ole Gahr [Vis alle 7 forfattere av denne artikkelen] (2025). Contrasting latitudinal patterns in spawning phenology of three gadoids in the northeast Atlantic. ICES Journal of Marine Science. ISSN 1054-3139. 82(10), s. 1–13. doi: 10.1093/icesjms/fsaf186. Fulltekst i vitenarkiv Vis sammendrag

  The timing of phytoplankton spring blooms varies geographically, typically with later blooms occurring at higher latitudes. This is expected to lead to a latitudinal gradient in timing of spawning of species that depend on synchronizing spawning with the spring bloom to optimize larval feeding opportunities. Here, we contrast and compare spatial variation in spawning phenology of cod (Gadus morhua), haddock (Melanogrammus aeglefinus), and saithe (Pollachius virens) in three areas spanning from the North Sea in the south to the Barents Sea in the north. Data on macroscopically determined gonadal development stages were obtained from fish collected by research surveys and by fishing vessels. We found evidence of active spawning over extensive areas, including areas outside the currently recognized spawning grounds. Among the three species, haddock spawned the latest in all areas, and saithe earliest, except in the North Sea, where cod spawned first. Size-related differences in peak spawning were only observed in the south for cod and saithe, and in the north for haddock, with smaller fish spawning later. However, the results on size were associated with high uncertainty. Consistent with the expectation, the peak of the spawning season for cod and haddock was later at higher latitudes. In contrast, the latitudinal variation in spawning time in saithe was limited. Our results illustrate the need for both species and region-specific investigations of spawning phenology, as it may affect recruitment dynamics, especially under climate change, and have implications for survey design, stock assessment, and ultimately, population persistence.

• Langangen, Øystein Ole Gahr; Olsen, Tora; Ottersen, Geir; Eriksen, Elena & Durant, Joël M. (2025). Consequences of climate-related poleward shifts in cod spawning. Canadian Journal of Fisheries and Aquatic Sciences. ISSN 0706-652X. 82, s. 1–11. doi: 10.1139/cjfas-2024-0380. Fulltekst i vitenarkiv Vis sammendrag

  With climate change, many species alter their distribution according to local climate velocities. For marine fish, suitable areas for spawning in a warming climate may be less favorable for the resulting offspring, e.g., the sensitive larval and juvenile phases. The size of the young-of-the-year before their first winter is an important factor for subsequent survival, year class formation, and population dynamics of many fish species and populations. Here, we investigate how the change in spawning location of the Barents Sea population of Atlantic cod ( Gadus morhua) may affect the mean individual size of the offspring in their first fall. We demonstrate how the observed climate warming-related northerly shift in spawning location may lead to smaller sizes of young-of-the-year cod. Our analysis reveals a positive effect of sea temperature on offspring size and a nonlinear effect of spawning stock biomass (SSB) with smaller offspring size at both the high and low end of SSB. Decreased offspring size is likely to affect population dynamics through reduced recruitment success, and we hypothesize that it may have been part of the reason for this population's recent reduction in biomass.

• Stige, Leif Christian; Fall, Johanna Jennifer Elisabeth & Langangen, Øystein Ole Gahr (2025). The Roles of Environmental Change and Fishing in Norwegian Coastal Cod Stock Dynamics. Fisheries Oceanography. ISSN 1054-6006. 34(4). doi: 10.1111/fog.12728. Fulltekst i vitenarkiv Vis sammendrag

  Disentangling the roles of fishing and environmental factors in fish stock dynamics is fundamental for sustainable fisheries management. The stock biomass of Norwegian coastal cod north of 67°N has varied between periods of increase and decline in recent decades, with declining biomass since 2014. Here we developed a population model for the dynamics of this stock. The model was statistically fitted to survey indices of abundance- and length-at-age and environmental covariates from 1995 to 2022. The final model included a negative effect of saithe biomass on survival to age 1, positive temperature effects on growth to ages 2 and 3, density dependence in survival and growth, size-dependent maturation, and negative fishery effects on adult survival. Hindcast scenario simulations, where environmental influences or mortality from fishing were fixed, were used to disentangle the roles of recruitment, growth and adult survival for stock fluctuations in recent decades. The changes in stock biomass appear mainly to be driven by adult survival and hence fishing. The fishery effects on stock biomass were mostly the direct effects of biomass removal, with no indication of fishery-caused recruitment failure at the spatial scale represented by the model.

• Durant, Joël; Dupont, Nicolas; Ono, Kotaro & Langangen, Øystein Ole Gahr (2024). Interaction between three key species in the sea ice-reduced Arctic Barents Sea system. Proceedings of the Royal Society of London. Biological Sciences. ISSN 0962-8452. 291(2032). doi: 10.1098/rspb.2024.1408. Fulltekst i vitenarkiv Vis sammendrag

  Population dynamics depend on trophic interactions that are affected by climate change. The rise in sea temperature is associated with the disappearance of sea ice in the Arctic. In the Arctic part of the Barents Sea, Atlantic cod, capelin and polar cod are three fish populations that interact and are confronted with climate-induced sea ice reductions. The first is a major predator in the system, while the last two are key species in Arctic and sub-Arctic ecosystems, respectively. There are still many unknowns regarding how predicted environmental change may influence the joint dynamics of these populations. Using time series from a 32 yearlong survey, we developed a state-space model that jointly modelled the dynamics of cod, capelin and polar cod. Using a hindcast scenario approach, we projected the effect of reduced sea ice on these populations. We show that the impact of sea ice reduction and concomitant sea temperature increase may lead to a decrease of polar cod abundance at the benefit of capelin but n

• Olsen, Tora; Stige, Leif Christian; Dupont, Nicolas; Durant, Joël & Langangen, Øystein Ole Gahr (2024). Repeated large declines in the Barents Sea capelin population are associated with different ecosystem conditions. ICES Journal of Marine Science. ISSN 1054-3139. 81(8), s. 1584–1593. doi: 10.1093/icesjms/fsae101. Fulltekst i vitenarkiv Vis sammendrag

  Although fluctuations in the biomass of small pelagic fish are strong and rapid, they are largely investigated as individual events or generalized to what is common across several species and populations. The Barents Sea capelin population, which experienced four large decreases in biomass between 1980 and 2016, can be used to compare the similarity of causes and sequence of events leading up to large biomass decline in the same population. Here, we used an age-structured state-space model to investigate the possible contributions from somatic growth, food availability, temperature, predation by Atlantic cod, herring interactions, and/or fisheries on the capelin biomass declines. We recalculated total biomass while perturbing these effects and assessed the change in total biomass. We did not find a single, common cause for the dramatic biomass declines. Although median biomass increased in several of the scenarios, a large decline in total biomass was mostly retained. Atlantic cod predation and food availability influenced adult capelin biomass. Food availability mainly impacted the first period, while the importance of Atlantic cod predation increased from the first period to the third period. Reducing larval mortality from herring interactions or temperature did not suppress the biomass declines. Recruitment failures were maintained due to a combination of process errors and uncertainty in the effect of spawning biomass, and thus fisheries, on age 0 abundance. The impact of each perturbation depended on the specific ecosystem context at the time of the biomass decline.

• Olsen, Tora; Frøysa, Håvard Guldbrandsen; Yaragina, Natalya; Titelman, Josefin; Durant, Joël & Langangen, Øystein Ole Gahr (2024). Predator biomass affects west–east shifts in Barents Sea capelin (Mallotus villosus) spawning ground use. Fisheries Oceanography. ISSN 1054-6006. 33(6). doi: 10.1111/fog.12685. Fulltekst i vitenarkiv Vis sammendrag

  Spawning locations of migratory fish influence the environmental and oceanographic fate of eggs and larvae. However, we have an incomplete understanding of how and why realized spawning locations vary. We quantified the yearly variation in spawning location for Barents Sea capelin (Mallotus villosus) within the recognized spawning areas along the coast of northern Norway and Murman. Furthermore, we tested whether water temperature, sea ice cover, predation and capelin fisheries are associated with spawning locations. Estimated spatial variation in spawning longitude used data from Soviet-Russian ichthyoplankton surveys from 1959 to 1993 and a bootstrap procedure. The mean spawning area along the coast was then calculated by combining larvae observations with backtracking a larvae drift model from potential spawning areas. We fitted a generalized additive model (GAM) to assess the effects of environmental conditions on these drift-corrected spawning longitudes. Capelin's mean yearly spawning location varied substantially across the study period. An increase in immature cod biomass, a major predator of capelin, during the spawning migration period in winter and early spring corresponds to more eastward capelin spawning. The eastward shift in mean spawning longitude seems to reach an eastern limit at high immature cod biomass. Sea temperature, sea ice cover and capelin fisheries were not associated with mean capelin spawning longitude. We show that the realized spawning areas for capelin are related to cod biomass, potentially through cod predation depleting and/or acting as a migration barrier for mature capelin.

• Ferreira, Ana Sofia de Araújo; Langangen, Øystein Ole Gahr; Yaragina, Nathalia A.; Prokopchuk, Irina P. & Durant, Joël (2024). How the spatio-temporal overlap of cod, haddock, and capelin larvae affects their recruitment in the Norwegian–Barents Sea system. Marine Ecology Progress Series. ISSN 0171-8630. 734, s. 79–90. doi: 10.3354/meps14564. Fulltekst i vitenarkiv Vis sammendrag

  The Norwegian-Barents Sea (NBS) system is very productive; however, the extent to which this productivity varies remains uncertain for many species. Cod Gadus morhua , haddock Melanogrammus aeglefinus , and capelin Mallotus villosus are 3 key species that overlap in a large portion of the NBS ecosystem. In this hypothesis-driven study, we investigated the interaction of these 3 species during their pelagic larval stage and assessed the impact of their spatio-temporal overlap on survival. We hypothesised that the spatio-temporal overlap between the larvae of the 3 species, as they likely drift together, influences their survival at later stages. We calculated the spatio-temporal overlap of each pair of the 3 studied species and tested it against their recruitment. We utilised a multiple linear regression model with temperature, spawning stock biomass, and spatio-temporal overlap among larvae, their food, and competitors as explanatory variables; this model explained ∼43, ∼24, and ∼73% of the variability in cod, haddock, and capelin recruitment, respectively. The overlaps between larval cod and competing species and/or food have a negative influence on cod recruitment, whereas the sign of the relationships pertaining to haddock and capelin varies. These results improve our understanding of how these fish species use their critical habitats to face emerging environmental stressors.

• Jourdain, Eve Marie; Karoliussen, Richard; Martin, Sarah Louise; Langangen, Øystein Ole Gahr; Robeck, Todd & Borgå, Katrine [Vis alle 8 forfattere av denne artikkelen] (2024). Social and genetic connectivity despite ecological variation in a killer whale network. Proceedings of the Royal Society of London. Biological Sciences. ISSN 0962-8452. 291(2021). doi: 10.1098/rspb.2024.0524. Fulltekst i vitenarkiv Vis sammendrag

  Philopatric kin-based societies encourage a narrow breadth of conservative behaviours owing to individuals primarily learning from close kin, promoting behavioural homogeneity. However, weaker social ties beyond kin, and across a behaviourally diverse social landscape, could be sufficient to induce variation and a greater ecological niche breadth. We investigated a network of 457 photo-identified killer whales from Norway (548 encounters in 2008–2021) with diet data available (46 mixed-diet individuals feeding on both fish and mammals, and 411 exclusive fish-eaters) to quantify patterns of association within and between diet groups, and to identify underlying correlates. We genotyped a subset of 106 whales to assess patterns of genetic differentiation. Our results suggested kinship as main driver of social bonds within and among cohesive social units, while diet was most likely a consequence reflective of cultural diffusion, rather than a driver. Flexible associations within and between ecologically diverse social units led to a highly connected network, reducing social and genetic differentiation between diet groups. Our study points to a role of social connectivity, in combination with individual behavioural variation, in influencing population ecology in killer whales.

• Durant, Joël; Holt, Rebecca Emma & Langangen, Øystein Ole Gahr (2024). Large biomass reduction effect on the relative role of climate, fishing, and recruitment on fish population dynamics. Scientific Reports. 14(1). doi: 10.1038/s41598-024-59569-4. Fulltekst i vitenarkiv Vis sammendrag

  Many species around the world have collapsed, yet only some have recovered. A key question is what happens to populations post collapse. Traditionally, marine fish collapses are linked to overfishing, poor climate, and recruitment. We test whether the effect on biomass change from these drivers remains the same after a collapse. We used a regression model to analyse the effect of harvesting, recruitment, and climate variability on biomass change before and after a collapse across 54 marine fish populations around the world. The most salient result was the change in fishing effect that became weaker after a collapse. The change in sea temperature and recruitment effects were more variable across systems. The strongest changes were in the pelagic habitats. The resultant change in the sensitivity to external drivers indicates that whilst biomass may be rebuilt, the responses to variables known to affect stocks may have changed after a collapse. Our results show that a general model applied to many stocks provides useful insights, but that not all stocks respond similarly to a collapse calling for stock-specific models. Stocks respond to environmental drivers differently after a collapse, so caution is needed when using pre-collapse knowledge to advise on population dynamics and management.

• Abe, Keita; Diekert, Florian Klaus; Melsom, Arne & Langangen, Øystein Ole Gahr (2024). Do fishers follow fish displaced by climate warming? npj Ocean Sustainability. 3(1), s. 1–7. doi: 10.1038/s44183-024-00063-9. Fulltekst i vitenarkiv Vis sammendrag

  Climate change is associated with altered oceanographic conditions that tend to shift the geographical distributions of fish. To assess the impact of climate change on fisheries, one must go beyond projections of catch potential and understand how fishers respond to moving target species. Many previous studies have explicitly or implicitly assumed that fishers follow fish that are displaced by climate warming. Here, we evaluate this assumption by analyzing a long-term, large-scale yet high-resolution dataset combined with a detailed oceanographic model. Our study case is the Atlantic cod (Gadhus morhua) fishery in Norway, one of the largest whitefish fisheries in the world, with little technological or judicial constraints on the potential spatial response of fishers. An oceanographic model is used to predict the areas that have been suitable for Atlantic cod spawning over the two last decades. We compare whether these areas overlap with actually observed fishing locations. While the areas that are suitable for spawning clearly predict how much fish are caught per trip, the suitability of an area does not predict how many vessels fish in a given area at a given point in time. In contrast, the number of vessels in the previous week and the previous year explain the current number of vessels in that area. Hence, future projections of climate change effects should account for the rich and nuanced behavioral responses of humans to project climate change effects on fisheries.

• Johannesen, Edda; Frøysa, Håvard Guldbrandsen; Langangen, Øystein Ole Gahr & Vikebø, Frode Bendiksen (2024). Northeast Arctic haddock (Melanogrammus aeglefinus) spawning grounds and drift to nursery areas in the Barents Sea. Fisheries Oceanography. ISSN 1054-6006. 33(6). doi: 10.1111/fog.12694. Fulltekst i vitenarkiv Vis sammendrag

  The Northeast Arctic stock of haddock (Melanogrammus aeglefinus) is the most northerly stock of the species. It is one of the largest haddock stocks but not the most studied. Similar to the more extensively studied sympatric cod (Gadus morhua) and herring (Clupea harengus) stocks, it relies on Atlantic currents for egg and larvae transportation from spawning grounds to nursery areas in the Barents Sea. The exact locations of the spawning grounds have not been known and the maps that have been published differ substantially. Here, we present results from the first survey dedicated to exploring the spawning habitat of Northeast Arctic (NEA) haddock, and a particle drift and larvae growth model with physical forcing from a high-resolution ocean model. Gadoid eggs, later identified as haddock by DNA analyses, as well as spawning individuals were sampled at the survey. Spawning haddock was found in Atlantic water in temperatures 3.6–6.2°C and depths of 240–560 m. Sampled eggs were mostly in early development stages, suggesting that they were largely spawned in the surveyed area. Using the drift model, we found that most of the modelled particles released as eggs at our surveyed area ended up in the nursery area in the Barents Sea. A crude estimate of the spawning stock biomass based on the survey suggests that the most updated spawning ground map of NEA haddock might be too restricted in its extent. Particle drift and growth with physical forcing for the years 2012–2022 indicated small interannual variation in aberrant drift, and that growth driven by ambient temperature could be important in explaining interannual variation in haddock recruitment.

• Dupont, Nicolas; Durant, Joël; Langangen, Øystein Ole Gahr & Stige, Leif Christian (2024). Changes in prey-predator interactions in an Arctic food web under climate change. Progress in Oceanography. ISSN 0079-6611. 229, s. 1–16. doi: 10.1016/j.pocean.2024.103380. Fulltekst i vitenarkiv Vis sammendrag

  Global warming affects marine ecosystems by changing environmental conditions, ecosystem structure, and ecosystem functioning. In parts of the Arctic, increased sea temperature and decreased sea ice have led to a poleward expansion of boreal species and increased their interactions with native Arctic species. To investigate and quantify the changing interactions in an Arctic marine food web under new environmental conditions, we studied the interactions between key prey fish species in the seasonally ice-covered parts of the Barents Sea: adult polar cod (Boreogadus saida) and capelin (Mallotus villosus) and one of the major predators in the system: Atlantic cod (Gadus morhua). For this, we compared the predictive performance of threshold models predicting the abundance of adult polar cod as a function of Atlantic cod. Each model was associated with a hypothesis describing prey-predator interactions in different environmental conditions defined by threshold values of summer sea-ice or capelin stock biomass. The best predictive model showed that the predation effect of Atlantic cod on polar cod was strongest in years of low summer sea ice cover and low capelin stock biomass. Our results exemplified that Arctic species such as polar cod may experience increased predation pressure under climate change from boreal species such as Atlantic cod. These effects depend, however, not only on changes in abiotic drivers of species distributions, but also on food-web interactions involving mid-trophic level species such as capelin.

• Langangen, Øystein Ole Gahr & Durant, Joël (2024). Persistence of fish populations to longer, more intense, and more frequent mass mortality events. Global Change Biology. ISSN 1354-1013. 30(3), s. 1–10. doi: 10.1111/gcb.17251. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Over the last decades, mass mortality events have become increasingly common across taxa with sometimes devastating effects on population biomass. In the aquatic environment, fish are sensitive to mass mortality events, particularly at the early life stages that are crucial for population dynamics. However, it has recently been shown for fish, that a single mass mortality event in early life typically does not lead to population collapse. Moreover, the frequency and intensity of extreme events that can cause mass mortality, such as marine heatwaves, are increasing. Here, we show that increasing frequency and intensity of mass mortality events may lead to population collapse. Since the drivers of mass mortality events are diverse, and often linked to climate change, it is challenging to predict the frequency and severity of future mass mortality events. As an alternative, we quantify the probability of population collapse depending on the frequency and intensity as well as the duration of mass mortality events. Based on 39 fish species, we show that the probability of collapse typically increases with increasing frequency, intensity, and duration of the mortality events. In addition, we show that the collapse depends on key traits such as natural mortality, recruitment variation, and density dependence. The presented framework provides quantitative estimates of the sensitivity of fish species to these increasingly common extreme events, which paves the way for potential mitigation actions to alleviate adverse impacts on harvested fish populations across the globe.

• Durant, Joël; Holt, Rebecca Emma; Ono, Kotaro & Langangen, Øystein Ole Gahr (2023). Predatory walls may impair climate warming-associated population expansion. Ecology. ISSN 0012-9658. 104(9), s. 1–8. doi: 10.1002/ecy.4130. Fulltekst i vitenarkiv Vis sammendrag

  Climate change has a profound impact on species distribution and abundanceglobally, as well as local diversity, which affects ecosystem functioning. In par-ticular, changes in population distribution and abundance may lead tochanges in trophic interactions. Although species can often shift their spatialdistribution when suitable habitats are available, it has been suggested thatpredator presence can be a constraint on climate-related distribution shifts.We test this using two well-studied and data-rich marine environments.Focusing on a pair of sympatric fishes, Atlantic haddock Melanogrammus aeglefinus and cod Gadus morhua, we study the effect of the presence andabundance of the latter on the former distribution. We found that the distribu-tion of cod and increased abundance may limit the expansion of haddock tonew areas and could consequently buffer ecosystem changes due to climatechange. Though marine species may track the rate and direction of climateshifts, our results demonstrate that the presence of predators may limit theirexpansion to thermally suitable habitats. By integrating climatic and ecologicaldata at scales that can resolve predator–prey relationships, this analysis dem-onstrates the usefulness of considering trophic interactions to gain a morecomprehensive understanding and to mitigate the effects of climate change onspecies distributions.

• Buttay, Lucie; Ohlberger, Jan Philipp & Langangen, Øystein Ole Gahr (2023). Management strategies can buffer the effect of mass mortality in early life stages of fish. Journal of Applied Ecology. ISSN 0021-8901. 60(10), s. 2056–2065. doi: 10.1111/1365-2664.14489. Fulltekst i vitenarkiv Vis sammendrag

  1. Mass mortality (MM) events affecting early life stages of fish can have strong and long-term consequences for population abundance and demography as well as the economic activity supported by exploited stocks. Adaptive fishery manage-ment may help mitigate economic impacts and ensure sustainable resource use following a MM event. 2. Using a state-space life cycle model, we simulated ‘what-if’ scenarios of MM on Northeast Arctic cod (Gadus morhua) eggs and larvae. We compared the expected catches, total stock biomass (TSB), and interannual variability in catches over a period of 10 years after the simulated disturbance. We further evaluated a range of management mitigation strategies, namely reductions in fishing mortality of varying duration (1–10 years) and intensity (no fishing reduction to full ban). 3. A large range of reductions in fishing led to an increase in expected catches over 10 years compared to no reductions, especially when applied immediately after the perturbation and when the cod population was characterized by a high mean age and high TSB. Severe fishing reductions can increase catches substantially but are associated with high interannual variability. Fishing reductions of moder-ate intensity applied between 1 and 4 years would allow to increase catches with only a slight increase in interannual variability. 4. Our findings demonstrate the potential benefits of an adaptive approach to fish-eries management and highlight that mitigation actions may ensure the sustain-able exploitation of fish stocks in the wake of unexpected disturbances. 5. Synthesis and application. Mass mortality events during early life stages of fish can potentially have substantial and long-term effects on the population. Mitigation is more efficient when the affected population has a diverse age structure and when the mitigation strategy is applied immediately after the perturbation. Severe re-duction in fishing mortality is an efficient measure to increase the expected aver-age catch but is associated with high interannual variability. Fishing reduction of moderate intensity applied during 1–4 years after the event also increases the expected average catch with only slightly higher interannual variability in catches.

• Skjæraasen, Jon Egil; Karlsen, Ørjan; Langangen, Øystein Ole Gahr; Meier, Sonnich; Dunlop, Katherine Mary & Meeren, Terje van der [Vis alle 14 forfattere av denne artikkelen] (2022). Attraction of cod Gadus morhua from coastal spawning grounds to salmon farms. Aquaculture Environment Interactions. ISSN 1869-215X. 14, s. 229–242. doi: 10.3354/aei00440. Fulltekst i vitenarkiv Vis sammendrag

  Wild fish aggregate at aquaculture net-pens, but the underlying mechanisms are not fully understood. This study examined how salmon farms attract coastal Atlantic cod Gadus morhua from their inshore spawning grounds. Acoustic receivers were deployed at 5 known cod spawning grounds and 6 salmon Salmo salar farms located at varying distances from these grounds in a mid-Norway study site. Cod were caught at each spawning ground annually from 2017-2019, fitted with acoustic transmitters and released (n = 535). A total of 289 tagged cod (54%) were detected at the salmon farms, with more cod detected at farms closest to the focal spawning grounds and at operational farms. The latter result is likely linked to the availability of feeding opportunities at farm locations. Those cod that were detected by the receivers spent less time at farms farther from their release locations. For the farm-associated cod, 70% were detected for <1 wk at the farms. However, 48 cod spent >1 mo close to the farms, with 1 individual staying 720 d underneath the farm. A total of 135 cod visited 2 or more farms, with farms in proximity more connected in terms of inter-farm movement. Some of the cod utilizing these local spawning grounds likely have considerable dietary input from salmon feed.

• Dupont, Nicolas; Vikebø, Frode Bendiksen & Langangen, Øystein Ole Gahr (2022). Assessing the patchiness of early life stage of a fish stock (Gadus morhua) and its contribution to the stock recruitment. Frontiers in Marine Science. 9, s. 1–10. doi: 10.3389/fmars.2022.932169. Fulltekst i vitenarkiv Vis sammendrag

  Patchiness, defined as spatial heterogeneity in distribution of organisms, is a common phenomenon in zooplankton including ichtyoplankton. In heterogeneous landscapes, depending on the scale of prey and predatory distributions, individuals in patches may experience distinct differences in the survival rate compared to individuals distributed more homogeneously outside patches. In this study, we focused on drifting eggs and larvae of Northeast Arctic (NEA) cod, one of the largest exploited fish stock in the world. The eggs and larvae are largely distributed along the north-western coast of Norway and northern Russia. We ask to what degree individuals are located in patches contribute to the species recruitment. For this purpose, we developed a patch recognition method to detect the existence of patches in particle tracking simulations using a connected-component labeling algorithm. We then assessed the contribution of individuals in detected patches to the total recruitment. Our results showed that depending on year, day of year, and resolution scale for detection of patches, recruits present in patches can vary between 0.6% and 38.7% with an average of 20.4% of total recruitment. The percentage decreased with increasing day of year in the drifting season but increased with decreasing patch resolution scale, down to the finest investigated scale of 8 km. On the basis of these results, we advise field recruitment studies of NEA cod to at least resolve an 8-km spatial scale to capture effects of spatial heterogeneity in the survival rate on the species recruitment.

• Christiansen, Svenja; Langangen, Øystein Ole Gahr; Titelman, Josefin; Vøllestad, Leif Asbjørn & Kaartvedt, Stein (2022). Three-dimensional swimming behavior and activity of a mesopelagic fish. Limnology and Oceanography. ISSN 0024-3590. 67, s. 2677–2690. doi: 10.1002/lno.12230. Fulltekst i vitenarkiv Vis sammendrag

  Knowledge of the three-dimensional behavior and activity patterns of mesopelagic fishes is crucial for understanding their role in ecological and biogeochemical processes in the ocean. We here show that the use of submerged stationary echosounders enables detailed analysis of three-dimensional swimming behavior of small mesopelagic fishes in situ. We explore a 4-month continuous dataset containing more than 100,000 nocturnal tracks of juvenile Maurolicus muelleri, obtained from a surface-facing, moored echosounder in a Norwegian fjord. We extend previous observations of vertical nocturnal swimming and step-wise (stop-and-go) dusk and dawn migrations with the corresponding horizontal and turning behavior. In the middle of the night, the M. muelleri mainly drifted with the currents. Horizontal swimming speeds were then usually below 0.5 body length s−1 but increased substantially during the step-wise dusk and dawn vertical migration. Most vertical relocations were followed by a change in horizontal direction. Our observations are in line with hypotheses of growth maximization in the juvenile fish, and of reducing detection and encounters with predators.

• Yaragina, Natalia A.; Stige, Leif Christian & Langangen, Øystein Ole Gahr (2022). Bycatch data from ichthyoplankton surveys reveal long-term trends in gelatinous zooplankton in the Norwegian and Barents Seas. ICES Journal of Marine Science. ISSN 1054-3139. 79(3), s. 868–881. doi: 10.1093/icesjms/fsab225. Fulltekst i vitenarkiv
• Ohlberger, Jan; Langangen, Øystein Ole Gahr & Stige, Leif Christian (2022). Age structure affects population productivity in an exploited fish species. Ecological Applications. ISSN 1051-0761. 32(5). doi: 10.1002/eap.2614. Fulltekst i vitenarkiv Vis sammendrag

  Long-term changes in the age and size structure of animal populations are well documented, yet their impacts on population productivity are poorly understood. Fishery exploitation can be a major driver of changes in population age–size structure because fisheries significantly increase mortality and often selectively remove larger and older fish. Climate change is another potential driver of shifts in the demographic structure of fish populations. Northeast Arctic (NEA) cod is the largest population of Atlantic cod (Gadus morhua) and one of the world's most important commercial fish stocks. This population has experienced considerable changes in population age–size structure over the past century, largely in response to fishing. In this study, we investigate whether changes in spawner age structure have affected population productivity in NEA cod, measured as recruits per spawning stock biomass, over the past 75 years. We find evidence that shifts in age structure toward younger spawners negatively affect population productivity, implying higher recruitment success when the spawning stock is composed of older individuals. The positive effect of an older spawning stock is likely linked to maternal effects and higher reproductive output of larger females. Our results indicate a threefold difference in productivity between the youngest and oldest spawning stock that has been observed since the 1950s. Further, our results suggest a positive effect of environmental temperature and a negative effect of intraspecific cannibalism by older juveniles on population productivity, which partly masked the effect of spawner age structure unless accounted for in the model. Collectively, these findings emphasize the importance of population age structure for the productivity of fish populations and suggest that harvest-induced demographic changes can have negative feedbacks for fisheries that lead to a younger spawning stock. Incorporating demographic data into harvest strategies could thus facilitate sustainable fishery management.

• Durant, Joël; Ono, Kotaro & Langangen, Øystein Ole Gahr (2022). Empirical evidence of nonlinearity in bottom up effect in a marine predator-prey system. Biology Letters. ISSN 1744-9561. 18(11). doi: 10.1098/rsbl.2022.0309. Fulltekst i vitenarkiv Vis sammendrag

  Strength of species interaction may have profound effects on population dynamics. Empirical estimates of interaction strength is often based on the assumption that the interaction strengths are constant. Barents Sea cod and capelin are two fish populations for which such interaction has been acknowledged and used, under the assumption of constant interaction strength, when studying their population dynamics. However, species interaction can often be non-linear in marine ecosystems and might profoundly change our understanding of food chains. Analysing 37 years long survey time series in the Arcto-Boreal Barents Sea with a state-space modelling framework, we demonstrate that the effect of capelin on cod is not linear but shifts depending on capelin abundance: while capelin is beneficial for cod population at high abundance, below the threshold, it becomes less important for cod. Our analysis therefore shows the importance of investigating non-linearity in species interaction and may contribute to an improved understanding on species assemblages.

• Planque, Benjamin; Aarflot, Johanna Myrseth; Buttay, Lucie; Carroll, JoLynn; Fransner, Sara Filippa Krusmynta & Hansen, Cecilie [Vis alle 17 forfattere av denne artikkelen] (2022). A standard protocol for describing the evaluation of ecological models. Ecological Modelling. ISSN 0304-3800. 471. doi: 10.1016/j.ecolmodel.2022.110059. Fulltekst i vitenarkiv Vis sammendrag

  Numerical models of ecological systems are increasingly used to address complex environmental and resource management questions. One challenge for scientists, managers, and stakeholders is to appraise how well suited these models are to answer questions of scientific or societal relevance, that is, to perform, communicate, or access transparent evaluations of ecological models. While there have been substantial developments to support standardised descriptions of ecological models, less has been done to standardise and to report model evaluation practices. We present here a general protocol designed to guide the reporting of model evaluation. The protocol is organised in three major parts: the objective(s) of the modelling application, the ecological patterns o relevance and the evaluation methodology proper, and is termed the OPE (objectives, patterns, evaluation) protocol. We present the 25 questions of the OPE protocol which address the many aspects of the evaluation process and then apply them to six case studies based on a diversity of ecological models. In addition to standardising and increasing the transparency of the model evaluation process, we find that going through the OPE protocol helps modelers to think more deeply about the evaluation of their models. From this last point, we suggest that it would be highly beneficial for modelers to consider the OPE early in the modelling process, in addition to using it as a reporting tool and as a reviewing tool. Standardisation Best practice Ecological patterns Skill assessment Transparency

• Langangen, Øystein Ole Gahr; Ohlberger, Jan Philipp; Stige, Leif Christian; Patin, Rémi; Buttay, Lucie & Stenseth, Nils Christian [Vis alle 8 forfattere av denne artikkelen] (2022). Effects of early life mass mortality events on fish populations. Fish and Fisheries. ISSN 1467-2960. 24(1), s. 176–186. doi: 10.1111/faf.12718. Fulltekst i vitenarkiv Vis sammendrag

  Mass mortality events are ubiquitous in nature and can be caused by, for example, diseases, extreme weather and human perturbations such as contamination. Despite being prevalent and rising globally, how mass mortality in early life causes population-level effects such as reduced total population biomass, is not fully explored. In particular for fish, mass mortality affecting early life may be dampened by compensatory density-dependent processes. However, due to large variations in year-class strength, potentially caused by density-independent variability in survival, the impact at the population level may be high in certain years. We quantify population-level impacts at two levels of mass mortality (50% and 99% additional mortality) during early life across 40 fish species using age-structured population dynamics models. The findings from these species-specific models are further supported by an analysis of detailed stock-specific models for three of the species. We find that population impacts are highly variable between years and species. Short-lived species that exhibit a low degree of compensatory density dependence and high interannual variation in survival experience the strongest impacts at the population level. These quantitative and general relationships allow predicting the range of potential impacts of mass mortality events on species based on their life history. This is critical considering that the frequency and severity of mass mortality events are increasing worldwide.

• Nater, Chloe Rebecca; Stubberud, Marlene Wæge; Langangen, Øystein Ole Gahr; Rustadbakken, Atle; Moe, S. Jannicke & Ergon, Torbjørn [Vis alle 8 forfattere av denne artikkelen] (2022). Towards a future without stocking: harvest and river regulation determine long-term population viability of migratory salmonids. Climate Research (CR). ISSN 0936-577X. 86, s. 37–52. doi: 10.3354/cr01644. Fulltekst i vitenarkiv Vis sammendrag

  Freshwater species are particularly vulnerable to emerging threats linked to climate change because they are often already heavily impacted by habitat destruction, pollution, and exploitation. For many harvested populations of freshwater fish, these combined impacts have been mitigated for decades through stocking with captive-bred individuals. However, stocking may lead to loss of genetic variation, which may be crucial for adaptation under climate change. Exploration of sustainable alternatives is therefore paramount. We used a female-based integral projection model (IPM) to assess the consequences of terminating a long-term stocking programme for a population of landlocked, migratory brown trout Salmo trutta, and to evaluate relative effectiveness of alternative management strategies involving harvest regulations and river habitat improvement. The IPM classified individuals by body size, life history stage, and location relative to a hydropower dam, and was parameterised with 50 yr of individual-based data, supplemented with literature values. Model simulations indicated a strong population decline of 22-29% per year without stocking, much of which was attributed to high harvest mortality. Consequently, drastic reductions in fishing pressure were predicted to be necessary to ensure population viability without stocking. Mitigation measures reducing mortality associated with the hydropower dam or restoring spawning areas could further contribute to population viability when combined with changes in harvest regulations. Our results thus emphasise that large changes in management strategies, such as termination of long-term stocking programmes, require a thorough assessment of potential consequences and alternative mitigation strategies using data and models, or, at the very least, a precautionary approach under consideration of on-going climate change. Migratory salmonid · Salmo trutta · Integral projection model · Harvest · Fishing · Stocking · Dam · Hydropower · Trout

• Nilsen, Ina; Hansen, Cecilie; Kaplan, Isaac; Holmes, Elizabeth & Langangen, Øystein Ole Gahr (2022). Exploring the role of Northeast Atlantic cod in the Barents Sea food web using a multi-model approach. Fish and Fisheries. ISSN 1467-2960. 23(5), s. 1083–1098. doi: 10.1111/faf.12671. Fulltekst i vitenarkiv Vis sammendrag

  It is commonly accepted that no ecosystem model is the ‘best’, but rather that ecosystem models should be used in ensembles. This is also the case for the Barents Sea ecosystem, where we have used two different ecosystem models to explore the role of the top-predator Northeast Arctic (NEA) stock of Atlantic cod (Gadus morhua, Gadidae) in the food web. The two models differ in complexity; Gompertz being less complex in terms of food web (7 components) and processes compared to the complex Nordic and Barents Seas Atlantis model (53 components). On the other hand, Gompertz provides thousands of stochastic realizations for each scenario, whereas Atlantis provides only one deterministic simulation. To compare the response to changes in NEA cod on two key prey species, capelin (Mallotus villosus, Osmeridae) and polar cod (Boreogadus saida, Gadidae), we perturbed the historical fishing pressure by ±50% and used the same NEA cod biomass in both models. Even though the links between NEA cod and the prey species are similar in the two models, the results from the study reveal that indirect effects through other food-web components might be as important as direct predator–prey interactions. Differences in spatial structure and overlap between species also influence the species response to the perturbations. In this study, we focus on the mechanisms that drives the changes in the models, and advise on potential consequences for fisheries management. The two models can complement each other, and the differences between them point to areas where more knowledge is needed.

• Durant, Joël; Aarvold, Leana & Langangen, Øystein Ole Gahr (2021). Stock collapse and its effect on species interactions: Cod and herring in the Norwegian-Barents Seas system as an example. Ecology and Evolution. 11, s. 16993–17004. doi: 10.1002/ece3.8336. Fulltekst i vitenarkiv Vis sammendrag

  Both the Norwegian Spring Spawning herring (Clupea harengus) and the Northeast Arctic (NEA) cod (Gadus morhua) are examples of strong stock reduction and decline of the associated fisheries due to overfishing followed by a recovery. Cod and herring are both part of the Barents Sea ecosystem, which has experienced major warming events in the early (1920–1940) and late 20th century. While the collapse or near collapse of these stocks seems to be linked to an instability created by overfishing and climate, the difference of population dynamics before and after is not fully understood. In particular, it is unclear how the changes in population dynamics before and after the collapses are associated with biotic interactions. The combination of the availability of unique long-term time series for herring and cod makes it a well-suited study system to investigate the effects of collapse. We examine how species interactions may differently affect the herring and cod population dynamic before and after a collapse. Particularly we explore, using a GAM modeling approach, how herring could affect cod and vice versa. We found that the effect of cod biomass on herring that was generally positive (i.e., covariation) became negative after the collapse (i.e., predation or competition). Likewise a change occurred for the cod, the juvenile herring biomass that had no effect before the collapse had a negative effect after. Our results indicate that the population collapses may alter the inter-specific interactions and response to abiotic environmental changes. While the stocks are at similar abundance levels before and after the collapses, the system is potentially different in its functioning and may require different management action.

• Skjæraasen, Jon Egil; Karlsen, Ørjan; Langangen, Øystein Ole Gahr; Meeren, Terje van der; Keeley, Nigel B. & Myksvoll, Mari Skuggedal [Vis alle 12 forfattere av denne artikkelen] (2021). Impact of salmon farming on Atlantic cod spatio-temporal reproductive dynamics. Aquaculture Environment Interactions. ISSN 1869-215X. 13, s. 399–412. doi: 10.3354/AEI00415. Fulltekst i vitenarkiv Vis sammendrag

  Salmon farming in marine net pens is a major activity in many temperate regions. This industry may affect coastal ecosystems in several ways, such as with waste pollution and parasite spillover. Less is known about the extent to which salmon farming disrupts the use of inshore spawning grounds by wild fish, such as the Atlantic cod Gadus morhua . Acoustic telemetry was therefore used to explore cod space use during the spawning season in a coastal region in mid-Norway with multiple salmon farms. Acoustic receivers were placed in clusters at 5 known cod spawning grounds and 6 nearby salmon farms. Data from 481 adult cod caught at the spawning grounds during 2017-2019 and equipped with acoustic telemetry transmitters were analysed. Overall, fewer fish were detected at farms than spawning grounds, even when accounting for distance from release point. Individual cod residency (days detected / duration of spawning period) was generally higher at the spawning grounds close to farms but low at the farms themselves, with little apparent spawning at the farm localities. In contrast, spawning was clearly occurring at the nearby spawning grounds, with cod spending weeks (n = 316) or months (n = 158) there during the spawning period. Males had longer residence times at spawning grounds than females, likely linked to the cod mating system. Overall, we found little support for the assertion that salmon farms disrupt inshore spawning dynamics of cod using nearby spawning grounds presently, either by attracting spawners to farms or causing fish to leave these grounds.

• Dupont, Nicolas; Durant, Joël; Gjøsæter, Harald; Langangen, Øystein Ole Gahr & Stige, Leif Christian (2021). Effects of sea ice cover, temperature and predation on the stock dynamics of the key Arctic fish species polar cod Boreogadus saida. Marine Ecology Progress Series. ISSN 0171-8630. 677, s. 141–159. doi: 10.3354/meps13878. Fulltekst i vitenarkiv Vis sammendrag

  Climate change has large effects on population dynamics of fish species in high latitude ecosystems. Arctic fish stocks experience multiple pressures with changing abiotic living conditions and increased competition and predation from boreal species. However, there are many unknowns regarding how environmental change influences the dynamics of those populations. Here, we focused on the Barents Sea polar cod, a pan-Arctic zooplanktivorous key fish species physiologically and ecologically adapted to the presence of sea ice. We developed an age-resolved Bayesian state-space model of the dynamics of polar cod based on 30 yr of survey data (1986-2015). Using this model, we quantified how inter-annual changes in abundance were associated with abiotic variables (temperature and sea ice cover) and biotic variables (prey biomasses and a predation index). Using the model output, we used a hindcast scenario approach to investigate to which degree the observed variations in total population size were related to the abiotic or biotic variables. Our results showed that variation in abundance of young polar cod (ages 0 and 1) was best explained by abiotic variables while variation in the older age groups (ages 3 and 4) was best explained by predation. Hindcast scenarios showed that the abiotic variables had a more evident effect than predation on population dynamics, but none of the variables we considered could explain the drastic population decline observed in recent years. Our work shows the advantages of studying age-specific responses as a stepping stone to understand changes at the population level.

• Stubberud, Marlene Wæge; Nater, Chloe Rebecca; Vindenes, Yngvild; Vøllestad, Leif Asbjørn & Langangen, Øystein Ole Gahr (2021). Low impact of first-time spawners on population growth in a brown trout population. Climate Research (CR). ISSN 0936-577X. 86, s. 65–78. doi: 10.3354/cr01645. Fulltekst i vitenarkiv Vis sammendrag

  For species with individual variation in reproductive success, experience in breeding and the distribution of different breeders is important for population productivity and viability. Human impacts, such as climate change and harvesting, can alter this distribution and thus population dynamics. Here, we investigated the effect of spawning experience on population growth in a population of migratory brown trout Salmo trutta subject to stressors including migration barriers, harvesting, and climate change. We described the population dynamics with a structured integral projection model that differentiates between first-time and repeat spawners. We then took a scenario-based approach to test to which extent spawning experience has a positive effect on the population growth of brown trout by running 3 different model simulations: a baseline scenario with no changes to the reproductive output of the population, a non-selective scenario in which the reproductive output of all spawners was reduced, and a selective scenario where the reproductive output of only first-time spawners was reduced. We found that the reproductive output of repeat spawners is more important than that of first-time spawners for population growth, in line with other studies. Moreover, the contribution of first-time spawners to the population growth through their own survival is more important than their contribution to growth through reproduction. To ensure the continued existence of the study population, survival of first-time spawners and reproductive success of repeat spawners should be prioritised. More generally, including breeding experience adds more mechanistic detail, which ultimately can aid management and conservation efforts. Integral projection model · Iteroparity · Spawning experience · Management · Selective harvesting · Population dynamics · Brown trout · Salmo trutta · Dam

• Melsom, Arne; Kvile, Kristina Øie; Dagestad, Knut Frode; Brostrøm, Gøran & Langangen, Øystein (2021). Exploring drift simulations from ocean circulation experiments: Application to cod eggs and larval drift. Climate Research (CR). ISSN 0936-577X. 86, s. 145–162. doi: 10.3354/cr01652. Fulltekst i vitenarkiv Vis sammendrag

  Drift models are commonly used to study the transport of early life stages of fish and other marine organisms. Various approaches may be applied to examine the distribution and variability of ocean trajectory pathways. In the present study, we compare results using passive Eulerian tracers and Lagrangian float trajectories that are embedded in numerical models. We supplement this analysis by applying an offline model for drift computations. The contrasts in the results from the various configurations are mainly due to differences in drift depth. Simulations were performed using horizontal resolutions of 4 and 0.8 km. The higher-resolution experiment gives somewhat more realistic results for the drift time from Lofoten to the Tromsøflaket bank at the southwestern entrance of the Barents Sea. Furthermore, differences in results between simulation years are much larger than the differences that arise from the choice of model configuration. Climate variability at high latitudes on a multi-decadal time scale is dominated by large interannual variability superimposed on an underlying moderate warming trend. We conclude that a properly configured offline drift model using hourly or 2-hourly results from a simulation with a horizontal resolution of 1 km or finer is the best approach for investigations of trajectory pathways. The flexibility of an offline drift model is also highly advantageous in biological contexts, as it easily allows for a variety of ways in which behavioural characteristics can be parameterized, including descriptions that are defined after the ocean circulation simulation has been executed.

• Vikebø, Frode Bendiksen; Broch, Ole Jacob; Endo, Clarissa Akemi Kajiya; Frøysa, Håvard G; Carroll, JoLynn & Juselius, Jonas [Vis alle 7 forfattere av denne artikkelen] (2021). Northeast Arctic Cod and Prey Match-Mismatch in a High-Latitude Spring-Bloom System. Frontiers in Marine Science. 8, s. 1–13. doi: 10.3389/fmars.2021.767191. Fulltekst i vitenarkiv Vis sammendrag

  By combining an ocean model, a nutrient-phytoplankton-zooplankton-detritus-model and an individual-based model for early life stages of Northeast Arctic cod we systematically investigate food limitations and growth performance for individual cod larvae drifting along the Norwegian coast from spawning grounds toward nursery areas in the Barents Sea. We hypothesize that there is food shortage for larvae spawned early and late in the 2-monthlong spawning period, and to a larger degree to the north and south of the main spawning grounds in the Lofoten. Model results for three contrasting years (1995, 2001, and 2002) show that spawning early in the season at spawning grounds in the Lofoten and farther north is favorable for larval growth close to their size- and temperature-dependent potential. Still, both early and late spawned larvae experience slower growth than individuals originating closer to the time of peak spawning late March/early April. The reasons are low temperatures and shortage in suitable prey, respectively, and this occurs more frequent in areas of strong currents about 1–2 months post hatching. In particular, late spawned larvae grow relatively slow despite higher temperatures later in the season because they are outgrown by their preferred prey.

• Langangen, Øystein & Stige, Leif Christian (2021). Shedding light on the link between the spatial distribution of eggs and survival in Northeast Arctic cod. Fisheries Oceanography. ISSN 1054-6006. 30, s. 429–436. doi: 10.1111/fog.12528. Fulltekst i vitenarkiv Vis sammendrag

  Spreading the offspring in space and time may offer bet-hedging benefits by buffering environmental influences on parts of the offspring distribution. It has previously been shown that high mean age and size of spawners in the Northeast Arctic stock of Atlantic cod (Gadus morhua) is positively associated with high abundance and wide spatiotemporal distribution of eggs. However, little support has been found for a wide egg distribution acting as a buffer against environmental influences by leading to higher mean or less variable recruitment. The lacking relation between egg distribution and recruitment can be due to data limitation or alternatively that wide egg distributions may be of limited relevance for survival in later stages. To investigate these two alternatives, we analysed the output from a mechanistic coupled physical–biological drift model that projected the abundance of juvenile cod on the nursing grounds in the Barents Sea from empirically estimated egg distributions. The projections were performed with initial egg distributions reflecting the expected distributions at high and low mean weight of the spawners. We found that the extended distribution at the fringes only marginally contributed to recruitment, as a large proportion of the extra eggs at high mean age and size of spawners did not reach suitable nursing areas or failed to grow to a suitable size by the end of the season. In conclusion, our results suggest that the buffering effect of a geographically wide distribution of spawning products is insignificant compared to other effects on recruitment.

• Färber, Leonie Anette; Gemert, Rob van; Langangen, Øystein; Durant, Joel Marcel & Andersen, Ken Hasle (2020). Population variability under stressors is dependent on body mass growth and asymptotic body size. Royal Society Open Science. 7(2), s. 1–11. doi: 10.1098/rsos.192011. Fulltekst i vitenarkiv Vis sammendrag

  The recruitment and biomass of a fish stock are influenced by their environmental conditions and anthropogenic pressures such as fishing. The variability in the environment often translates into fluctuations in recruitment, which then propagate throughout the stock biomass. In order to manage fish stocks sustainably, it is necessary to understand their dynamics. Here, we systematically explore the dynamics and sensitivity of fish stock recruitment and biomass to environmental noise. Using an age-structured and trait-based model, we explore random noise (white noise) and autocorrelated noise (red noise) in combination with low to high levels of harvesting. We determine the vital rates of stocks covering a wide range of possible body mass (size) growth rates and asymptotic size parameter combinations. Our study indicates that the variability of stock recruitment and biomass are probably correlated with the stock's asymptotic size and growth rate. We find that fast-growing and large-sized fish stocks are likely to be less vulnerable to disturbances than slow-growing and small-sized fish stocks. We show how the natural variability in fish stocks is amplified by fishing, not just for one stock but for a broad range of fish life histories.

• Dupont, Nicolas; Durant, Joel Marcel; Langangen, Øystein; Gjøsæter, Harald & Stige, Leif Christian (2020). Sea ice, temperature, and prey effects on annual variations in mean lengths of a key Arctic fish, Boreogadus saida, in the Barents Sea. ICES Journal of Marine Science. ISSN 1054-3139. 77(5), s. 1796–1805. doi: 10.1093/icesjms/fsaa040. Fulltekst i vitenarkiv Vis sammendrag

  Oceanographic conditions in the Arctic are changing, with sea ice cover decreasing and sea temperatures increasing. Our understanding of the effects on marine populations in the area is, however, limited. Here, we focus on the Barents Sea stock of polar cod (Boreogadus saida). Polar cod is a key fish species for the transfer of energy from zooplankton to higher trophic levels in the Arctic food web. We analyse the relationships between 30-year data series on the length-at-age of polar cod cohorts (ages 0–4) and sea surface temperature, sea ice concentration, prey biomasses, predator indices, and length-at-age the previous year using multiple linear regression. Results for several ages showed that high length-at-age is significantly associated with low sea ice concentration and high length-at-age the previous year. Only length-at-age for age 1 shows a positive significant relationship with prey biomass. Our results suggest that retreating sea ice has positive effects on the growth of polar cod in the Barents Sea despite previous observations of a stagnating stock biomass and decreasing stock abundance. Our results contribute to identifying mechanisms by which climate variability affects the polar cod population, with implications for our understanding of how future climate change may affect Arctic ecosystems.

• Romagnoni, Giovanni; Kvile, Kristina Øie; Dagestad, Knut Frode; Eikeset, Anne Maria; Kristiansen, Trond & Stenseth, Nils Christian [Vis alle 7 forfattere av denne artikkelen] (2020). Influence of larval transport and temperature on recruitment dynamics of North Sea cod (Gadus morhua) across spatial scales of observation. Fisheries Oceanography. ISSN 1054-6006. 29(4), s. 324–339. doi: 10.1111/fog.12474. Fulltekst i vitenarkiv Vis sammendrag

  The survival of fish eggs and larvae, and therefore recruitment success, can be critically affected by transport in ocean currents. Combining a model of early‐life stage dispersal with statistical stock–recruitment models, we investigated the role of larval transport for recruitment variability across spatial scales for the population complex of North Sea cod (Gadus morhua). By using a coupled physical–biological model, we estimated the egg and larval transport over a 44‐year period. The oceanographic component of the model, capable of capturing the interannual variability of temperature and ocean current patterns, was coupled to the biological component, an individual‐based model (IBM) that simulated the cod eggs and larvae development and mortality. This study proposes a novel method to account for larval transport and success in stock–recruitment models: weighting the spawning stock biomass by retention rate and, in the case of multiple populations, their connectivity. Our method provides an estimate of the stock biomass contributing to recruitment and the effect of larval transport on recruitment variability. Our results indicate an effect, albeit small, in some populations at the local level. Including transport anomaly as an environmental covariate in traditional stock–recruitment models in turn captures recruitment variability at larger scales. Our study aims to quantify the role of larval transport for recruitment across spatial scales, and disentangle the roles of temperature and larval transport on effective connectivity between populations, thus informing about the potential impacts of climate change on the cod population structure in the North Sea.

• Durant, Joel Marcel; Ono, Kotaro; Stenseth, Nils Christian & Langangen, Øystein (2020). Nonlinearity in interspecific interactions in response to climate change: Cod and haddock as an example. Global Change Biology. ISSN 1354-1013. 26(10), s. 5554–5563. doi: 10.1111/gcb.15264. Fulltekst i vitenarkiv Vis sammendrag

  Climate change has profound ecological effects, yet our understanding of how trophic interactions among species are affected by climate change is still patchy. The sympatric Atlantic haddock and cod are co‐occurring across the North Atlantic. They compete for food at younger stages and thereafter the former is preyed by the latter. Climate change might affect the interaction and coexistence of these two species. Particularly, the increase in sea temperature (ST) has been shown to affect distribution, population growth and trophic interactions in marine systems. We used 33‐year long time series of haddock and cod abundances estimates from two data sources (acoustic and trawl survey) to analyse the dynamic effect of climate on the coexistence of these two sympatric species in the Arcto‐Boreal Barents Sea. Using a Bayesian state‐space threshold model, we demonstrated that long‐term climate variation, as expressed by changes of ST, affected species demography through different influences on density‐independent processes. The interaction between cod and haddock has shifted in the last two decades due to an increase in ST, altering the equilibrium abundances and the dynamics of the system. During warm years (ST over ca. 4°C), the increase in the cod abundance negatively affected haddock abundance while it did not during cold years. This change in interactions therefore changed the equilibrium population size with a higher population size during warm years. Our analyses show that long‐term climate change in the Arcto‐Boreal system can generate differences in the equilibrium conditions of species assemblages.

• Nater, Chloe R; Vindenes, Yngvild; Aass, Per; Cole, Diana; Langangen, Øystein & Moe, S. Jannicke [Vis alle 10 forfattere av denne artikkelen] (2020). Size‐ and stage‐dependence in cause‐specific mortality of migratory brown trout. Journal of Animal Ecology. ISSN 0021-8790. 89(9), s. 2122–2133. doi: 10.1111/1365-2656.13269. Fulltekst i vitenarkiv Vis sammendrag

  Evidence‐based management of natural populations under strong human influence frequently requires not only estimates of survival but also knowledge about how much mortality is due to anthropogenic vs. natural causes. This is the case particularly when individuals vary in their vulnerability to different causes of mortality due to traits, life history stages, or locations. Here, we estimated harvest and background (other cause) mortality of landlocked migratory salmonids over half a century. In doing so, we quantified among‐individual variation in vulnerability to cause‐specific mortality resulting from differences in body size and spawning location relative to a hydropower dam. We constructed a multistate mark–recapture model to estimate harvest and background mortality hazard rates as functions of a discrete state (spawning location) and an individual time‐varying covariate (body size). We further accounted for among‐year variation in mortality and migratory behaviour and fit the model to a unique 50‐year time series of mark–recapture–recovery data on brown trout (Salmo trutta) in Norway. Harvest mortality was highest for intermediate‐sized trout, and outweighed background mortality for most of the observed size range. Background mortality decreased with body size for trout spawning above the dam and increased for those spawning below. All vital rates varied substantially over time, but a trend was evident only in estimates of fishers' reporting rate, which decreased from over 50% to less than 10% throughout the study period. We highlight the importance of body size for cause‐specific mortality and demonstrate how this can be estimated using a novel hazard rate parameterization for mark–recapture models. Our approach allows estimating effects of individual traits and environment on cause‐specific mortality without confounding, and provides an intuitive way to estimate temporal patterns within and correlation among different mortality sources.

• Skjæraasen, Jon Egil; Korsbrekke, Knut; Dingsør, Gjert Endre; Langangen, Øystein; Opdal, Anders Frugård & Jørgensen, Christian (2020). Large annual variation in the amount of skipped spawning for female Northeast Arctic haddock Melanogrammus aeglefinus. Fisheries Research. ISSN 0165-7836. 230. doi: 10.1016/j.fishres.2020.105670. Fulltekst i vitenarkiv Vis sammendrag

  Although the phenomenon of skipped spawning has been described in numerous fishes, time-series are scarce. We used the presence of post-ovulatory follicles in histological gonad slides from females not developing oocytes for Northeast Arctic (NEA) haddock Melanogrammus aeglefinus from 2009 to 2012 to construct a length-based statistical model giving the probability that a non - developing female was skipping spawning, as opposed to not being sexually mature. This model was then applied on demographic winter survey data from the Barents Sea from 1989 to 2014. This indicated large annual variation in skipping numbers. Comparing these survey estimates to the total annual ICES stock numbers, we found that skipping peaked in the years 1994–1996 and 2009–2014, when the median yearly estimate of skipped spawners was 20–45 % of all females aged ≥ 3 years. In contrast, only ∼ 3 % of females at age ≥ 3 years skipped spawning in 2007. The proportional representation of skipped spawners at the stock level appeared linked to stock energy reserves with more skipping occurring when energy levels were low. Skipping also became more frequent with increasing population age, i.e. when immatures were less abundant, although the very largest/oldest fish tended to spawn. Because the proportion of NEA haddock that skips spawning is variable and can be high, understanding variation in this phenomenon and its drivers may improve population dynamic models.

• Ono, Kotaro; Langangen, Øystein & Stenseth, Nils Christian (2019). Improving risk assessments in conservation ecology. Nature Communications. 10, s. 1–7. doi: 10.1038/s41467-019-10700-4. Fulltekst i vitenarkiv Vis sammendrag

  Conservation efforts and management decisions on the living environment of our planet often rely on the results from statistical models. Yet, these models are imperfect and quantification of risk associated with the estimate of management-relevant quantities becomes crucial in providing robust advice. Here we demonstrate that estimates of risk themselves could be substantially biased but by combining data fitting with an extensive simulation–estimation procedure, one can back-calculate the correct values. We apply the method to 627 time series of population abundance across four taxa using the Gompertz state-space model as an example. We find that the risk of large bias in population status estimate increases with the species’ growth rate, population variability, weaker density dependence, and shorter time series, across taxa. We urge scientists dealing with conservation and management to adopt a similar approach to ensure a more accurate estimate of risk measures and contribute towards a precautionary approach to management.

• Durant, Joel Marcel; Molinero, Juan Carlos; Ottersen, Geir; Reygondeau, Gabriel; Stige, Leif Christian & Langangen, Øystein (2019). Contrasting effects of rising temperatures on trophic interactions in marine ecosystems. Scientific Reports. 9:15213, s. 1–9. doi: 10.1038/s41598-019-51607-w. Fulltekst i vitenarkiv Vis sammendrag

  In high-latitude marine environments, primary producers and their consumers show seasonal peaks of abundance in response to annual light cycle, water column stability and nutrient availability. Predatory species have adapted to this pattern by synchronising life-history events such as reproduction with prey availability. However, changing temperatures may pose unprecedented challenges by decoupling the predator-prey interactions. Here we build a predator-prey model accounting for the full life-cycle of fish and zooplankton including their phenology. The model assumes that fish production is bottom-up controlled by zooplankton prey abundance and match or mismatch between predator and prey phenology, and is parameterised based on empirical findings of how climate influences phenology and prey abundance. With this model, we project possible climate-warming effects on match-mismatch dynamics in Arcto-boreal and temperate biomes. We find a strong dependence on synchrony with zooplankton prey in the Arcto-boreal fish population, pointing towards a possible pronounced population decline with warming because of frequent desynchronization with its zooplankton prey. In contrast, the temperate fish population appears better able to track changes in prey timing and hence avoid strong population decline. These results underline that climate change may enhance the risks of predator-prey seasonal asynchrony and fish population declines at higher latitudes.

• Stige, Leif Christian; Rogers, Lauren; Neuheimer, Anna B.; Hunsicker, Mary E.; Yaragina, Natalia A. & Ottersen, Geir [Vis alle 9 forfattere av denne artikkelen] (2019). Density‐ and size‐dependent mortality in fish early life stages. Fish and Fisheries. ISSN 1467-2960. 20(5), s. 962–976. doi: 10.1111/faf.12391. Fulltekst i vitenarkiv Vis sammendrag

  The importance of survival and growth variations early in life for population dynamics depends on the degrees of compensatory density dependence and size dependence in survival at later life stages. Quantifying density‐ and size‐dependent mortality at different juvenile stages is therefore important to understand and potentially predict the recruitment to the population. We applied a statistical state‐space modelling approach to analyse time series of abundance and mean body size of larval and juvenile fish. The focus was to identify the importance of abundance and body size for growth and survival through successive larval and juvenile age intervals, and to quantify how the dynamics propagate through the early life to influence recruitment. We thus identified both relevant ages and mechanisms (i.e. density dependence and size dependence in survival and growth) linking recruitment variability to early life dynamics. The analysis was conducted on six economically and ecologically important fish populations from cold temperate and sub‐arctic marine ecosystems. Our results underscore the importance of size for survival early in life. The comparative analysis suggests that size‐dependent mortality and density‐dependent growth frequently occur at a transition from pelagic to demersal habitats, which may be linked to competition for suitable habitat. The generality of this hypothesis warrants testing in future research.

• Stubberud, Marlene Wæge; Vindenes, Yngvild; Vøllestad, Asbjørn; Winfield, Ian J.; Stenseth, Nils Christian & Langangen, Øystein (2019). Effects of size‐ and sex‐selective harvesting: An integral projection model approach. Ecology and Evolution. 9(22), s. 12556–12570. doi: 10.1002/ece3.5719. Fulltekst i vitenarkiv Vis sammendrag

  Harvesting is often size‐selective, and in species with sexual size dimorphism, it may also be sex‐selective. A powerful approach to investigate potential consequences of size‐ and/or sex‐selective harvesting is to simulate it in a demographic population model. We developed a population‐based integral projection model for a size‐ and sex‐structured species, the commonly exploited pike (Esox lucius). The model allows reproductive success to be proportional to body size and potentially limited by both sexes. We ran all harvest simulations with both lower size limits and slot limits, and to quantify the effects of selective harvesting, we calculated sex ratios and the long‐term population growth rate (λ). In addition, we quantified to what degree purely size‐selective harvesting was sex‐selective, and determined when λ shifted from being female to male limited under size‐ and sex‐selective harvesting. We found that purely size‐selective harvest can be sex‐selective, and that it depends on the harvest limits and the size distributions of the sexes. For the size‐ and sex‐selective harvest simulations, λ increased with harvest intensity up to a threshold as females limited reproduction. Beyond this threshold, males became the limiting sex, and λ decreased as more males were harvested. The peak in λ, and the corresponding sex ratio in harvest, varied with both the selectivity and the intensity of the harvest simulation. Our model represents a useful extension of size‐structured population models as it includes both sexes, relaxes the assumption of female dominance, and accounts for size‐dependent fecundity. The consequences of selective harvesting presented here are especially relevant for size‐ and sex‐structured exploited species, such as commercial fisheries. Thus, our model provides a useful contribution toward the development of more sustainable harvesting regimes.

• Ohlberger, Jan Philipp; Langangen, Øystein; Winfield, Ian J & Vindenes, Yngvild (2019). The importance of variation in offspring body size for stability in cannibalistic populations. Oikos. ISSN 0030-1299. 129(1), s. 59–69. doi: 10.1111/oik.05507. Fulltekst i vitenarkiv Vis sammendrag

  Animals exhibit remarkable intraspecific variation in phenotypic traits such as body size. Understanding how such trait variation affects population and ecosystem dynamics is critically important, because future environmental change and human impacts are expected to alter phenotypic trait distributions. In species with seasonal reproduction, offspring size variation within cohorts is ubiquitous, yet we know little about its implications for population stability. In addition, long‐term monitoring data indicate that changes in offspring size variation occur at ecologically relevant time scales. Here, we study the consequences of changing offspring size variation by developing and analysing an integral projection model (IPM). Our model accounts for size‐dependent cannibalism as well as additional density regulation occurring during the first year. The model is parameterized using literature values and long‐term monitoring data for pike Esox lucius, a common fish predator in temperate freshwater ecosystems, but the general model structure applies to a wide range of size‐structured organisms. Our analyses demonstrate that a wide size distribution of offspring promotes stable dynamics, whereas narrow distributions can be destabilizing because cannibalism increases the annual variation in mean offspring mortality. Our results indicate that the stabilizing effect of offspring size variation is likely an important property of size‐structured organisms with seasonal reproduction and cannibalistic behaviour. This work highlights the importance of intracohort trait variation and describes how variation in body size can shape the dynamics of animal populations.

• Färber, Leonie Anette; Durant, Joel Marcel; Vindenes, Yngvild & Langangen, Øystein (2018). Increased early offspring growth can offset the costs of long-distance spawning migration in fish. Marine Ecology Progress Series. ISSN 0171-8630. 600, s. 141–150. doi: 10.3354/meps12662. Fulltekst i vitenarkiv Vis sammendrag

  For long-distance migrations to pay off for individual fish, the energetic costs must be counterbalanced by benefits. Understanding the fitness trade-offs associated with migration is essential for our ability to sustainably manage migratory species. Here, we investigated such trade-offs associated with the spawning migration of Northeast Arctic cod Gadus morhua, a stock of high historical and commercial value known to be able to migrate more than 1000 km southward to reach suitable spawning grounds. Reaching the more distant spawning grounds requires more energy and hence leaves less energy available for processes such as egg production. Previous studies have indicated that increased larval survival (e.g. from rapid early development in warmer southern waters) may offset the parental costs of migration. However, it was suggested that spatial variability in survival of early life stages might cancel out this survival benefit. As an alternative, we ask if the fitness benefit of long-distance migration may reside in increased offspring growth. Using an integral projection model incorporating effects of body length and migration distance, we quantified the increase in offspring growth needed to offset parental costs of long-distance migration on fitness. Our results suggest that a 12.5% length in crease of juvenile cod is required to offset parental costs of long-distance migration. This is within the estimated growth benefit of 20% suggested by drift models of early life stages of cod. These results highlight the potential importance of offspring growth as another factor explaining the benefit of long distance migration, broadening our knowledge on spawning migration.

• Langangen, Øystein; Färber, Leonie Anette; Stige, Leif Christian; Diekert, Florian Klaus; Barth, Julia Maria Isis & Matschiner, Michael [Vis alle 11 forfattere av denne artikkelen] (2018). Ticket to spawn: Combining economic and genetic data to evaluate the effect of climate and demographic structure on spawning distribution in Atlantic cod. Global Change Biology. ISSN 1354-1013. 25(1), s. 134–143. doi: 10.1111/gcb.14474. Fulltekst i vitenarkiv Vis sammendrag

  Climate warming and harvesting affect the dynamics of species across the globe through a multitude of mechanisms, including distribution changes. In fish, migrations to and distribution on spawning grounds are likely influenced by both climate warming and harvesting. The Northeast Arctic (NEA) cod (Gadus morhua) performs seasonal migrations from its feeding grounds in the Barents Sea to spawning grounds along the Norwegian coast. The distribution of cod between the spawning grounds has historically changed at decadal scales, mainly due to variable use of the northern and southern margins of the spawning area. Based on historical landing records, two major hypotheses have been put forward to explain these changes: climate and harvesting. Climate could affect the distribution through, for example, spatial habitat shifts. Harvesting could affect the distribution through impacting the demographic structure. If demographic structure is important, theory predicts increasing spawner size with migration distance. Here, we evaluate these hypotheses with modern data from a period (2000–2016) of increasing temperature and recovering stock structure. We first analyze economic data from the Norwegian fisheries to investigate geographical differences in size of spawning fish among spawning grounds, as well as interannual differences in mean latitude of spawning in relation to changes in temperature and demographic parameters. Second, we analyze genetically determined fish sampled at the spawning grounds to unambiguously separate between migratory NEA cod and potentially smaller sized coastal cod of local origin. Our results indicate smaller spawners farther away from the feeding grounds, hence not supporting the hypothesis that harvesting is a main driver for the contemporary spawning ground distribution. We find a positive correlation between annual mean spawning latitude and temperature. In conclusion, based on contemporary data, there is more support for climate compared to harvesting in shaping spawning ground distribution in this major fish stock in the North Atlantic Ocean.

• Langangen, Øystein; Stige, Leif Christian; Kvile, Kristina Øie; Yaragina, Natalia A.; Skjæraasen, Jon Egil & Vikebø, Frode Bendiksen [Vis alle 7 forfattere av denne artikkelen] (2018). Multi-decadal variations in spawning ground use in Northeast Arctic haddock (Melanogrammus aeglefinus). Fisheries Oceanography. ISSN 1054-6006. 27(5), s. 435–444. doi: 10.1111/fog.12264. Fulltekst i vitenarkiv
• Kvile, Kristina Øie; Romagnoni, Giovanni; Dagestad, Knut-Frode; Langangen, Øystein & Kristiansen, Trond (2018). Sensitivity of modelled North Sea cod larvae transport to vertical behaviour, ocean model resolution and interannual variation in ocean dynamic. ICES Journal of Marine Science. ISSN 1054-3139. 75(7), s. 2413–2424. doi: 10.1093/icesjms/fsy039. Fulltekst i vitenarkiv Vis sammendrag

  Transport with ocean currents affects the spatial distribution and survival of fish eggs and larvae and thereby population connectivity. Biophysical models are commonly used to understand these dynamics. Advancements such as implementing vertical swimming behaviour and higher resolution ocean circulation models are known to improve model performance, however, the relative importance of vertical behaviour vs. ocean model resolution is elusive. Here, we use North Sea cod (Gadus morhua) as a case study to assess how vertical movement, ocean model resolution and interannual variation in ocean dynamics influence drift patterns and population connectivity. We couple a fine (1.6 km, 3 h) and coarser (4 km, 24 h) ocean model to an individual-based model for cod eggs and larvae, and compare simulations with and without vertical movement of eggs and larvae. The results are moderately influenced by vertical movement and ocean model resolution but differ substantially between years. While ocean model resolution is consistently more influential than vertical movement, the effect of vertical movement strongly depends on the spatiotemporal scale of the analyses. This study highlights which aspects of biophysical modelling of connectivity that most critically affect the results, allowing better investing computational resources and proposing goal-based guidelines for future studies.

• Stige, Leif Christian; Kvile, Kristina Øie; Bogstad, Bjarte & Langangen, Øystein (2018). Predator-prey interactions cause apparent competition between marine zooplankton groups. Ecology. ISSN 0012-9658. 99(3), s. 632–641. doi: 10.1002/ecy.2126. Fulltekst i vitenarkiv Vis sammendrag

  Predator‐mediated apparent competition is an indirect negative interaction between two prey species mediated by a shared predator. Quantifying such indirect ecosystem effects is methodologically challenging but important for understanding ecosystem functioning. Still, there are few examples of apparent competition from pelagic marine environments. Using state‐space statistical modeling, we here provide evidence for apparent competition between two dominant zooplankton groups in a large marine ecosystem, i.e., krill and copepods in the Barents Sea. This effect is mediated by a positive association between krill biomass and survival of the main planktivorous fish in the Barents Sea, capelin Mallotus villosus, and a negative association between capelin and copepod biomasses. The biomass of Atlantic krill species is expected to increase in the Barents Sea due to ongoing climate change, thereby potentially negatively affecting copepods through apparent competition. By demonstrating and quantifying apparent competition in a large marine ecosystem, our study paves the way for more realistic projections of indirect ecosystem effects of climate change and harvesting.

• Stige, Leif Christian; Ottersen, Geir; Yaragina, Nathalia A.; Vikebø, Frode Bendiksen; Stenseth, Nils Christian & Langangen, Øystein (2018). Combined effects of fishing and oil spills on marine fish: Role of stock demographic structure for offspring overlap with oil. Marine Pollution Bulletin. ISSN 0025-326X. 129(1), s. 336–342. doi: 10.1016/j.marpolbul.2018.02.056. Fulltekst i vitenarkiv Vis sammendrag

  It has been proposed that the multiple pressures of fishing and petroleum activities impact fish stocks in synergy, as fishing-induced demographic changes in a stock may lead to increased sensitivity to detrimental effects of acute oil spills. High fishing pressure may erode the demographic structure of fish stocks, lead to less diverse spawning strategies, and more concentrated distributions of offspring in space and time. Hence an oil spill may potentially hit a larger fraction of a year-class of offspring. Such a link between demographic structure and egg distribution was recently demonstrated for the Northeast Arctic stock of Atlantic cod for years 1959–1993. We here estimate that this variation translates into a two-fold variation in the maximal proportion of cod eggs potentially exposed to a large oil spill. With this information it is possible to quantitatively account for demographic structure in prospective studies of population effects of possible oil spills.

• Nater, Chloé Rebecca; Rustadbakken, Atle; Ergon, Torbjørn; Langangen, Øystein; Moe, S. Jannicke & Vindenes, Yngvild [Vis alle 8 forfattere av denne artikkelen] (2018). Individual heterogeneity and early life conditions shape growth in a freshwater top predator. Ecology. ISSN 0012-9658. 99(5), s. 1011–1017. doi: 10.1002/ecy.2178. Fulltekst i vitenarkiv Vis sammendrag

  Body size can have profound impacts on survival, movement, and reproductive schedules shaping individual fitness, making growth a central process in ecological and evolutionary dynamics. Realized growth is the result of a complex interplay between life history schedules, individual variation, and environmental influences. Integrating all of these aspects into growth models is methodologically difficult, depends on the availability of repeated measurements of identifiable individuals, and consequently represents a major challenge in particular for natural populations. Using a unique 30‐yr time series of individual length measurements inferred from scale year rings of wild brown trout, we develop a Bayesian hierarchical model to estimate individual growth trajectories in temporally and spatially varying environments. We reveal a gradual decrease in average juvenile growth, which has carried over to adult life and contributed to decreasing sizes observed at the population level. Commonly studied environmental drivers like temperature and water flow did not explain much of this trend and overall persistent and among‐year individual variation dwarfed temporal variation in growth patterns. Our model and results are relevant to a wide range of questions in ecology and evolution requiring a detailed understanding of growth patterns, including conservation and management of many size‐structured populations.

• Skjæraasen, Jon Egil; Devine, Jennifer Ann; Godiksen, Jane Aanestad; Fonn, Merete; Otterå, Håkon Magne & Kjesbu, Olav Sigurd [Vis alle 9 forfattere av denne artikkelen] (2017). Timecourse of oocyte development in saithe Pollachius virens. Journal of Fish Biology. ISSN 0022-1112. 90(1), s. 109–128. doi: 10.1111/jfb.13161. Fulltekst i vitenarkiv Vis sammendrag

  Wild caught North Sea saithe Pollachius virens were monitored for growth, sex steroid profiles and oocyte development pre-spawning and measured for egg size and group fecundity during the spawning season in the laboratory. Vitellogenesis commenced in late October–early November, at a leading cohort size (CL) of c. 250 µm, after which oocytes grew rapidly in size until spawning started in February. Notably, a distinct cortical alveoli stage was virtually absent with yolk granules observed in developing oocytes at the very beginning of vitellogenesis. Little atresia was observed pre-spawning, but atretic re-absorption of remnant oocytes containing yolk granules was found in all females immediately post-spawning. As expected, concentrations of sex steroids, oestradiol-17β (females), testosterone (both sexes) and 11-ketotestosterone (both sexes), increased pre-spawning before dropping post-spawning. The present experiment provides the first validation of sex steroid levels in P. virens. Post-ovulatory follicles were visible in histological sections from female gonads 9–11 months post-spawning, but then disappeared. Spawning commenced around a CL of c. 750 µm (700–800 µm). Hydrated oocytes (eggs) measured between 1·04 and 1·31 mm (mean = 1·18 mm) with decreasing sizes towards the end of spawning. The average estimated realized fecundity was c. 0·84 million eggs (median female total length, LT = 60 cm). Spawning lasted from 13 February to 29 March.

• Stige, Leif Christian; Yaragina, Natalya; Langangen, Øystein; Bogstad, Bjarte; Stenseth, Nils Christian & Ottersen, Geir (2017). Effect of a fish stock's demographic structure on offspring survival and sensitivity to climate. Proceedings of the National Academy of Sciences of the United States of America (PNAS). ISSN 0027-8424. 114(6), s. 1347–1352. doi: 10.1073/pnas.1621040114. Fulltekst i vitenarkiv Vis sammendrag

  Commercial fishing generally removes large and old individuals from fish stocks, reducing mean age and age diversity among spawners. It is feared that these demographic changes lead to lower and more variable recruitment to the stocks. A key proposed pathway is that juvenation and reduced size distribution causes reduced ranges in spawning period, spawning location, and egg buoyancy; this is proposed to lead to reduced spatial distribution of fish eggs and larvae, more homogeneous ambient environmental conditions within each year-class, and reduced buffering against negative environmental influences. However, few, if any, studies have confirmed a causal link from spawning stock demographic structure through egg and larval distribution to year class strength at recruitment. We here show that high mean age and size in the spawning stock of Barents Sea cod (Gadus morhua) is positively associated with high abundance and wide spatiotemporal distribution of cod eggs. We find, however, no support for the hypothesis that a wide egg distribution leads to higher recruitment or a weaker recruitment–temperature correlation. These results are based on statistical analyses of a spatially resolved data set on cod eggs covering a period (1959−1993) with large changes in biomass and demographic structure of spawners. The analyses also account for significant effects of spawning stock biomass and a liver condition index on egg abundance and distribution. Our results suggest that the buffering effect of a geographically wide distribution of eggs and larvae on fish recruitment may be insignificant compared with other impacts.

• Langangen, Øystein; Olsen, Erik Joel Steinar; Stige, Leif Christian; Ohlberger, Jan Philipp; Yaragina, Natalia A. & Vikebø, Frode Bendiksen [Vis alle 9 forfattere av denne artikkelen] (2017). The effects of oil spills on marine fish: Implications of spatial variation in natural mortality. Marine Pollution Bulletin. ISSN 0025-326X. 119(1), s. 102–109. doi: 10.1016/j.marpolbul.2017.03.037. Fulltekst i vitenarkiv Vis sammendrag

  The effects of oil spills on marine biological systems are of great concern, especially in regions with high biological production of harvested resources such as in the Northeastern Atlantic. The scientific studies of the impact of oil spills on fish stocks tend to ignore that spatial patterns of natural mortality may influence the magnitude of the impact over time. Here, we first illustrate how spatial variation in natural mortality may affect the population impact by considering a thought experiment. Second, we consider an empirically based example of Northeast Arctic cod to extend the concept to a realistic setting. Finally, we present a scenario-based investigation of how the degree of spatial variation in natural mortality affects the impact over a gradient of oil spill sizes. Including the effects of spatial variations in natural mortality tends to widen the impact distribution, hence increasing the probability of both high and low impact events.

• Langangen, Øystein; Ohlberger, Jan; Stige, Leif C.; Durant, Joel M.; Ravagnan, Elisa & Stenseth, Nils C. [Vis alle 7 forfattere av denne artikkelen] (2017). Cascading effects of mass mortality events in Arctic marine communities. Global Change Biology. ISSN 1354-1013. 23(1), s. 283–292. doi: 10.1111/gcb.13344. Fulltekst i vitenarkiv Vis sammendrag

  Mass mortality events caused by pulse anthropogenic or environmental perturbations (e.g., extreme weather, toxic spills or epizootics) severely reduce the abundance of a population in a short time. The frequency and impact of these events are likely to increase across the globe. Studies on how such events may affect ecological communities of inter-acting species are scarce. By combining a multispecies Gompertz model with a Bayesian state-space framework, we quantify community-level effects of a mass mortality event in a single species. We present a case study on a community of fish and zooplankton in the Barents Sea to illustrate how a mass mortality event of different intensities affecting the lower trophic level (krill) may propagate to higher trophic levels (capelin and cod). This approach is especially valuable for assessing community-level effects of potential anthropogenic-driven mass mortality events, owing to the ability to account for uncertainty in the assessed impact due to uncertainty about the ecological dynamics. We hence quantify how the assessed impact of a mass mortality event depends on the degree of precaution considered. We suggest that this approach can be useful for assessing the possible detrimental outcomes of toxic spills, for example oil spills, in relatively simple communities such as often found in the Arctic, a region under increasing influence of human activities due to increased land and sea use.

• Kvile, Kristina Øie; Langangen, Øystein; Prokopchuk, Irina P.; Stenseth, Nils Christian & Stige, Leif Christian (2016). Disentangling the mechanisms behind climate effects on zooplankton. Proceedings of the National Academy of Sciences of the United States of America (PNAS). ISSN 0027-8424. 113(7), s. 1841–1846. doi: 10.1073/pnas.1525130113. Fulltekst i vitenarkiv
• Vindenes, Yngvild; Langangen, Øystein; Winfield, Ian J. & Vøllestad, Leif Asbjørn (2016). Fitness consequences of early life conditions and maternal size effects in a freshwater top predator. Journal of Animal Ecology. ISSN 0021-8790. 85(3), s. 692–704. doi: 10.1111/1365-2656.12489. Fulltekst i vitenarkiv
• Kvile, Kristina Øie; Stige, Leif Christian; Prokopchuk, Irina P. & Langangen, Øystein (2016). A statistical regression approach to estimate zooplankton mortality from spatiotemporal survey data. Journal of Plankton Research. ISSN 0142-7873. 38(3), s. 624–635. doi: 10.1093/plankt/fbw028. Fulltekst i vitenarkiv

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• Langangen, Øystein Ole Gahr; Olsen, Tora; Durant, Joël; Ottersen, Geir & Eriksen, Elena (2024). Poleward shift in spawning leads to smaller Atlantic cod offspring. Fulltekst i vitenarkiv
• Langangen, Øystein Ole Gahr; Olsen, Tora; Durant, Joël; Ottersen, Geir & Eriksen, Elena (2024). Poleward shift in spawning leads to smaller Atlantic cod offspring. Fulltekst i vitenarkiv
• Abe, Keita; Diekert, Florian Klaus; Melsom, Arne & Langangen, Øystein Ole Gahr (2024). Author Correction: Do fishers follow fish displaced by climate warming? npj Ocean Sustainability. 3(1). doi: 10.1038/s44183-024-00066-6. Fulltekst i vitenarkiv
• Planque, Benjamin; Aarflot, Johanna Myrseth; Buttay, Lucie; Carroll, JoLynn; Fransner, Sara Filippa Krusmynta & Eide, Cecilie Hansen [Vis alle 17 forfattere av denne artikkelen] (2023). A standard protocol for describing the evaluation of ecological models. Fulltekst i vitenarkiv
• Langangen, Øystein Ole Gahr & Durant, Joël (2023). Mass mortality and population dynamics of fish. Fulltekst i vitenarkiv
• Dupont, Nicolas; Durant, Joël; Langangen, Øystein Ole Gahr & Stige, Leif Christian (2023). Survival of adult polar cod (Boreogadus saida) in the context of “borealization” of the Barents Sea. Fulltekst i vitenarkiv
• Dupont, Nicolas; Durant, Joël; Langangen, Øystein Ole Gahr & Stige, Leif Christian (2023). Effects of abrupt ecological change on the Barents Sea Arctic food-web: predictions from a food-web state-space model. Fulltekst i vitenarkiv
• Durant, Joël; Ono, Kotaro & Langangen, Øystein Ole Gahr (2023). Empirical evidence of non-linearity in the interaction between fish stocks in the Barents Sea. Fulltekst i vitenarkiv Vis sammendrag

  The strength of species interactions can have a significant impact on population dynamics. Empirical estimates of interaction strength are often based on the assumption that the interaction strength is constant. Several fish species interact in the Barents Sea and their population dynamics are typically modelled under the assumption of constant interaction strength. However, interactions between species are often non-linear in marine ecosystems and this could fundamentally change our understanding of food webs’ functioning. Here, we present two examples of such nonlinear interactions in the Barents Sea, between cod Gadus morhua and capelin Mallotus villosus, and between cod and haddock Melanogrammus aeglefinus. Analysis of long survey time series in the Arcto-boreal Barents Sea within a state-space modelling framework showed that the effect of capelin on cod is not linear but varies with capelin abundance. Similarly, interactions between cod and haddock have changed over the past two decades due to rising ocean temperatures, altering the equilibrium abundances and the dynamics of the system. Our analyses demonstrate that long-term climate change in the Arcto-boreal system leads to differences in equilibrium conditions for species communities and demonstrates the importance of investigating nonlinearities in species interactions, leading to a better understanding of species communities and species assemblages.

• Dupont, Nicolas; Durant, Joël; Langangen, Øystein Ole Gahr & Stige, Leif Christian (2022). Change in the Barents Sea Arctic food-web dynamic based on biotic and abiotic environmental factors. Fulltekst i vitenarkiv
• Langangen, Øystein Ole Gahr & Stige, Leif Christian (2022). The role of large spawners for population productivity in Northeast Arctic cod. Fulltekst i vitenarkiv
• Dupont, Nicolas; Durant, Joël; Gjøsæter, Harald; Langangen, Øystein Ole Gahr & Stige, Leif Christian (2021). Using time series to predict the future: quantifying the effects of borealization on polar cod (Boreogadus saida). Fulltekst i vitenarkiv
• Dupont, Nicolas; Durant, Joel Marcel; Langangen, Øystein; Gjøsæter, Harald & Stige, Leif Christian (2019). Environmental effects on population annual mean length of polar cod (Boreogadus saida) in the Barents Sea. Fulltekst i vitenarkiv
• Dupont, Nicolas; Durant, Joel Marcel; Langangen, Øystein; Gjøsæter, Harald & Stige, Leif Christian (2019). Environmental effects on population annual mean length of polar cod (Boreogadus saida) in the Barents Sea. Fulltekst i vitenarkiv
• Stige, Leif Christian; Rogers, Lauren; Neuheimer, Anna B.; Hunsicker, Mary E.; Yaragina, Natalia A. & Ottersen, Geir [Vis alle 9 forfattere av denne artikkelen] (2019). Density- and size-dependent mortality in fish early life stages. Fulltekst i vitenarkiv
• Stige, Leif Christian; Durant, Joel Marcel & Langangen, Øystein (2019). Effects of climate change, fishing and oil on marine ecosystems. Fulltekst i vitenarkiv
• Stige, Leif Christian; Langangen, Øystein; Durant, Joel Marcel; Knutsen, Halvor; Olsen, Esben Moland & Ottersen, Geir (2019). Forskning på torsk. Biolog. ISSN 0801-0722. 37(2), s. 46–49. Fulltekst i vitenarkiv
• Stenseth, Nils Christian; Stige, Leif Christian & Langangen, Øystein (2018). Kunnskapsbasert forvaltning i et varmere klima. Aftenposten (morgenutg. : trykt utg.). ISSN 0804-3116. s. 16–16. Fulltekst i vitenarkiv
• Stige, Leif Christian; Langangen, Øystein & Stenseth, Nils Christian (2018). Skreien ikke utenfor livsfare. Dagens næringsliv. ISSN 0803-9372. s. 32–32. Fulltekst i vitenarkiv
• Langangen, Øystein (2018). Long-term effects of local scale OIL pollution on fish Populations and COMmunities in the Lofoten-Barents Sea system (OILCOM). Fulltekst i vitenarkiv
• Färber, Leonie Anette; Camilla, Sguotti; Durant, Joel Marcel; Langangen, Øystein; Otto, Saskia A & Möllmann, Christian (2018). Detecting catastrophic transitions – the case of North Atlantic herring. Fulltekst i vitenarkiv
• Deris, Leana; Langangen, Øystein & Durant, Joel Marcel (2018). Effect of cod (Gadus morhua) predation on juvenile herring (Clupea harengus) in the Barents Sea. Fulltekst i vitenarkiv Vis sammendrag

  The Barents Sea is a shelf sea sustaining a highly productive ecosystem including several commercially important fish stocks among which the Northeast Arctic (NEA) cod (Gadus morhua), Norwegian Spring Spawning (NSS) herring (Clupea harengus) and the Barents Sea (BS) capelin (Mallotus villosus) are key fish species in this ecosytem. NSS herring is currently one of the largest herring stocks in the world, and it is both an important prey (for e.g. cod and marine mammals) and predator species (for e.g. planktonic early life stages of fish and zooplankton). It spawns in the Norwegian Sea in February-March and deposits eggs on the sea bottom. After hatching, larvae and 0-group appear in the upper water layer and drift by the coastal current to enter the Barents Sea where they spend their first 3 to 4 years of life before migrating to the north Atlantic as adult. It is during its juvenile life stage that the herring is susceptible to predation by the large stock of NEA cod. Here, we develop a state-space model to estimate the effect of cod predation on young herring mortality. We used the obtained estimates to quantify at the population level the joint effect of fishing and cod predation on young herring. Our results indicate a significant effect of cod and illustrate how management of NSS herring benefits from jointly considering cod and fishing.

• Färber, Leonie Anette; Sguotti, Camilla; Durant, Joel Marcel; Langangen, Øystein; Otto, Saskia A & Möllmann, Christian (2018). Detecting catastrophic transitions – the case of North Atlantic herring. Fulltekst i vitenarkiv
• Deris, Leana; Langangen, Øystein & Durant, Joel Marcel (2018). Effect of stock collapse on predator prey relationships. Fulltekst i vitenarkiv
• Stige, Leif Christian; Ottersen, Geir; Yaragina, Natalia A.; Stenseth, Nils Chr. & Langangen, Øystein (2017). Effect of a fish stock’s demographic structure on offspring exposure to possible oil spills. Fulltekst i vitenarkiv
• Langangen, Øystein (2017). Multi-decadal variations in spawning ground use in Northeast Arctic haddock. Fulltekst i vitenarkiv
• Dupont, Nicolas; Stige, Leif Christian & Langangen, Øystein (2017). Effects of heterogeneous spatial mortality on recruitment of Northeast Arctic cod (Gadus morhua) in the Barents Sea. Fulltekst i vitenarkiv
• Durant, Joel Marcel; Langangen, Øystein & Dupont, Nicolas (2017). Barents Sea cod larvae survival and match-mismatch. Fulltekst i vitenarkiv
• Bellier, Edwige; Langangen, Øystein & Ohlberger, Jan Philipp (2017). Stage-dependent interactions of two harvested competitors. Fulltekst i vitenarkiv
• Färber, Leonie Anette; Durant, Joel Marcel; Vindenes, Yngvild & Langangen, Øystein (2017). Benefit of long distance migration for Barents Sea cod. Fulltekst i vitenarkiv
• Langangen, Øystein (2017). Ticket to spawn: combining economic and genetic data to evaluate the role of climate and harvesting on temporal shifts in spawning location. Fulltekst i vitenarkiv
• Kvile, Kristina Øie; Stige, Leif Christian; Prokopchuk, Irina P. & Langangen, Øystein (2016). A statistical regression approach to estimate zooplankton mortality from spatiotemporal survey data. Fulltekst i vitenarkiv
• Kvile, Kristina Øie; Dalpadado, Padmini; Fiksen, Øyvind; Langangen, Øystein; Opdal, Anders Frugård & Prokopchuk, Irina P. [Vis alle 8 forfattere av denne artikkelen] (2016). Climate responses of Calanus finmarchicus in a high latitude system. Fulltekst i vitenarkiv
• Stige, Leif Christian; Kvile, Kristina Øie & Langangen, Øystein (2016). Zooplankton and fish: revealing key processes in predator‑prey dynamics in the Barents Sea. Fulltekst i vitenarkiv
• Klöcker, Antonia; Junge, Claudia; Queiroz, Nuno; Vihtakari, Mikko & Langangen, Øystein Ole Gahr (2025). On the move: The movement ecology of sharks in high-latitude waters and Beyond. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Animal movement is a pervasive phenomenon that shapes and connects ecosystems. In the ocean, the movements of apex and mesopredators like sharks play a vital role in structuring the distribution and behaviour of lower trophic Levels and maintaining habitat connectivity both vertically and horizontally. However, their life-history traits and often high mobility make sharks particularly vulnerable to human-induced threats such as overfishing, habitat degradation, and climate change. Understanding where, when, how, and why sharks move is essential for developing effective and adaptive management strategies to ensure their long-term survival. This dissertation explored the movement ecology of two ecologically distinct shark species occurring in Norwegian waters: the bentho-pelagic, mesopredatory spurdog and the pelagic, plankton-feeding basking shark. Using satellite telemetry, this work examined both the vertical and horizontal dimensions of movement and habitat use across different seasons and environments. Beyond characterizing spatiotemporal movement patterns, this work investigated potential internal and external drivers, including pregnancy, morphological adaptations, as well as physical and biological resource availability and distribution, to gain insights into the underlying mechanisms of shark movement. Paper I examined seasonal habitat use and vertical movement patterns of 19 pregnant spurdogs in Norwegian fjords. The study revealed consistent diel vertical migration and deep-water use in winter and spring, with individuals shifting to shallower, warmer waters and reducing diel vertical migration in summer and autumn. Complementary seasonal variations in likely foraging-related activity bursts suggested winter movements were primarily foraging-driven, whereas behavioural thermoregulation may have dominated before parturition later in the year. Paper II assessed the potential for regional endothermy in basking sharks, a trait that allows a few cold-adapted fast-swimming pelagic fish like the porbeagle to retain heat in specific body regions. Contrary to expectations, a comparative morphological analysis confirmed the absence of red muscle endothermy in basking sharks. Instead, it revealed vascular arrangements that suggest cranial and, notably, visceral endothermy in the species. Together with partial red muscle internalization and its large body size, these adaptations likely facilitate an active lifestyle in cold-water environments. Paper III investigated habitat-dependent vertical movement in two basking Sharks across broad thermal, spatial, and temporal scales. Tracked individuals utilized waters with previously unrecorded low temperatures, both in the short- and longterm, likely facilitated by the morphological adaptations identified in Paper II. Further, the tracked individuals exhibited distinct vertical movement patterns in response to the local environment. The strong association with isolumes, thermoclines, bathymetry, and bioluminescent layers indicated that basking sharks actively track prey across diverse habitats. Paper IV analyzed the horizontal movements of 13 high-latitude basking Sharks throughout the annual cycle, documenting large-scale return migrations between areas of summer residency along the Norwegian Shelf and lower-latitude oceanic waters in the North Atlantic occupied during winter and spring. Their absence in known high-use areas at intermediate latitudes (i.e., UK and Ireland) and more extensive latitudinal migrations compared to their lower-latitude counterparts in the Northeast Atlantic suggest a distinct migration strategy amongst these ‘boreal’ individuals, likely driven by pronounced seasonal fluctuations in resource availability. Collectively, these studies provide new insights into the movement ecology of spurdog and basking sharks at the high-latitude edge of their distribution. The findings highlight the value of long-term, high-resolution telemetry and biologging in understanding drivers that enable these mobile predators to navigate complex and dynamic ocean environments. With ongoing technological advances, the rise of big data approaches, and the adoption of more integrated and open science practices, movement ecology is poised to evolve into a more mechanistic and predictive discipline. As anthropogenic pressures intensify, including the accelerating Atlantification in the Arctic, this transformation will be crucial for developing adaptive, evidence-based management strategies to safeguard these vulnerable ocean sentinels – now and in the future.

• Carroll, JoLynn; Skeie, Geir Morten; Vikebø, Frode Bendiksen; Broch, Ole Jacob; Frøysa, Håvard Guldbrandsen & Howell, Daniel [Vis alle 12 forfattere av denne artikkelen] (2023). SYMBIOSES III: Final report. SYMBIOSES - Advancing simulation technologies for ecosystem-based assessment of major oil spills in ecologically sensitive and environmentally challenging areas. Fulltekst i vitenarkiv
• Nilsen, Ina; Eide, Cecilie Hansen; Olsen, Are; Langangen, Øystein Ole Gahr & Kaplan, Isaac (2023). Dealing with uncertainty in ecosystem models along three axes; resolution, forcing and projections. Universitetet i Bergen. ISSN 9788230843871. Fulltekst i vitenarkiv Vis sammendrag

  With the imminent threat of climate change, there is an urgent need to understand how warmer temperatures will affect marine ecosystems. Models provide the best tools to study the future, but although great efforts have been made to understand the impacts of warming temperatures, there is still large uncertainties related to the model projections. The uncertainties can arise from structural uncertainty of the ecosystem model, uncertainties regarding climate projections, or uncertainties related to how species will respond to future climate changes. In this thesis, we therefore apply ecosystem models of varying complexity, climate models of varying resolution and climate projections under various emission scenarios to understand and quantify uncertainty. By assessing the uncertainty, we highlight consistent results that suggest higher confidence, and areas where differences in the results suggest that more research is needed. The results from this thesis are divided into three research papers. The first paper deals with structural uncertainty regarding model complexity, as we explore the role of Northeast Atlantic cod (Gadus Morhua) in the Barents Sea food web by using a multi-model approach. We apply two ecosystem models of different complexity; the minimalistic Gompertz model and the highly complex Atlantis model, to study how capelin and polar cod respond to changing levels of cod. We highlight consistent results across the models and identify diverging results due to differences in spatial structure and number of foodweb components, and conclude that for fishery management purposes, the two models can complement each other. The second paper deals with uncertainty regarding the resolution of climate projections used to force ecosystem models, where we apply physics (temperature, salinity, volume transport and sea ice) from a regional model (Nemo-NAA10km) and its driving global climate model (NorESM2) to the Nordic and Barents Seas Atlantis ecosystem model (NoBa). We found that few higher trophic level (TL>3) species were affected by using forcing from a global versus a regional model, and there was a general agreement in future biomass trends and distribution patterns. Yet, our results showed how a slight difference in temperature can have dramatic consequences for specific species and demonstrate that species projection uncertainty could arise from poor representation of the physical forcing, as well as due to uncertainty in the ecosystem model parameterization. In the third and final paper, we deal with the uncertainty regarding ecosystem responses to future climate changes. We apply physics from three different climate projections (SPP1-2.6, SSP2-4.5 and SSP5-8.5) to study the impact of rising temperatures in the Nordic and Barents Seas using NoBa. To account for uncertainty in the response of phytoplankton and zooplankton to future climate change, we included variation in phyto- and zooplankton growth levels. We identify potential winners and losers in a warming climate and focus on the underlying mechanisms that drives the changes in the model, including spatial differences, thermal tolerance, and species interactions. Through our work we have demonstrated the value of using ecosystem models of varying complexity, climate models of varying resolution and climate projections under various emission scenarios to quantify uncertainty regarding model projections. By investigating uncertainty along these three axes, we learn more about the models and the mechanisms that drives the changes and provide valuable insight for management and future ecosystem studies.

• Planque, Benjamin; Aarflot, Johanna Myrseth; Buttay, Luice; Fransner, Filippa; Hansen, Cecilie & Husson, Berengere [Vis alle 15 forfattere av denne artikkelen] (2020). Best practices for ecological model evaluation II: Workshop Report. Septentrio Academic Publishing. Fulltekst i vitenarkiv

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