Stein Kaartvedt

• Kaartvedt, Stein (2026). A career studying fjords and oceans in unconventional ways. ICES Journal of Marine Science. ISSN 1054-3139. 83(3), p. 1–11. doi: 10.1093/icesjms/fsag030. Full text in Research Archive
• 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. Full text in Research Archive Show summary

  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.

• García-Seoane, Eva; Gjøsæter, Harald; Dalpadado, Padmini & Kaartvedt, Stein (2025). Why is the Arabian Sea a hotspot for myctophids? Fisheries Research. ISSN 0165-7836. 288. doi: 10.1016/j.fishres.2025.107453. Full text in Research Archive Show summary

  Myctophids are the dominant mesopelagic fishes in most of the world’s oceans. They play a key ecological role and also have a potential for harvest. We here synthesize general traits of myctophid biology and address the success and life history strategy of the dominant myctophid species in the mesopelagic hotspot of the Arabian Sea. In this ocean, the biomass of myctophids is particularly high, but dominated by only a few species. With low abundance of competing small pelagic fish, much of the high production is channeled via zooplankton to the stocks of mesopelagic fishes. The ability of the successful species to tolerate low oxygen levels facilitates exploitation of these productive waters and reduces competition for prey. Simultaneously, the hypoxic waters provide refuge from predators. The myctophids in the Arabian Sea feed in warm waters at night and inhabit cooler, yet relatively warm waters during daytime. We hypothesize that this temperature regime combined with a high abundance of prey provides a favorable combination of foraging and digestion. To this adds that the location near the Equator provides a year-long favorable regular shift between length of day and night for foraging in upper waters and digestion in deep waters, though digestion may be constrained by low oxygen concentrations. Myctophid species from tropical habitats have shorter life spans than species from colder regions. We suggest that in tropical habitats, such as the Arabian Sea, extended spawning seasons are common for myctophids whereas restricted spawning seasons are more common in cold-water habitats. The Gulf of Oman, which is the upwelling region with the highest primary production in Indian Ocean, is recognized to have the greatest potential for any commercial exploitation of mesopelagic fish.

• Salvanes, Anne Gro Vea; Gallo, Natalya; Solås, Martine Røysted; Saltalamacchia, Francesco; Aksnes, Dag Lorents & Darelius, Elin Maria K. [Show all 19 contributors for this article] (2025). Deep Fjords Are Excellent Natural Infrastructure for Climate Impact Studies. Fish and Fisheries. ISSN 1467-2960. 26(2), p. 270–277. doi: 10.1111/faf.12879. Full text in Research Archive Show summary

  Fjords provide valuable research opportunities for marine scientists. They are excellent natural infrastructure for climate impact studies associated with hypoxic episodes and consequences for mesopelagic and deep-sea ecosystems involving oceanographic circulation processes and basin water renewals. Repeated sampling from the same populations is possible, making fjords excellent systems for developing time series of data for climate impact studies. We provide an overview of the 14 years of data from Norwegian West Coast fjords, focusing on Masfjorden, and report major findings from Oslofjorden in Eastern Norway, exhibiting recurrent hypoxia in the basin waters. We document that the oxygen levels in Masfjorden decreased rapidly by over 60% at 450 m depth in < 8 years, which is much faster than the average rate of deoxygenation in the global ocean. We also discuss the increase in the deep-sea and low-light-adapted coronate jellyfish Periphylla periphylla in view of altered optical conditions of the basin water potentially related to deoxygenation. We argue that fjords like Masfjorden and Oslofjorden are not only macrocosms for ecological processes but also are likely an accelerated version of deep oceans with respect to climate impacts.

• Kaartvedt, Stein; Christiansen, Svenja & Røstad, Anders (2024). Mesopelagic fishes in the Red Sea. In Rasul, Najeeb M. A. & Stewart, Ian C.F. (Ed.), Coral Reefs and Associated Marine Fauna Around the Arabian Peninsula. CRC Press. ISSN 9781032342900. p. 93–100. doi: 10.1201/9781003321392-10. Full text in Research Archive Show summary

  The Red Sea twilight zone (200-1000 m) is a unique environment characterized by extraordinarily high temperatures, low zooplankton concentrations, and strata with low oxygen contents. This harsh habitat houses few species of mesopelagic fishes, yet individuals are numerous, and distinct mesopelagic acoustic scattering layers occur throughout the ocean. The scattering fish layers extend down to 600-800 m during the day. The fishes carry out rapid nocturnal vertical migrations to upper waters, with limited seasonal variations. However, behavioural plasticity includes examples of strikingly short-term and rapid darts into upper waters at dusk and dawn. Such behaviour may relate to the rapid shifts between day and night at low latitudes, the respective light sensitivities of the fish species and varying presence of predators. Foraging mainly happens in upper waters at night, yet the fishes are active at day in their mesopelagic habitat. Excretion in the daytime habitat fuels microbial processes, generating a hotspot for heterotrophic prokaryotes. The fishes’ role as prey in the Red Sea food chain remains to be established. The Red Sea mesopelagic fishes appear to spawn throughout the year, though apparently with minima in mid-summer.

• Christiansen, Svenja; Bråthen, Mabel Pernille; Moksnes, Fredrik Lund; Schmedling, Charlotte Aurora; Thorsby, Sara Jegstad & Titelman, Josefin [Show all 7 contributors for this article] (2024). Extensive avoidance behaviour of krill from predators and nets. ICES Journal of Marine Science. ISSN 1054-3139. 81(9), p. 1764–1773. doi: 10.1093/icesjms/fsae123. Full text in Research Archive Show summary

  We used echosounders to assess krill (Meganyctiphanes norvegica) responses to sampling gear and predators. Towing pelagic trawls above autonomous submerged, upward-looking echosounders revealed instantaneous diving by the krill during daytime but not at night. The krill dived at 10–30 cm s−1 with a near-simultaneous response throughout the krill layer below the gear (>10 m). Traversing the paths of the previous sampling revealed long-lasting voids in the krill distribution during the daytime. Such voids were less apparent at night. Backscatter increased in the wake of the gear during nocturnal sampling, possibly due to changes in individual backscatter related to variation in swimming behaviour. During daytime, krill responded to fish schools with instantaneous coherent diving, spanning tens of meters at speeds of 10–27 cm s−1. Coordinated and far-ranging responses indicate that even loose krill aggregations may facilitate predator avoidance by enabling efficient signal transmission between individuals. The krill antipredator behaviour instigated marked daytime krill patchiness. Krill patchiness will vary relative to the predator regime and reflect recent encounters.

• Aksnes, Dag Lorents; Løtvedt, Anita Stene; Lindemann, Christian; Calleja, Maria Ll.; Morán, Xosé Anxelu G. & Kaartvedt, Stein [Show all 7 contributors for this article] (2023). Effects of migrating mesopelagic fishes on the biological carbon pump. Marine Ecology Progress Series. ISSN 0171-8630. 717, p. 107–126. doi: 10.3354/meps14373. Full text in Research Archive Show summary

  Mesopelagic fishes and invertebrates contribute to the biological carbon pump (BCP) through direct and indirect effects on the gravitational, diffusive, and migrant (active) fluxes. Here, we analyzed the effect of these organisms on total carbon export and sequestration using an idealized depth-resolved food-web model. We constrained a baseline scenario with observations from the Red Sea where mesopelagic fishes perform extensive diel vertical migration (DVM), presumably making them particularly important in carbon export. Our results are consistent with previous studies suggesting that mesopelagic fishes fuel a hotspot of heterotrophic activity in the mesopelagic zone of the Red Sea. While the supply of new nutrients to the euphotic zone is of first order in framing total carbon export and sequestration, we found 2 modulating effects of fish DVM. First, DVM enhances carbon sequestration because active carbon flux attenuates less than the gravitational and diffusive fluxes, and also because migrators fuel the gravitational flux deep into the mesopelagic zone. Secondly, through microbial food-web cascades, fish predation affects the relative contribution of the active and the passive fluxes with non-linear, less intuitive consequences for carbon export and sequestration. These food-web cascades involve a surprisingly large variation in the depth of the euphotic zone that is associated with a ‘diatom nutrient trap’ at the bottom of the euphotic zone.

• Kaartvedt, Stein; Christiansen, Svenja & Titelman, Josefin (2023). Mid-summer fish behavior in a high-latitude twilight zone. Limnology and Oceanography. ISSN 0024-3590. 68(7), p. 1654–1669. doi: 10.1002/lno.12374. Full text in Research Archive Show summary

  Abstract The behavior of the mesopelagic fish Benthosema glaciale was studied at 60°N in mid‐summer. We hypothesized that diel vertical migration (DVM) is constrained by short and dusk nights (surface illumination &gt; 10 −2   μ mol m −2 s −1 ) and that individuals are active at depth during the long summer days. Submerged echosounders provided high‐resolution data throughout the water column. During the day, a part of the population ascended toward the increasing daylight. Short vertical relocations were followed by minutes of vertical inactivity. Swimming included horizontal turns and loops associated with the vertical steps. Normal DVM was initiated ~ 4 h before sunset and reflected independent individual decisions. The fish initially ascended stepwise but switched to mostly straight upwards swimming attaining 3–4 cm s −1 . Their vertical speed was faster than the slow ascent of isolumes and even the deepest living fish potentially could reach upper layers shortly after sunset. However, many individuals aborted their ascent and returned to depth before the darkest time of the night, while others returned downward closer to sunrise. The daytime swimming and individual variability in diel migration behavior have implications for encounters with prey and predators in the twilight zone and the biological carbon pump. A principal conclusion is that mesopelagic fishes can modify their behavior and migration patterns to suit a wide range of changing conditions.

• Aldanondo, Naroa; Kaartvedt, Stein & Irigoien, Xabier (2023). Growth patterns of two Red Sea mesopelagic fishes. Marine Biology. ISSN 0025-3162. 170(1), p. 1–9. doi: 10.1007/s00227-022-04144-6. Full text in Research Archive
• 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, p. 2677–2690. doi: 10.1002/lno.12230. Full text in Research Archive Show summary

  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.

• Kaartvedt, Stein; Christiansen, Svenja; Røstad, Anders & Aksnes, Dag Lorents (2022). Mesopelagic fishes in a hurry at low latitudes. Marine Ecology Progress Series. ISSN 0171-8630. 694, p. 149–156. doi: 10.3354/meps14114. Full text in Research Archive
• Benguria, Beatriz Sobradillo; Christiansen, Svenja; Røstad, Anders & Kaartvedt, Stein (2022). Individual daytime swimming of mesopelagic fishes in the world's warmest twilight zone. Deep Sea Research Part I: Oceanographic Research Papers. ISSN 0967-0637. 190. doi: 10.1016/j.dsr.2022.103897. Full text in Research Archive Show summary

  We assessed the activity and swimming patterns of mesopelagic fishes in the Red Sea using bottom-moored, upward-facing echosounders deployed at 555 and 700 m depth. The vertically migrating mesopelagic scattering layer descended close to the bottom during daytime. This permitted assessment of behavior at mesopelagic depths by applying acoustic target tracking for individuals traversing the acoustic beam. Swimming activity did not fit the notion of torpid behavior in the daytime habitat. The fishes were moving continuously, with a prevailing downward direction before noon and upward after, though individuals were swimming in both directions at all times. They moreover were swimming horizontally at estimated speeds of ∼2.1 cm s−1, suggesting ∼0.5–1 body length s−1, intermittently turning. High activity at high temperatures suggests high respiration at depth, considered a key element for the active carbon pump.

• Kaartvedt, Stein; Ugland, Karl I.; Heuschele, Jan David & Solberg, Ingrid (2021). Coordinated gas release among the physostomous fish sprat (Sprattus sprattus). Scientific Reports. 11(1). doi: 10.1038/s41598-021-92585-2. Full text in Research Archive Show summary

  Previous experimental studies suggest that the production of sound associated with expelling gas from an open swimbladder may play a role in communication. This would suggest non-random gas release. We used deployed echosounders to study patterns of gas release among a ford population of sprat (Sprattus sprattus). The echosounder records concurrently revealed individual fsh and their release of gas. The gas release primarily occurred at night, partly following recurrent temporal patterns, but also varying between nights. In testing for non-randomness, we formulated a datadriven simulation approach. Non-random gas release scaled with the length of the analyzed time intervals from 1 min to 6 h, and above 30 min the release events in more than 50% of the intervals were signifcantly connected.

• Langbehn, Tom; Aksnes, Dag Lorents; Kaartvedt, Stein; Fiksen, Øyvind; Ljungström, Anna Jeja Gabriella & Jørgensen, Christian (2021). Poleward distribution of mesopelagic fishes is constrained by seasonality in light. Global Ecology and Biogeography. ISSN 1466-822X. 31(3), p. 546–561. doi: 10.1111/geb.13446. Full text in Research Archive Show summary

  Aim: Mesopelagic fishes have a near-global distribution in the upper 1,000 m from tropical to sub-Arctic oceans across temperature regimes. Yet, their abundance decreases poleward and viable populations seem excluded from high latitudes. Why? Location: North Atlantic between 50–85°N, with implications for high-latitude oceans globally. Time period: Present-day. Major taxa studied: Diel vertically migrating (DVM) mesopelagic fishes. Methods: We use a mechanistic, state-dependent life-history model to characterize DVM mesopelagic fishes. This model links light-dependent encounters and temperature-dependent physiology, allowing optimal DVM strategies to emerge. We run the model along a latitudinal gradient with increasing seasonality in light and track individual fitness-related measures, that is, survival and surplus energy, through the annual cycle to make predictions about population consequences. Results: Mesopelagic fishes thrive in the oceans’ twilight zone, and many are dependent on periods of darkness for safe foraging near the surface, before migrating back to depth during daytime. When daylight lasts for 24 hr during the Arctic summer, these fish are trapped in deep waters void of prey because it is never safe to forage in the shallow waters where zooplankton prey are found. Hence, they are left with two poor options, starvation at depth or depredation while foraging. Our model predicts surplus energy, vital for reproduction and growth, to halve from 50–85°N and annual survival to drop by two-thirds over a narrow range of 10° of latitude around the Arctic Circle. Thus, low recruitment and high predation mortality during summer make polar waters population sinks for mesopelagic fishes because of the extreme seasonality in light. Main conclusions: At high latitudes, foraging mesopelagic fishes are exposed to sunlight in upper waters also at night. This makes them easy prey for visual predators, which limits their poleward distribution. Our findings highlight the importance to think beyond temperature to explain high-latitude range limits.

• Christiansen, Svenja; Klevjer, Thor Aleksander; Røstad, Anders; Aksnes, Dag Lorents & Kaartvedt, Stein (2021). Flexible behaviour in a mesopelagic fish (Maurolicus muelleri). ICES Journal of Marine Science. ISSN 1054-3139. 78(5), p. 1623–1635. doi: 10.1093/icesjms/fsab075. Full text in Research Archive Show summary

  Variability of mesopelagic scattering layers is often attributed to environmental conditions or multi-species layer composition. Yet, little is known about variation in behaviour among the individuals forming scattering layers. Based on a 10 months high-resolution dataset from stationary echosounders in a Norwegian fjord, we here assess short-term and long-term behaviour of a single mesopelagic fish species, the pearlside Maurolicus muelleri. The daytime vertical extension of the monospecific pearlside scattering layers spanned four orders of magnitude ambient light in the autumn and winter and less than one order of magnitude in summer. While the main layers tracked relatively stable light levels over daytime, some individuals actively crossed light gradients of up to 1.5 orders of magnitude. This included individuals that moved between scattering layers, and apparently bold individuals that made regular upward excursions beyond the main population distribution. During the daytime, M. muelleri mitigated the risk of predation by forming tight groups in the upper scattering layer and, at light levels >10−6 µmol m−2 s−1, by instantly diving into deeper waters upon encounters with predators. Our observations suggest that individual, and probably state-dependent, decisions may extend the pearlsides’ vertical distribution, with implications for predator–prey interactions.

• Kaartvedt, Stein; Røstad, Anders & Titelman, Josefin (2021). Sleep walking copepods? Calanus diapausing in hypoxic waters adjust their vertical position during winter. Journal of Plankton Research. ISSN 0142-7873. 43(2), p. 199–208. doi: 10.1093/plankt/fbab004. Full text in Research Archive Show summary

  Abstract While hypoxia is generally associated with negative connotations, some animals may also take advantage of reduced oxygen concentrations. However, the dynamics of such processes for zooplankton are poorly understood. We made continuous acoustic studies of Calanus helgolandicus overwintering in hypoxic waters (Oslofjorden, Norway). Their apparent minimum oxygen tolerance was 0.2–0.3 mL O2 L−1 at 8°C. The copepods adjusted their vertical distribution in concert with the upward progression of hypoxia as oxygen contents declined in the course of winter. The hypoxic overwintering habitat largely excluded potential predators and mortality appeared low in early winter. As the copepod distribution shallowed in phase with declining oxygen contents at depth, mortality increased. In contrast to recent predictions, C. helgolandicus had sufficient energy reserves to sustain long-term overwintering. Termination of the overwintering phase in spring was gradual but appeared to accelerate during the development of the spring bloom. Enhanced oceanic deoxygenation with climate change may affect seasonally migrating copepods in unpredictable ways.

• Kaartvedt, Stein; Røstad, Anders; Christiansen, Svenja & Klevjer, Thor Aleksander (2020). Diel vertical migration and individual behavior of nekton beyond the ocean's twilight zone. Deep Sea Research Part I: Oceanographic Research Papers. ISSN 0967-0637. 160. doi: 10.1016/j.dsr.2020.103280. Full text in Research Archive Show summary

  Diel vertical migration (DVM) is normally limited to the upper 1000 m. However, the use of ship-borne and stationary submerged echosounders in the Red Sea unveiled consistent formation of daytime patches extending to 1200-1300 m, the patches dissolving at night when individuals migrated shallower. The diel vertical migration was not synchronized, and in the evening 4–5 h passed from the first to the last individual leaving their continuously dark daytime habitat. The mean ascent and descent speeds were ~15 cm s−1. Individuals were active upon return to deep water in the morning, some swimming rapidly up and down at speeds of up to ~25 cm s−1, with abrupt changes in swimming direction. Patch formation was swift, in spite of dilute overall population density, suggesting remote signaling to team up with conspecifics. Vertical swimming subsided as patches became established and persisted at 2–3 cm s−1 within patches. The acoustic target strength of the individuals peaked at ~ -55 dB (38 kHz) and was constant regardless of vertical swimming directions. Based on the behavioral and acoustic evidence squids (Sthenoteuthis sp) are likely acoustic targets.

• Kaartvedt, Stein; Røstad, Anders; Opdal, Anders Frugård & Aksnes, Dag Lorents (2019). Herding mesopelagic fish by light. Marine Ecology Progress Series. ISSN 0171-8630. 625, p. 225–231. doi: 10.3354/meps13079. Full text in Research Archive Show summary

  To assess organisms forming mesopelagic scattering layers in the Red Sea, we took advantage of their reactions to light. We used a remotely operated vehicle (ROV) equipped with LED lamps for herding the acoustic targets down to the bottom (700 m), while concurrently monitoring the event by shipborne and deployed echosounders as well as video footage from the ROV. In essence, entire mesopelagic scattering layers at 38 kHz were moved downwards until the organisms became trapped and concentrated at the bottom and identified as fish from video images. However, responses to the artificial light source appeared to include both repulsion and attraction. An individual-based model reproduced the herding event by assuming a dichotomous response to light where targets close to the light source are attracted, while targets further away are repulsed. We hypothesize that attraction is associated with the artificial light acting as a point source (beam light), while the repulsion is associated with the artificial light acting as diffuse light.

• Christiansen, Svenja; Titelman, Josefin & Kaartvedt, Stein (2019). Nighttime Swimming Behavior of a Mesopelagic Fish. Frontiers in Marine Science. 6, p. 1–12. doi: 10.3389/fmars.2019.00787. Full text in Research Archive Show summary

  Nocturnal migration of mesopelagic fish into surface waters is well-documented. Yet, although there is increasing evidence of individual-based deviations from average population migrations and of the importance of small-scale individual behavior for predator-prey interactions and energetic cycling, little is known about what mesopelagic animals do when in upper waters. Using high-resolution data from an upward-facing, moored, split-beam echosounder we analyzed the night-time individual vertical swimming behavior of pearlsides (Maurolicus muelleri) over one winter. The population behavior is characterized by migration to the surface after sunset, “midnight-sinking” and another migration to the surface in the morning, followed by return to the daytime habitat. Focusing on individuals unveiled diverse behavioral patterns during different phases of the migration. After ascending to upper layers at dusk, M. muelleri leaves the surface waters, not by sinking, but by actively swimming in a step-wise pattern characterized by relocations alternated by pauses. Following the descent, vertical swimming is sustained at lower levels. Around midnight, the vertical swimming direction changes from predominantly downward to upward. Several hours before dawn, the fish start ascending toward the surface in a step-wise pattern. During population ascent in the afternoon and descent in the morning, some individuals at the fringes of schools migrate without intermittent pauses. This study documents the feasibility of using submerged, stationary echosounders in unveiling the individual behavior of mesopelagic fish.

• Kaartvedt, Stein; Langbehn, Tom & Aksnes, Dag Lorents (2019). Enlightening the ocean’s twilight zone. ICES Journal of Marine Science. ISSN 1054-3139. 76(4), p. 803–812. doi: 10.1093/icesjms/fsz010. Full text in Research Archive
• Langbehn, Tom; Aksnes, Dag Lorents; Kaartvedt, Stein; Fiksen, Øyvind & Jørgensen, Christian (2019). Light comfort zone in a mesopelagic fish emerges from adaptive behaviour along a latitudinal gradient. Marine Ecology Progress Series. ISSN 0171-8630. 623, p. 161–174. doi: 10.3354/meps13024. Full text in Research Archive Show summary

  Throughout the oceans, small fish and other micronekton migrate between daytime depths of several hundred meters and near-surface waters at night. These diel vertical migrations of mesopelagic organisms structure pelagic ecosystems through trophic interactions, and are a key element in the biological carbon pump. However, depth distributions and migration amplitude vary greatly. Suggested proximate causes of the migration such as oxygen, temperature, and light often correlate and therefore the causal underpinnings have remained unclear. Using mesopelagic fishes and the Norwegian Sea as a study system, we developed a dynamic state variable model that finds optimal migration patterns that we validate with acoustic observations along a latitudinal gradient. The model describes predation risk and bioenergetics, and maximizes expected energy surplus, a proxy for Darwinian fitness. The model allows us to disentangle the drivers of migration and make predictions about depth distribution and related fitness consequences along a latitudinal trajectory with strong gradients in environmental drivers and vertical distribution of scattering layers. We show that the model-predicted vertical migration of mesopelagic fishes matches that observed along this transect. For most situations, modelled mesopelagic fish behaviour can be well described by a light comfort zone near identical to that derived from observations. By selectively keeping light or temperature constant, the model reveals that temperature, in comparison with light, has little effect on depth distribution. We find that water clarity, which limits how deeply light can penetrate into the ocean, structures daytime depths, while surface light at night controlled the depth of nocturnal ascents.

• Calleja, Maria Ll; Ansari, Mohd I; Røstad, Anders; Silva, Luis; Kaartvedt, Stein & Irigoien, Xabier [Show all 7 contributors for this article] (2018). The mesopelagic scattering layer: A hotspot for heterotrophic prokaryotes in the Red Sea twilight zone. Frontiers in Marine Science. 5, p. 1–9. doi: 10.3389/fmars.2018.00259. Full text in Research Archive Show summary

  The vast majority of marine dissolved organic carbon (DOC), the largest reservoir of reduced carbon on Earth, is believed to accumulate in the abyssal layers of the ocean over timescales of decades to millennia. However, evidence is growing that small animals that migrate vertically every day from the surface to mesopelagic layers are significantly contributing to the active vertical flux of organic matter. Whether that represents an important source of carbon available for microbial production and respiration at the mesopelagic realm, and its contribution to oceanic carbon budgets and energy flows, is yet to be explored. Here we present data suggesting that Red Sea migrating animals may produce an overlooked source of labile DOC (used at a mean rate of 2.1 μmol C L−1 d−1) that does not accumulate but fuels the metabolism in the twilight zone, generating a disregarded hotspot for heterotrophic prokaryotes.

• Hozumi, Aya; Kaartvedt, Stein; Røstad, Anders; Berumen, Michael L; Cochran, Jesse E.M. & Jones, Burton H. (2018). Acoustic backscatter at a Red Sea whale shark aggregation site. Regional Studies in Marine Science. 20, p. 23–33. doi: 10.1016/j.rsma.2018.03.008. Full text in Research Archive
• Hozumi, Aya; Hong, Pei-Ying; Kaartvedt, Stein; Røstad, Anders & Jones, Burton H. (2018). Water quality, seasonality, and trajectory of an aquaculture-wastewater plume in the Red Sea. Aquaculture Environment Interactions. ISSN 1869-215X. 10, p. 61–77. doi: 10.3354/AEI00254. Full text in Research Archive
• Kaartvedt, Stein & Titelman, Josefin (2018). Planktivorous fish in a future Arctic Ocean of changing ice and unchanged photoperiod. ICES Journal of Marine Science. ISSN 1054-3139. 75(7), p. 2312–2318. doi: 10.1093/icesjms/fsx248. Full text in Research Archive
• Solberg, Ingrid & Kaartvedt, Stein (2017). The diel vertical migration patterns and individual swimming behavior of overwintering sprat Sprattus sprattus. Progress in Oceanography. ISSN 0079-6611. 151, p. 49–61. doi: 10.1016/j.pocean.2016.11.003. Full text in Research Archive Show summary

  Sprat DVM Dusk and dawn Hypoxia Diving behavior Nocturnal behavior Swimming speed

• de, Fanny Busserolles; Cortesi, Fabio; Helvik, Jon Vidar; Davies, Wayne I. L.; Templin, Rachel M. & Sullivan, Robert K. P. [Show all 12 contributors for this article] (2017). Pushing the limits of photoreception in twilight conditions: The rod-like cone retina of the deep-sea pearlsides. Science Advances. 3(11), p. 1–13. doi: 10.1126/sciadv.aao4709. Full text in Research Archive Show summary

  Most vertebrates have a duplex retina comprising two photoreceptor types, rods for dim-light (scotopic) vision and cones for bright-light (photopic) and color vision. However, deep-sea fishes are only active in dim-light conditions; hence, most species have lost their cones in favor of a simplex retina composed exclusively of rods. Although the pearlsides, Maurolicus spp., have such a pure rod retina, their behavior is at odds with this simplex visual system. Contrary to other deep-sea fishes, pearlsides are mostly active during dusk and dawn close to the surface, where light levels are intermediate (twilight or mesopic) and require the use of both rod and cone photoreceptors. This study elucidates this paradox by demonstrating that the pearlside retina does not have rod photoreceptors only; instead, it is composed almost exclusively of transmuted cone photoreceptors. These transmuted cells combine the morphological characteristics of a rod photoreceptor with a cone opsin and a cone phototransduction cascade to form a unique photoreceptor type, a rod-like cone, specifically tuned to the light conditions of the pearlsides’ habitat (blue-shifted light at mesopic intensities). Combining properties of both rods and cones into a single cell type, instead of using two photoreceptor types that do not function at their full potential under mesopic conditions, is likely to be the most efficient and economical solution to optimize visual performance. These results challenge the standing paradigm of the function and evolution of the vertebrate duplex retina and emphasize the need for a more comprehensive evaluation of visual systems in general.

• Bozman, Andrea; Titelman, Josefin; Kaartvedt, Stein; Eiane, Ketil & Aksnes, Dag Lorents (2017). Jellyfish distribute vertically according to irradiance. Journal of Plankton Research. ISSN 0142-7873. 39(2), p. 280–289. doi: 10.1093/plankt/fbw097. Full text in Research Archive Show summary

  We tested the hypothesis that the coronate jellyfish Periphylla periphylla distributes vertically according to a preferential range of absolute light intensities. The study was carried out in Lurefjorden, Norway, a fjord characterized by mass occurrences of this jellyfish. We collected data on the vertical distribution of P. periphylla medusa during day, dusk and night periods from video observations by a remotely operated vehicle in relation to estimated ambient light levels. Our results suggest that large P. periphylla (average size in catches ~9 cm diameter) avoided total irradiance levels above 5×10−3 µmol quanta m−2 s−1. Nearly two-thirds of the population stayed above irradiance of 10−7 µmol quanta m−2 s−1 during daytime, while some individuals occupied much darker water. Thus, part of the population appeared to distribute vertically and undertake diel vertical migration (DVM) according to a preferential range of light intensities.

• Kaartvedt, Stein; Røstad, Anders & Aksnes, Dag Lorents (2017). Changing weather causes behavioral responses in the lower mesopelagic. Marine Ecology Progress Series. ISSN 0171-8630. 574, p. 259–263. doi: 10.3354/meps12185. Full text in Research Archive
• Aksnes, Dag Lorents; Røstad, Anders; Kaartvedt, Stein; Martinez, Udane; Duarte, Carlos M. & Irigoien, Xabier (2017). Light penetration structures the deep acoustic scattering layers in the global ocean. Science Advances. 3(5). doi: 10.1126/sciadv.1602468. Full text in Research Archive Show summary

  The deep scattering layer (DSL) is a ubiquitous acoustic signature found across all oceans and arguably the dominant feature structuring the pelagic open ocean ecosystem. It is formed by mesopelagic fishes and pelagic invertebrates. The DSL animals are an important food source for marine megafauna and contribute to the biological carbon pump through the active flux of organic carbon transported in their daily vertical migrations. They occupy depths from 200 to 1000 m at daytime and migrate to a varying degree into surface waters at nighttime. Their daytime depth, which determines the migration amplitude, varies across the global ocean in concert with water mass properties, in particular the oxygen regime, but the causal underpinning of these correlations has been unclear. We present evidence that the broad variability in the oceanic DSL daytime depth observed during the Malaspina 2010 Circumnavigation Expedition is governed by variation in light penetration. We find that the DSL depth distribution conforms to a common optical depth layer across the global ocean and that a correlation between dissolved oxygen and light penetration provides a parsimonious explanation for the association of shallow DSL distributions with hypoxic waters. In enhancing understanding of this phenomenon, our results should improve the ability to predict and model the dynamics of one of the largest animal biomass components on earth, with key roles in the oceanic biological carbon pump and food web.

• Banguera-Hinestroza, E; Eikrem, W; Mansour, H; Solberg, I; Cúrdia, J & Holtermann, K [Show all 8 contributors for this article] (2016). Seasonality and toxin production of Pyrodinium bahamense in a Red Sea lagoon. Harmful Algae. ISSN 1568-9883. 55, p. 163–171. doi: 10.1016/j.hal.2016.03.002. Full text in Research Archive
• Folkvord, Arild; Gundersen, Geir; Albretsen, Jon; Asplin, Lars; Kaartvedt, Stein & Giske, Jarl (2016). Impact of hatch date on early life growth and survival of Mueller’s pearlside (Maurolicus muelleri) larvae and life-history consequences. Canadian Journal of Fisheries and Aquatic Sciences. ISSN 0706-652X. 73, p. 163–176. doi: 10.1139/cjfas-2015-0040. Full text in Research Archive
• Folkvord, Arild; Gundersen, Geir; Albretsen, Jon; Asplin, Lars; Kaartvedt, Stein & Giske, Jarl (2016). Impact of hatch-date on early life growth and survival of Mueller's pearlside (Maurolicus Muelleri) larvae and life-history consequences. Canadian Journal of Fisheries and Aquatic Sciences. ISSN 0706-652X. 73(2), p. 163–176. doi: 10.1139/cjfas-2015-0040. Full text in Research Archive Show summary

  Growth and survival of Maurolicus muelleri larvae in Herdlefjorden, Norway, were investigated by daily otolith increment analysis. While high egg densities were generally observed throughout the spawning season, three cohorts each with a narrow window of hatching dates were identified. The first of these cohorts was characterized by low growth and poor morphometric condition and disappeared from the fjord during autumn. High-resolution drift modeling indicated that Herdlefjorden had a net export of larvae and negligible import in the period cohort 1 disappeared. Yet, the advective loss rate of larvae was not considered high enough to explain the near complete disappearance of the first cohort. An otolith-based growth chronology indicated that growth conditions in Herdlefjorden improved noticeably around mid-September and remained favorable the following month. The analysis of daily otolith increments could thus be used to document within-season variability in larval growth and survival. The low and variable survival due to short-term fluctuations in environmental conditions indicate that multiple batch spawning is an adequately evolved life-history strategy for marine planktivorous fish such as M. muelleri.

• Ariza, A; Landeira, J.M.; Escanez, A.; Wienerroither, Rupert; Soto, N. Aguilar de & Røstad, Anders [Show all 8 contributors for this article] (2016). Vertical distribution, composition and migratory patterns of acoustic scattering layers in the Canary Islands. Journal of Marine Systems. ISSN 0924-7963. 157, p. 82–91. doi: 10.1016/j.jmarsys.2016.01.004. Full text in Research Archive
• Klevjer, Thor A.; Irigoien, Xabier; Røstad, Anders; Fraile-Nuez, Eugenio; Benítez-Barrios, Verónica María & Kaartvedt, Stein (2016). Large scale patterns in vertical distribution and behaviour of mesopelagic scattering layers. Scientific Reports. 6. doi: 10.1038/srep19873. Full text in Research Archive Show summary

  Recent studies suggest that previous estimates of mesopelagic biomasses are severely biased, with the new, higher estimates underlining the need to unveil behaviourally mediated coupling between shallow and deep ocean habitats. We analysed vertical distribution and diel vertical migration (DVM) of mesopelagic acoustic scattering layers (SLs) recorded at 38 kHz across oceanographic regimes encountered during the circumglobal Malaspina expedition. Mesopelagic SLs were observed in all areas covered, but vertical distributions and DVM patterns varied markedly. The distribution of mesopelagic backscatter was deepest in the southern Indian Ocean (weighted mean daytime depth: WMD 590 m) and shallowest at the oxygen minimum zone in the eastern Pacific (WMD 350 m). DVM was evident in all areas covered, on average ~50% of mesopelagic backscatter made daily excursions from mesopelagic depths to shallow waters. There were marked differences in migrating proportions between the regions, ranging from ~20% in the Indian Ocean to ~90% in the Eastern Pacific. Overall the data suggest strong spatial gradients in mesopelagic DVM patterns, with implied ecological and biogeochemical consequences. Our results suggest that parts of this spatial variability can be explained by horizontal patterns in physical-chemical properties of water masses, such as oxygen, temperature and turbidity.

• Seibel, Brad A.; Schneider, Jillian L.; Kaartvedt, Stein; Wishner, Karen F. & Daly, Kendra L. (2016). Hypoxia tolerance and metabolic suppression in oxygen minimum zone euphausiids: implications for ocean deoxygenation and biogeochemical cycles. Integrative and Comparative Biology. ISSN 1540-7063. 56(4), p. 510–523. doi: 10.1093/icb/icw091. Full text in Research Archive
• Prihartato, Perdana K.; Irigoien, Xabier; Genton, Marc G. & Kaartvedt, Stein (2016). Global effects of moon phase on nocturnal acoustic scattering layers. Marine Ecology Progress Series. ISSN 0171-8630. 544, p. 65–75. doi: 10.3354/meps11612. Full text in Research Archive
• Wiebe, Peter H.; Bucklin, Ann; Kaartvedt, Stein; Røstad, Anders & Blanco-Bercial, Leocadio (2016). Vertical distribution and migration of euphausiid species in the Red Sea. Journal of Plankton Research. ISSN 0142-7873. 38(4), p. 888–903. doi: 10.1093/plankt/fbw038. Full text in Research Archive
• Kaartvedt, Stein; Antunes, andré; Røstad, Anders; Klevjer, Thor A. & Vestheim, Hege Finsås (2016). Zooplankton at deep Red Sea brine pools. Journal of Plankton Research. ISSN 0142-7873. 38(3), p. 679–684. doi: 10.1093/plankt/fbw013. Full text in Research Archive
• Røstad, Anders; Kaartvedt, Stein & Aksnes, Dag Lorents (2016). Light comfort zones of mesopelagic acoustic scattering layers in two contrasting optical environments. Deep Sea Research Part I: Oceanographic Research Papers. ISSN 0967-0637. 113, p. 1–6. doi: 10.1016/j.dsr.2016.02.020. Full text in Research Archive Show summary

  We make a comparison of the mesopelagic sound scattering layers (SLs) in two contrasting optical environments; the clear Red Sea and in murkier coastal waters of Norway (Masfjorden). The depth distributions of the SL in Masfjorden are shallower and narrower than those of the Red Sea. This difference in depth distribution is consistent with the hypothesis that the organisms of the SL distribute according to similar light comfort zones (LCZ) in the two environments. Our study suggest that surface and underwater light measurements ranging more than 10 orders of magnitude is required to assess the controlling effects of light on SL structure and dynamics.

• Prihartato, Perdana K.; Aksnes, Dag Lorents & Kaartvedt, Stein (2015). Seasonal patterns in the nocturnal distributionand behavior of the mesopelagic fish Maurolicus muelleri at high latitudes. Marine Ecology Progress Series. ISSN 0171-8630. 521, p. 189–200. doi: 10.3354/meps11139. Full text in Research Archive
• Calbet, Albert; Agersted, Mette Dalgaard; Kaartvedt, Stein; Møhl, Malene; Møller, Eva Friis & Enghoff-Poulsen, Søren [Show all 12 contributors for this article] (2015). Heterogeneous distribution of plankton within the mixed layer and its implications for bloom formation in tropical seas. Scientific Reports. 5, p. 1–14. doi: 10.1038/srep11240. Full text in Research Archive Show summary

  Intensive sampling at the coastal waters of the central Red Sea during a period of thermal stratification, prior to the main seasonal bloom during winter, showed that vertical patches of prokaryotes and microplankton developed and persisted for several days within the apparently density uniform upper layer. These vertical structures were most likely the result of in situ growth and mortality (e.g., grazing) rather than physical or behavioural aggregation. Simulating a mixing event by adding nutrient-rich deep water abruptly triggered dense phytoplankton blooms in the nutrient-poor environment of the upper layer. These findings suggest that vertical structures within the mixed layer provide critical seeding stocks that can rapidly exploit nutrient influx during mixing, leading to winter bloom formation.

• Solberg, Ingrid; Røstad, Anders & Kaartvedt, Stein (2015). Ecology of overwintering sprat (Sprattus sprattus). Progress in Oceanography. ISSN 0079-6611. 138, p. 116–135. doi: 10.1016/j.pocean.2015.08.003. Full text in Research Archive Show summary

  We used moored upward-facing echosounders in combination with field campaigns to address the overwintering ecology of the clupeid sprat (Sprattus sprattus) throughout four separate winters in a Norwegian fjord. The stationary echosounders were cabled to shore and provided continuous measurements at a temporal resolution of seconds. The long-term coverage of several winters enabled study of the sprat behavior in relation to different biotic parameters like abundance, vertical distribution and taxonomic composition of potential prey and predators, as well as environmental conditions like ice-free vs. ice-covered waters and hypoxic- vs. normoxic conditions. Also the size distribution of the sprat differed significantly between years. The majority of the large-size classes had empty stomachs, particularly prominent in one winter. Otherwise, the diet of the sprat seemed to vary according to the fluctuating mesozooplankton community, yet with calanoid copepods being the most common prey in the sprat stomachs all winters. Krill were not common prey apart for the largest sprat in one winter, but particularly large concentrations of krill appeared to mitigate predation pressure from gadoids, which then preferred krill as prey. During daytime, sprat distribution and swimming behavior varied according to the oxygen conditions. Solitary swimming in near-bottom-waters (~150 m) prevailed in moderate hypoxia (30% O2 saturation) as opposed to schooling in mid-waters when the deep waters were oxygen depleted (0–7% O2 saturation). Nevertheless, a bimodal vertical distribution with an additional part of the sprat population distributed in upper waters was common in all years. The sprat carried out diel vertical migration (DVM) in all winters, but the patterns varied, and included both normal and asynchronous DVM, including fish with a somewhat deeper nocturnal than daytime distribution. Moreover, individual sprat carried out short and rapid excursions to the surface during the night in all years, likely for gulping atmospheric air. Ice conditions imposed a behavioral response with the sprat moving to shallower depths after the ice covering. The varied ecology and behavior observed throughout the course of four consecutive years underlines the importance of conducting long-term studies for the understanding of overwintering strategies. Overall, this study provided unique insight into the dynamic conditions that a population of fish may encounter while overwintering, providing novel information on a scarcely described phase in the life history of fish at high latitudes.

• Kaartvedt, Stein; Ugland, Karl Inne; Klevjer, Thor A.; Røstad, Anders; Titelman, Josefin & Solberg, Ingrid (2015). Social behaviour in mesopelagic jellyfish. Scientific Reports. 5, p. 1–8. doi: 10.1038/srep11310. Full text in Research Archive Show summary

  Gelatinous organisms apparently play a central role in deep pelagic ecosystems, but lack of observational methodologies has restricted information on their behaviour. We made acoustic records of diel migrating jellyfish Periphylla periphylla forming small, ephemeral groups at the upper fringe of an acoustic scattering layer consisting of krill. Groups of P. periphylla were also documented photographically using a remotely operated vehicle (ROV). Although the adaptive value of group formation remains speculative, we clearly demonstrate the ability of these jellyfishes to locate and team up with each other.

• Vestheim, Hege & Kaartvedt, Stein (2015). A deep sea community at the Kebrit brine pool in the Red Sea. Marine Biodiversity. ISSN 1867-1616. p. 1–7. doi: 10.1007/s12526-015-0321-0. Full text in Research Archive
• Ugland, Karl Inne; Aksnes, Dag Lorents; Klevjer, Thor A.; Titelman, Josefin & Kaartvedt, Stein (2014). Lévy night flights by the jellyfish Periphylla periphylla. Marine Ecology Progress Series. ISSN 0171-8630. 513, p. 121–130. doi: 10.3354/meps10942. Full text in Research Archive
• Godø, Olav Rune; Handegard, Nils Olav; Browman, Howard; Macaulay, Gavin; Kaartvedt, Stein & Giske, Jarl [Show all 9 contributors for this article] (2014). Marine ecosystem acoustics (MEA): Quantifying processes in the sea at the spatio-temporal scales on which they occur. ICES Journal of Marine Science. ISSN 1054-3139. 71(8), p. 2357–2369. doi: 10.1093/icesjms/fsu116. Full text in Research Archive
• Vestheim, Hege; Røstad, Anders; Klevjer, Thor A.; Solberg, Ingrid & Kaartvedt, Stein (2014). Vertical distribution and diel vertical migration of krill beneath snow-covered ice and in ice-free waters. Journal of Plankton Research. ISSN 0142-7873. 36(2), p. 503–512. doi: 10.1093/plankt/fbt112. Full text in Research Archive
• Solberg, Ingrid & Kaartvedt, Stein (2014). Surfacing behavior and gas release of the physostome sprat (Sprattus sprattus) in ice-free and ice-covered waters. Marine Biology. ISSN 0025-3162. 161(2), p. 285–296. doi: 10.1007/s00227-013-2334-1. Full text in Research Archive Show summary

  Upward-facing echosounders that provided continuous, long-term measurements were applied to address the surfacing behavior and gas release of the physostome sprat (Sprattus sprattus) throughout an entire winter in a 150-m-deep Norwegian fjord. During ice-free conditions, the sprat surfaced and released gas bubbles at night with an estimated surfacing rate of 3.5 times per fish day−1. The vertical swimming speeds during surfacing were considerably higher (~10 times) than during diel vertical migrations, especially when returning from the surface, and particularly when the fjord was not ice covered. The sprat released gas a few hours after surfacing, suggesting that the sprat gulped atmospheric air during its excursions to the surface. While the surface activity increased after the fjord became ice covered, the records of gas release decreased sharply. The under-ice fish then displayed a behavior interpreted as “searching for the surface” by repeatedly ascending toward the ice, apparently with limited success of filling the swim bladder. This interpretation was supported by lower acoustic target strength in ice-covered waters. The frequent surfacing behavior demonstrated in this study indicates that gulping of atmospheric air is an important element in the life of sprat. While at least part of the population endured overwintering in the ice-covered habitat, ice covering may constrain those physostome fishes that lack a gas-generating gland in ways that remain to be established.

• Kaartvedt, Stein (2014). The Submarine Volcano Eruption off El Hierro Island: Effects on the Scattering Migrant Biota and the Evolution of the Pelagic Communities. PLOS ONE. 9(7). doi: 10.1371/journal.pone.0102354. Full text in Research Archive
• Hessen, Dag Olav & Kaartvedt, Stein (2014). Top-down cascades in lakes and oceans: Different perspectives but same story? Journal of Plankton Research. ISSN 0142-7873. 36(4), p. 914–924. doi: 10.1093/plankt/fbu040. Full text in Research Archive
• Ariza, A; Kaartvedt, Stein; Røstad, A; Garijo, JC; Aristegui, J & Fraile-Nuez, E [Show all 7 contributors for this article] (2014). The submarine volcano eruption off El Hierro Island: Effects on the scattering migrant biota and the evolution of the pelagic communities. PLOS ONE. doi: 10.1371/journal.pone.0102354. Full text in Research Archive
• Irigoien, Xabier; Klevjer, Thor A.; Røstad, Anders; Martinez, U; Boyra, G & Acuna, JL [Show all 14 contributors for this article] (2014). Large mesopelagic fishes biomass and trophic efficiency in the open ocean. Nature Communications. doi: 10.1038/ncomms4271. Full text in Research Archive Show summary

  With a current estimate of ~1,000 million tons, mesopelagic fishes likely dominate the world total fishes biomass. However, recent acoustic observations show that mesopelagic fishes biomass could be significantly larger than the current estimate. Here we combine modelling and a sensitivity analysis of the acoustic observations from the Malaspina 2010 Circumnavigation Expedition to show that the previous estimate needs to be revised to at least one order of magnitude higher. We show that there is a close relationship between the open ocean fishes biomass and primary production, and that the energy transfer efficiency from phytoplankton to mesopelagic fishes in the open ocean is higher than what is typically assumed. Our results indicate that the role of mesopelagic fishes in oceanic ecosystems and global ocean biogeochemical cycles needs to be revised as they may be respiring ~10% of the primary production in deep waters.

• Skjoldal, Hein Rune; Wiebe, Peter; Postel, Lutz; Knutsen, Tor; Kaartvedt, Stein & Sameoto, Douglas D. (2013). Intercomparison of zooplankton (net) sampling systems: Results from the ICES/GLOBEC sea-going workshop. Progress in Oceanography. ISSN 0079-6611. 108, p. 1–42. doi: 10.1016/j.pocean.2012.10.006. Full text in Research Archive
• Vestheim, Hege Finsås; Brucet, Sandra & Kaartvedt, Stein (2013). Vertical distribution, feeding and vulnerability to tactile predation in Metridia longa (Copepoda, Calanoida). Marine Biology Research. ISSN 1745-1000. 9(10), p. 949–957. doi: 10.1080/17451000.2013.793806. Full text in Research Archive
• Røstad, Anders & Kaartvedt, Stein (2013). Seasonal and diel patterns in sedimentary flux of krill fecal pellets recorded by an echo sounder. Limnology and Oceanography. ISSN 0024-3590. 58(6), p. 1985–1997. doi: 10.4319/lo.2013.58.6.1985. Full text in Research Archive
• Solberg, Ingrid; Kaartvedt, Stein & Klevjer, Thor A. (2013). The use of echosounders for long-term studies of the overwintering ecology of sprat (Sprattus sprattus), 2013 MTS/IEEE OCEANS - Bergen. IEEE (Institute of Electrical and Electronics Engineers). ISSN 9781479900015. doi: 10.1109/OCEANS-Bergen.2013.6608015. Full text in Research Archive Show summary

  Upward-facing echosounders, were used to study the overwintering ecology of sprat during four winters in a Norwegian fjord. The echosounders provided continuous data at a temporal resolution of seconds and enabled studies of individual swimming behavior of sprat in addition to population behavior. The long-term coverage of several winters enabled us to study how the sprat responded to different environmental conditions, like ice-free waters versus ice covered waters and hypoxic conditions versus well-oxygenated waters. The studies unveiled that the overwintering strategies of the sprat are flexible, varying in accordance with environmental conditions.

• Bagøien, Espen; Melle, Webjørn Raunsgård & Kaartvedt, Stein (2012). Seasonal development of mixed layer depths, nutrients, chlorophyll and Calanus finmarchicus in the Norwegian Sea – A basin-scale habitat comparison. Progress in Oceanography. ISSN 0079-6611. 103, p. 58–79. doi: 10.1016/j.pocean.2012.04.014. Full text in Research Archive
• Kaartvedt, Stein; Staby, Arved & Aksnes, Dag Lorents (2012). Efficient trawl avoidance by mesopelagic fishes causes large underestimation of their biomass. Marine Ecology Progress Series. ISSN 0171-8630. 456, p. 1–6. doi: 10.3354/meps09785. Full text in Research Archive
• Kaartvedt, Stein; Klevjer, Thor A. & Aksnes, Dag Lorents (2012). Internal wave-mediated shading causes frequent vertical migrations in fishes. Marine Ecology Progress Series. ISSN 0171-8630. 452, p. 1–10. doi: 10.3354/meps09688. Full text in Research Archive
• Godø, Olav Rune; Samuelsen, Annette; Macaulay, Gavin John; Patel, Ruben; Hjøllo, Solfrid Sætre & Horne, John [Show all 8 contributors for this article] (2012). Mesoscale Eddies Are Oases for Higher Trophic Marine Life. PLOS ONE. 7(1). doi: 10.1371/journal.pone.0030161. Full text in Research Archive Show summary

  Mesoscale eddies stimulate biological production in the ocean, but knowledge of energy transfers to higher trophic levels within eddies remains fragmented and not quantified. Increasing the knowledge base is constrained by the inability of traditional sampling methods to adequately sample biological processes at the spatio-temporal scales at which they occur. By combining satellite and acoustic observations over spatial scales of 10 s of km horizontally and 100 s of m vertically, supported by hydrographical and biological sampling we show that anticyclonic eddies shape distribution and density of marine life from the surface to bathyal depths. Fish feed along density structures of eddies, demonstrating that eddies catalyze energy transfer across trophic levels. Eddies create attractive pelagic habitats, analogous to oases in the desert, for higher trophic level aquatic organisms through enhanced 3-D motion that accumulates and redistributes biomass, contributing to overall bioproduction in the ocean. Integrating multidisciplinary observation methodologies promoted a new understanding of biophysical interaction in mesoscale eddies. Our findings emphasize the impact of eddies on the patchiness of biomass in the sea and demonstrate that they provide rich feeding habitat for higher trophic marine life.

• Klevjer, Thor A.; Torres, Daniel J. & Kaartvedt, Stein (2012). Distribution and diel vertical movements of mesopelagic scattering layers in the Red Sea. Marine Biology. ISSN 0025-3162. 159(8), p. 1833–1841. doi: 10.1007/s00227-012-1973-y. Full text in Research Archive
• Dypvik, Eivind; Røstad, Anders & Kaartvedt, Stein (2012). Seasonal variations in vertical migration of glacier lanternfish, Benthosema glaciale. Marine Biology. ISSN 0025-3162. 159, p. 1673–1683. doi: 10.1007/s00227-012-1953-2. Full text in Research Archive
• Solberg, Ingrid; Klevjer, Thor A. & Kaartvedt, Stein (2012). Continuous acoustic studies of overwintering sprat Sprattus sprattus reveal flexible behavior. Marine Ecology Progress Series. ISSN 0171-8630. 464, p. 245–256. doi: 10.3354/meps09877. Full text in Research Archive Show summary

  The clupeid fish Sprattus sprattus was studied in a 150 m deep Norwegian fjord throughout an entire overwintering period during which the fjord froze over and a major water renewal occurred. A bottom-mounted (upward-facing) echosounder provided continuous highresolution data and enabled studies of swimming speed and behavior of individual sprat in addition to population behavior. The continuous acoustic studies were supplemented with intermittent field campaigns. The sprat displayed different behavioral modes with changing environmental conditions. During the first part of the winter, the majority of the population occurred in deep waters during both day and night, yet exhibited a shallower night-time distribution. Individual sprat swam alternately up and down, a ‘rise and sink’ behavior likely a compensation for negative buoyancy because of swim bladder compression. Because feeding was negligible in deep waters, the swimming pattern was not inferred as prey search behavior. Another part of the population schooled at shallower depths during the day and carried out vertical migration to upper waters at night. However, individuals were observed as they switched between these behavioral groups. A sudden change in both swimming behavior and vertical distribution occurred as the fjord became ice covered. Near-bottom ‘rise and sink’ swimming was replaced by schooling in midwater during the day, and the sprat aggregated in dense layers near the surface at night. We suggest that the ice made the sprat shift their antipredator strategy from hiding at depth to hiding in schools in the darker waters below the ice. This long-term acoustic study has shown that sprat have a flexible behavioral repertoire, displaying different overwintering strategies within a population, depending on environmental conditions.

• Dypvik, Eivind; Klevjer, Thor A. & Kaartvedt, Stein (2012). Inverse vertical migration and feeding in glacier lanternfish (Benthosema glaciale). Marine Biology. ISSN 0025-3162. 159(2), p. 443–453. doi: 10.1007/s00227-011-1822-4. Full text in Research Archive
• Klevjer, Thor A. & Kaartvedt, Stein (2011). Krill (Meganyctiphanes norvegica) swim faster at night. Limnology and Oceanography. ISSN 0024-3590. 56(3), p. 765–774. doi: 10.4319/lo.2011.56.3.0765. Full text in Research Archive
• Kaartvedt, Stein; Titelman, Josefin; Røstad, Anders & Klevjer, Thor A. (2011). Beyond the average: Diverse individual migration patterns in a population of mesopelagic jellyfish. Limnology and Oceanography. ISSN 0024-3590. 56(6), p. 2189–2199. doi: 10.4319/lo.2011.56.6.2189. Full text in Research Archive
• Staby, Arved; Røstad, Anders & Kaartvedt, Stein (2011). Long-term acoustical observations of the mesopelagic fish Maurolicus muelleri reveal novel and varied vertical migration patterns. Marine Ecology Progress Series. ISSN 0171-8630. 441, p. 241–255. doi: 10.3354/meps09363. Full text in Research Archive
• Utne-Palm, Anne Christine; Salvanes, Anne Gro Vea; Currie, Bronwen; Kaartvedt, Stein; Nilsson, Göran Erik & Braithwaite, Victoria Anne [Show all 19 contributors for this article] (2010). Trophic Structure and Community Stability in an Overfished Ecosystem. Science. ISSN 0036-8075. 329(5989), p. 333–336. doi: 10.1126/science.1190708. Full text in Research Archive
• Wiebe, Peter H.; Chu, Dezhang; Kaartvedt, Stein; Hundt, Anna Hoelter; Melle, Webjørn & Ona, Egil [Show all 7 contributors for this article] (2010). The acoustic properties of Salpa thompsoni. ICES Journal of Marine Science. ISSN 1054-3139. 67(3), p. 583–593. doi: 10.1093/icesjms/fsp263. Full text in Research Archive
• Dupont, Nicolas; Klevjer, Thor A.; Kaartvedt, Stein & Aksnes, Dag Lorents (2009). Diel vertical migration of the deep-water jellyfish Periphylla periphylla simulated as individual responses to absolute light intensity. Limnology and Oceanography. ISSN 0024-3590. 54(5), p. 1765–1775. Full text in Research Archive
• Vestheim, Hege & Kaartvedt, Stein (2009). Vertical migration, feeding and colouration in the mesopelagic shrimp Sergestes arcticus. Journal of Plankton Research. ISSN 0142-7873. 31(11), p. 1427–1435. doi: 10.1093/plankt/fbp077. Full text in Research Archive
• Klevjer, Thor A.; Kaartvedt, Stein & Båmstedt, Ulf (2009). In situ behaviour and acoustic properties of the deep living jellyfish Periphylla periphylla. Journal of Plankton Research. ISSN 0142-7873. 31(8), p. 793–803. doi: 10.1093/plankt/fbp036. Full text in Research Archive
• Aksnes, Dag Lorents; Dupont, Nicolas; Staby, Arved; Fiksen, Øyvind; Kaartvedt, Stein & Aure, Jan (2009). Coastal water darkening and implications for mesopelagic regime shifts in Norwegian fjords. Marine Ecology Progress Series. ISSN 0171-8630. 387, p. 39–49. doi: 10.3354/meps08120. Full text in Research Archive Show summary

  The light regime of the water column has a strong structuring effect on aquatic food webs and it has been previously hypothesized that coastal water darkening has increased the success of tactile predators relative to visual predators such as fish. Due to a general lack of time-series of optical parameters, we applied a proxy for light attenuation that depends on salinity and dissolved oxygen at a time of the year when chlorophyll concentrations were low. We present evidence that coastal waters of Norway have darkened as a result of freshening over the period 1935 to 2007. Our results suggest that locations where coastal water penetrates into deep basins have been prone to water column darkening, particularly if dissolved oxygen has also declined. We have estimated that salinity and oxygen variation on the ranges 33 to 35 PSU and 1 to 6 ml O2 l–1 are associated with up to 8 orders of magnitude difference in photon flux at 200 m depth in a water column devoid of pigments. Our results suggest such darkening needs to be considered in analyses of mesopelagic regime shifts involving mass occurrence of the jellyfish Periphylla periphylla.

• Kaartvedt, Stein; Røstad, Anders; Klevjer, Thor A. & Staby, Arved (2009). Use of bottom-mounted echo sounders in exploring behavior of mesopelagic fishes. Marine Ecology Progress Series. ISSN 0171-8630. 395, p. 109–118. doi: 10.3354/meps08174. Full text in Research Archive
• Kaartvedt, Stein; Røstad, Anders & Klevjer, Thor A. (2009). Sprat Sprattus sprattus can exploit low oxygen waters for overwintering. Marine Ecology Progress Series. ISSN 0171-8630. 390, p. 237–249. doi: 10.3354/meps08196. Full text in Research Archive
• Broms, Cecilie; Melle, Webjørn & Kaartvedt, Stein (2009). Oceanic distribution and life cycle of Calanus species in the Norwegian Sea and adjacent waters. Deep-Sea Research Part II: Topical Studies in Oceanography. ISSN 0967-0645. 56(21-22), p. 1910–1921. doi: 10.1016/j.dsr2.2008.11.005. Full text in Research Archive
• Kaartvedt, Stein (2008). Photoperiod may constrain the effect of global warming in arctic marine systems. Journal of Plankton Research. ISSN 0142-7873. 30(11), p. 1203–1206. doi: 10.1093/plankt/fbn075. Full text in Research Archive
• Kaartvedt, Stein; Torgersen, Thomas; Klevjer, Thor A.; Røstad, Anders & Devine, Jennifer A. (2008). Behavior of individual mesopelagic fish in acoustic scattering layers of Norwegian fjords. Marine Ecology Progress Series. ISSN 0171-8630. 360, p. 201–209. doi: 10.3354/meps07364. Full text in Research Archive Show summary

  Mesopelagic acoustic scattering layers (SLs) in 2 fjords were studied from a stationary research vessel. Diel vertical movements of SLs were assessed by hull-mounted transducers, while in situ behavior of individuals constituting the SLs was resolved by a submerged echo sounder. The study focused on SLs made up of the lightfish Maurolicus muelleri and the lanternfish Benthosema glaciale. Individual fish migrated in a pronounced stepwise manner, alternating between vertical movements and stationary phases both during ascent and descent. Mean lengths of steps varied between 2.01 and 0.40 m, and mean duration of stationary phases between 69 and 36 s for fish in different SLs. Such travel-pause behavior concords with saltatory search, where fish scan the water for prey during the stationary phases, relocate and scan a new water parcel. Little activity was recorded among individuals in deep water, apart from infrequent, short shifts in vertical distribution. This study shows that stationary submerged echo sounders can provide detailed information on in situ behavior of mesopelagic fish.

• Kaartvedt, Stein; Klevjer, Thor A.; Torgersen, Thomas; Sørnes, Tom Asbjørn & Røstad, Anders (2007). Diel vertical migration of individual jellyfish (Periphylla periphylla). Limnology and Oceanography. ISSN 0024-3590. 52(3), p. 975–983. doi: 10.4319/lo.2007.52.3.0975. Full text in Research Archive
• Olsen, Esben Moland; Melle, Webjørn; Kaartvedt, Stein; Holst, J C & Mork, KA (2007). Spatially structured interactions between a migratory pelagic predator, the Norwegian spring-spawning herring Clupea harengus L., and its zooplankton prey. Journal of Fish Biology. ISSN 0022-1112. 70, p. 799–815. doi: 10.1111/j.1095-8649.2007.01342.x. Full text in Research Archive
• Vestheim, Hege Finsås & Kaartvedt, Stein (2006). Plasticity in coloration as an antipredator strategy among zooplankton. Limnology and Oceanography. ISSN 0024-3590. 51(4), p. 1931–1934. Full text in Research Archive
• Røstad, Anders; Kaartvedt, Stein; Klevjer, Thor A. & Melle, Webjørn (2006). Fish are attracted to vessels. ICES Journal of Marine Science. ISSN 1054-3139. 63, p. 1431–1437. Full text in Research Archive
• Klevjer, Thor A. & Kaartvedt, Stein (2006). In situ target strength and behaviour of northern krill (Meganyctiphanes norvegica). ICES Journal of Marine Science. ISSN 1054-3139. 63, p. 1726–1735. Full text in Research Archive

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• Kaartvedt, Stein; Christiansen, Svenja & Titelman, Josefin (2024). Internal waves and fine-scale zooplankton sampling: Comment on Dewar-Fowler et al. (2023). Marine Ecology Progress Series. ISSN 0171-8630. 734, p. 173–175. doi: 10.3354/meps14577. Full text in Research Archive
• Langbehn, Tom; Ljungström, Anna Jeja Gabriella; Fiksen, Øyvind; Aksnes, Dag; Kaartvedt, Stein & Jørgensen, Christian (2023). Model evidence for photic barriers to poleward range shifts. Full text in Research Archive
• Langbehn, Tom; Ljungström, Anna Jeja Gabriella; Fiksen, Øyvind; Aksnes, Dag Lorents; Kaartvedt, Stein & Jørgensen, Christian (2023). Insights from mechanistic models: understanding causes and consequences of mesopelagic diel vertical migration across latitudes. Full text in Research Archive
• Salvanes, Anne Gro Vea; Aksnes, Dag Lorents; Folkvord, Arild; Gallo, Natalya; Kaartvedt, Stein & Levin, Lisa A. [Show all 12 contributors for this article] (2022). West Norwegian sill fjords as natural infrastructures for investigating consequences of ocean deoxygenation on fish. Full text in Research Archive
• Salvanes, Anne Gro Vea; Gallo, Natalya; Folkvord, Arild; Staby, Arved; Solås, Martine Røysted & Saltalamacchia, Francesco [Show all 15 contributors for this article] (2021). The HypOnFjordFish project - effects of oxygen loss on fish in West Norwegian fjords. Full text in Research Archive
• Kaartvedt, Stein & Hylland, Ketil (2021). Oslofjorden er langt fra død. Full text in Research Archive
• Langbehn, Tom; Aksnes, Dag Lorents; Kaartvedt, Stein; Fiksen, Øyvind & Jørgensen, Christian (2019). Poleward distribution of mesopelagic fish is constrained by seasonality in light. Full text in Research Archive
• Langbehn, Tom; Jørgensen, Christian; Fiksen, Øyvind; Varpe, Øystein; Aksnes, Dag Lorents & Kaartvedt, Stein (2019). From light, to vision, to species distribution. Full text in Research Archive
• Langbehn, Tom; Aksnes, Dag Lorents; Kaartvedt, Stein; Fiksen, Øyvind & Jørgensen, Christian (2019). Poleward distribution of mesopelagic fish is constrained by seasonality in light. Full text in Research Archive
• Langbehn, Tom; Aksnes, Dag Lorents; Kaartvedt, Stein; Fiksen, Øyvind & Jørgensen, Christian (2019). Poleward distribution of mesopelagic fish is constrained by seasonality in light. Full text in Research Archive
• Langbehn, Tom; Aksnes, Dag Lorents; Kaartvedt, Stein; Fiksen, Øyvind & Jørgensen, Christian (2019). Light and the ecology of mesopelagic fish at high latitudes. Full text in Research Archive
• Solberg, Ingrid; Kaartvedt, Stein; Klevjer, Thor A. & Røstad, Anders (2016). Livet under sjøisen - brislingens adferd før- og etter islegging. Full text in Research Archive
• Røstad, Anders; Kaartvedt, Stein & Aksnes, Dag Lorents (2016). Erratum to “Light comfort zones of mesopelagic acoustic scattering layers in two contrasting optical environments” (Deep-Sea Research (2016) 113(Part I) (1–6) (S0967063715300224) (10.1016/j.dsr.2016.02.020)). Deep Sea Research Part I: Oceanographic Research Papers. ISSN 0967-0637. 114, p. 162–164. doi: 10.1016/j.dsr.2016.05.001. Full text in Research Archive
• Solberg, Ingrid; Kaartvedt, Stein & Klevjer, Thor A. (2013). The use of echosounders for long-term studies of the overwintering ecology of sprat (Sprattus sprattus). Full text in Research Archive
• Iversen, Svein Arnholt; Melle, Webjørn Raunsgård; Bagøien, Espen; chu, d; Edvardsen, Bente & Ellertsen, Bjørnar [Show all 25 contributors for this article] (2008). The Antarctic krill and ecosystem survey with r/v GO Sars, in 2008. Full text in Research Archive
• Misund, Ole Arve; Gammelsrød, Tor; Kaartvedt, Stein; Mikelborg, Øystein; Moksness, Erlend & Røttingen, Ingolf (2006). Forskningsfaglig begrunnelse for fornying av forskningsfartøy. Havforskningsinstituttet. Full text in Research Archive

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