Rebecca E. Holt

• Durant, Joël M.; Langangen, Øystein Ole Gahr; Dolgov, Andrey V.; Lindstrøm, Ulf Ove; Krasnov, Yuri & Nikolaeva, Natalia G. [Show all 7 contributors for this article] (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. Full text in Research Archive Show summary

  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.

• 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. Full text in Research Archive Show summary

  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.

• 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), p. 1–8. doi: 10.1002/ecy.4130. Full text in Research Archive Show summary

  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.

• Ottersen, Geir & Holt, Rebecca Emma (2022). Long-term variability in spawning stock age structure influences climate–recruitment link for Barents Sea cod. Fisheries Oceanography. ISSN 1054-6006. 32(1), p. 91–105. doi: 10.1111/fog.12605. Full text in Research Archive Show summary

  Fish populations may spawn a vast number of offspring, while only a small and highly variable fraction of a new cohort survives long enough to enter into the fisheries as recruits. It is intuitive that the size and state of the spawning stock, the adult part of the fish population, is important for recruitment. Additionally, environmental conditions can greatly influence survival through vulnerable early life stages until recruitment. To understand what regulates recruitment, an essential part of fish population dynamics, it is thus necessary to explain the impact of fluctuations in both spawning stock and environment, including interactions. Here, we examine if the connection between the environment and recruitment is affected by the state of the spawning stock, including biomass, mean age and age diversity. Specifically, we re-evaluate the hypothesis stating that recruitment from a spawning stock dominated by young fish and few age classes is more vulnerable to environmental fluctuations. We expand upon earlier work on the Barents Sea stock of Atlantic cod, now with data series extended in time both backwards and forwards to cover the period 1922–2019. While our findings are correlative and cannot prove a specific cause and effect mechanism, they support earlier work and strengthen the evidence for the hypothesis above. Furthermore, this study supports that advice to fisheries management should include considerations of environmental status.

• Townhill, Bryony; Holt, Rebecca Emma; Bogstad, Bjarte; Durant, Joël; Pinnegar, John K. & Dolgov, Andrey V. [Show all 9 contributors for this article] (2021). Diets of the Barents Sea cod (Gadus morhua) from the 1930s to 2018. Earth System Science Data. ISSN 1866-3508. 13(3), p. 1361–1370. doi: 10.5194/essd-13-1361-2021. Full text in Research Archive Show summary

  A new dataset on the diet of Atlantic cod in the Barents Sea from the 1930s to the present day has been compiled to produce one of the largest fish diet datasets available globally. Atlantic cod is one of the most ecologically and commercially important fish species in the North Atlantic. The stock in the Barents Sea is by far the largest, as a result of both successful management and favourable environmental conditions since the early 2000s. As a top predator, cod plays a key role in the Barents Sea ecosystem. The species has a broad diet consisting mainly of crustaceans and teleost fish, and both the amount and type of prey vary in space and time. The data – from Russia, Norway and the United Kingdom – represent quantitative stomach content records from more than 400 000 fish and qualitative data from 2.5 million fish. Many of the data are from joint collaborative surveys between Norway and Russia. The sampling was conducted throughout each year, allowing for seasonal, annual and decadal comparisons to be made. Visual analysis shows cod diets have changed considerably from the start of the dataset in the 1930s to the present day. There was a large proportion of herring in the diets in the 1930s, whereas in more recent decades capelin, invertebrates and other fish dominate. There are also significant interannual asynchronous fluctuations in prey, particularly capelin and euphausiids. Combining these datasets can help us understand how the environment and ecosystems are responding to climatic changes, and what influences the diet and prey switching of cod. Trends in temperature and variability indices can be tested against the occurrence of different prey items, and the effects of fishing pressure on cod and prey stocks on diet composition could be investigated. The dataset will also enable us to improve parametrization of food web models and to forecast how Barents Sea fisheries may respond in the future to management and to climate change. The Russian data are available through joint projects with the Polar Branch of the Russian Federal Research Institute of Fisheries and Oceanography (VNIRO).

• Holt, Rebecca Emma; Hvingel, Carsten; Agnalt, Ann-Lisbeth; Dolgov, Andrey V.; Hjelset, Ann Merete & Bogstad, Bjarte (2021). Snow crab (Chionoecetes opilio), a new food item for North-east Arctic cod (Gadus morhua) in the Barents Sea. ICES Journal of Marine Science. ISSN 1054-3139. 78(2), p. 491–501. doi: 10.1093/icesjms/fsaa168. Full text in Research Archive Show summary

  The snow crab (Chionoecetes opilio) is a newly established species in the Barents Sea, increasing in both distribution and abundance in recent years. We explore the potential importance of North-east Arctic cod (Gadus morhua) predation in structuring the snow crab population expansion in the Barents Sea, through an analysis of cod stomach content data from 2003 to 2018. Spatio-temporal patterns of snow crab within cod diet are assessed across years, between seasons, as well as ontogenetic trends, including predator–prey size relationships. Snow crab represents a new prey item for cod and in recent years the most dominant demersal crustacean species in cod diet. The proportion of snow crab within cod diet increases over time. Primarily 60–110 cm cod prey upon snow crab, mainly during the summer/autumn season. Our results support the idea that the snow crab is still expanding, shifting distribution north and westwards. Cod has the potential to regulate the snow crab population but is unlikely to be in direct competition with the fishery in the Barents Sea. This work suggests that cod can be used as biological sampling tools together with other monitoring programmes to elucidate how new species may affect predator–prey and food-web dynamics within an ecosystem context.

• Woods, P J; Macdonald, J I; Bárðarson, H; Bonanomi, S.; Boonstra, W. J. & Cornell, G. [Show all 29 contributors for this article] (2021). A review of adaptation options in fisheries management to support resilience and transition under socio-ecological change. ICES Journal of Marine Science. ISSN 1054-3139. 79(2), p. 463–479. doi: 10.1093/icesjms/fsab146. Full text in Research Archive
• Holt, Rebecca Emma; Bogstad, Bjarte; Durant, Joel Marcel; Dolgov, Andrey & Ottersen, Geir (2019). Barents Sea cod (Gadus morhua) diet composition: long-term interannual, seasonal, and ontogenetic patterns. ICES Journal of Marine Science. ISSN 1054-3139. 76(6), p. 1641–1652. doi: 10.1093/icesjms/fsz082. Full text in Research Archive Show summary

  Atlantic cod (Gadus morhua) is an ecologically and commercially important species in the North-Atlantic region. Cod is a top predator and information on its trophic ecology is integral for understanding predator–prey relationships and food-web dynamics. We present an analysis of the trophic patterns of Barents Sea (BS) cod using a unique 33-year time-series of stomach-content data from 1984 to 2016. We assessed patterns in diet (prey) composition across years, between seasons, as well as ontogenetic trends in diet, including predator–prey size relationships. Ontogenetic shifts in diet were observed, with fish becoming more important prey with increasing cod size. A very early onset of piscivory was found in <20 cm cod. Cannibalism was found in cod > 20 cm and increased with size. Juvenile cod exhibit a tendency towards consuming prey up to 33% of their body length, whereas larger cod feed on all prey sizes, resulting in asymmetric predator–prey size distributions. Diet varied significantly during 1984–2016, consistent with changes in both prey, cod abundance, and distribution. Seasonal differences were observed; capelin dominated the winter diet, whereas cod, polar cod, and other fish species were prevalent in summer/autumn months. This work represents an important step towards understanding trophic linkages that determine BS ecosystem dynamics.

• Holt, Rebecca Emma; Brown, Christopher J.; Schlacher, Thomas A.; Sheldon, Fran; Balcombe, Stephen R. & Connolly, Rod M. (2017). Species traits and connectivity constrain stochastic community re-assembly. Scientific Reports. 7(1). doi: 10.1038/s41598-017-14774-2. Full text in Research Archive
• Holt, Rebecca Emma; Woods, P.J; Ferreira, A.S.A; Bardarson, H; Bonanomi, Sara & Boonstra, W.J [Show all 25 contributors for this article] (2017). Avoiding pitfalls in interdisciplinary education. Climate Research (CR). ISSN 0936-577X. 74(2), p. 121–129. doi: 10.3354/cr01491. Full text in Research Archive
• Pedersen, Martin Wæver; Kokkalis, Alexandros; Bardarson, Hermann; Bonanomi, Sara; Boonstra, Wiebren & Butler, William E. [Show all 23 contributors for this article] (2016). Trends in marine climate change research in the Nordic region since the first IPCC report. Climatic Change. ISSN 0165-0009. 134(1), p. 147–161. doi: 10.1007/s10584-015-1536-6. Full text in Research Archive
• Holt, Rebecca & Jørgensen, Christian (2015). Climate change in fish: Effects of respiratory constraints on optimal life history and behaviour. Biology Letters. ISSN 1744-9561. 11:20141032(2). doi: 10.1098/rsbl.2014.1032. Full text in Research Archive Show summary

  The difference between maximum metabolic rate and standard metabolic rate is referred to as aerobic scope, and because it constrains performance it is suggested to constitute a key limiting process prescribing how fish may cope with or adapt to climate warming. We use an evolutionary bioenergetics model for Atlantic cod (Gadus morhua) to predict optimal life histories and behaviours at different temperatures. The model assumes common trade-offs and predicts that optimal temperatures for growth and fitness lie below that for aerobic scope; aerobic scope is thus a poor predictor of fitness at high temperatures. Initially, warming expands aerobic scope, allowing for faster growth and increased reproduction. Beyond the optimal temperature for fitness, increased metabolic requirements intensify foraging and reduce survival; oxygen budgeting conflicts thus constrain successful completion of the life cycle. The model illustrates how physiological adaptations are part of a suite of traits that have coevolved.

• Boonstra, Wiebren; Ottosen, Katharina M.; Ferreira, Ana Sofia de Araújo; Richter, Andries Peter; Rogers, Lauren & Pedersen, Martin Wæver [Show all 24 contributors for this article] (2015). What are the major global threats and impacts in marine environments? Investigating the contours of a shared perception among marine scientists from the bottom-up. Marine Policy. ISSN 0308-597X. 60, p. 197–201. doi: 10.1016/j.marpol.2015.06.007. Full text in Research Archive
• Holt, Rebecca E. & Jørgensen, Christian (2014). Climate warming causes life-history evolution in a model for Atlantic cod (Gadus morhua). Conservation Physiology. 2, p. cou050–cou050. doi: 10.1093/conphys/cou050. Full text in Research Archive Show summary

  Climate change influences the marine environment, with ocean warming being the foremost driving factor governing changes in the physiology and ecology of fish. At the individual level, increasing temperature influences bioenergetics and numerous physiological and life-history processes, which have consequences for the population level and beyond. We provide a statedependent energy allocation model that predicts temperature-induced adaptations for life histories and behaviour for the North-East Arctic stock (NEA) of Atlantic cod (Gadus morhua) in response to climate warming. The key constraint is temperature- dependent respiratory physiology, and the model includes a number of trade-offs that reflect key physiological and ecological processes. Dynamic programming is used to find an evolutionarily optimal strategy of foraging and energy allocation that maximizes expected lifetime reproductive output given constraints from physiology and ecology. The optimal strategy is then simulated in a population, where survival, foraging behaviour, growth, maturation and reproduction emerge. Using current forcing, the model reproduces patterns of growth, size-at-age, maturation, gonad production and natural mortality for NEA cod. The predicted climate responses are positive for this stock; under a 2°C warming, the model predicted increased growth rates and a larger asymptotic size. Maturation age was unaffected, but gonad weight was predicted to more than double. Predictions for a wider range of temperatures, from 2 to 7°C, show that temperature responses were gradual; fish were predicted to grow faster and increase reproductive investment at higher temperatures. An emergent pattern of higher risk acceptance and increased foraging behaviour was also predicted. Our results provide important insight into the effects of climate warming on NEA cod by revealing the underlying mechanisms and drivers of change. We show how temperatureinduced adaptations of behaviour and several life-history traits are not only mediated by physiology but also by trade-offs with survival, which has consequences for conservation physiology.

• Holt, Rebecca Emma; Foggo, Andy; Neat, Francis C. & Howell, Kerry L. (2013). Distribution patterns and sexual segregation in chimaeras: Implications for Conservation and management. ICES Journal of Marine Science. ISSN 1054-3139. 70, p. 1198–1205. doi: 10.1093/icesjms/fst058. Full text in Research Archive
• Jørgensen, Christian & Holt, Rebecca (2013). Natural mortality: Its ecology, how it shapes fish life histories, and why it may be increased by fishing. Journal of Sea Research. ISSN 1385-1101. 75, p. 8–18. doi: 10.1016/j.seares.2012.04.003. Full text in Research Archive Show summary

  A stronger focus on natural mortality may be required to better understand contemporary changes in fish life histories and behaviour and their responses to anthropogenic drivers. Firstly, natural mortality is the selection under which fish evolved in the first place, so a theoretical understanding of effects of natural mortality alone is needed. Secondly, due to trade-offs, most organismal functions can only be achieved at some cost in terms of survival. Several trade-offs might need to be analysed simultaneously with effects on natural mortality being a common currency. Thirdly, there is scattered evidence that natural mortality has been increasing, some would say dramatically, in some fished stocks, which begs explanations. Fourthly, natural mortality most often implies transfer of mass and energy from one species to another, and therefore has foodweb and ecosystem consequences. We therefore analyse a model for evolution of fish life histories and behaviour, where state-dependent energy-allocation and growth strategies are found by optimization. Natural mortality is split into five different components, each specified as the outcome of individual traits and ecological trade-offs: a fixed baseline mortality; size-dependent predation; risk-dependent growth strategy; a fixed mortality when sexually mature; and mortality increasing with reproductive investment. The analysis is repeated with and without fishing. Each component of natural mortality has consequences for optimal life history strategies. Beyond earlier models, we show i) how the two types of reproductive mortality sometimes have similar and sometimes contrasting effects on life history evolution, ii) how ecosystem properties such as food availability and predation levels have stronger effects on optimal strategies than changing other mortality components, and iii) how expected changes in risk-dependent growth strategies are highly variable depending on the type of mortality changed.

• Howell, Kerry L.; Holt, Rebecca Emma; Endrino, I, P & Stewart, Heather (2011). When the species is also a habitat: comparing the predictively modelled distributions of Lophelia pertusa and the reef habitat it forms. Biological Conservation. ISSN 0006-3207. 144, p. 2656–2665. doi: 10.1016/j.biocon.2011.07.025. Full text in Research Archive

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• Candela, Andrea Campos; Morgan, Rachael Louise; Holt, Rebecca Emma; Humanes, Adriana; Andreassen, Anna Haukedal & Beldade, Ricardo [Show all 10 contributors for this article] (2023). In the heat of the night: hypoxia shapes ecology and physiology of coral reefs. Full text in Research Archive
• Ottersen, Geir & Holt, Rebecca Emma (2023). Spawning stock age structure and climate combined cause long-term fluctuations in recruitment – demonstrated for the Norwegian spring-spawning herring stock? Full text in Research Archive
• Ottersen, Geir & Holt, Rebecca Emma (2022). Spawning stock age structure and climate combined cause long term fluctuations in Barents Sea cod recruitment. Full text in Research Archive
• Ottersen, Geir & Holt, Rebecca Emma (2022). Barents Sea cod recruitment less affected by temperature variability when spawning stock includes old, large fish. Full text in Research Archive
• Candela, Andrea Campos; Morgan, Rachael Louise; Holt, Rebecca Emma; Jutfelt, Fredrik & Jørgensen, Christian (2022). In the heat of the night: the role of hypoxia on coral reefs. Full text in Research Archive
• Ottersen, Geir & Holt, Rebecca Emma (2021). Hva spiser torsken - modellstudier på torskens diett og næringsnettet i Barentshavet. Full text in Research Archive
• Holt, Rebecca Emma; Bogstad, Bjarte; Durant, Joël; Dolgov, Andrey V. & Ottersen, Geir (2019). Erratum: Barents Sea cod (Gadus morhua) diet composition: Long-term interannual, seasonal, and ontogenetic patterns (ICES Journal of Marine Science (2019) DOI: 10.1093/icesjms/fsz082). ICES Journal of Marine Science. ISSN 1054-3139. 76(6), p. 1936–1936. doi: 10.1093/icesjms/fsz117. Full text in Research Archive
• Holt, Rebecca Emma; Durant, Joel Marcel; Ottersen, Geir & Bogstad, Bjarte (2017). Cod diet and food web dynamics: What can we learn from the past? Full text in Research Archive
• Holt, Rebecca Emma (2016). CoDINA: Cod DIet and food web dyNAmics. Full text in Research Archive
• Ross, Stine Dalmann; Holt, Rebecca Emma; Deurs, Mikael Van & Andersen, N, G (2014). How does prey quality affect life-history traits under different temperature regimes? Full text in Research Archive
• Jørgensen, Christian; Peck, MA; Antognarelli, F; Azzurro, E; Burrows, MT & Cheung, WWL [Show all 17 contributors for this article] (2012). Conservation physiology of marine fishes: advancing the predictive capacity of models. Biology Letters. ISSN 1744-9561. 8(6), p. 900–903. doi: 10.1098/rsbl.2012.0609. Full text in Research Archive
• Holt, Rebecca (2015). Climate-Induced Evolution of the Behaviour and Life-History Strategies of Atlantic Cod (Gadus morhua). A Modelling Approach. Universitetet i Bergen. Full text in Research Archive Show summary

  Climate change influences the marine environment, with ocean warming being the foremost driving factor governing changes in the physiology and ecology of fish. At the individual level temperature can directly influence numerous physiology processes in marine fish as well as bioenergetics, having consequences for life-history processes and population level consequences. A primary mechanism through which temperature can affect fish is through respiratory physiology as it has consequences for performance and fitness. The difference between maximum metabolic rate and standard metabolic rate is referred to as aerobic scope, and is suggested to constitute a key limiting process prescribing how fish may cope with or adapt to climate warming. Field data and experiments have demonstrated an array of effects and influences of temperature on Atlantic cod and other marine fish species. This PhD research explores how temperature can influence the life-history and behaviour of Atlantic cod through a theoretical modelling approach. A state-dependent energy allocation model was developed to predict temperature-induced adaptations for the life-histories and behaviour of Atlantic cod, in response to climate warming. The model was parameterised primarily for the North-East Arctic cod stock and then extended to facilitate parametrisation of a further 19 stocks, enabling intra-specific stock comparisons across the North-Atlantic. The model is based on life-history theory, which addresses how an individual should schedule its life to maximise its expected lifetime reproductive output. Energy allocation and investment into growth, maturation and reproduction depends on a number of physiological and ecological factors, such as respiratory physiology and aerobic scope, constraints on body size, environmental variability in prey availability, spawning migration distance, and mortality. These constraints and mechanisms are well justified to the physiology and ecology of the species. By using state-dependent dynamic programming techniques, a model can find optimal energy allocation between growth and reproduction at the individual level as well as simulate lifetime trajectories in a population model under the external driver of increasing temperature. The model predictions provide a number of results important for conservation physiology and fisheries management of Atlantic cod and marine fish species more generally. Firstly the model shows that when predicting the effects of climate change on fish, one should view the species physiology and ecology as a whole, being aware that organisms possess a suite of traits that have co-evolved. Some methods within the literature for predicting the effects of climate warming can be one-sided, focussing on purely physiological or ecological responses, omitting evolutionary considerations. The effects of climate warming on fish are complex, thus an integrated whole organism approach is needed. Secondly, we find for the North-East Arctic cod stock, the effects of climate warming are positive. Increased temperatures were predicted to select for faster lifehistory strategies, increasing growth rates, larger size-at-age and higher reproductive output. Thirdly, our model fills in the gap between respiratory physiology as described by the Oxygen- and capacity-limited-thermal tolerance theory (OCLTT) and how it may influence fitness. We find that optimal temperatures for growth and fitness lie well below that for aerobic scope; and agree in accordance with a recent literature debate, that aerobic scope is a poor predictor of fitness at high temperatures. This has implications for climate change effects, conservation physiology, and the OCLTT theory. Fourthly, we have identified key mechanisms and potential drivers pertaining to the life-history variation across 20 cod stocks in the North-Atlantic. We find that the drivers of intra-specific variation in Atlantic cod could be explained by differences in temperature, seasonality and spawning migration distance between the stocks. Furthermore, when predicting the effects of climate warming on these stocks, the model illustrates how climate warming may influence the life-history strategies of cod in diverse ways. We find that cold-water stocks may fare better and be more resilient under increasing temperature scenarios than their warm-water counterparts. Lastly this PhD research provides a unique life-history modelling tool that can be adapted and extended. Due to the mechanistic modelling framework, the model could be adapted to other marine teleosts, particularly other boreal or temperate species. As we include an explicit respiratory physiology component, it also allows flexibility to include additional drivers, such as the effects of hypoxia on increased respiratory costs as well as the effects of fishing mortality. The integrating effects of multiple drivers are already prominent in the current literature and our mechanistic model could take this further to assess effects on life-history traits.

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