Barents Sea Cod Research Articles

An exploratory analysis of warming effects on wealth in the Barents Sea fisheries

The Barents Sea and Arctic regions in general are facing significant impacts from climate change. A major concern is increasing temperatures. I explore four different scenarios of temperature impacts on the Barents Sea Atlantic cod (Gadus morhua), its fishery, and the level of natural capital – wealth – that the fish stock represents. My framework incorporates interaction with the main prey, capelin, under uncertainty in the population dynamics equations and under two relevant management regimes. One regime is based on maximum sustainable yield (MSY) and one is based on ecosystem-based management (EBM). Because of uncertainty over how warming will impact the Barents Sea cod, the investigated scenarios include both positive and negative impacts on parameters in the population dynamics. Wealth under MSY is robust to growth rate impacts, but with impacts on the carrying capacity wealth will change significantly. The EBM regime would increase wealth in all scenarios. Wealth estimates supports appropriate accounting prices for the natural capital stocks—prices that reflect modeled ecosystem and temperature dynamics.

Empirical evidence of nonlinearity in bottom up effect in a marine predator-prey system.

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

Fishery and the state of the Barents and Norwegian Seas cod stock

The assessment of the stock status and prospects for the fishery of the Barents Sea cod is characterized by significant uncertainty caused by the peculiarities of the age structure, which often led to significant differences in the results obtained using different models. As a result, this caused intense discussions both within the framework of the International Council for the Exploration of the Sea (ICES) and at the meetings of the Joint Norwegian-Russian Fisheries Commission. In 2021, at a specialized ICES workshop («benchmark» group), the input data and configuration of the main assessment model - SAM, were refined, related to the inclusion of additional information on older age groups. As a result, two models currently used by the ICES working group to estimate cod stock (SAM and TISVPA), which are very different in their approaches, showed that the number of generations born in 2004–2005 significantly decreased, which led to a decrease in cod biomass. An assessment of the prospects for the state of the stock, performed both on the basis of calculations using the SAM model and on the basis of an alternative approach using the AMO (Atlantic Multidecadal Oscillation Index), showed a high probability of further decline and stabilization of the spawning stock biomass (and catches) in the next 2–3 years, after which restoration of spawning stock biomass is likely due to sufficiently abumdante generations born in 2011 and 2014.

Long-term variability in spawning stock age structure influences climate-recruitment link for Barents Sea cod.

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.

Brexit and consequences for quota sharing in the Barents Sea cod fishery

The fishery for Northeast Atlantic cod (Gadus morhua) in the Barents Sea is the largest cod fishery in the world. For historical reasons, the EU has cod quotas in both the Norwegian Economic Zone (NEZ) and the Svalbard Protection Zone (SPZ) that are allocated among the member states according to an allocation key that has remained constant for decades. Of the EU countries, until the end of 2020 the UK had the largest quota share, followed by Spain. In this paper we analyse how Brexit may lead to changes in quota sharing between Norway, the UK and the EU. Norway and the UK are have agreed on a framework fisheries agreement, however, they have so far failed to reach agreement on quota sharing and access for 2021. Furthermore, there is now conflict between Norway and the EU when it comes to cod quotas in the SPZ, the outcome of which is uncertain.

Marine Stewardship Council (MSC) Certification of Arctic Fisheries: Processes and Outcomes

Certification according to private sustainability standards (ecolabelling) has become an important addition to public fisheries management in recent years. The major global ecolabel in terms of comprehensiveness and coverage is the Marine Stewardship Council (MSC) Fisheries Standard. Under the MSC Standard, the status of the fishery’s target stocks, its impact on the wider ecosystem and the effectiveness of its management system are assessed. Becoming and remaining certified requires continuous behavioural adaptation from fisheries through a fine-meshed system of conditions attached to certification. In this article, MSC certification of two clusters of fisheries in Arctic waters is discussed, one large- and one small-scale. In the Barents Sea cod and haddock fisheries, the main obstacle to certification has been the fisheries’ impact on endangered, threatened and protected (ETP) species and bottom habitats, and in order to remain certified beyond the first five-year certification period, the fishing companies have had to introduce a number of voluntary measures beyond what is required by law. In the local lumpfish fisheries in Greenland, Iceland and Norway, conditions attached to certification have been related to the effects of these fisheries on seabirds and marine mammals. Here essential parts of a management regime, such as biological reference points and harvest control rules, have come about as a direct result of MSC certification. MSC certification is no panacea, but it seems to have found a niche as a supplement to national legislation and international agreements.

Otolith growth and zone formation during first maturity and spawning of Atlantic cod (Gadus morhua)

Specific impacts of somatic growth, sexual maturation, and spawning events on otolith zone formation in Atlantic cod (Gadus morhua) were assessed in a 33-month tank experiment, using Barents Sea cod and Norwegian coastal cod. High and low feeding ration combinations were used to mimic environmental stressors in the field. For both stocks, apparent macrostructural “spawning zones” in otoliths are registered in statutory stock monitoring programs to estimate age at maturity, thus adding key information to stock biomass assessments. We found that substantial energy investments in reproduction caused reductions in otolith growth and altered proportional width between translucent and opaque zones. These effects, however, were only statistically significant among individuals with high reproductive investments, while otoliths from individuals with low investments did not differ from the otoliths for immatures. Reproduction may thus not necessarily induce spawning zones, and alternatively, spawning zones may not necessarily reflect reproduction. Altogether, this suggests that the individual energy level, as a premise for metabolic activity, plays a key role in the formation of such zones and thus is related to environmental conditions.

Barents Sea cod (Gadus morhua) diet composition: long-term interannual, seasonal, and ontogenetic patterns

Barents Sea cod (Gadus morhua) diet composition: long-term interannual, seasonal, and ontogenetic patterns

Barents Sea cod (Gadus morhua) diet composition: long-term interannual, seasonal, and ontogenetic patterns

Abstract 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.

Seafood from a changing Arctic

We review current knowledge about climate change impacts on Arctic seafood production. Large-scale changes in the Arctic marine food web can be expected for the next 40–100 years. Possible future trajectories under climate change for Arctic capture fisheries anticipate the movement of aquatic species into new waters and changed the dynamics of existing species. Negative consequences are expected for some fish stocks but others like the Barents Sea cod (Gadus morhua) may instead increase. Arctic aquaculture that constitutes about 2% of global farming is mainly made up of Norwegian salmon (Salmo salar) farming. The sector will face many challenges in a warmer future and some of these are already a reality impacting negatively on salmon growth. Other more indirect effects from climate change are more uncertain with respect to impacts on the economic conditions of Arctic aquaculture.

Climate change, fisheries management and fishing aptitude affecting spatial and temporal distributions of the Barents Sea cod fishery

Climate change is expected to influence spatial and temporal distributions of fish stocks. The aim of this paper is to compare climate change impact on a fishery with other factors impacting the performance of fishing fleets. The fishery in question is the Northeast Arctic cod fishery, a well-documented fishery where data on spatial and temporal distributions are available. A cellular automata model is developed for the purpose of mimicking possible distributional patterns and different management alternatives are studied under varying assumptions on the fleets’ fishing aptitude. Fisheries management and fishing aptitude, also including technological development and local knowledge, turn out to have the greatest impact on the spatial distribution of the fishing effort, when comparing the IPCC’s SRES A1B scenario with repeated sequences of the current environmental situation over a period of 45 years. In both cases, the highest profits in the simulation period of 45 years are obtained at low exploitation levels and moderate fishing aptitude.

Forecasting future recruitment success for Atlantic cod in the warming and acidifying Barents Sea.

Productivity of marine fish stocks is known to be affected by environmental and ecological drivers, and global climate change is anticipated to alter recruitment success of many stocks. While the direct effects of environmental drivers on fish early life stage survival can be quantified experimentally, indirect effects in marine ecosystems and the role of adaptation are still highly uncertain. We developed an integrative model for the effects of ocean warming and acidification on the early life stages of Atlantic cod in the Barents Sea, termed SCREI (Simulator of Cod Recruitment under Environmental Influences). Experimental results on temperature and CO2 effects on egg fertilization, egg and larval survival and development times are incorporated. Calibration using empirical time series of egg production, temperature, food and predator abundance reproduces age-0 recruitment over three decades. We project trajectories of recruitment success under different scenarios and quantify confidence limits based on variation in experiments. A publicly accessible web version of the SCREI model can be run under www.oceanchange.uni-bremen.de/;SCREI. Severe reductions in average age-0 recruitment success of Barents Sea cod are projected under uncompensated warming and acidification toward the middle to end of this century. Although high population stochasticity was found, considerable rates of evolutionary adaptation to acidification and shifts in organismal thermal windows would be needed to buffer impacts on recruitment. While increases in food availability may mitigate short-term impacts, an increase in egg production achieved by stock management could provide more long-term safety for cod recruitment success. The SCREI model provides a novel integration of multiple driver effects in different life stages and enables an estimation of uncertainty associated with interindividual and ecological variation. The model thus helps to advance toward an improved empirical foundation for quantifying climate change impacts on marine fish recruitment, relevant for ecosystem-based assessments of marine systems under climate change.

Ontogenetic development of otolith shape during settlement of juvenile Barents Sea cod (Gadus morhua)

Abstract This study documents how settlement of juvenile Atlantic cod (Gadus morhua) in the Barents Sea affects otolith growth and morphology. A simple method to objectively discriminate between age 0 and age 1 cod sampled in late summer was demonstrated by using only two otolith morphometric descriptors: area and perimeter. In the pre-settled 0-group cod, otolith lobe formation clearly increased with fish size, resulting in high otolith crenulation. This trend was disrupted during settlement, resulting in noticeably less crenulated otoliths of the settled 1-group cod sampled in winter. Combined observations of otolith shape, fish size, and body condition suggest that environmental factors associated with settlement during autumn, particularly reduced food intake, directly affect lobe formation leading to less crenulated otoliths. Comparably reduced body condition and otolith crenulation of 0-group cod in bottom trawls (vs. pelagic trawls) may indicate early settlement or vertical exploratory behaviour in the Barents Sea Ecosystem Survey (in August–September) and, thus, an underrepresentation of 0-group cod from pelagic trawling.

Effect of a fish stock's demographic structure on offspring survival and sensitivity to climate

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

Reproductive strategy of a migratory fish stock: implications of spatial variations in natural mortality

We investigate how the reproductive strategy in a migratory marine fish may be influenced by spatial variations in mortality in early life stages. In particular, we examine how spawning time and location affect offspring survival and growth. A drift model for early life stages (eggs to age 1) of the Barents Sea cod (Gadus morhua) is combined with empirical estimates of spatial variation in mortality at two different life stages. We examine seasonal and interannual differences in survival and growth in offspring originating from two spawning grounds, with the central site requiring higher migration distance, and hence cost, than the northern site. When accounting for spatially explicit mortality fields, central and northern spawned offspring have about equal survival, as do early and late spawned offspring. Furthermore, central spawned offspring grow faster and are likely to reach a larger size compared with northern spawned offspring. Our results indicate that the fitness benefit of southward migration in the Barents Sea cod is not mainly due to higher early survival of offspring, but rather due to effects of offspring acquiring a larger size.

How Many Fish Need to Be Measured to Effectively Evaluate Trawl Selectivity?

The aim of this study was to provide practitioners working with trawl selectivity with general and easily understandable guidelines regarding the fish sampling effort necessary during sea trials. In particular, we focused on how many fish would need to be caught and length measured in a trawl haul in order to assess the selectivity parameters of the trawl at a designated uncertainty level. We also investigated the dependency of this uncertainty level on the experimental method used to collect data and on the potential effects of factors such as the size structure in the catch relative to the size selection of the gear. We based this study on simulated data created from two different fisheries: the Barents Sea cod (Gadus morhua) trawl fishery and the Mediterranean Sea multispecies trawl fishery represented by red mullet (Mullus barbatus). We used these two completely different fisheries to obtain results that can be used as general guidelines for other fisheries. We found that the uncertainty in the selection parameters decreased with increasing number of fish measured and that this relationship could be described by a power model. The sampling effort needed to achieve a specific uncertainty level for the selection parameters was always lower for the covered codend method compared to the paired-gear method. In many cases, the number of fish that would need to be measured to maintain a specific uncertainty level was around 10 times higher for the paired-gear method than for the covered codend method. The trends observed for the effect of sampling effort in the two fishery cases investigated were similar; therefore the guidelines presented herein should be applicable to other fisheries.

Feeding of the Barents Sea fish on large bivalvians

Stomach contents of major commercial fish species in the Barents Sea (cod Gadus mohrua, had-dock Melanogrammus aeglefinus, and striped catfish Anarhichas minor) include the soft parts of large bivalvians—serripes Serripes groenlandicus and Iceland scallop Chlamys islandica. The remains of serripes in the stomachs of fish are represented by muscular legs, while those of scallop by muscle and gonads. It is assumed that fish are likely to tear the soft parts of serripes during movement of a mollusk on the ground. Fish can consume the body of scallops only as scallop processing wastes or after shell crushing by fishing gear at bottom trawling and dredging.

Effect of the lifting panel on selectivity of a compulsory grid section (Sort-V) used by the demersal trawler fleet in the Barents Sea cod fishery

In this study we investigated the effect of the lifting panel on the selectivity of a compulsory grid section (Sort-V) used by the demersal trawler fleet in the Barents Sea cod fishery. Ideally, every fish passing through the grid section of the gear would make contact with the grid and be retained or released depending on size. However, in reality some fish may not be able to make contact with the grid or may actively avoid making contact with it. The purpose of the lifting panel is to make the grid contact probability, Cgrid (the proportion of fish that contact the grid), as high as possible. We found that the presence of the lifting panel has a significant effect on Cgrid, as the proportion of fish that came in contact with the grid was reduced from 82% when the lifting panel was present to 67% when the lifting panel was removed. For cod that were making contact with the grid, the mean selectivity parameters, L50grid and SRgrid, were 51.06cm (50.26–51.87cm) and 7.91cm (7.33–8.48cm), respectively, which is in agreement with values estimated for similar grid sections in previous studies. This study highlights the importance of the lifting panel for increasing the effectiveness of the grid section tested.

Resilience and tipping points of an exploited fish population over six decades.

Complex natural systems with eroded resilience, such as populations, ecosystems and socio-ecological systems, respond to small perturbations with abrupt, discontinuous state shifts, or critical transitions. Theory of critical transitions suggests that such systems exhibit fold bifurcations featuring folded response curves, tipping points and alternate attractors. However, there is little empirical evidence of fold bifurcations occurring in actual complex natural systems impacted by multiple stressors. Moreover, resilience of complex systems to change currently lacks clear operational measures with generic application. Here, we provide empirical evidence for the occurrence of a fold bifurcation in an exploited fish population and introduce a generic measure of ecological resilience based on the observed fold bifurcation attributes. We analyse the multivariate development of Barents Sea cod (Gadus morhua), which is currently the world's largest cod stock, over six decades (1949-2009), and identify a population state shift in 1981. By plotting a multivariate population index against a multivariate stressor index, the shift mechanism was revealed suggesting that the observed population shift was a nonlinear response to the combined effects of overfishing and climate change. Annual resilience values were estimated based on the position of each year in relation to the fitted attractors and assumed tipping points of the fold bifurcation. By interpolating the annual resilience values, a folded stability landscape was fit, which was shaped as predicted by theory. The resilience assessment suggested that the population may be close to another tipping point. This study illustrates how a multivariate analysis, supported by theory of critical transitions and accompanied by a quantitative resilience assessment, can clarify shift mechanisms in data-rich complex natural systems.

Bayesian fitting of probabilistic maturation reaction norms to population-level data

Probabilistic maturation reaction norms (PMRNs) are an important tool for studying fisheries-induced evolution and environmental effects on life history. To date there has been no way to fit a PMRN to population-level fisheries data; instead individual-level data must be used. This limits the stocks and time periods that can be studied.We introduce a Bayesian method for fitting PMRNs to population-level data. The method is verified against both an existing result and simulated data, and applied to historical Barents Sea cod data which combines observations of population-level variation in age, size and maturity status from Russia and Norway.The method shows a clear and rapid trend towards greater probability of maturation at smaller lengths in the Barents Sea cod.The new model fitting algorithm allows us to study historic changes in life history despite the lack of individual-level data seen in much long term data. Access to more data will aid the study of evolutionary hypotheses in a wide range of organisms.