Life-history evolution in response to foraging risk, modelled for Northeast Arctic cod (Gadus morhua)

Foraging behaviour is known to be a key element in ecology and evolution. Increased foraging intensity increases energy intake, which is useful for growth and reproduction but comes at the cost of higher mortality risk due to increased exposure to predators. Here, we investigate these trade-offs through an individual-based, mechanistic modelling framework adapted to the Northeast Arctic Cod. The model incorporates a series of life-history traits, survival trade-offs, and heritability, which allow evolution to occur and optimal strategies to emerge due to individual trait combinations and their fitness consequences. By altering the relationship between foraging intensity and mortality risk, we find that increased risk causes evolution towards lower foraging effort leading to lower growth and in turn, earlier maturation and a faster pace of life. These results build on previous studies by demonstrating behavioural evolution without direct anthropogenic stressors. Natural mortality among fish is poorly understood, and these results highlight an interesting point of further research that could help future modelling approaches make more accurate assumptions about natural mortality and its components.publishedVersio

Large annual variation in the amount of skipped spawning for female Northeast Arctic haddock Melanogrammus aeglefinus

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

Unclear associations between small pelagic fish and jellyfish in several major marine ecosystems

During the last 20 years, a series of studies has suggested trends of increasing jellyfish (Cnidaria and Ctenophora) biomass in several major ecosystems worldwide. Some of these systems have been heavily fished, causing a decline among their historically dominant small pelagic fish stocks, or have experienced environmental shifts favouring jellyfish proliferation. Apparent reduction in fish abundance alongside increasing jellyfish abundance has led to hypotheses suggesting that jellyfish in these areas could be replacing small planktivorous fish through resource competition and/or through predation on early life stages of fish. In this study, we test these hypotheses using extended and published data of jellyfish, small pelagic fish and crustacean zooplankton biomass from four major ecosystems within the period of 1960 to 2014: the Southeastern Bering Sea, the Black Sea, the Northern California Current and the Northern Benguela. Except for a negative association between jellyfish and crustacean zooplankton in the Black Sea, we found no evidence of jellyfish biomass being related to the biomass of small pelagic fish nor to a common crustacean zooplankton resource. Calculations of the energy requirements of small pelagic fish and jellyfish stocks in the most recent years suggest that fish predation on crustacean zooplankton is 2–30 times higher than jellyfish predation, depending on ecosystem. However, compared with available historical data in the Southeastern Bering Sea and the Black Sea, it is evident that jellyfish have increased their share of the common resource, and that jellyfish can account for up to 30% of the combined fish-jellyfish energy consumption. We conclude that the best available time-series data do not suggest that jellyfish are outcompeting, or have replaced, small pelagic fish on a regional scale in any of the four investigated ecosystems. However, further clarification of the role of jellyfish requires higher-resolution spatial, temporal and taxonomic sampling of the pelagic community.publishedVersio

Panel-based Assessment of Ecosystem Condition of the North Sea Shelf Ecosystem

The System for Assessment of Ecological Condition, coordinated by the Norwegian Environment Agency, is intended to form the foundation for evidence-based assessments of the ecological condition of Norwegian terrestrial and marine ecosystems not covered by the EU Water Framework Directive. The reference condition is defined as “intact ecosystems”, i.e., a condition that is largely unimpacted by modern industrial anthropogenic activities. An ecosystem in good ecological condition does not deviate substantially from this reference condition in structure, functions or productivity. This report describes the first operational assessment of the ecological condition of the marine shelf ecosystem in the Norwegian sector of the North Sea and Skagerrak. The assessment method employed is the Panel-based Assessment of Ecosystem Condition (PAEC1) and the current assessment has considered to what extent the North Sea and Skagerrak shelf ecosystem deviates from the reference condition2 by evaluating change trajectories.Panel-based Assessment of Ecosystem Condition of the North Sea Shelf EcosystempublishedVersio

Panel-based Assessment of Ecosystem Condition of the North Sea Shelf Ecosystem - Appendices

publishedVersio

Dynamics of spawning migrations in Northeast Arctic cod

The Northeast Arctic (NA) cod is known to undertake long southbound spawning migrations from their feeding grounds in the Barents Sea, to various spawning grounds along the Norwegian coast. Hence it’s native Norwegian name “skrei”, meaning to move or to travel. The spawned eggs and larvae subsequently drift northwards along with the prevailing currents, eventually reaching the Barents Sea as juveniles. From commercial fisheries statistics we see that these spawning grounds once spanned across nearly 2000 km of the west coast of Norway, from Finnmark in the north to Vest-Agder at the very south. Today the latitudinal range of the spawning grounds is significantly reduced, with only a marginal part of the stock spawning as far south as Møre, some 1500 km from the Barents Sea. Coarse estimates from the Institute of Marine Research suggest that around 90 - 95 % of the spawning stock spawn from Lofoten and northwards, utilizing only a third of their original expanse. Also, recent scientific surveys have found that during the last decade, the NA cod have to a great extent stopped spawning at their major traditional grounds in Vestfjorden, Lofoten, and have relocated to more northbound areas. Historically, scientists have shown little interest in the changes in spawning grounds, but due to the major impact this recent shift has had on local fishermen and landing ports, the topic currently attracts noticeable interest from fisheries scientists as well as climate scientists. The NA cod’s previous widespread spawning distribution, together with its historical and contemporary northbound shifts has spurred two major research questions addressed in this thesis. First, why does spawning take place over such a vast geographical area, implying that certain parts of the population undergo spawning migrations several thousands of kilometres further than their conspecifics? Secondly, what has caused the northbound shift in spawning grounds? From general evolutionary theory, we would expect that individuals undertaking longer spawning migrations, thus leaving less time to forage in the Barents Sea, as well as increasing their energetic cost of migration, would achieve a comparable return benefit. If not, such a life history strategy could simply not hold through the course of natural selection. The benefit need not target the spawning individual directly, but can also be mediated through increased fitness to their progeny, essentially through increased survival probability. Using a simplistic assumption that fitness benefit increases linearly with migration distance, we developed a model simulating a population of individuals which finds optimal solutions to the trade-off between growth and reproduction, depending on physiological condition and ecological constraints. Overall, the model predict that larger individuals and individuals in better condition gain higher fitness benefit from longer spawning migrations compared to smaller and less fit individuals. These findings are partly due to a nonlinear relationship between hydrodynamic friction and individual size, meaning that relatively, larger fish spend less energy on swimming compared to smaller individuals. More interestingly, when simulating historical fishing pressure at the spawning grounds, there is selection for large late-maturing fish and longer migrations, whilst a contemporary trawl fishery, typically located at the feeding areas in the Barents Sea, select for small and early-maturing fish with shorter optimal migration distances. The latter case is consistent with observational studies, and indicates that fisheries’ induced evolution have not only lowered the maturation age of NA cod, but may also be causing the northbound shift in spawning ground distribution. To test the validity of our initial assumption that southerly spawning grounds are in fact associated with higher fitness benefits, we employed different general circulation models to track virtual fish eggs and larvae released at various spawning grounds along the Norwegian coast. From their drift trajectories towards the Barents Sea, we found that eggs and larvae released from more southerly spawning grounds experienced higher average temperature exposures, generally thought to promote faster growth and consequently reduce mortality in early life stages. However, the southernmost spawning grounds generally also experienced more retention in local fjord systems. In addition, seasonal and inter-annual variation in drift trajectories, as well as overall temperature exposure, growth and survival was evident, indicating that climatic conditions may also play a role for offspring success. Overall, the latitudinal effect on larval temperature exposure was significantly stronger than the climatic variability. Finally, by utilizing empirical data from commercial catch statistics dating back to 1866, our initial theory, that shifts in spawning grounds are caused by a size-selective industrial trawl fishery in the Barents Sea, was tested against alternative explanatory factors such as density dependence and climate change. In total, 104 years of landing data were compiled for the entire Norwegian coast, revealing large fluctuations in spawning ground distributions, but also showing trends towards more northbound spawning after the 1920s. Climatic variation was found only partially to explain the variation, whilst rapidly increasing landings from the trawl fishery in the Barents Sea starting around 1923, clearly coincided with the northbound shifts in spawning grounds

Relationships between spawning ground identity, Latitude and early life thermal exposure in Northeast Arctic Cod

from its feeding grounds in the Barents Sea to various spawning banks along the Norwegian coast. Prior to the 1990s these banks were located on a wide latitudinal range from Finnmark (~71º N) to Møre (~63º N), or even to the south-western parts of Norway (~60º N), with the highest densities around Lofoten (~69º N). The migration is energetically costly, but may be profitable if offspring experience warmer water, higher growth rates and lower mortality. To investigate if such a temperature- benefit-hypothesis is plausible, we utilize a regional oceanographic model system (ROMS) and a particle tracking model to trace the drift of particles (virtual cod larvae) released at six important spawning grounds along a north-south gradient. We did this for two years with contrasting oceanographic conditions, and we assume the integrated ambient temperature of each particle determines growth potential during the northbound drift. In the model, particles released at the most southerly bank generally do experience significantly higher temperatures than particles released at more northern spawning grounds. This is caused by a combination of higher sea-temperatures and higher retention above and around the southern spawning ground. However, particles released at the important spawning grounds in Vestfjorden are exposed to the lowest temperatures of all. Our results suggest that offspring temperature exposure is not simply a function of latitude, but that other factors such as retention, larval prey availability and potential energetic costs of parents may modify the profitability of the spawning migration

Effects of temperature and food availability on larval cod survival: A model for behaviour in vertical gradients

Recruitment success in living marine resources is variable due to high survival variability of early life stages. Improving our understanding of how environmental and ecological factors mechanistically interact and influence larval fish growth and survival is necessary to better predict year-class strength and expected physiological and behavioural responses to climate warming. We use a state-dependent optimality model for the behaviour of larval Atlantic cod Gadus morhua to analyse trade-offs related to growth and survival. Temperature-dependent maximum growth rates and vertical profiles of temperature and stochastic prey availability are used as inputs within a mechanistic modelling framework that finds optimal behavioural strategies of vertical migration and foraging activity. The fitness criterion used is maximization of survival probability until the larvae reach a given body size (15 mm). Detailed descriptions of predation, physiology, growth and survival of larval cod emerge from simulations of the optimal strategies. The model shows that the effect of temperature on survival is complex. Increasing temperature may lead to faster growth and higher survival, but only when there is sufficient food. In poor food environments, higher temperatures make larvae more susceptible to predation as they take higher risks to satisfy their metabolic costs. Overall, these results suggest that larval Atlantic cod, especially those from warmer-water stocks, may experience reduced survival and recruitment in climate-change scenarios that predict both elevated temperatures and reduced food supply

Long-term stability in modelled zooplankton influx could uphold major fish spawning grounds on the Norwegian continental shelf

Early life stages of fish spawned on the Norwegian continental shelf have long been suggested to depend on eggs and nauplii from the crustacean zooplankton Calanus finmarchicus, for survival. Calanus overwinters in the deep basins of the Norwegian Sea, and gravid females must be advected onto the shelf prior to spawning if eggs and nauplii larvae are to serve as food for fish larvae. In this study, cross-shelf advection of calanus is simulated over 52 years (1960-2011), using a numerical ocean model coupled with an individual-based model. The results suggest that cross-shelf transport of calanus is surprisingly stable across years, and that transport is particularly concentrated immediately upstream of the two major spawning areas for the Northeast Arctic cod and the Norwegian spring spawning herring, namely Lofoten and Møre, respectively. Two large topographical features, the Træna trough and the Norwegian trench, appear to be funnelling calanus onto the shelf in these two areas. This could suggest that the fish spawning grounds outside Møre and Lofoten are, in part, maintained due to stable inter-annual food supply in spring.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author