Common trends in recruitment dynamics of north-east Atlantic fish stocks and their links to environment, ecology and management

Recruitment dynamics are challenging to assess or predict because of the many underlying drivers that vary in their relevance over time and space. Stock size, demographic and trait composition, condition and distribution of spawning fish and the spatio‐temporal dynamics of trophic and environmental interactions all influence recruitment processes. Exploring common patterns among stocks and linking them to potential drivers may therefore provide insights into key mechanisms of recruitment dynamics. Here, we analysed stock‐recruitment data of 64 stocks from the north‐east Atlantic Ocean for common trends in variation and synchrony among stocks using correlation, cluster and dynamic factor analyses. We tested common trends in recruitment success for relationships with large‐scale environmental processes as well as stock state indicators, and we explored links between recruitment success and demographic, environmental and ecological variables for a subset of individual stocks. The results revealed few statistically significant correlations between stocks but showed that underlying common trends in recruitment success are linked to environmental indices and management indicators. Statistical analyses confirmed previously suggested relationships of environmental–ecological factors such as the subpolar gyre and Norwegian coastal current with specific stocks, and indicated a large relevance of spawning stock biomass and demographics, as well as predation, whereas other suggested relationships were not supported by the data. Our study shows that despite persistent challenges in determining drivers of recruitment due to poor data quality and unclear mechanisms, combining different data analysis techniques can improve our understanding of recruitment dynamics in fish stocks.publishedVersio

Body size adaptions under climate change: Zooplankton community more important than temperature or food abundance in model of a zooplanktivorous fish

One of the most well-studied biogeographic patterns is increasing body size with latitude, and recent body size declines in marine and terrestrial organisms have received growing attention. Spatial and temporal variation in temperature is the generally invoked driver but food abundance and quality are also emphasized. However, the underlying mechanisms are not clear and the actual cause is likely to differ both within and among species. Here, we focused our attention on drivers of body size in planktivorous fish that forage through vision. This group of organisms plays a central role in marine ecosystems by linking the energy flow from lower to higher trophic levels. Using a model that incorporates explicit mechanisms for vision-based feeding and physiology, we investigated the influence on optimal body size of several biotic (prey size, prey energy content, and prey biomass concentration) and abiotic (temperature, latitude, and water clarity) factors known to affect foraging rates and bioenergetics. We found prey accessibility to be the most influential factor for body size, determined primarily by prey size but also by water clarity, imposing visual constraints on prey encounters and thereby limiting feeding rates. Hence, for planktivores that forage through vision, an altered composition of the prey field could have important implications for body size and for the energy available for reproduction and other fitness-related tasks. Understanding the complicated effects of climate change on zooplankton communities is thus crucial for predicting impacts on planktivorous fish, as well as consequences for energy flows and body sizes in marine systems.publishedVersio

Environmental stressors may cause unpredicted, notably lagged life-history responses in adults of the planktivorous Atlantic herring

Here we challenge traditional views on the direction of change in teleost body condition and reproductive traits in response to abiotic and biotic factors by studying the data-rich, planktivorous Norwegian spring-spawning herring (NSSH), a member of the abundant Atlantic herring (Clupea harengus) stock complex. To test potential influential factors, we focused on the last twenty years, i.e. a period with ocean warming, a transient but significant drop in zooplankton biomass, and accelerating interspecific competition resulting from primarily Atlantic mackerel (Scomber scombrus) entering these high-latitude waters in large quantities, “the new mackerel era” in the Nordic Seas. Adult NSSH concurrently allocated relatively less to growth in length than weight resulting in higher body condition. Growth likely decreased in warmer waters under stiff prey competition to support reproductive costs. Condition and reproductive responses were not only immediate but were also lagged by three seasons, corresponding to the period when new oocytes are produced. Furthermore, fecundity increased in warmer waters while egg size dropped. Hence, fine-tuned trade-off mechanisms were apparent and varied. We demonstrate that evaluations of reproductive trade-offs based on pooled data are misleading; poor- and good-condition NSSH followed different reproductive trajectories. These findings emphasize difficult-to-predict trends in life-history traits should be tracked longitudinally by the individuals and their aggregate cohort, as they are linked to complex overarching environmental phenomena, like ecosystem carrying capacity and climate fluctuations.publishedVersio

A warmer environment can reduce sociability in an ectotherm

The costs and benefits of being social vary with environmental conditions, so individuals must weigh the balance between these trade-offs in response to changes in the environment. Temperature is a salient environmental factor that may play a key role in altering the costs and benefits of sociality through its effects on food availability, predator abundance, and other ecological parameters. In ectotherms, changes in temperature also have direct effects on physiological traits linked to social behaviour, such as metabolic rate and locomotor performance. In light of climate change, it is therefore important to understand the potential effects of temperature on sociality. Here, we took the advantage of a ‘natural experiment’ of threespine sticklebacks from contrasting thermal environments in Iceland: geothermally warmed water bodies (warm habitats) and adjacent ambient-temperature water bodies (cold habitats) that were either linked (sympatric) or physically distinct (allopatric). We first measured the sociability of wild-caught adult fish from warm and cold habitats after acclimation to a low and a high temperature. At both acclimation temperatures, fish from the allopatric warm habitat were less social than those from the allopatric cold habitat, whereas fish from sympatric warm and cold habitats showed no differences in sociability. To determine whether differences in sociability between thermal habitats in the allopatric population were heritable, we used a common garden breeding design where individuals from the warm and the cold habitat were reared at a low or high temperature for two generations. We found that sociability was indeed heritable but also influenced by rearing temperature, suggesting that thermal conditions during early life can play an important role in influencing social behaviour in adulthood. By providing the first evidence for a causal effect of rearing temperature on social behaviour, our study provides novel insights into how a warming world may influence sociality in animal populations.publishedVersio

A warmer environment can reduce sociability in an ectotherm

The costs and benefits of being social vary with environmental conditions, so individuals must weigh the balance between these trade-offs in response to changes in the environment. Temperature is a salient environmental factor that may play a key role in altering the costs and benefits of sociality through its effects on food availability, predator abundance, and other ecological parameters. In ectotherms, changes in temperature also have direct effects on physiological traits linked to social behaviour, such as metabolic rate and locomotor performance. In light of climate change, it is therefore important to understand the potential effects of temperature on sociality. Here, we took the advantage of a ‘natural experiment’ of threespine sticklebacks from contrasting thermal environments in Iceland: geothermally warmed water bodies (warm habitats) and adjacent ambient-temperature water bodies (cold habitats) that were either linked (sympatric) or physically distinct (allopatric). We first measured the sociability of wild-caught adult fish from warm and cold habitats after acclimation to a low and a high temperature. At both acclimation temperatures, fish from the allopatric warm habitat were less social than those from the allopatric cold habitat, whereas fish from sympatric warm and cold habitats showed no differences in sociability. To determine whether differences in sociability between thermal habitats in the allopatric population were heritable, we used a common garden breeding design where individuals from the warm and the cold habitat were reared at a low or high temperature for two generations. We found that sociability was indeed heritable but also influenced by rearing temperature, suggesting that thermal conditions during early life can play an important role in influencing social behaviour in adulthood. By providing the first evidence for a causal effect of rearing temperature on social behaviour, our study provides novel insights into how a warming world may influence sociality in animal populations

Characteristics of meiofauna in extreme marine ecosystems: a review

Extreme marine environments cover more than 50% of the Earth’s surface and offer many opportunities for investigating the biological responses and adaptations of organisms to stressful life conditions. Extreme marine environments are sometimes associated with ephemeral and unstable ecosystems, but can host abundant, often endemic and well-adapted meiofaunal species. In this review, we present an integrated view of the biodiversity, ecology and physiological responses of marine meiofauna inhabiting several extreme marine environments (mangroves, submarine caves, Polar ecosystems, hypersaline areas, hypoxic/anoxic environments, hydrothermal vents, cold seeps, carcasses/sunken woods, deep-sea canyons, deep hypersaline anoxic basins [DHABs] and hadal zones). Foraminiferans, nematodes and copepods are abundant in almost all of these habitats and are dominant in deep-sea ecosystems. The presence and dominance of some other taxa that are normally less common may be typical of certain extreme conditions. Kinorhynchs are particularly well adapted to cold seeps and other environments that experience drastic changes in salinity, rotifers are well represented in polar ecosystems and loriciferans seem to be the only metazoan able to survive multiple stressors in DHABs. As well as natural processes, human activities may generate stressful conditions, including deoxygenation, acidification and rises in temperature. The behaviour and physiology of different meiofaunal taxa, such as some foraminiferans, nematode and copepod species, can provide vital information on how organisms may respond to these challenges and can provide a warning signal of anthropogenic impacts. From an evolutionary perspective, the discovery of new meiofauna taxa from extreme environments very often sheds light on phylogenetic relationships, while understanding how meiofaunal organisms are able to survive or even flourish in these conditions can explain evolutionary pathways. Finally, there are multiple potential economic benefits to be gained from ecological, biological, physiological and evolutionary studies of meiofauna in extreme environments. Despite all the advantages offered by meiofauna studies from extreme environments, there is still an urgent need to foster meiofauna research in terms of composition, ecology, biology and physiology focusing on extreme environments

A warmer environment can reduce sociability in an ectotherm

The costs and benefits of being social vary with environmental conditions, so individuals must weigh the balance between these trade-offs in response to changes in the environment. Temperature is a salient environmental factor that may play a key role in altering the costs and benefits of sociality through its effects on food availability, predator abundance, and other ecological parameters. In ectotherms, changes in temperature also have direct effects on physiological traits linked to social behaviour, such as metabolic rate and locomotor performance. In light of climate change, it is therefore important to understand the potential effects of temperature on sociality. Here, we took the advantage of a 'natural experiment' of threespine sticklebacks from contrasting thermal environments in Iceland: geothermally warmed water bodies (warm habitats) and adjacent ambient-temperature water bodies (cold habitats) that were either linked (sympatric) or physically distinct (allopatric). We first measured the sociability of wild-caught adult fish from warm and cold habitats after acclimation to a low and a high temperature. At both acclimation temperatures, fish from the allopatric warm habitat were less social than those from the allopatric cold habitat, whereas fish from sympatric warm and cold habitats showed no differences in sociability. To determine whether differences in sociability between thermal habitats in the allopatric population were heritable, we used a common garden breeding design where individuals from the warm and the cold habitat were reared at a low or high temperature for two generations. We found that sociability was indeed heritable but also influenced by rearing temperature, suggesting that thermal conditions during early life can play an important role in influencing social behaviour in adulthood. By providing the first evidence for a causal effect of rearing temperature on social behaviour, our study provides novel insights into how a warming world may influence sociality in animal populations.</p

Long-term changes in life-history traits of Norwegian spring-spawning herring

Life-history traits are key determinants of populations&rsquo; dynamics. Those traits are susceptible to natural selection and are therefore sensitive to the mortality regime and the selection pattern a population experience. Nowadays, almost all natural populations are affected by anthropogenic activities (e.g. urbanisation, loss or segmentation of habitats). More specifically, fish stocks, reproducing in the wild and coping with environmental fluctuations, have been exploited with industrialized efficiency for decades. The effects of fisheries on exploited stocks life-history traits have been widely studied and there is currently no doubt that fisheries-induced evolution is taking place in many of them. Fisheries-induced evolution of life-history traits constitutes a challenge for management, since genetic changes are difficult to reverse, and can lead to a loss of productivity and resilience of the exploited stock. Considering the amount of evidence supporting fisheries-induced evolution, it is therefore surprising that little of it is found in an intensively exploited pelagic fish, the Norwegian spring-spawning herring. Exploited for centuries, the stock collapsed in the late 1960&rsquo;s because of overfishing and took more than 15 years to recover. The fishery went from an open-access fishery to a TAC regulated fishery combined with management regulations such as a minimum landing size of 25 cm. A weak decrease in age at maturation, maybe attributable to fisheries-induced evolution, has been detected, while other traits haven&rsquo;t been studied yet. The goal of this thesis is to investigate how traits other than age and length at maturation have been affected by the fishing pressure in Norwegian spring-spawning herring. In addition, it gives an overview of how the selection pressures Norwegian Spring-spawning herring is subjected to may have changed during the last century and explain the weakness of the trends observed. Using multivariate linear (mixed effect) models together with data spanning 80 years, significant residual trends, potentially attributable to genetic changes, were found for the reproductive investment (increase, Paper II) and adult growth (decrease, Paper IV). No significant trend could be found in juvenile growth (Paper IV). However, the trends observed are weak and year to year variation is still mainly driven by environmental factors. In the case they stem from genetic changes, three possibilities could explain their weakness: (1) They are partly masked by phenotypic plasticity. (2) Changes in the selectivity experienced (Paper III) could slow down evolution rates. (3) The current selection pattern (Paper III) is driving the long-term trend. In addition, we showed that fishing can induce changes in natural mortality by selecting specific behaviours (Paper I), potentially leading to biased estimates for stock size assessment. However, it is not known how much of this pertains to Norwegian spring-spawning herring and warrants more research. The consequences of the observed trends in Norwegian spring-spawning herring lifehistory traits for the stock&rsquo;s dynamics are not very clear yet. Even though slower growth could lead to a loss of productivity, consequences would not be as drastic as in stocks were age and size at maturation are largely reduced. Considering the current knowledge about fisheries-induced evolution, it seems that the modern management measures for the Norwegian spring-spawning herring stock are the most desirable and that the potential evolution shown in this thesis is of little consequence for the stock&rsquo;s management compared to environmental variability. Close monitoring of the stock is however necessary to avoid or mitigate any detrimental effect fisheries-induced evolution could have in the future on the stock&rsquo;s productivity and, most importantly, recovery potential

Evolutionary effects of fishing gear on foraging behavior and life-history traits

Fishing gears are designed to exploit the natural behaviors of fish, and the concern that fishing may cause evolution of behavioral traits has been receiving increasing attention. The first intuitive expectation is that fishing causes evolution toward reduced boldness because it selectively removes actively foraging individuals due to their higher encounter rate and vulnerability to typical gear. However, life‐history theory predicts that fishing, through shortened life span, favors accelerated life histories, potentially leading to increased foraging and its frequent correlate, boldness. Additionally, individuals with accelerated life histories mature younger and at a smaller size and therefore spend more of their life at a smaller size where mortality is higher. This life‐history evolution may prohibit increases in risk‐taking behavior and boldness, thus selecting for reduced risk‐taking and boldness. Here, we aim to clarify which of these three selective patterns ends up being dominant. We study how behavior‐selective fishing affects the optimal behavioral and life‐history traits using a state‐dependent dynamic programming model. Different gear types were modeled as being selective for foraging or hiding/resting individuals along a continuous axis, including unselective fishing. Compared with unselective harvesting, gears targeting hiding/resting individuals led toward evolution of increased foraging rates and elevated natural mortality rate, while targeting foraging individuals led to evolution of decreased foraging rates and lower natural mortality rate. Interestingly, changes were predicted for traits difficult to observe in the wild (natural mortality and behavior) whereas the more regularly observed traits (length‐at‐age, age at maturity, and reproductive investment) showed only little sensitivity to the behavioral selectivity.publishedVersio

Evolutionary effects of fishing gear on foraging behavior and life-history traits

Fishing gears are designed to exploit the natural behaviors of fish, and the concern that fishing may cause evolution of behavioral traits has been receiving increasing attention. The first intuitive expectation is that fishing causes evolution toward reduced boldness because it selectively removes actively foraging individuals due to their higher encounter rate and vulnerability to typical gear. However, life‐history theory predicts that fishing, through shortened life span, favors accelerated life histories, potentially leading to increased foraging and its frequent correlate, boldness. Additionally, individuals with accelerated life histories mature younger and at a smaller size and therefore spend more of their life at a smaller size where mortality is higher. This life‐history evolution may prohibit increases in risk‐taking behavior and boldness, thus selecting for reduced risk‐taking and boldness. Here, we aim to clarify which of these three selective patterns ends up being dominant. We study how behavior‐selective fishing affects the optimal behavioral and life‐history traits using a state‐dependent dynamic programming model. Different gear types were modeled as being selective for foraging or hiding/resting individuals along a continuous axis, including unselective fishing. Compared with unselective harvesting, gears targeting hiding/resting individuals led toward evolution of increased foraging rates and elevated natural mortality rate, while targeting foraging individuals led to evolution of decreased foraging rates and lower natural mortality rate. Interestingly, changes were predicted for traits difficult to observe in the wild (natural mortality and behavior) whereas the more regularly observed traits (length‐at‐age, age at maturity, and reproductive investment) showed only little sensitivity to the behavioral selectivity