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The Barents Sea stock of capelin (Mallotus villosus) has suffered three major collapses (>90% reduction) since 1985 due to recruitment failures. As capelin is a key species in the area, these population collapses have had major ecosystem consequences. By analysing data on spawner biomass and three recruitment stages (larvae, 0-group, and 1-year-olds), we suggest that much of the recruitment failures are caused by predation from herring (Clupea harengus) and 0-group and adult Northeast Arctic cod (Gadus morhua). Recruitment is furthermore positively correlated with sea temperatures in winter and spring. Harvesting of maturing capelin on their way to the spawning grounds reduced the abundance of larvae significantly, but this reduction to a large extent is compensated for later in life, as mortality is strongly density-dependent between the larval stage and age 1. Altogether, our study indicates a very high importance of trophic interactions, consistent with similar findings in other high-latitude marine ecosystems.

In the Barents Sea, capelin is a key food item for the North-East Arctic cod stock. This capelin stock has had very unstable population dynamics since 1985, with recruitment failures leading to three major collapses of the stock (>90% reduction of the stock size), resulting in decreased growth and survival of cod. Here we analyze in detail how predation and harvest affects the recruitment of capelin, using data on three different stages (i.e., larvae, zero-group and 1-year-olds) through the first 1.5 years of the capelin's life. We demonstrate that both herring predation (on capelin larvae) and cod predation (both on spawners and on offspring) has had major negative effects on capelin recruitment. Mortality is furthermore demonstrated to be strongly density-dependent, and is lower when temperatures are high – probably due to higher food availability for capelin. Harvesting maturing capelin on the way to the spawning grounds did affect the production of larvae, at least during the first half of the 1980s. However, the reduced production of larvae appears to a large extent to have been compensated by decreased density-dependent mortality on later life-stages, resulting in only minor effects on the abundance as 1-year-olds. Altogether, our study points to the importance of trophic interactions in determining the dynamic structure in high-latitude marine ecosystems. Keywords: stock collapse, predation, trophic interactions, harvesting, density-dependent mortality, Generalized Additive Models (GAM)

Open access article Worldwide depletion of fish stocks has led fisheries managers to become increasingly concerned about rebuilding and recovery planning. To succeed, factors affecting recovery dynamics need to be understood, including the role of fisheries-induced evolution. Here we investigate a stock’s response to fishing followed by a harvest moratorium by analyzing an individual-based evolutionary model parameterized for Atlantic cod Gadus morhua from its northern range, representative of long-lived, late-maturing species. The model allows evolution of life-history processes including maturation, reproduction, and growth. It also incorporates environmental variability, phenotypic plasticity, and density-dependent feedbacks. Fisheries-induced evolution affects recovery in several ways. The first decades of recovery were dominated by demographic and density-dependent processes. Biomass rebuilding was only lightly influenced by fisheries-induced evolution, whereas other stock characteristics such as maturation age, spawning stock biomass, and recruitment were substantially affected, recovering to new demographic equilibria below their preharvest levels. This is because genetic traits took thousands of years to evolve back to preharvest levels, indicating that natural selection driving recovery of these traits is weaker than fisheries-induced selection was. Our results strengthen the case for proactive management of fisheries-induced evolution, as the restoration of genetic traits altered by fishing is slow and may even be impractical.

Atlantic cod (Gadus morhua, L.) stocks off Newfoundland and Labrador underwent severe depletions that started in the late 1980s and led to moratoria in the early 1990s. Fisheries closures are still applied in most of the areas where cod is found. A very low stock level, probably resulting from the combined effects of overfishing and unfavorable environmental conditions, has been accompanied with modifications of life history traits of the fish. Among reproductive traits, a decrease in both age and size at first reproduction has been observed. A partial genetical determinism of these shifts is supported by earlier analyses of probabilistic maturation reaction norms, revealing a consistent trend towards earlier maturation but also strong short-term fluctuations. In this study, we elucidate the effect of changes in individual body condition on short and long-term changes in maturation of cod. Laboratory studies have demonstrated that condition of individuals can influence the number and quality of offspring in cod. Here we use probabilistic maturation reaction norms with three explanatory dimensions (age, size and condition) to examine to what extent changes in age- and size-dependent maturation probabilities are linked with changes in condition, as measured by the hepato-somatic index.

Fishing is very likely to create selective pressures inducing adaptive changes in the life histories of harvested stocks. Using field data and adequate statistical methods, such alterations can be demonstrated. However, in order to understand underlying causes and to evaluate alternative management practices, past selective pressures must be quantified and predictions of future evolutionary changes are needed. In this respect, modelling the ecological and evolutionary dynamics of exploited stocks is a critical challenge. To illustrate this point, we studied the evolution of age and size at maturation induced by fishing using adaptive dynamics theory – a framework that allows modelling long-term evolution of quantitative traits under density- and frequency-dependent selection. Specifically, we investigated the evolutionary implications of alternative management policies. As a novel contribution to the discussion of fisheries-induced adaptive change, we showed that frequency-dependent selection, arising from fishing mortality under some particular management policies, can not only reduce the age and size at maturation and thus stock biomass, but can ultimately even induce the extinction of entire stocks. The potential for such phenomena of ‘evolutionary suicide’ is overlooked in models that do not incorporate life history evolution. Our findings thus highlight the importance of considering evolutionary trends in the management of exploited stocks.