Evolution of growth in Gulf of St Lawrence cod?

Fishing is often size selective such that the likelihood of capture increases with body size. It has therefore been postulated that fishing could favour evolution of slower growth because smaller size would reduce exposure to fishing gear (e.g. Ricker 1981). A recent study by Swain et al. (2007; hereafter referred to as SSH) makes a valuable attempt to demonstrate such an effect on length-at-age of southern Gulf of St Lawrence cod (Gadus morhua). The strength of their study lies in an innovative combination of three elements. First, as the evolving trait, they used length-at-age 4 years, an age at which cod are representatively sampled but have experienced little fishing mortality. Confounding demographic effects of size-selective fishing were therefore avoided. Second, they had time series of temperature and population density, both possibly affecting length-at-age through phenotypic plasticity. Finally, and as the most innovative element, they linked their approach to quantitative genetics theory. Using a modified breeder's equation, they modelled changes in length-at-age 4 as a function of genetic and environmental components: ΔL4=h2S+βΔE+ϵ. Here ΔL4 and ΔE are differences in length-at-age 4 and environment, respectively, between the focal cohort and its parent generation. S is the selection differential (difference in mean length-at-age 4 between fish observed at age 4 and those observed at reproducing ages). Estimated heritability h2 and parameter β are regression coefficients, and ϵ is a normally distributed error term with zero mean. SSH assumed that the environment can be described by changes in population density Δd and temperature Δt. The key point is that a significant effect of S on ΔL4 would indicate an evolutionary response in length-at-age 4. SSH's statistically favoured regression model was one including both S and Δd; they concluded that the data suggested an evolutionary response to fishing. Of course, as SSH readily pointed out, one cannot rule out the existence of alternative and untested factors. Here, we comment on some caveats in the analysis by SSH. We do not challenge their novel approach, but question some key assumptions and the strength of their conclusions

Linking shrimp recruitment and environmental variability in French Guiana

The shrimp fishery (Farfantepenaeus subtilis and F. brasiliensis) was the first fishery in value in French Guiana during the 1990s. However, two successive drops in recruitment in 1999 and 2006 led to decreases in stock and hence landings of shrimp. Recruitment levels are today at their lowest. Prior analyses suggest a minor influence of harvesting and fish predation on the failure in stock rebuilding. Environment instead appears to play a pivotal role here. In a region where few oceanographic surveys have been carried out, remote sensing has proven a powerful tool to track variations in the environment. Series of water temperature, turbidity, suspended matters and chlorophyll a concentrations were analysed and compared to the series of shrimp recruitment in order to: 1) Detect the occurrence of temporal trends or potential regime shifts in waters off French Guiana, 2) Assess the relationships between these environmental factors and recruitment success of F. subtilis. While trends can be observed in some environmental factors, no clear link can be established between a single variable and the recent decline in shrimp recruitment

Body condition and evolution of maturation of Atlantic cod in Newfoundland

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

Temporal stability of the maturation schedule of capelin

Capelin in the Barents Sea are primarily harvested in a terminal fishery that targets maturing individuals. Theory predicts that, in a semelparous population (i.e., one in which reproduction is seasonal, synchronous, and followed by parental mortality), an unselective, terminal fishery (i.e., one in which most of the fish that are not caught will not have a new spawning opportunity) does not generate strong selection for changed age and size at maturation. The probabilistic maturation reaction norm (PMRN) method was applied to test this prediction and to detect possible temporal changes in length at maturation of Barents Sea capelin between 1978 and 2008. Maturation reaction norms suggest that maturation is age-independent in capelin, but that males require a larger size to attain the same maturation probability as females. No temporal trends in length at maturation could be detected, thus confirming the theoretical prediction. Furthermore, none of the candidate environmental variables tested to explain the temporal variability in length at maturation (water temperature and capelin biomass) consistently showed a significant correlation with the PMRN midpoints

Evolutionary impact assessment: Accounting for the evolutionary consequences of fishing in an ecosystem approach to fisheries management

Managing fisheries resources to maintain healthy ecosystems is one of the main goals of the ecosystem approach to fisheries (EAF). While a number of international treaties call for the implementation of EAF, there are still gaps in the underlying methodology. One aspect that has received substantial scientific attention recently is fisheries-induced evolution (FIE). Increasing evidence indicates that intensive fishing has the potential to exert strong directional selection on life-history traits, behaviour, physiology, and morphology of exploited fish. Of particular concern is that reversing evolutionary responses to fishing can be much more difficult than reversing demographic or phenotypically plastic responses. Furthermore, like climate change, multiple agents cause FIE, with effects accumulating over time. Consequently, FIE may alter the utility derived from fish stocks, which in turn can modify the monetary value living aquatic resources provide to society. Quantifying and predicting the evolutionary effects of fishing is therefore important for both ecological and economic reasons. An important reason this is not happening is the lack of an appropriate assessment framework. We therefore describe the evolutionary impact assessment (EvoIA) as a structured approach for assessing the evolutionary consequences of fishing and evaluating the predicted evolutionary outcomes of alternative management options. EvoIA can contribute to EAF by clarifying how evolution may alter stock properties and ecological relations, support the precautionary approach to fisheries management by addressing a previously overlooked source of uncertainty and risk, and thus contribute to sustainable fisheries