Douglas P. Swain

Evolutionary response to size-selective mortality in an exploited fish population

Many collapsed fish populations have failed to recover after a decade or more with little fishing. This may reflect evolutionary change in response to the highly selective mortality imposed by fisheries. Recent experimental work has demonstrated a rapid genetic change in growth rate in response to size-selective harvesting of laboratory fish populations. Here, we use a 30-year time-series of back-calculated lengths-at-age to test for a genetic response to size-selective mortality in the wild in a heavily exploited population of Atlantic cod (Gadus morhua). Controlling for the effects of density- and temperature-dependent growth, the change in mean length of 4-year-old cod between offspring and their parental cohorts was positively correlated with the estimated selection differential experienced by the parental cohorts between this age and spawning. This result supports the hypothesis that there have been genetic changes in growth in this population in response to size-selective fishing. Such changes may account for the continued small size-at-age in this population despite good conditions for growth and little fishing for over a decade. This study highlights the need for management regimes that take into account the evolutionary consequences of fishing.

Genetic variation in life-history reaction norms in a marine fish.

Neither the scale of adaptive variation nor the genetic basis for differential population responses to the environment is known for broadcast-spawning marine fishes. Using a common-garden experimental protocol, we document how larval growth, survival and their norms of reaction differ genetically among four populations of Atlantic cod (Gadus morhua). These traits, and their plastic responses to food and temperature, differed across spatial scales at which microsatellite DNA failed to detect population structure. Divergent survival reaction norms indicate that warm-water populations are more sensitive to changes in food, whereas cold-water populations are more sensitive to changes in temperature. Our results suggest that neither the direction nor the magnitude of demographic responses to environmental change need be the same among populations. Adaptive phenotypic plasticity, previously undocumented in marine fishes, can significantly influence the probability of recovery and persistence of collapsed populations by affecting their ability to respond to natural and anthropogenic environmental change.

Impacts of environmental change and direct and indirect harvesting effects on the dynamics of a marine fish community

We describe dramatic shifts in the species composition of the marine fish community of the southern Gulf of St. Lawrence using a 35-year time series of catch rates in an annual bottom-trawl survey. We attempt to understand the causes of these changes using a traits-based approach that relates the similarity among species in their abundance trends to similarities in their ecological traits. We selected traits based on a priori beliefs of how each should reflect susceptibility to changes in a different external factor potentially affecting the community. We found evidence for an effect of ocean climate and top-down effects of fishing and seal predation, but not for bottom-up effects of prey availability on adult fishes. Mean body length in the community decreased dramatically in the 1990s. This reflected the removal of large-bodied fishes by fishing and sharp increases in the abundance of small fishes. The biomass of small fish was inversely correlated with an index of predation on those fish by larger fish...

Life-history evolution and elevated natural mortality in a population of Atlantic cod (Gadus morhua).

Fisheries‐induced evolution has been hypothesized to delay the recovery of collapsed fish stocks through effects on their productivity. The cod stock in the southern Gulf of St. Lawrence (SGSL) collapsed in the early 1990s and has shown no recovery since then, due mainly to high natural mortality of adult cod. Age and size at maturation of SGSL cod decreased sharply over time in cohorts produced in the 1950s and 1960s, likely reflecting an evolutionary response to intensified fishing, and have remained low since then, despite severe reductions in fishing mortality over the past 15 years. A predicted consequence of early maturation is increased natural mortality due to higher costs to reproduction. Early maturation may be a cause of increases in natural mortality of SGSL cod in the 1970s but does not appear to be related to the much larger increases since then. Instead, the current high natural mortality of SGSL cod appears to be primarily a cause, rather than a consequence, of the continued early maturation in this population, now replacing fishing mortality as the agent of selection favouring early maturity. This striking example of the failure to reverse fisheries‐induced evolution by relaxing fishing pressure emphasizes the need for management strategies that minimize the chances of harvest‐induced genetic change.

Countergradient variation in body shape between two populations of Atlantic cod (Gadus morhua).

Variation in morphological traits is generally thought to be cogradient, with environmental effects on phenotypic expression reinforcing genetic differences between populations. We compared body shape between two populations of Atlantic cod (Gadus morhua). Striking shape differences occurred between juveniles from the two populations when reared in a common laboratory environment. However, no difference in body shape occurred between wild-reared juveniles from the two populations, suggesting that the genetic differences between populations were obscured by opposing effects of the environmental differences experienced in the wild. We suggest that much of the genetic diversity in body shape of fishes may be cryptic, with stabilizing selection for the same optimal phenotype resulting in genetic divergence between populations subject to contrasting environmental influences.

Microevolution in time and space: SNP analysis of historical DNA reveals dynamic signatures of selection in Atlantic cod.

Little is known about how quickly natural populations adapt to changes in their environment and how temporal and spatial variation in selection pressures interact to shape patterns of genetic diversity. We here address these issues with a series of genome scans in four overfished populations of Atlantic cod (Gadus morhua) studied over an 80‐year period. Screening of >1000 gene‐associated single‐nucleotide polymorphisms (SNPs) identified 77 loci that showed highly elevated levels of differentiation, likely as an effect of directional selection, in either time, space or both. Exploratory analysis suggested that temporal allele frequency shifts at certain loci may correlate with local temperature variation and with life history changes suggested to be fisheries induced. Interestingly, however, largely nonoverlapping sets of loci were temporal outliers in the different populations and outliers from the 1928 to 1960 period showed almost complete stability during later decades. The contrasting microevolutionary trajectories among populations resulted in sequential shifts in spatial outliers, with no locus maintaining elevated spatial differentiation throughout the study period. Simulations of migration coupled with observations of temporally stable spatial structure at neutral loci suggest that population replacement or gene flow alone could not explain all the observed allele frequency variation. Thus, the genetic changes are likely to at least partly be driven by highly dynamic temporally and spatially varying selection. These findings have important implications for our understanding of local adaptation and evolutionary potential in high gene flow organisms and underscore the need to carefully consider all dimensions of biocomplexity for evolutionarily sustainable management.

Environmental and Genetic Influences on Stock Identification Characters

Publisher Summary Characters used to identify fish stocks can be divided into three groups: those that are purely genetic, those that are purely environmental, and those that may reflect both genetic and environmental variation. Early studies characterized stocks on the basis of phenotypic variation in life-history, meristic, morphometric, and life history traits. These characters are quantitative genetic traits, typically controlled by many genes and affected by the environment in which those genes are expressed. They also are generally related to fitness and thus, molded by natural and sexual selection, they reflect local adaptation. Several molecular genetic techniques have been developed to directly examine genetic variation within and between groups (e.g., protein allozymes, mitochondrial DNA, and microsatellite DNA). These genetic markers usually are assumed to be neutral or nearly neutral to selection, although this is not always the case. Finally, environmental markers, such as the elemental composition of otoliths, have also been used to delineate stock structure in recent decades. These markers, signatures of the habitats or areas occupied at each life history stage, are usually thought to reflect purely environmental differences between groups of fish, although it has been suggested that genetic effects also may contribute to differences between individuals, stocks, or species. This chapter focuses on a comparison between the “traditional” approaches to stock identification using phenotypic characters, in particular meristic, morphometric, and life history traits, and the newer approaches using molecular genetic markers.

Accounting for length- and depth-dependent diel variation in catchability of fish and invertebrates in an annual bottom-trawl survey

Diel variation in the catchability of 51 species of fish and 13 invertebrate taxa from an annual bottom-trawl survey of the southern Gulf of St. Lawrence was examined using generalized linear (Poisson) models. Two approaches were used: comparative fishing based on spatially paired day and night tows, and statistical control of spatio-temporal effects in regular survey data. The two approaches provided remarkably similar results. Furthermore, species-specific results were consistent between survey time periods where different vessels were used, and were quite comparable to results from other studies conducted in separate geographic areas. Where sufficient data existed, we considered both length and depth dependencies in diel variation in catchability. We commonly found significant length dependency for groundfish species and the pervasive pattern was for higher nighttime catches of small fish, the magnitude of the effect generally decreasing with increasing length. In contrast, water depth had a much less important impact, except for a few species. When considered jointly, however, patterns in length/depth dependency were not always clear. Given length-dependent geographic and bathymetric patterns in species distribution, the potential for confounding length and depth dependencies exists and correction factors for diel variation in catchability need to be applied cautiously.

Evolution of growth in Gulf of St Lawrence cod: reply to Heino et al.

In a recent paper ([Swain et al . 2007][1]), we examined the evidence for a genetic response to size-selective mortality in southern Gulf of St Lawrence cod, using data on back-calculated length at age and a quantitative genetics model. We used the model![Formula][2] (1.1)where Δ L 4 and Δ E

Comment on "Slow adaptation in the face of rapid warming leads to collapse of the Gulf of Maine cod fishery".

Pershing et al. (Science, 13 November 2015, p. 809) concluded that recent warming in the Gulf of Maine contributed to the collapse of Gulf of Maine cod. We argue that this conclusion is based on a flawed analysis of the population dynamics of this cod stock. We believe that understanding the potential role of climate change in the collapse of this stock requires more defensible analyses.