Adaptive phenotypic plasticity in Atlantic salmon and Atlantic cod and its relevance to risk assessments of aquaculture

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Risks to wild populations resulting from interbreeding with farmed escapees depend primarily on the probability of spawning between groups, the level of genetic differentiation between farmed and wild individuals, and the abundance of wild populations relative to conservation targets. The present work pertains to the second of these factors. In addition to the divergence that breeding designs and domestication selection can generate, genetic dissimilarities will also depend on the magnitude and type of genetic differentiation that exists among populations in the wild. Here, using common-garden experimental protocols, we describe experiments on Atlantic salmon (Salmo salar) and Atlantic cod (Gadus morhua) that have documented genetic differences phenotypic plasticity. Multi-generational crosses between farmed and wild salmon have revealed (i) adaptive variability in plasticity among populations and (ii) negative fitness consequences to wild salmon resulting from interbreeding, notably from changes to developmental reaction norms. In Northwest Atlantic cod, larval survival and growth, and their plastic responses to changes in food supply and temperature, differ across spatial scales at which analyses of microsatellite DNA failed to detect population structure. Based on the Atlantic salmon research, these results are consistent with the hypotheses that (i) cod populations differ significantly in how they respond to environmental change and that (ii) these differences in plasticity are genetically based and may be a product of local adaptation. Disruption to reaction norms and adaptive phenotypic plasticity, resulting from interbreeding between wild individuals and farmed escapees, merits considerably greater attention in risk assessments of aquaculture than it has to date