Modelling the interactive effects of selective fishing and environmental change on Atlantic salmon demogenetics

1. Changes in life-history traits have been observed in many fish species over past decades. This led to the 'fisheries-induced evolution' hypothesis proposing that fisheries may be causing genetic changes to populations through selective harvesting. Another hypothesis, which is not mutually exclusive, is that observed changes are due to phenotypic plasticity in response to environmental changes. 2. Using an individual-based demogenetic model, we investigate the relative importance of selective fishing and environmental change scenarios on the Atlantic salmon Salmo salar. 3. In simulation experiments, results show that poor oceanic growth conditions resulting from environmental change drove mainly phenotypic responses, such as a shift towards a multiple-sea-winter life history accompanied by a decline in population size. These changes were attributable to the longer time needed to reach maturation and the resulting increase in cumulative mortality during the oceanic phase. 4. Increased selective fishing against multiple-sea-winter fish mainly induced an evolutionary effect in the form of a lower maturation threshold in females, increasing the proportion of one sea-winter fish. The maturation threshold of males was not modified by selective fishing due to their earlier reproduction and return after a single winter at sea, thereby avoiding most of the selective effects of fishing. 5. Policy implications. The results suggest that given the present configuration of traditional fisheries, fishing is likely to worsen the effects of oceanic environmental change. Management strategies avoiding targeting multiple-sea-winter fish may need to be considered in order to ensure the populations' resilience to poor oceanic conditions for growth. (Résumé d'auteur