Evolution in salmon life-history induced by direct and indirect effects of fishing

Understanding the drivers of evolution is a fundamental aim in biology. However, identifying the evolutionary impacts of human activities, both direct and indirect, is challenging because of lack of temporal data and limited knowledge of the genetic basis of most traits1. Atlantic salmon is a species exposed to intense anthropogenic pressures during its anadromous life cycle2. Previous research has shown that salmon age at maturity has evolved towards earlier maturation over the last 40 years, with an 18% decrease3 in the allele associated with late maturation at the large-effect vgll3 locus4; but the drivers of this change remain unknown. Here, we link genetic and phenotypic changes in a large Atlantic salmon population with salmon prey species biomass in the Barents Sea, temperature, and fishing effort in order to identify drivers of age at maturity evolution. We show that age at maturity evolution is associated with two different types of fisheries induced evolution acting in opposing directions: an indirect effect linked with commercial harvest of a salmon prey species (capelin) at sea (selection against late maturation), and a direct effect due to temporal changes in net fishing pressure in the river (surprisingly, selection against early maturation). Although the potential for direct and indirect evolutionary effects of fishing have been acknowledged, empirical evidence for induced changes at the genetic level has been lacking5. As capelin are primarily harvested to produce fish meal and oil for aquaculture6, we hereby identify an indirect path by which Atlantic salmon aquaculture may negatively affect wild populations.