Why age and size at maturity have changed in Pacific salmon

Abstract

Over the last few decades, the size at which Pacific salmon Oncorhynchus spp. attains maturity has decreased in many populations, whereas the age at maturity has increased. Both fisheries-induced evolution and environmentally-induced phenotypic plasticity could contribute to the changing age and size at maturity of Pacific salmon. We evaluated the potential for genetic changes in the maturation schedule of Japanese chum salmon using the probabilistic maturation reaction norm (PMRN) method. We found that the recent decrease in size at maturity, and increase in age at maturity, of Japanese chum salmon can be largely attributed to a phenotypic response to a reduced growth rate, but that fisheries-induced evolution should not be ruled out. Recent claims concerning fisheries-induced evolution of the maturation schedule are based on the decline in the age-specific body size at which the probability of maturing is 50%, a feature of PMRNs. However, the PMRN could change with changing environmental conditions. Therefore, a genetic change cannot be diagnosed only by the PMRN method. © Inter-Research 2007.