What we can learn from studies of fishery selection on age and size at maturation in Alaskan salmon

Fishing is also often size selective, altering the distribution of life history traits such as size and age at maturation, and can have significant ecological and evolutionary effects on exploited populations. We quantified six decades of fishery selection on a commercially and biologically important stock, Bristol Bay, Alaska sockeye salmon (Oncorhynchus nerka) along with concurrent trends in age and size at maturation. Analyses revealed that larger than average fish have been caught in most years and that selection differed between males and females. Also, these fish have declined in age and length at age over time. We evaluated if these trait changes represent genetic effects caused by selective fishing or phenotypic plasticity due to changes in environmental conditions. We compared length at age declines with fishery selection patterns and environmental indices and used probabilistic maturation reaction norms and quantitative genetics models to examine these questions and found that both fishing and environmental factors have contributed. Management actions should be taken to reduce adverse effects of size-selective fishing while understanding that trends in age and size may continue due to environmental factors. These findings can be applied to improve the management of Columbia River Chinook salmon stocks.