The demographic structure of populations is affected by life-history strategies and how these interact with natural and anthropogenic drivers such as exploitation, climate change, and biotic interactions. Recent changes in the age- and size-structure of fish populations have been reported in a variety of species, yet the underlying causes remain poorly understood. Previous work suggests that the mean size and age of North American Chinook salmon populations are declining; patterns of concern because Chinook are highly valued for their exceptional size and because the loss of the largest and oldest individuals may cause reduced population productivity. Using long-term data series from wild and hatchery populations, we quantified changes in the demographic structure of Chinook salmon over the past four decades across the entire Northeast Pacific Ocean, from California through western Alaska.
Results/Conclusions
Our results show that wild and hatchery Chinook are becoming smaller and younger throughout most of the Pacific coast. Proportions of older age-classes have decreased over time in most regions. Simultaneously, the size-at-age of older fish (e.g. ocean-4 and 5) has declined while the size-at-age of younger fish (e.g. ocean-1) has increased. Size trends were strongest in western Alaska, but weak or non-existent at the southern end of their range. While it remains to be explored whether the trends are caused by changes in climate, species interactions, or fishing practices, our results suggest that the selective removal of large fish has likely contributed to an ecological and potentially evolutionary change toward reduced average body sizes.