Effects of warming climate and competition in the ocean for life-histories of Pacific salmon

Abstract

The life-histories of exploited fish species, such as Pacific salmon, are vulnerable to a wide variety of anthropogenic stressors including climate change, selective exploitation and competition with hatchery releases for finite foraging resources. However, these stressors may generate unexpected changes in life-histories due to developmental linkages when species complete their migratory life cycle in different habitats. We used multivariate time-series models to quantify changes in the prevalence of different life-history strategies of sockeye salmon from Bristol Bay, Alaska, over the past half-century—specifically, how they partition their lives between freshwater habitats and the ocean. Climate warming has decreased the time spent by salmon in their natal freshwater habitat, as climate-enhanced growth opportunities have enabled earlier migration to the ocean. Migration from freshwater at a younger age, and increasing competition from wild and hatchery-released salmon, have tended to delay maturation toward the salmon spending an additional year feeding in the ocean. Models evaluating the effects of size-selective fishing on these patterns had only small support. These stressors combine to reduce the size-at-age of fish vulnerable to commercial fisheries and have increasingly favoured a single-age class, potentially affecting the age class complexity that stabilizes this highly reliable resource.