COS 85-1 - Habitat modulates population-level responses of salmon growth in freshwater to a century of climatic change and competition

Background/Question/Methods

The impacts of climate change are widespread and globally threaten natural systems. Within regions, heterogenous physical landscapes can differentially filter climate and lead to local response diversity. While freshwater lakes are responding rapidly to climate change, these habitats also exhibit a diversity of thermal responses that are influenced by their unique morphology, which in turn can differentially affect the growth and survival of sensitive biota such as fishes. Salmonids are an important group of cold-water fishes that have complex life histories shaped largely by heterogenous freshwater habitats, and are highly sensitive to warming temperatures. Here we use modern genetic tools and a mixed-effects model to study the population-level response of sockeye salmon (Oncorhynchus nerka) rearing in multiple freshwater lakes of Canada’s Skeena River watershed, and explore the moderating effect of habitat on growth over a century of change in regional temperature and competition.

Results/Conclusions

Our results demonstrate that freshwater growth has broadly increased over the last century, although population-level responses were unique to individual habitats. Intraspecific competition showed the strongest negative effect on the growth of fish across populations, where our model estimated a 8% decrease in growth for every 1SD increase in fish abundance. While growth tended to be lower in years with high temperatures, habitat played an important role in modulating the negative effect of temperature. Specifically, growth was positively associated with rising temperatures in relatively deep ( >50 m) nursery lakes, whereas warmer temperatures were associated with decreased growth in shallow lakes. The influence of temperature also was modulated by glacier extent whereby the growth of fish from lakes situated in watersheds with little (i.e., < 5%) glacier cover remained unchanged with rising temperatures, but decreased with rising temperatures for fish in lakes within more glaciated watersheds. Maintaining the integrity of an array of freshwater habitats likely will help foster a diverse climate-response portfolio for important fish species, which in turn can ensure that salmon watersheds are resilient to future environmental change.