An Assessment of 50 Years of Climate Warming on Growth of Juvenile Sockeye Salmon from the Alaska Peninsula

Summer growth is critical for juvenile sockeye salmon (Oncorhynchus nerka) survival and can determine the duration of freshwater residence and timing of outmigration. Growth is typically a density dependent process that is modified by lake productivity and temperature. In an Alaska Peninsula lake experiencing warming air temperatures and declining lake volume over the past 50 years, we assessed the importance of density dependence, temperature, and resident fish competition as well as their interactions for juvenile sockeye growth. We assessed a range of competing models that accounted for a variety of density and environmental effects using standard model selection procedures (AICc). Despite large variability in mean length at the end of the growing season, there was a positive trend in mean length between 1961 and 2010, a time period characterized by ~2°C warming and ~50% loss of rearing habitat. Across the suite of models considered, year (proxy for lake volume) received a high weight across all models and surprisingly had a positive effect on growth. Juvenile sockeye density negatively affected growth while spring temperatures had a positive effect. The interaction between density and temperature showed relatively high weighting across all models. It appears that the positive effects of year and temperature on juvenile sockeye salmon growth currently outweigh the negative effects of density dependence. However, potentially increased mid-summer emigration rates may account for weakened density dependence and thus a broader approach may be needed to assess the effects of habitat change on juvenile sockeye production at the watershed scale.