Linking Growth of Juvenile Sockeye Salmon to Habitat Temperature in Alaskan Lakes

Abstract

We examined the influence of temperature on the size of age-1 smolts of sockeye salmon Oncorhynchus nerka relative to that of food availability and density dependence across a variety of Alaskan lakes. We analyzed data from 36 lakes providing 134 lake years (annual means) of data. These lakes represent clear, organically stained, and glacially turbid lake types. They produce age-1 sockeye salmon smolts exhibiting a threefold variation in average length (52–145 mm) and a 30-fold variation in average weight (1–30 g). We used simple regression analysis to test for linkages between age-1 smolt size and the independent variables zooplankton biomass (mg/m²), smolt density (number/km²), length of the growing season, and mean water column temperature. Zooplankton biomass (ZB) was the strongest single predictor of smolt size, accounting for 52% of the variation in mean age-1 smolt fork length (FL) and weight (W). Individually, smolt density (SMD) accounted for only 10% of the variance in age-1 smolt size; however, mean water column temperature (T_s ) explained 24% of the variation in FL and 19% of the variation in W. Smolt size was unrelated to the length of the growing season (S); however, growth rate indexed by length varied inversely with S while growth in terms of weight did not. Subsequently, we combined all factors into a multivariate model of age-1 smolt size and the population's biological and thermal environment. Taken together, the factors ZB, SMD, and T_s accounted for 70% of the variation in FL and W. The influence of temperature on smolt size appeared to be more direct (through its effect on metabolic rates) than indirect (through limiting food availability (plankton)). Our results may influence fishery management objectives and serve as a template to project future trends in juvenile sockeye salmon growth under different climatic conditions.