Assessing the production of sockeye salmon (Oncorhynchus nerka) at Chilkat Lake, Southeast Alaska, using current trophic conditions and the paleolimnologic sediment record

Abstract

Anadromous sockeye salmon are the only Pacific salmon that require an extensive rearing period in a lake environment. Recent studies suggest changes in lake nutrient inputs are directly related to the magnitude of annual spawning populations (escapement), and can be quantified by sediment stable isotope analysis of nitrogen ¹⁵N). Marine nitrogen supplied by returned salmon is enriched in ¹⁵N relative to the freshwater nitrogen, which is derived from atmospheric N₂ (ð¹⁵N=0 ⁰/₀₀). Recent (1987-1996) limnological investigations of Chilkat Lake, Alaska, matched to brood year escapement estimates, indicate that sockeye marine-derived phosphorus and nitrogen contributed up to 87% and 20%, respectively, of the lake nutrient pool. Since salmon populations fluctuate annually, the resulting marine derived nutrient (MDN) inputs and ð¹⁵N also fluctuate. Sedimentary ð¹⁵N is positively correlated (r²=0.8605, p=0.0076, df=5) with observed escapement. Additionally, ð¹⁵N is positively related to lake paleoproductivity and inversely correlated with high concentrations of CaCO₃ and glacial silt. The frequency and duration of these CaCO₃ inputs are believed to be the result of climate related glacial meltwater intrusions from the Tsirku River. Thus, fluctuations in MDN can be shown to influence the overall sockeye salmon production at Chilkat Lake by controlling lake productivity.