Long-Term Perspective on Salmon Abundance and Effects on Aquatic Ecosystems

A long-term perspective on variability in Pacific salmon abundance prior to the impacts of widespread commercial fishing and habitat alteration is possible from δ¹⁵N analysis of lake sediment cores. Such methods are relevant for sockeye salmon systems, as they utilize lake habitat for spawning and rearing. Comparisons of lake sediment δ¹⁵N records to climate and paleoclimate records as well as catch and escapement data can help assess impacts from such processes on salmon productivity. In addition, analysis of sediment proxies of limnologic conditions as well as primary producers and zooplankton provide insight into how lake ecosystems respond to changes in escapement level over time. Sockeye salmon lake sedimentary δ¹⁵N records document substantial variability in sockeye abundance over decadal to millennial timescales since the end of the last ice age, ~12,000 years ago. Analysis of sediments from a range of different lake types and regions reveal the existence of common patterns within and between different salmon producing regions, consistent with impacts associated with changes in the ocean-climate system. Many systems show declines in δ¹⁵N over the last ~100 years that likely reflect declining escapement levels. The importance of salmon-derived nutrients (SDN) to lake and salmon productivity varies strongly across systems and is usually proportional to the relative importance of SDN to a systems nutrient budget, which in general is correlated with δ¹⁵N. In lakes where SDN is a substantial component of nutrient loading, temporal variability in δ¹⁵N is often highly correlated to measures of lake productivity. Consideration of contemporary and paleo-sedimentary zooplankton data, however, suggests that food web interactions are important in regulating the effects of SDN on salmon productivity. Thus the management of sockeye salmon systems, such as in setting escapement goals, should ideally be based on knowledge on long-term variability in limnologic, nutrient and ecologic conditions of the given system, and might preferably be adaptive to climatic and oceanic conditions.