Interdecadal and Centennial Variability in Pacific Salmon Abundance Over 500 Years

Substantial evidence now exists for the importance of interdecadal modes of variation in salmon abundance driven in part by climatic oscillations. However, our knowledge of temporal variation in salmon abundance is limited in most cases to inferences based on observational data from the last century. How interdecadal modes of variation fit into a longer temporal context remains largely unknown, and yet this knowledge may provide important insight into how salmon populations will respond to climatic changes in the future.

Nitrogen stable isotopes preserved in lake sediments can be used to reconstruct pre-historical abundances of anadromous sockeye salmon. We analyzed time series of stable nitrogen isotopes from 21 sockeye salmon nursery lakes across southwestern and southcentral Alaska to characterize spatial and temporal patterns of variation in salmon abundance over the past 500 years. This is the most extensive analysis of pre-historical patterns of variation in salmon abundance to date. We found that while some stocks varied on interdecadal time scales (50-90 year cycles), matching observed patterns in sockeye salmon fishery catches from the 1900s, most stocks were dominated by lower frequency variation, with prolonged periods of high or low abundance lasting over a century. While the mechanisms driving these low-frequency fluctuations are unclear, it is important to note that low-frequency shifts in salmon abundance were not coherent among lakes. This indicates that exposure to regional climatic conditions has not synchronized salmon production across this region over centennial time scales, and suggests that salmon stocks are unlikely to show coherent responses to future climate change across southwestern and southcentral Alaska.