COS 22-6 - A century of ecosystem change in the North Pacific Ocean: using isotopes to investigate the trophic dynamics of sockeye salmon

Background/Question/Methods

Changes in the trophic dynamics of marine ecosystems are driven by climate variability and increasingly unpredictable ocean conditions. To develop effective conservation management goals and protocols for key species, we must understand how the trophic relationships in marine ecosystems are influenced by variation in climate and ocean environment. In this study, we conducted stable isotope analysis on fisheries-collected salmon scales from a 100-year archive to investigate how the marine trophic ecology of sockeye salmon (Onchorynchus nerka) has changed throughout the last century (1913 – 2021). Specifically, we fit a series of hierarchical models to examine if environmental and climate indices, such as Sea Surface Temperature and Pacific Decadal Oscillation, could explain temporal variation in salmon scale δ¹⁵N, δ¹³C, and δ³⁴S. Nitrogen isotope compositions of consumer tissues are used extensively to estimate trophic positions, while carbon isotope ratios are often used to determine dietary source and foraging location. Sulfur isotope ratios, which also reflect isotopic compositions at the food web base, may add clarification to interpretation of isotope measurements.

Results/Conclusions

We found considerable inter-annual variation for all isotopes between 1913 and 2021, with δ¹³C ranging from -18.86 to -15.92‰, δ¹⁵N ranging from 8.84 to 14.05‰, and δ³⁴S ranging from 15.53 to 17.85‰. Carbon, nitrogen, and sulfur values all showed patterns of depletion in their heavier isotopes later in the first half of the century, and δ¹³C of scales decreased throughout time even after accounting for the global atmospheric increase in fossil-fuel derived CO2. Fish size and age were positively correlated with δ³⁴S of scales, likely due to increased time spent in the marine environment. Potential relationships exist between climate indices and temporal isotope variability, with abrupt changes in mean annual nitrogen and carbon isotope scale composition occurring near periods of temperature regime shift in the North Pacific Ocean. These results suggest considerable variation in foraging ecology and ocean conditions experienced by sockeye salmon throughout the last 100 years and suggest important links between environmental variation and marine ecosystem dynamics.