RETROSPECTIVE ISOTOPIC ANALYSES OF STELLER SEA LION TOOTH ANNULI AND SEABIRD FEATHERS: A CROSS-TAXA APPROACH TO INVESTIGATING REGIME AND DIETARY SHIFTS IN THE GULF OF ALASKA

Stable isotope (δ¹⁵N and δ¹³C) values of individual tooth annuli of female Steller sea lions ( n = 120) collected from the 1960s through the 1980s were used for retrospective analyses of temporal changes in food webs in the Gulf of Alaska and North Pacific Ocean. We also examined isotopically contour feathers of tufted puffins ( n = 135) and crested auklets ( n = 37) through this period to test for broader isotopic patterns indicative of whole food web changes. Steller sea lions decreased slightly in δ¹³C and increased in δ¹⁵N values, suggesting an increasing trophic level and change in foraging location or oceanographic isotopic signature. Steller sea lion first and second tooth annuli were enriched in ¹⁵N and depleted in ¹³C compared with subsequent annuli, indicating the effects of maternal influence through weaning. The general pattern of increasing δ¹⁵N values among Steller sea lions supports previous conclusions regarding a reduction or redistribution of forage fishes and an increase of demersal and semi-demersal species in the North Pacific ecosystem. There were no significant changes in δ¹⁵N values for either bird species. However, δ¹³C values in both bird species again suggested changes in foraging location or a shift in oceanographic currents.