Temporal records of δ¹³C and δ¹⁵N in North Pacific pinnipeds: inferences regarding environmental change and diet

Abstract

Sea lion and seal populations in Alaskan waters underwent various degrees of decline during the latter half of the twentieth century and the cause(s) for the declines remain uncertain. The stable carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) isotope ratios in bone collagen from wild Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus) and harbor seals (Phoca vitulina) from the Bering Sea and Gulf of Alaska were measured for the period 1951–1997 to test the hypothesis that a change in trophic level may have occurred during this interval and contributed to the population declines. A significant change in δ¹⁵N in pinniped tissues over time would imply a marked change in trophic level. No significant change in bone collagen δ¹⁵N was found for any of the three species during the past 47 years in either the Bering Sea or the Gulf of Alaska. However, the ¹⁵N in the Steller sea lion collagen was significantly higher than both northern fur seals and harbor seals. A significant decline in δ¹³C (almost 2 ‰ over the 47 years) was evident in Steller sea lions, while a declining trend, though not significant, was evident in harbor seals and northern fur seals. Changes in foraging location, in combination with a trophic shift, may offer one possible explanation. Nevertheless, a decrease in δ¹³C over time with no accompanying change in δ¹⁵N suggests an environmental change affecting the base of the foodweb rather than a trophic level change due to prey switching. A decline in the seasonal primary production in the region, possibly resulting from decreased phytoplankton growth rates, would exhibit itself as a decline in δ¹³C. Declining production could be an indication of a reduced carrying capacity in the North Pacific Ocean. Sufficient quantities of optimal prey species may have fallen below threshold sustaining densities for these pinnipeds, particularly for yearlings and subadults who have not yet developed adequate foraging skills.