Stable isotope assessment of temporal and geographic differences in feeding ecology of northern fur seals (Callorhinus ursinus)

Abstract

The feeding ecology of northern fur seals (Callorhinus ursinus) and their primary prey was examined by analyzing their skin tissues for stable-carbon and nitrogen isotope content. The Pribilof Island northern fur seal population was designated as depleted in 1988 because their numbers were, and continue to be, below their optimum sustainable population. The primary cause for this decline is thought to be a decrease in their prey base in the eastern Bering Sea. Determination of fur seal feeding ecology is important so that disturbances in prey availability can be further studied. This study was able to augment current northern fur seal dietary knowledge using stable-carbon and nitrogen isotope analysis. Parturient and lactating female and juvenile male skin tissue and prey items were analyzed. Female and male tissues were collected in July, August and November, and July and August, 1997, respectively, from both St. Paul and St. George Islands. July/August samples represented spring migratory diet, while November samples illustrated breeding season diet. Females were found to be more enriched isotopically in ¹⁵N and ¹³C over juvenile males during their migration. November females on St. Paul island were enriched in ¹⁵N over July/August females, while St. George females were not significantly enriched. July/August females on St. George Island were more enriched in ¹³C over November females, while St. Paul females were not significantly enriched. There were no differences in ¹⁵N or ¹³C enrichment for migratory animals of the some sex traveling to either island, and there was no difference in ¹⁵N between islands for November females. There was a significant difference in ð¹³C values between islands for November females with St. Paul being more enriched. Prey analysis showed step-wise enrichments for two species and prey stable isotope values were compared to fur seal values for an estimation of dietary content. Females appear to be feeding at a higher trophic level and feeding more coastally than males during migration. Migratory females appear to be feeding more coastally than breeding season females.