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

We investigated the feeding ecology and foraging location of migrating and nursing northern fur seal (Callorhinus ursinus) adult females and migrating juvenile males from the Pribilof Islands, Alaska, using carbon (delta C-13) and nitrogen (delta N-15) isotope analysis of fur seal skin and whole potential prey. Post-parturient and lactating females had mean delta N-15 values significantly (0.8%) higher than pregnant, migratory females, and delta C-13 values that were not significantly different. Two opportunistically collected, migrating, nulliparous females had mean delta C-13 values 1.1% lower than migrating, pregnant females, and delta N-15 values that were not different. Pregnant, migratory females had mean delta C-13 values significantly (similar to1.5%) higher than migratory juvenile males, and mean delta N-15 values significantly (similar to0.6-1.6%) higher than migratory juvenile males. The exception was one group of juvenile males from St. Paul Island with mean delta N-15 values that were not significantly different from migrating females. The mean delta N-15 values of pregnant females indicate they were feeding at a higher trophic level than juvenile males during migration. The higher mean delta C-13 values for pregnant females suggest they were feeding coastally during the spring migration, while juvenile males and nulliparous females were feeding offshore. The higher delta N-15 values for post-parturient, lactating females over migrating, pregnant females point to either a trophic shift in diet over time, or a more likely N-15-enrichment due to negative nitrogen balance caused by the nutritional stress of lactation and the feeding/fasting regime experienced by females. Similar mean delta C-13 values for migrating and breeding-season females indicate that both groups were feeding in coastal, on-shelf domains during their respective time periods. Similar mean delta N-15 values for nulliparous and pregnant females indicate they were feeding at similar trophic levels despite indications of feeding in separate ecosystems during migration. Using a delta N-15 shift of 2-3% per trophic level, we made general inferences about the trophic levels at which northern fur seals were feeding. The interpretation of our delta N-15 data indicates that migrating pregnant females, lactating females and the majority of migrating juvenile males consumed prey with mean delta N-15 values between 14.2% and 15.2%, 15.1% and 16.1%, and 13.6% and 14.6%, respectively. Probable fur seal prey was analyzed as well. Walleye pollock showed progressive N-15 and C-13-enrichments with age. Mean delta N-15 and delta C-13 values of 3- to 4-year-old fish were similar to6.0% and 1.1% higher, respectively, than values for 0-age pollock. Atka mackerel also showed isotopic enrichment with age. The delta N-15 and delta C-13 values of large fish were 0.8% and 0.3% higher, respectively, than values for smaller fish.