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

Abstract

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 (δ¹³C) and nitrogen (δ¹⁵N) isotope analysis of fur seal skin and whole potential prey. Post-parturient and lactating females had mean δ¹⁵N values significantly (0.8‰) higher than pregnant, migratory females, and δ¹³C values that were not significantly different. Two opportunistically collected, migrating, nulliparous females had mean δ¹³C values 1.1‰ lower than migrating, pregnant females, and δ¹⁵N values that were not different. Pregnant, migratory females had mean δ¹³C values significantly (~1.5‰) higher than migratory juvenile males, and mean δ¹⁵N values significantly (~0.6–1.6‰) higher than migratory juvenile males. The exception was one group of juvenile males from St. Paul Island with mean δ¹⁵N values that were not significantly different from migrating females. The mean δ¹⁵N values of pregnant females indicate they were feeding at a higher trophic level than juvenile males during migration. The higher mean δ¹³C values for pregnant females suggest they were feeding coastally during the spring migration, while juvenile males and nulliparous females were feeding offshore. The higher δ¹⁵N values for post-parturient, lactating females over migrating, pregnant females point to either a trophic shift in diet over time, or a more likely ¹⁵N-enrichment due to negative nitrogen balance caused by the nutritional stress of lactation and the feeding/fasting regime experienced by females. Similar mean δ¹³C 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 δ¹⁵N 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 δ¹⁵N 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 δ¹⁵N data indicates that migrating pregnant females, lactating females and the majority of migrating juvenile males consumed prey with mean δ¹⁵N 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 ¹⁵N and ¹³C-enrichments with age. Mean δ¹⁵N and δ¹³C values of 3- to 4-year-old fish were ~6.0‰ and 1.1‰ higher, respectively, than values for 0-age pollock. Atka mackerel also showed isotopic enrichment with age. The δ¹⁵N and δ¹³C values of large fish were 0.8‰ and 0.3‰ higher, respectively, than values for smaller fish.