MEPS 236:289-300 (2002)  -  doi:10.3354/meps236289

ABSTRACT: We investigated changes in trophic level and feeding location over time in juvenile male northern fur seals Callorhinus ursinus from the Pribilof Islands, Alaska, using stable nitrogen (δ¹⁵N) and carbon (δ¹³C) isotope analyses of their fur, muscle, blubber, brain, liver, and kidney tissues. Mean δ¹⁵N values were non-uniform between tissues and ranged from 14.9‰ (fur) to 17.1‰ (lipid extracted blubber). Mean δ¹³C values also varied with tissue type and ranged from -24.7‰ (non-lipid extracted blubber) to -17.5‰ (fur). Mean isotope values of tissues clustered into groups coincident with their estimated protein and isotope turnover times, with fur representing the most remote incorporation of isotopic data, followed by muscle (δ¹⁵N = 15.1 to 15.6‰; δ¹³C ª -18.1‰), brain (δ¹⁵N ª 17.0‰; δ¹³C = -18.4 to -18.1‰), blubber (δ¹⁵N = 17.1‰; δ¹³C = -19.7‰ to -18.1‰), kidney (δ¹⁵N ª 16.4‰; δ¹³C = -18.5 to -18.2‰), and liver (δ¹⁵N = 16.0‰ to 16.2‰; δ¹³C = -18.4‰ to -18.2‰). Mean kidney and liver δ¹⁵N values (~16.3‰) indicated that juvenile males from St. Paul and St. George Islands were feeding at the same trophic level during summer 1997. Mean kidney and liver δ¹³C values suggested that juvenile males from St. George Island (δ¹³C = -18.2‰) were feeding at the Bering Sea shelf break, while juvenile males from St. Paul Island (δ¹³C = -18.5‰) were feeding more on the continental shelf. Comparing δ¹⁵N ratios of fur with δ¹⁵N values from all other tissues allowed us to estimate that juvenile males were at their lowest trophic level when they were at the youngest age (~2 yr old) targeted in this study. Mean δ¹⁵N values from tissues collected opportunistically from 2 nulliparous females ( δ¹⁵N values ranged from 16.1‰ for muscle to 18.0‰ for blubber) and 2 post-parturient females (δ¹⁵N values ranged from 16.1‰ for muscle to 18.9‰ for blubber) suggested that, at all times, females were feeding at higher trophic levels than juvenile males. Lipid-extracted blubber samples from juvenile males had much higher δ¹⁵N values (17.1‰) and δ¹³C values (-19.7 to -18.1‰) than non-lipid extracted blubber (~16.0 and ~-24.7‰, respectively), underscoring the importance of lipid removal when analyzing tissues for stable isotopes.

KEY WORDS: Stable isotopes · Northern fur seals · Feeding ecology · Foraging · Migration · Trophic dynamics