Stable nitrogen and carbon isotope ratios in multiple tissues of the northern fur seal Callorhinus ursinus: implications for dietary and migratory reconstructions

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 ( 15 N) and carbon ( 13 C) isotope analyses of their fur, muscle, blubber, brain, liver, and kidney tissues. Mean 15 N values were non-uniform between tissues and ranged from 14.9 (fur) to 17.1 (lipid extracted blubber). Mean 13 C 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 ( 15 N = 15.1 to 15.6 ; 13 C -18.1 ), brain ( 15 N 17.0 ; 13 C = -18.4 to -18.1 ), blubber ( 15 N = 17.1 ; 13 C = -19.7 to -18.1 ), kidney ( 15 N 16.4 ; 13 C = -18.5 to -18.2 ), and liver ( 15 N = 16.0 to 16.2 ; 13 C = -18.4 to -18.2 ). Mean kidney and liver 15 N 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 13 C values suggested that juvenile males from St. George Island ( 13 C = -18.2 ) were feeding at the Bering Sea shelf break, while juvenile males from St. Paul Island ( 13 C = -18.5 ) were feeding more on the continental shelf. Comparing 15 N ratios of fur with 15 N 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 15 N values from tissues collected opportunistically from 2 nulliparous females ( 15 N values ranged from 16.1 for muscle to 18.0 for blubber) and 2 postparturient females ( 15 N 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 15 N values (17.1 ) and 13 C 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.