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 (delta(15)N) and carbon (delta(13)C) isotope analyses of their fur, muscle, blubber, brain, liver, and kidney tissues. Mean delta(15)N values were non-uniform between tissues and ranged from 14.9 parts per thousand (fur) to 17.1 parts per thousand (lipid extracted blubber). Mean VC values also varied with tissue type and ranged from -24.7 parts per thousand (non-lipid extracted blubber) to -17.5parts per thousand (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 (delta(15)N = 15.1 to 15.6parts per thousand; delta(13)C = -18.1parts per thousand), brain (delta(15)N = 17.0parts per thousand; delta(13)C = -18.4 to -18.1 parts per thousand), blubber (delta(15)N = 17.1 parts per thousand; delta(13)C = -19.7 parts per thousand to -18.1 parts per thousand), kidney (delta(15)N = 16.4 parts per thousand; delta(13)C = -18.5 to -18.2parts per thousand), and liver (delta(15)N = 16.0parts per thousand to 16.2 parts per thousand; delta(13)C = -18.4 %o to -18.2parts per thousand). Mean kidney and liver delta(15)N values (similar to16.3parts per thousand) 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 delta(13)C values suggested that juvenile males from St. George Island (delta(13)C = -18.2parts per thousand) were feeding at the Bering Sea shelf break, while juvenile males from St. Paul Island (delta(13)C = -18.5parts per thousand) were feeding more on the continental shelf. Comparing delta(15)N ratios of fur with delta(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 (similar to2 yr old) targeted in this study. Mean delta(15)N values from tissues collected opportunistically from 2 nulliparous females (delta(15)N values ranged from 16.1parts per thousand for muscle to 18.0parts per thousand for blubber) and 2 post-parturient females (delta(15)N values ranged from 16.1 parts per thousand for muscle to 18.9 parts per thousand 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 delta(15)N values,(17.1 %o) and VC values (-19.7 to -18.1parts per thousand) than non-lipid extracted blubber (-16.0 and similar to-24.7parts per thousand, respectively), underscoring the importance of lipid removal when analyzing tissues for stable isotopes.