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Marine Ecology Progress Series

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

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

Carolyn M. Kurle1,*, Graham A. J. Worthy2,**

1National Marine Mammal Laboratory, Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA, 7600 Sand Point Way NE, Seattle, Washington 98115, USA
2Physiological Ecology and Bioenergetics Laboratory, Texas A&M University at Galveston, 5001 Avenue U, Suite 105, Galveston, Texas 77551, USA
Present addresses: *Ecology and Evolutionary Biology, University of California, 100 Shaffer Road, Santa Cruz, California 95060, USA. E-mail: **Department of Biology, University of Central Florida, 4000 Central Florida Boulevard, Orlando, Florida 32816, USA

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

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