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MEPS 236:289-300 (2002)
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Abstract
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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: kurle@biology.ucsc.edu **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 (d15N) and
carbon (d13C) isotope analyses of their fur, muscle, blubber, brain, liver, and kidney tissues. Mean d15N values were non-uniform between tissues and ranged from 14.9‰
(fur) to 17.1‰ (lipid extracted blubber). Mean d13C 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 (d15N = 15.1 to 15.6‰; d13C º -18.1‰), brain (d15N º 17.0‰; d13C = -18.4 to -18.1‰), blubber
(d15N = 17.1‰; d13C = -19.7‰ to -18.1‰), kidney (d15N º 16.4‰; d13C = -18.5 to -18.2‰), and liver (d15N = 16.0‰ to 16.2‰; d13C = -18.4‰ to -18.2‰). Mean kidney and liver d15N 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 d13C
values suggested that juvenile males from St. George Island (d13C = -18.2‰) were feeding at the Bering Sea shelf break, while juvenile males from St. Paul Island (d13C =
-18.5‰) were feeding more on the continental shelf. Comparing d15N ratios of fur with d15N 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 d15N values from tissues collected opportunistically from 2 nulliparous females (d15N values ranged from 16.1‰ for muscle to 18.0‰ for blubber) and 2 post-parturient females (d15N 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 d15N values (17.1‰) and d13C 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
Full text in pdf format
Published in MEPS Vol.
236
(2002) on July 3
Print ISSN: 0171-8630; Online ISSN: 1616-1599.
Copyright © Inter-Research, Oldendorf/Luhe, 2002
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