Stable nitrogen and carbon isotope ratios (13C/12C and 15N/14N, respectively) of plasma, serum, clotted red blood cells, and unclotted red blood cells from six captive northern fur seals (Callorhinus ursinus) and two fish species in their diet, Atlantic herring (Clupea harengus) and Icelandic capelin (Mallotus villosus), were measured. The δ15N values from the fur seal blood components ranged from 15.5 ± 0.1‰ (mean ± SE; all RBCs) to 16.7 ± 0.1‰ (plasma), and δ13C values ranged from –18.3 ± 0.1‰ (serum) to –17.5 ± 0.1‰ (clotted RBCs). Fur seal blood components had higher δ15N and δ13C values than their diet. Mean enrichments of 15N between fur seal RBCs and plasma/serum and their prey were +4.1 and +5.2‰, respectively, while mean 13C enrichments were +0.6‰ (serum), +1.0‰ (plasma), and +1.3 to +1.4‰ (all RBCs). Fur seal blood components did not differ in δ15N or δ13C between the sexes. One female (Baabs) was pregnant at the time of sampling and was resampled 4 months later, when she was approximately 3 months post parturient. The δ15N values of the blood components taken from Baabs during pregnancy were all higher by 0.6‰ (unclotted RBCs) to 1.3‰ (plasma) than those of samples taken during lactation, while her δ13C values were nearly the same (RBCs) for the two time periods or lower during lactation by 0.5‰ (serum) and 0.7‰ (plasma). This study was the first to analyze isotope ratios of blood components from captive fur seals and their prey. The subsequent establishment of fractionation values between captive fur seals and their diet can be used to interpret trophic level and habitat usage of otariids feeding in the wild.

We tested the effects of sex, tissue, and diet on stable isotope discrimination factors (Δ(13)C and Δ(15)N) for six tissues from rats fed four diets with varied C and N sources, but comparable protein quality and quantity. The Δ(13)C and Δ(15)N values ranged from 1.7-4.1‰ and 0.4-4.3‰, respectively. Females had higher Δ(15)N values than males because males grew larger, whereas Δ(13)C values did not differ between sexes. Differences in Δ(13)C values among tissue types increased with increasing variability in dietary carbon sources. The Δ(15)N values increased with increasing dietary δ(15)N values for all tissues except liver and serum, which have fast stable isotope turnover times, and differences in Δ(15)N values among tissue types decreased with increasing dietary animal protein. Our results demonstrate that variability in dietary sources can affect Δ(13)C values, protein source affects Δ(15)N values even when protein quality and quantity are controlled, and the isotope turnover rate of a tissue can influence the degree to which diet affects Δ(15)N values.

We investigated the feeding ecology and foraging location of migrating and nursing northern fur seal (Callorhinus ursinus) adult females and migrating juvenile males from the Pribilof Islands, Alaska, using carbon (δC) and nitrogen (δN) isotope analysis of fur seal skin and whole potential prey. Post-parturient and lactating females had mean δN values significantly (0.8‰) higher than pregnant, migratory females, and δC values that were not significantly different. Two opportunistically collected, migrating, nulliparous females had mean δC values 1.1‰ lower than migrating, pregnant females, and δN values that were not different. Pregnant, migratory females had mean δC values significantly (~1.5‰) higher than migratory juvenile males, and mean δN values significantly (~0.6-1.6‰) higher than migratory juvenile males. The exception was one group of juvenile males from St. Paul Island with mean δN values that were not significantly different from migrating females. The mean δN values of pregnant females indicate they were feeding at a higher trophic level than juvenile males during migration. The higher mean δC values for pregnant females suggest they were feeding coastally during the spring migration, while juvenile males and nulliparous females were feeding offshore. The higher δN values for post-parturient, lactating females over migrating, pregnant females point to either a trophic shift in diet over time, or a more likely N-enrichment due to negative nitrogen balance caused by the nutritional stress of lactation and the feeding/fasting regime experienced by females. Similar mean δC values for migrating and breeding-season females indicate that both groups were feeding in coastal, on-shelf domains during their respective time periods. Similar mean δN values for nulliparous and pregnant females indicate they were feeding at similar trophic levels despite indications of feeding in separate ecosystems during migration. Using a δN shift of 2-3‰ per trophic level, we made general inferences about the trophic levels at which northern fur seals were feeding. The interpretation of our δN data indicates that migrating pregnant females, lactating females and the majority of migrating juvenile males consumed prey with mean δN values between 14.2‰ and 15.2‰, 15.1‰ and 16.1‰, and 13.6‰ and 14.6‰, respectively. Probable fur seal prey was analyzed as well. Walleye pollock showed progressive N andC-enrichments with age. Mean δN and δC values of 3- to 4-year-old fish were ~6.0‰ and 1.1‰ higher, respectively, than values for 0-age pollock. Atka mackerel also showed isotopic enrichment with age. The δN and δC values of large fish were 0.8‰ and 0.3‰ higher, respectively, than values for smaller fish.

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