We measured stable carbon ( 13 C/ 12 C) and nitrogen ( 15 N/ 14 N) isotope ratios in nail, whiskers, skin, hair, and blood of captive harp seals (Pagophilus groenlandicus), harbour seals (Phoca vitulina), and ringed seals (Phoca hispida) held on a constant diet of herring (Clupea harengus) for at least 2 years. In addition, isotope ratios were measured in the muscle and liver of two harp seals, and in the lung, kidney, heart, and spleen of a single harp seal that had died in captivity. Isotopic fractionation values between dietary herring (δ 13 C: -20.3 ± 0.7‰; δ 15 N: 13.0 ± 0.4‰, n = 33) and these tissues ranged for nitrogen from +1.7 to +3.1‰ for blood and liver, respectively, and for carbon from +0.6 to +3.2‰ for liver and whiskers, respectively. No differences in isotopic fractionation values among species or age groups were detected. These values will permit more accurate dietary reconstructions on the basis of isotopic analysis of the tissues of seals and other marine mammals. For two captive harp seals, carbon and nitrogen isotope values showed small variation along the length of six whiskers (range of standard deviations for δ 13 C: 0.21 to 0.57‰, for δ 15 N: 0.27 to 0.45‰) but showed some evidence of being inversely correlated.Résumé : Nous avons mesuré des rapports d'isotopes stables pour le carbone ( 13 C/ 12 C) et l'azote ( 15 N/ 14 N) dans les griffes, les moustaches, la peau, le poil et le sang de phoques captifs, soit des phoques du Groenland (Pagophilus groenlandicus), des phoques communs (Phoca vitulina) et des phoques annelé (Phoca hispida), soumis à un régime alimentaire constant de hareng (Clupea harengus) pendant au moins 2 ans. De plus, on a mesuré les rapports isotopiques dans le muscle et le foie de deux phoques du Groenland, ainsi que dans les poumons, les reins, le coeur et la rate d'un phoque du Groenland mort en captivité. Les plages des valeurs de séparation isotopique observées pour le hareng du régime alimentaire (δ 13 C : -20,3 ± 0,7‰; δ 15 N : 13,0 ± 0,4‰, n = 33) et pour ces tissus étaient comprises entre +1,7 et 3,1‰ (sang et foie, respectivement), dans le cas de l'azote, et dans celui du carbone, ces plages étaient comprises entre +0,6 et +3,2‰ (foie et moustaches, respectivement). On n'a décelé aucune différence, pour ce qui est des valeurs de séparation isotopique, entre des espèces ou groupes d'âges. Ces valeurs permettront une reconstruction plus précise du régime alimentaire, basées sur l'analyse isotopique des tissus des phoques et d'autres mammifères marins. Pour deux phoques du Groenland captifs, les valeurs du carbone et de l'azote présentaient de petites variations dans le sens de la longueur sur six moustaches (plage des écarts-types pour δ 13 C : 0,21 à 0,57‰; pour δ 15 N : 0,25 à 0,45‰), mais on pouvait aussi y voir certains signes de corrélation inverse. [Traduit par la Rédaction]

Zooplankton from the Bering, Chukchi, and Beaufort seas and a transect across the Arctic Ocean were collected from 369 stations on 18 cruises in the years 1985-1990 and 1993-1995. Carbon and nitrogen isotope ratio analyses were performed on the major taxonomic groups present-calanoid copepods, euphausiids and chaetognaths. The sampled waters around Alaska were divlded into 11 subregions based on water mass characteristics and the zooplankton statistically tested for significant differences in the isotope ratios. Within all regions, copepods were significantly more depleted in I3C than euphausiids (average 6I3C difference for copepods = -1.1 Ym than euphausiids), but showed no significant differences from euphausiids in 615N except in the eastern Alaskan Beaufort Sea where copepods were relatively enriched in "N. The greatest variability in isotope ratios was among geographic regions. All taxa tested were I3C-depleted In the eastern Beaufort Sea, the Arctic Ocean and in deep waters of the southwestern Bering Sea relative to the continental shelf waters of the Bering and Chukchi seas. The maximum enrichments were found in the southwestern Chukchi Sea and central Bering Sea shelf waters. The advection of water northward through the Bering Strait was evident as a plume of enriched zooplankton extending to the shelf break in the Arctic Ocean. In contrast, the 6I5N within taxa generally increased moving northward from the deep Bering Sea to the Chukchi Sea and eastward into the Beaufort Sea. The 6I5N values for chaetognaths were 2.5 to 3 %~ more enriched than copepods or euphausiids in all locations, consistent with their carnivorous diet. Comparisons of zooplankton isotope ratios among years and cruises within the same region revealed no significant differences. Low 6I5N and 6I3C values in zooplankton of the pelagic Bering Sea are presumed to result from the isotopic discrimination arising in the presence of high nutrient abundances and slow phytoplankton growth rates whereas depleted values in coastal waters of the Canadian Beaufort Sea presumably derive from Mackenzie River inputs of terrestrially derived carbon and nitrogenous nutrients with low I5N and I3C abundances. The geographic heterogeneity in isotope ratios over short distances indicates a need for caution in the interpretation of isotope ratios in marine mammals and birds with regard to trophic status and habitat usage.

Theory predicts that generalist predators will switch to alternative prey when preferred foods are not readily available. Studies on the feeding ecology of the American marten (Martes americana) throughout North America suggest that this mustelid is a generalist predator feeding largely on voles (Microtus sp.; Clethrionomys sp.). We investigated seasonal and annual changes in diets of martens in response to the changing abundance of small rodents (Peromyscus keeni, and Microtus longicaudus) on Chichagof Island, Southeast Alaska, using stable isotope analysis. We hypothesized that martens would feed primarily on small rodents during years with high abundance of these prey species, whereas during years of low abundance of prey, martens would switch to feed primarily on the seasonally available carcasses of salmon. We also hypothesized that home-range location on the landscape (i.e., access to salmon streams) would determine the type of food consumed by martens, and martens feeding on preferred prey would exhibit better body condition than those feeding on other foods. We live-captured 75 martens repeatedly, from mid-February to mid-December 1992-1994. We also obtained marten carcasses from trappers during late autumn 1991 and 1992, from which we randomly sub-sampled 165 individuals. Using stable isotope ratios and a multiple-source mixing model, we inferred that salmon carcasses composed a large portion of the diet of martens in autumn during years of low abundance of rodents (1991 and 1992). When small rodents were available in high numbers (1993 and 1994), they composed the bulk of the diet of martens in autumn, despite salmon carcasses being equally available in all years. Selection for small rodents occurred only in seasons in which abundance of small rodents was low. Logistic regression revealed that individuals with access to salmon streams were more likely to incorporate salmon carcasses in their diet during years of low abundance of small rodents. Using stable isotope analysis on repeated samples from the same individuals, we explored some of the factors underlying feeding habits of individuals under variable ecological conditions. We were unable to demonstrate that body weights of live-captured male and female martens differed significantly between individuals feeding on marine-derived or terrestrial diets. Therefore, martens, as true generalist predators, switched to alternative prey when their principal food was not readily available on a seasonal or annual basis. Although salmon carcasses were not a preferred food for martens, they provided a suitable alternative to maintain body condition during years when small rodents were not readily available.

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