We measured stable-carbon ( 1 3 C / 1 2~) and/or nitrogen (l5N/l4N) isotope ratios in 322 tissue samples (minus lipids) representing 43 species from primary producers through polar bears Ursus maritimus in the Barrow Strait-Lancaster Sound marine food web during July-August, 1988 to 1990. 613C ranged from -21.6 f 0.3%0 for particulate organic matter (POM) to -15.0 f 0.7%0 for the predatory amphipod Stegocephalus inflatus. 6 1 5~ was least enriched for POM (5.4 +. O.8%0), most enriched for polar bears (21.1 f 0.6%0), and showed a step-wise enrichment with trophic level of +3.8%0. We used this enrichment value to construct a simple isotopic food-web model to establish trophic relationships within thls marine ecosystem. This model confirms a food web consisting primanly of 5 trophic levels. b13C showed no discernible pattern of enrichment after the first 2 trophic levels, an effect that could not be attributed to differential lipid concentrations in food-web components. Although Arctic cod Boreogadus saida is a n important link between primary producers and higher trophic-level vertebrates during late summer, our isotopic model generally predicts closer links between lower trophic-level invertebrates and several species of seabirds and marine mammals than previously established.

The measurement of stable isotopes of nitrogen and carbon in tissues of marine consumers can provide trophic-level information that is a time-integrated approximation of assimilated diet. By measuring 6I5N and 613C concentrations in liver, muscle and bone collagen of 7 species of seabirds inhabiting the Barrow Strait-Lancaster Sound region, Northwest Territories. Canada, estimates of short-, intermediate-, and long-term trophic level (TL), respectively, were established. Trophic estimates were based on a simple model incorporating diet-tissue fractionation established through the captive rearing of piscivorous birds on known isotopic diets. 613C was not a useful indicator of trophic position in this marine food web but may indicate inshore or benthic vs offshore or pelagic feeding preferences in seabirds. 615N was a useful indicator and ranged from 12.7 f 0.3%0 (TL 3.3) in liver of dovekies Alle aUe to 19.1 + 0.8% in the bone collagen of glaucous gulls Larus hyperboreus.Consistent with their known diets, dovekies and common eiders Somateria mollissima showed lowest trophic positions for all tissues; black-legged kittiwakes Rissa tridactyla, thick-billed murres Uria lomvia, northern fulmars Fulmarus glacialis and black guillemots Cepphus grjfUe showed intermediate trophic positions (TL 3.9 to 4.3) and glaucous gulls showed highest trophic position (TL 4.5 to 4.8). In general, seabirds increased their consumption of lower-trophic-level invertebrates during the breeding season over their lifetime averages but fed their chicks diets consisting of high proportions of fish. Isotopic estimates suggest greater dependence on lower-trophic-level prey by murres, fulmars, kittiwakes and guillemots than previously established through convenhonal dietary techniques.

The food and feeding ecology of the 2 closely related species of prions Pachyptila belcheri and P. desolata was investigated over 3 consecutive chick-rearing periods at Iles Kerguelen, the only place where they nest sympatrically in large numbers. In all years, the 2 prion species fed on crustaceans, with a small proportion of mesopelagic fish and squid. The hyperiid amphipod Themisto gaudichaudii was consistently the dominant prey item, accounting for 76 and 70% by number, and 57 and 57% by reconstituted mass of the diet of P. belcheri and P. desolata, respectively. Prions, however, were segregated by feeding on different euphausiids, P. belcheri on Thysanoessa sp. (18% by number and 16% by mass) and P. desolata on Euphausia vallentini (9% by number and 15% by mass). P. desolata also caught more small prey such as copepods (9 vs <1% by number) and cypris larvae of Lepas australis (8 vs 3% by number) than P. belcheri, which can be related to the beak filtering apparatus present only in the former species. Biogeography of the prey and their state of digestion indicate that prions foraged in a wide variety of marine habitats, including the kelp belt, kelp rafts, and coastal, neritic and oceanic waters. Noticeable is the occurrence of E. superba in a significant number of food samples (15 and 10% for P. belcheri and P. desolata, respectively), suggesting feeding in distant foraging grounds in southern Antarctic waters, >1000 km from the breeding colonies, during the chick-rearing period. The stable-carbon and -nitrogen isotopic compositions of chick feathers were identical in both species, indicating no important trophic segregation during the breeding period, when adult birds are central-place foragers. The ratios were, however, different in adult feathers, suggesting moulting in Antarctic waters for P. belcheri and in subtropical waters for P. desolata, i.e. in distinct foraging areas when birds are not constrained to return to the colonies.

We used stable isotope techniques to analyze tissues of Keen's mice (Peromyscus keeni) and Townsend's voles (Microtus townsendii cowani) and a subset of prey items at Triangle Island, British Columbia, western Canada's largest seabird colony. Isotope analysis allowed us to investigate the importance of seabird prey in rodent diets in a system where seabirds and non-introduced rodents occur sympatrically. The δ15N values for terrestrial plants and terrestrial invertebrates on Triangle Island exceeded levels found in many terrestrial biomes and are typical of localities with high inputs of marine-derived N. We used multiple-source mixing models to estimate the relative inputs of potential prey items to vole and mouse diets. The δ13C and δ15N values of liver and muscle tissues of voles indicate that voles on Triangle Island derived their protein primarily from terrestrial plants, with some contribution by terrestrial invertebrates. In contrast, isotopic values of liver and muscle tissues of mice on Triangle Island indicated that mice prey primarily on seabird eggs and terrestrial invertebrates. Our results show that egg predation on Triangle Island is a general phenomenon in the mouse population, rather than occurring in only a few specialist feeders. Mice appear to feed on eggs once they become available and continue to utilize seabird prey, likely in the form of abandoned eggs or carcasses of chicks and adults, throughout the breeding season.

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