Trophic relationships among high Arctic seabirds: insights from tissue-dependent stable-isotope models

Abstract

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 StraitLancaster 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.