Seabirds as indicators of food web structure and ecosystem variability: qualitative and quantitative diet analyses using fatty acids

Abstract

The dynamics of predator-prey relationships, the structure of food webs, and the forag- ing behavior of individuals are critical to understanding animal ecology, interactions of predators with their prey, and effects of environmental variability on ecosystems. Like many other predators, seabirds are effective samplers of prey populations, and their diets can provide information about lower trophic levels over a range of spatial and temporal scales. Recognizing limitations of traditional methods of diet analysis, we validated the use of fatty acid (FA) signatures of subcutaneous adipose tissue biopsies for estimating diets of free-ranging seabirds. Calibration coefficients (CCs) for individ- ual FAs were determined from captive common murres (n = 13) fed a long-term, single-species diet. Quantitative FA signature analysis (QFASA), using these CCs, was then validated in murres (n = 26) and red-legged kittiwakes (n = 13) fed controlled mixed-species diets. FAs were analyzed from 426 free-ranging red-legged Rissa brevirostris and black-legged kittiwakes R. tridactyla, and common Uria aalge and thick-billed murres U. lomvia from the Bering Sea, 284 of which were also sampled for stomach contents analysis. Qualitatively, FA signatures revealed distinct separation of diets among all 4 species, and further separation by location and year. QFASA diet estimates were similar to those based on stomach contents, with diets of kittiwakes dominated by myctophids, while those of murres comprised a mixture of other forage species. QFASA estimates were indicative of regional habitat dif- ferences, and were consistent with other aspects of seabird ecology at our study sites. We conclude that seabird FAs provide important information about ecosystems, but this will likely depend on each species' foraging behavior and the complexities of the ecosystem it occupies.