Fatty acid signatures reveal fine scale structure of foraging distribution of harbor seals and their prey in Prince William Sound, Alaska

Abstract

Fatty acid signature analysis was used to investigate the diet and the spatial scales of foraging in harbor seals Phoca vitulina richardsi in Prince William Sound (PWS) and elsewhere in the Gulf of Alaska. Blubber samples collected in 1994 and 1995 from 104 harbor seals from PWS, Kodiak Island, and southeast Alaska were analyzed for fatty acid composition. A total of 163 potential prey samples representing 10 taxa were collected and individually analyzed for total fat content and fatty acid composition. Approximately 70 fatty acids and isomers were found in both harbor seals and their prey. Classification and regression tree analysis was used to classify seals and prey according to their fatty acid signatures. Large differences were found in the fatty acid composition of blubber from seals sampled at Kodiak, southeast Alaska and PWS, over a broad geographical scale of 400 to 800 km. Additionally, fatty acid signatures distinguished seals from different regions within PWS, as well as on fine-scale resolutions of specific haulout sites within 9 to 15 km of one another. These findings suggest that seals forage site-specifically. These conclusions are supported by prey fatty acid patterns, which also differed on similarly small spatial scales within PWS. Not only could prey species such as herring Clupea pallasi and pollock Theragra chalcogramma be differentiated from one another using fatty acid signatures, but they could also be distinguished by size-class and location within PWS, reflecting differences in diet with age and as well as with fine scale habitat. Results from this study are consistent with both satellite data from tagged harbor seals and stomach content analyses of forage fish species in PWS. Although preliminary, analyses suggest that large herring and pollock, as well as flatfish, may have dominated the diet of seals in southern PWS, whereas diets of seals in northern and eastern PWS may have been comprised more of small size classes of herring and pollock, and perhaps other items such as cephalopods, sandlance Ammodytes hexapterus, cod Gadus macrocephalus, and shrimp. We conclude that fatty acid signature analysis will be an important contribution to understanding marine food webs in estuarine and other marine environments.