FATTY-ACID SIGNATURES REVEAL FINE-SCALE STRUCTURE OF FORAGING DISTRIBUTION OF HARBOR SEALS AND THEIR PREY IN PRINCE WILLIAM SOUND, ALASKA

Fatty acid signature analysis was used to investigate the diet and the spatial scales of foraging in harbor seals Phoca vitulina richardsi i n Prince William Sound (PWS) and elsewhere in the Gulf of Alaska. Blub ber samples collected in 1994 and 1995 from 104 harbor seals from PWS, Kodiak Island, and southeast Alaska were analyzed for fatty acid comp osition. A total of 163 potential prey samples representing 10 taxa we re 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 tr ee analysis was used to classify seals and prey according to their fat ty acid signatures. Large differences were found in the fatty acid com position of blubber from seals sampled at Kodak, southeast Alaska and PWS, over a broad geographical scale of 400 to 800 km. Additionally, f atty acid signatures distinguished seals from different regions within PWS, as well as on fine-scale resolutions of specific haulout sites w ithin 9 to 15 km of one another. These findings suggest that seals for age site-specifically. These conclusions are supported by prey fatty a cid patterns, which also differed on similarly small spatial scales wi thin PWS. Not only could prey species such as herring Clupea pallasi a nd pollock Theragra chalcogramma be differentiated from one another us ing fatty acid signatures, but they could also be distinguished by siz e-class and location within PWS, reflecting differences in diet with a ge and as well as with fine-scale habitat. Results from this study are consistent with both satellite data from tagged harbor seals and stom ach content analyses of forage fish species in PWS. Although prelimina ry, analyses suggest that large herring and pollock, as well as flatfi sh, may have dominated the diet of seals in southern PWS, whereas diet s of seals in northern and eastern PWS may have been comprised more of small size classes of herring and pollock, and perhaps other items su ch as cephalopods, sandlance Ammodytes hexapterus, cod Gadus macroceph alus, and shrimp. We conclude that fatty acid signature analysis will be an important contribution to understanding marine food webs in estu arine and other marine environments.