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

Abstract

Fatty acld signature analysis was used to investigate the diet and the spatial scales of foraging in harbor seals Phoca vituljna 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 finescale 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 PM'S. 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 compr~sed more of small size classes of herring and pollock, and perhaps other items such as cephalopods, sandlance Arnmodytes 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.

You are what you eat: describing the foraging ecology of southern elephant seals (Mirounga leonina) using blubber fatty acids

“…The ability of FA profiles to reflect differences in diet structure depends on the degree of dietary specialization and the magnitude of geographical or temporal separation (Iverson et al 1997b;Brown et al 1999). For differences in diet between seals foraging in different zones to be detected by examining FA profiles, several scale-dependent conditions must be satisfied: (i) prey communities within these putatively distinct ecological zones must be measurably different from each other, with minimal over-lap of prey species among zones; (ii) prey species within each geographical zone must possess distinct FA profiles; and (iii) the combined FA profile of a number of dietary items of the predators from each zone must retain sufficient individuality to allow distinction between the animals that forage in each region.…”

“…FA signature analysis can theoretically be used to determine diet composition and also to provide a long-term indication of diet history (Iverson et al 1997a,b). Recent research using FA analysis to quantify diet in top-level marine predators has shown promise (Iverson et al 1997b;Raclot et al 1998;Brown et al 1999;Hooker et al 2001), although others have been less optimistic (Grahl-Nielsen 1999). Differences in FA signatures among populations, geographical regions and seasons have all been identified (Iverson et al 1997a;Walton et al 2000;Lea et al 2002;Walton & Pomeroy 2003), demonstrating that the technique can be used to detect changes in diet.…”

“…Differences in FA signatures among populations, geographical regions and seasons have all been identified (Iverson et al 1997a;Walton et al 2000;Lea et al 2002;Walton & Pomeroy 2003), demonstrating that the technique can be used to detect changes in diet. However, only Iverson et al (1997b) and Hooker et al (2001) have been able to attribute some of these differences to specific prey. Further, no study has determined the spatial and temporal variation in diet structure of individuals relative to foraging regions.…”

Tracing aquatic food webs using fatty acids: from qualitative indicators to quantitative determination

“…However, several studies have shown the reverse in the Pacific, with 20:1n-11 dominating 20:1n-9 in fish and invertebrates (Saito and Murata 1998) . On this basis, North Pacific and North Atlantic harbor seals ( Phoca vitulina ) consuming these prey can be readily distinguished (Iverson et al 1997) . Similarly, Smith et al (1996) used low levels of such marine-based FA, coupled with high levels of 18:2n-6, 18:3n-3, and 20:4n-6 (FA typical of primary producers in freshwater and terrestrial ecosystems, with low abundance in marine ecosystems), to distinguish freshwater from marine harbor seals.…”

“…FA represent the "building blocks" of lipids and are the largest constituent of neutral lipids (NL), such as triacylglycerols (TAG) and wax esters (WE), as well as of the polar phospholipids (PL). All FA consist of carbon atom chains, which are most commonly even-numbered and straight, containing 14-24 carbons and 0-6 double bonds, with (Iverson et al 1997(Iverson et al , 2006a FA are named as carbon number:number of double bonds and location ( n -x) of the double bond nearest the terminal methyl group, where all additional double bonds are separated by a -CH 2 -group (i.e., "methylene-interrupted"). Non-methylene interrupted FA (NMI FA) are separated by more than one methylene group; these are generally very small peaks and require special attention in identifying, and thus are listed separately ) .…”