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Use of Blubber Fatty Acid Profiles to Distinguish Dietary Differences Between Grey Seals Halichoerus Grypus from Two UK Breeding Colonies

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Use of Blubber Fatty Acid Profiles to Distinguish Dietary Differences Between Grey Seals Halichoerus Grypus from Two UK Breeding Colonies MARINE ECOLOGY PROGRESS SERIES Vol. 193: 201-208,2000 Published February 28 Mar Ecol Prog Ser Use of blubber fatty acid profiles to distinguish dietary differences between grey seals Halichoerus grypus from two UK breeding colonies 'Sea Mammal Research Unit, University of St Andrews, St Andrews KY16 8LB, Scotland, UK '~nstituteof Aquaculture, University of Stirling, Stirling FK9 4LA, Scotland, UK ABSTRACT: The population structure of grey seals Halichoerus grypus has previously been studied using molecular biological techniques. These works may be complemented by studies of certain eco- logical factors which play an important role in providing information on habitat usage, dietary behav- iour, local migrations, distribution and social structure. One such factor is the fatty acid composition of depot fats, which is strongly influenced by the fatty acid composition found in the prey. To examine the extent to which the fatty acid composition of seal depot fats can be used to distinguish between popu- lation~,blubber biopsy samples were obtained from female grey seals at 2 Scottish breeding sites, Rona (n = 23) and the Isle of May (n = 34) and the fatty acid compositions determined by gas chromatogra- phy. The patterns obtained were compared using both univariate and multivariate statistical proce- dures. Fatty acid profiles at the 2 breeding sites could be clearly differentiated, with only 1 seal out of 57 being rnis-classified. Thus, on the basis of blubber fatty acid composition, they may be regarded as 2 distinct foraging groups. KEY WORDS: Fatty acids . Grey seals - Population discrimination INTRODUCTION Female seals do not eat while onshore to give birth and suckle their young. They rely on their energy Approximately 110000 (one-third of the total world reserves (especially lipid) for maintenance and for milk population) grey seals Halichoerus grypus inhabit the production (Anderson 1990). Thus, in the months coastal waters of Britain (especially Scotland). Once a before breeding, females must eat sufficient amounts year, for a few weeks, seals gather at certain onshore of food to fully replenish their energy reserves, which sites for breeding. Seals at 2 Scottish breeding sites, are an important factor in successful reproduction namely Rona (59"06'N, 05" 50'W) and the Isle of May (Pomeroy et al. 1999). However, much remains un- (56"10fN,2'33'W) (Fig. 1) have been studied exten- known about the movements and foraging behaviour sively for more than 10 yr (Anderson & Fedak 1987, of most seals during these months. Satellite telemetry Pomeroy et al. 1999). Allen et al. (1995) tested the has provided useful information on the movements of a degree of breeding-site fidelity by screening over 1000 few grey seals (mainly males), but its widespread seals with 8 microsatellite loci. They found a mean application is restricted by its high cost. The technique inter-population distance F,, of 0.006, which is a low has been applied to seals from east Scotland (but not value and indicative of a high level of gene flow. Nev- Rona) and has shown that some seals tend to travel ertheless there is a highly significant difference (p < only relatively short distances to and from favoured 0.001) between the 2 colonies, so they can be regarded foraging sites while others can and do travel many as separate genetic populations (interbreeding units). hundreds of kilometres in a few days (McConnell et al. 1992, 1999). If long distance travel is extensive then seals from different breeding populations may inter- mingle at the same foraging areas and hence could be O Inter-Research 2000 Resale of full article not permitted Mar Ecol Prog Ser 193: 201-208. 2000 to investigate possible differences in diet and to complement the genetic work on popula- tion differentiation. If the seals from the 2 sites forage in different locations on different assemblages of prey then a difference would be expected in the profiles and they could be regarded as separate ecological 'populations'. Alternatively if there were extensive mixing at feeding grounds, then there would be little difference in profiles and no justification for differentiation. MATERIALS AND METHODS Tissue collection. In the course of related fieldwork (see Pomeroy et al. 1999 for details) female seals were captured and immobilised and blubber samples of about 0.5 g were collected using a 6 mm diameter disposable, single-use biopsy needle (Acuderm, Florida, USA). As fatty acid profiles can vary with the site on the body from which the sample is col- Fig. 1 Locations (latitude and longitude) of the 2 breeding colonies on lected (Kakela & Hyvarinen 1996) all samples on Rona and the Isle of May were collected from the mid-pelvic region. There are reports of some stratification of regarded as the same ecological 'population' (i.e.indi- blubber in grey seals (Fredheim et al. 1995) with viduals occupying the same location with similar forag- increasing levels of saturated fatty acids and longer ing behaviour). One way to investigate this is to study chain mono-unsaturated fatty acids (mainly 20:l and environmentally acquired characters which can pro- 22:l) and decreasing levels of short chain mono-unsat- vide information on habitat use, local migrations, for- urated fatty acids (mainly 14:l and 16:l) from outer to aging activities, distribution and social structure, but inner but little change in poly-unsaturated fatty acid not breeding behaviour (Avise 1994). Blubber fatty levels. All biopsy samples comprised a complete cross- acids are probably the most suitable such characters section of blubber from skin to muscle and were taken for studies on live animals since only small biopsy sam- around the start of the lactation period. All procedures ples are required. were performed under the appropnate UK Home Ackman et al. (1970) postulated that many distinc- Office licences. tive features of the fatty acid composition of marine As Rona and the Isle of May are remote locations, it organisms are passed through the food web and could was not possible to freeze the samples immediately be used to follow feeding patterns or identify popula- after collection. Samples (about 500 mg) were stored in tions. Marine oils typically contain a wide variety of chloroform:methanol (2:1, vol/vol) containing 0.05% fatty acids, many of which have their ultimate origin in butylated hydroxytoluene (BHT) as antioxidant until phytoplankton (Sargent et al. 1995),although blubber being returned to the mainland some 2 to 3 wk later, fatty acid profiles are strongly influenced by the diet where it was possible to store them at -20°C until but are not identical to it, since some endogenously analysis a few months later (Iverson et al. 1997b). synthesised fatty acids as well as modified dietary fatty Samples were taken from 23 breeding females at acids are also deposited (Iverson 1993).The technique Rona during the period 4 to 24 October 1996, and 34 has been used to distinguish between populdtions breedmg females at the Isle of May from 2 to 22 and/or dietary differences of several seal species (Ack- November 1996. man et al. 1971, West et al. 1979a, Grahl-Nielsen et al. Lipid extraction. Total lipid (essentially tnacylglyc- 1993, Kakela et al. 1993, Smith et al. 1996, Iverson el al. erol) was extracted from the blubber using the method 1997b),lobsters (Castell et al. 1995),fish (Iverson et al. of Folch et al. (1957). Thus, biopsy samples were 199713) and foxes (Pond et al. 1995). homogenised in 25 m1 chloroform:methanol (211,vol/ In the present study the fatty acid profiles of seals vol) containing 0.01 76 BHT as antioxidant and filtered. from Rona and the Isle of May were compared in order The organic phase was washed with 0.9% KC1 and Walton et al.: Dietary differences between Halichoerus grypus 203 dried over anhydrous sodium sulphate. An aliquot was produced consisting of a number of nodes and branches. taken to dryness and weighed. The top node contains the entire sample set. Each Fatty acid methylation/purification. Fatty acid me- remaining node contains a subset of the sample of the thyl esters were prepared and purified as described by node directly above it. The tree is binary because each Henderson et al. (1994) using l % (voVvo1) sulphuric node splits into 2 subsamples and the splits are deter- acid in methanol as the methylating reagent and incu- mined by whether the levels of a fatty acid (selected bation for 16 h at 50°C under nitrogen. The purified by the analysis as the best for this purpose) are above methyl esters were dissolved in hexane (1 mg ml-l). or below some cut-off value. The impurity value is a GC analysis. Fatty acid methyl esters (FAME) were measure of the levels of mis-classifications within the analysed by gas-liquid chromatography (glc) using a node. Canberra Packard 436 gas chromatograph equipped with a flame-ionization detector and fitted with a BP20 fused silica capillary column (30 m X 0.32 mm internal RESULTS diameter, S.G.E. Ltd, Milton Keynes, UK). Hydrogen was employed as the carrier gas and sample applica- The fatty acid compositions (% total fatty acids) of tion was by on-column injection. The temperature of the blubber of grey seals from Rona and the Isle of May the oven was programmed to rise from 50 to 150°C at are shown in Table 1. Overall the patterns were similar 35°C rnin-' and then to 225°C at 2.5'C rnin-' during with the 5 most abundant fatty acids 18:l(n-g),22:6 each analysis. Separated components were identified (n-3), 16:l(n-7), 16:O and 20:l(n-9) accounting for by reference to authentic standards and a well-charac- about 60% of the total.
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