Blubber fatty acid profiles indicate dietary resource partitioning between adult and juvenile southern elephant seals

Abstract

When resources are limited or patchy, a species may develop some degree of resource partitioning to reduce intra-specific competition. Development of intra-specific resource partitioning is more pronounced in species with clear phenotypic variation among individuals (e.g. age or sex). Southern elephant seals Mirounga leonina have pronounced sexual dimorphism and range widely in size and foraging range between juvenile and adult stages. However, hypothesized diet-based resource partitioning has been less clear due to difficulties in sampling diet while seals are away from breeding islands. We analysed fatty acids (FAs) from blubber of 122 juvenile seals and compared them to FA profiles from blubber of 52 adult females, and to FA profiles from 51 prey species (grouped as fish and squid) to examine evidence for diet-based resource partitioning in the seals. FA signature analysis revealed physiological and dietary differences between ages. Principle components of the 21 FAs from seal blubber and prey parts distinguished prey from seals, and clearly separated prey species into fish and squid classes. FA profiles from adult females differed to those from juveniles, with the former more 'squid-like' and the latter more 'fish-like'. Variation in FA profiles of seals was also apparent between sexes and during different seasons. Differences in diet between juveniles and adult females suggest resource partitioning occurs in response to large metabolic and physiological differences with age that limit juvenile dispersal and diving abilities. By consuming a different suite of prey species relative to adult females, juvenile southern elephant seals may reduce intra-specific competition.KEY WORDS: Fatty acid signature analysis · Southern elephant seal · Diet · Resource partitioning Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 384: [303][304][305][306][307][308][309][310][311][312] 2009 distinctive in individuals of different age and sex or life-history stage with obvious morphological differences (Polis 1984, Bolnick et al. 2003. Thus, intraspecific resource partitioning should be more pronounced in species with large phenotypic variation between age and sex classes.The sexually size dimorphic southern elephant seal Mirounga leonina is the largest of all the pinnipeds and exhibits pronounced morphological and physiological differences with age (Field et al. 2005b). Adult body size of males (up to 4 t) is 5 to 10 times that of females (0.5 t). This large body mass and an estimated population size of 757 000 ) make this species a major consumer of marine resources in the Southern Ocean, of which squid, and perhaps to a lesser extent fish, make up the bulk of the diet (Green & Burton 1993, Slip 1995. These factors, combined with the unpredictable and patchy environment of the Southern Ocean in which they spend most of their lives, make southern elephant seals prime candidates for the evolution of mechanisms that reduce competition through intra-specific resource part...

Diet of juvenile southern elephant seals reappraised by stable isotopes in whiskers

“…The relative low rates of juvenile survival imply that this age group is experiencing some set of circumstances that is different to the adult component of the population and to that of juvenile seals from stable populations. A difference in diet between juveniles and adult seals was confirmed in a recent study comparing fatty acids (FA) in the blubber of juvenile and adult seals (Newland et al 2009), which indicated that juvenile seals from Macquarie Island consumed predominantly fish, while the adults consumed more squid. However, the FA interpretation in that study was largely qualitative, and while suggesting that important age-based differences in diet exist, it did little to identify the specific details regarding diet which are required to interpret demographic differences.…”

“…6; Newland et al 2009). Having therefore eliminated squid as potential prey, we were then able to identify myctophids as the likely predominant prey species using SI analysis.…”

O' mother where wert thou? Maternal strategies in the southern elephant seal: a stable isotope investigation

“…Male harassment may also explain this pattern [38]. Isotopic analysis of archival tissues, such as whiskers or teeth, may prove useful to shed light on the ontogeny of this putative sexual segregation, a pattern documented in other seal species [57] and between juveniles of both sexes and adult females in southern elephant seals [58].…”

Cephalopoda as prey of juvenile Southern elephant seals at Isla 25 de Mayo/King George, South Shetland Islands

“…Cephalopods (squid and octopus) constitute a common prey in the diet of the southern elephant seal (clarke et al, 1982;rodhouSe et al, 1992;daneri et al, 2000), although fish have also been reported as a relatively important food item, at least seasonally (daneri & carlini, 2002;newland et al, 2009). The southern elephant seal has a key role as a top predator in the food webs of the southern marine ecosystem.…”