Differential resource allocation strategies in juvenile elephant seals in the highly seasonal Southern Ocean

Foraging behaviour of mammals, namely the change in distribution and trophic levels from juvenile stage to adulthood, can be investigated by measuring δ 13 C and δ 15 N stable isotopes for layers deposited in a growing tooth. For the first time, we describe geographic differences in the ontogeny of foraging strategies and in the niche partitioning process according to sex and age of a highly sexually dimorphic species: the southern elephant seal Mirounga leonina. Canines from 8 males and 6 females were analysed for δ 13 C and δ 15 N stable isotope signatures. To assess intra-individual variability, instead of analysing collagen we analysed the bulk dentine within each of the 4 growth layers deposited annually. The δ 13 C signature revealed that, in individuals of 1 to 4 yr of age, teeth of both males and females exhibited large intra-individual variation in δ 13 C, suggesting that juveniles were foraging over a broad range of marine habitats encompassing both sub-Antarctic and Antarctic waters. Four out of the 6 teeth taken from females were collected on individuals younger than 4 yr, preventing investigation of longer-term changes. A δ 13 C pattern emerged for males older than 4 yr: individuals became resident to either a subAntarctic (-17 ‰) or an Antarctic (-20 ‰, both values reported as deviations from the Vienna PeeDee Belemnite standard) foraging habitat, with a decrease in intra-individual variability. Up to the age of 4 yr, juvenile males were at a slightly higher trophic level than juvenile females, but by the age of 4 yr, while their δ 13 C signature revealed that they were faithful to their foraging habitat, males exhibited a significant increase in their trophic levels, as shown by their δ Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 439: [295][296][297][298][299][300][301][302][303][304][305] 2011 techniques of stomach lavaging or faecal analysis may provide high-resolution species-specific data directly related to digestion transit rate, and over a relatively short period. Stable isotopes, while lacking in high-resolution species-specific data over a short timeframe, can reveal surprising ecological features of a species by summarizing trophic information over a broader time and space window as compared to traditional dietary studies. Stable isotopic carbon and nitrogen analysis is now increasingly used to investigate the feeding habits of major consumers (Newsome et al. 2007a, Newsome et al. 2010). The δ 15 N measurements serve as indicators of consumer trophic position, as consumer tissues are typically enriched in 15 N relative to their food (DeNiro & Epstein 1981, McCutchan et al. 2003, with an average increase of 3.3 ± 0.26 ‰ between trophic levels (McCutchan et al. 2003). In contrast, δ 13C values vary little between trophic levels but are often used to indi cate the geographic source of prey items due to well documented variation in δ 13C with different photo synthetic processes (Hobson 1999). In the marine environment, δ C...

supporting

confidence: 93%

Stable carbon and nitrogen isotope variations in canine dentine growth layers of Kerguelen southern elephant seals

Martin¹,

Bentaleb²,

Steelandt³

et al. 2011

“…Both king penguins (Kooyman et al 1982, Moore et al 1999) and southern elephant seals (Slip 1997, Irvine et al 2000 are known to dive to great depths as they forage. In addition foraging ranges also overlap, with maximum distances of king penguins being 865 to 1984 km while juvenile southern elephant seals forage over average distances of 1432 to 2283 km (Bost et al 2004, Field et al 2007a). Thus if juvenile seals are consuming primarily the same prey as penguins, and given that there has been a 78 fold increase in the Macquarie Island penguin population from 1930 (3400 birds) to 1980 (218 000 birds), and that new colonies are currently being established on the island (van den Hoff et al 2009), it follows that there must have been a concurrent increase in competition for resources between seals and penguins.…”

Section: Implications For Demographics Of Southern Elephant Sealsmentioning

confidence: 99%

“…The Southern Ocean has demonstrated ecosystem perturbations due to climatic changes, mediated through variation in sea ice extent which impacts on overall productivity (Arrigo et al 1998) at a range of spatial and temporal scales. Therefore it is likely that variation in resource availability will affect predator foraging success and may differen-tially affect age, size or functional groups of a species (Field et al 2007a).…”

Section: Introductionmentioning

confidence: 99%

“…Since the 1960s there has been a major decline in southern elephant seal numbers, the extent, timing and nature of which was subject to considerable intra-population variability (Hindell 1991, Hindell et al 1994. The reason for the decline has remained unclear, although a mounting body of evidence suggests that changing environmental circumstances may have played a major role (Hindell 1991, Hindell et al 1994, Field et al 2007a. Indeed, demographic modelling has indicated that juvenile survival was lower in the declining populations compared to the stable or increasing populations implicating this as a factor influencing the population trends (Hindell 1991, Hindell et al 1994.…”

Section: Introductionmentioning

confidence: 99%

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

Newland¹,

Field²,

Cherel³

et al. 2011

Self Cite

Declines in marine predator populations have been attributed to anthropogenic activity and environmental change. Southern elephant seals Mirounga leonina are major consumers of biomass in the eastern region of the Southern Ocean and have been declining in numbers since the 1960s. Previous studies have identified evidence for habitat and diet partitioning over a range of spatial and temporal scales between juveniles and adults in the Macquarie Island population. We first analysed the stable isotopes (SI) of 6 entire vibrissae from a dead adult female southern elephant seal from Kerguelen Islands to determine moult and growth patterns. Secondly we analysed the SI from the vibrissae of 102 juvenile southern elephant seals to investigate diet. The results from the growth pattern analysis indicated that vibrissae do not grow or moult simultaneously. However, it is likely that at least part of the vibrissae will have been produced sometime during the most recent trip to sea and will give a broad indication of diet. The subsequent SI analysis confirmed that juveniles are consuming greater proportions of fish species, and identified myctophids as the primary component of juvenile diet. Myctophids are also consumed by king penguins Aptenodytes patagonicus which have greatly increased in numbers recently in the Macquarie Island area. This may have presented the juvenile southern elephant seals with increased competition and may influence survival.

“…During their time at sea, many marine mammals, and particularly phocid seals, accumulate large quantities of prey-derived lipid (Fedak et al 1994) which they store in body tissues, the majority (~84%) of which is stored in the blubber (Slip et al 1992). On land, changes in body composition can be measured with relative ease (Boyd et al 1993, Beck et al 2000, Field et al 2007, allowing for an estimate of gross body condition and mass gain over entire foraging trips. However, for species migrating long distances and spending months at sea (e.g.…”

Section: Introductionmentioning

confidence: 99%

Tracking changes in relative body composition of southern elephant seals using swim speed data

Thums¹,

Bradshaw²,

Hindell³

2008

Self Cite

Changes in buoyancy during an animal's time at sea are a powerful tool for inferring spatial and temporal foraging success. Buoyancy can be difficult to measure, but in some species of seal, drift components of dives can be used. We used swim speed data from adult female southern elephant seals Mirounga leonina using geo-locating velocity-time-depth recorders during 2004 postlactation (PL; n = 7) and 2002, 2004 and 2005 post-moult (PM; n = 18) foraging trips to detect periods of passive drifting during diving. In addition to the characteristic drift dives of elephant seals, drifting also occurred during putative foraging dives. We used generalised linear models (GLMs) to examine the relationship between body lipid content measured on land and several diving variables collected within a week of these measurements being taken. The strongest support (deviance explained = 90%) was for the model including drift rate (77%), seal length (12%) and descent rate (2%). Estimates of body lipid, based on the GLM, were predicted for each day of the foraging trips. Areas where seals increased their relative lipid content from one day to the next corresponded well with areas in which the seals spent the greatest amount of time. Inferring foraging success from positive changes in drift rate has so far been limited to elephant seals which perform characteristic drift dives, but the addition of swim speed data to detect short periods of stationary behaviour allows for this method to be expanded to a greater range of ocean predators.