Looking at the unseen: combining animal bio‐logging and stable isotopes to reveal a shift in the ecological niche of a deep diving predator

Abstract

Understanding how marine top predators exploit their environment is a central topic in marine ecology. Among all methodologies used to investigate this part of ecology, electronic devices are very useful to track animals' movements and foraging habitats, but they do not provide any dietary information. Stable isotopes provide information on trophic levels but remain imprecise to identify small spatial‐scale habitats. In this study, we combined the two approaches to obtain a synoptic view of the foraging behaviour variability of southern elephant seals Mirounga leonina. Our results suggested marked differences in distribution, diving behaviour, foraging habitats, trophic levels, and dietary habits of elephant seals according to their sex and age. Thus, we characterized main foraging habitats over the Kerguelen‐Heard Plateau and the Antarctic shelf for juvenile males, while females foraged mainly in oceanic waters of the Polar Frontal Zone and the Antarctic Zone. In addition, we highlighted the ontogeny of niche partitioning in this sexually dimorphic species. While females did not exhibit a major dietary shift in relation to their age and their breeding status, a different picture emerged for males. Young males had a trophic level identical to that of all females. However, at 3–4 yr of age, males showed a progressive increase in trophic level. The inter‐annual combination of bio‐logging and stable isotopes could provide a powerful tool to investigate possible shifts in ecological niche between years according to environmental changes.

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

“…Likewise, only 3 per cent of pups had a carbon isotope ratio higher than 219.5‰, which we speculated reflects extensive feeding on the Kerguelen plateau. If both foraging in Antarctic oceanic waters and on the Kerguelen plateau are so profitable, why do most females still bother to forage in the interfrontal zone [8,55]?…”

“…The Kerguelen plateau is an important foraging ground for adult males [8]. Breeding males are haulingout before females so that, before pupping, females could forage on the Kerguelen plateau without competing with or being harassed by males [38].…”

“…Carbon to nitrogen (C/N) ratios were checked, and when above 3.7, lipids were extracted using cyclohexane. Lipids are depleted in 13 C relative to proteins and carbohydrates [22] [8] used a mixture modelling approach to distinguish between large and small adults as assessed from their standard body length (snout-to-tail length, STL), and found a bimodal distribution of carbon stable isotope values in female adults and juvenile males. Kurtosis of the distribution of pup weaning mass tended to be negative, which was suggestive of bimodality (figure 1a,b and table 1; [23,24]).…”

“…Females only haul-out on land to breed in the early austral spring and to moult in the late summer. The bio-logging revolution has provided invaluable insights into the foraging ecology of these exceptional divers [7,8]. The main drawback of bio-logging studies is its costs: small sample size may weaken inferences [9].…”

“…In the case of southern elephant seal breeding on îles Kerguelen, Bailleul et al [8] identified two main foraging zones for females prior to hauling-out: the interfrontal zone (pelagic sub-Antarctic waters bounded by the sub-tropical front and the southern boundary of the Antarctic circumpolar current; electronic supplementary material, figure S1) and the Antarctic zone (area south of the southern boundary of the Antarctic circumpolar current; electronic supplementary material, figure S1). Based on tracking studies (n ¼ 44 postmoult trips of females), the estimated proportion of females committed to each strategy is 73 and 22 per cent for the interfrontal zone and Antarctica, respectively (the remaining 5% accounts for individuals who remained on the Kerguelen plateau, C. Guinet 2010, unpublished data).…”