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

Abstract

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.

Vibrissal growth parameters of southern elephant seals Mirounga leonina: obtaining fine-scale, time‑based stable isotope data

“…Vibrissae are often subsampled into 2 mm segments for isotopic analyses (e.g., Newland et al 2011;Walters et al 2014), and we therefore reported the average number of days represented by a 2 mm vibrissal segment to emphasize the temporal resolution of the biogeochemistry data obtainable from sequentially sampled vibrissae.…”

“…To ensure that the SI signature is represented similarly across different vibrissae grown at the same time, we compared the SI profile of two vibrissae collected simultaneously from a third individual (hereafter, Seal 3, 0.8 yo) and eight vibrissae collected from a deceased adult female SES (hereafter, Seal 4) (see Lewis et al (2006) and Newland et al (2011)). We controlled for different vibrissa lengths and growth rates in Seal 4 (Greaves et al 2004;Beltran et al 2015) by applying the growth rate of adult M. angustirostris from Beltran et al (2015), using the maximum length of each of the sampled vibrissae as the asymptotic length in the equation.…”

“…In contrast, the half-life of an isotopic tracer injected in captive harbour seals Phoca vitulina, was 47.1 days (Zhao & Schell 2004), although isotopic changes were visible after four days which implies that the marker incorporates rapidly into the protein synthesis of the growing vibrissae (Hirons et al 2001). As a result, vibrissae are increasingly utilised to study the diet of pinnipeds (Greaves et al 2004;Hückstädt et al 2011Hückstädt et al , 2012Newland et al 2011;Hindell et al 2012;Walters et al 2014). Nevertheless, the vibrissal growth rates of several phocid species remain to be determined, despite the recent interest in the utilisation of vibrissae to increase the temporal resolution of the dietary data (e.g., Hirons et al 2001;Greaves et al 2004;Zhao & Schell 2004;Hall-Aspland et al 2005;Beltran et al 2015).…”

“…Southern elephant seals (Mirounga leonina, hereafter SES) are the most abundant phocid in the sub-Antarctic region, and although their vibrissae have been used in dietary studies (Cherel et al 2008;Hückstädt et al 2011;Newland et al 2011;Walters et al 2014), the growth rate and shedding pattern (Ling 1966) of their vibrissae remains poorly understood.…”

“…Whether the vibrissae of SES grow continuously before being replaced (Ling 1966), or enter an inactive phase after reaching the maximum length (Newland et al 2011), remains debated.…”