Detecting alternate foraging ecotypes in Australian sea lion (Neophoca cinerea) colonies using stable isotope analysis

Abstract

Stable carbon (δ13C) and nitrogen (δ15N) isotopes are used frequently to describe the trophic ecology of top marine mammal predators. Australian sea lions (Neophoca cinerea) are one of the world's rarest otariid seals and exhibit the highest levels of natal site philopatry of any seal. We report the development of a screening technique to identify different foraging ecotypes and assess their relative frequencies in Australian sea lion breeding colonies using stable isotope ratios in pups. Geospatial and dive data from 15 adult females at three breeding colonies revealed alternate foraging strategies (inshore and offshore foraging) that were reflected in significant changes in δ13C and δ15N. Isotope fractionation from mother to pup was validated using paired whisker and blood serum samples with no significant difference between δ13C and δ15N enrichment of +1.27‰ (whiskers) and +1.92‰ (blood serum) from mothers to pups. Isotope ratios from whisker samples representing over 50% of pups born at three colonies revealed significant intercolony differences in maternal foraging ecotype frequencies. These results are unique in that ecological partitioning over such a small spatial scale has not been described in any other otariid species.

Age, body mass and environmental variation shape the foraging ontogeny of Galapagos sea lions

“…These findings are similar to the patterns of ontogenetic changes in stable isotope signatures of other mammal species (Balasse 2001, Newsome et al 2006, Dalerum et al 2007, York et al 2008. The enrichment of 1.7 ‰ in δ 15 N signatures of 6M GSL juveniles is similar to published measured or assumed fractionation between pinniped mothers and fully dependent offspring (Aurioles-Gamboa et al 2009, Habran et al 2010, Lowther & Goldsworthy 2011, indicating that 6M juveniles completely relied on maternal milk. Somewhat surprisingly, 12M juveniles showed similarly enriched δ 15 N values despite substantial diving ability, i.e.…”

“…Further, the mean difference between samples stored either dry or in ethanol of 0.1 and −0.06 ‰ for δ 15 N and δ 13 C, respectively, was below the range of published measurement errors of 0.3 and 0.1 ‰ for δ 15 N and δ 13 C, respectively (Habran et al 2010, Lowther & Goldsworthy 2011.…”

Creatures of habit: foraging habitat fidelity of adult female Australian sea lions

“…We employed the methods in Lowther & Goldsworthy (2010) to classify individual sea lions into significantly different foraging behaviours (ecotypes). To determine the broad-scale distribution of foraging effort, a fixed-kernel core range at the 50% utilisation distribution (UD-50) using least squares cross validation (LSCV) (Worton 1989, Börger et al 2008 was calculated for each sea lion over all trips using the R package 'adehabitat'.…”

“…In marine ecosystems a decreasing stable carbon ratio (δ 13 C) gradient has been identified with distance from shore (Hobson et al 1996, Post 2002. Isotope ratios provide ecological geotrophic data that has been used to describe temporal and individual variation in foraging location and diet outside the scope of traditional tracking methods using δ 13 C and δ 15 N from metabolically active (blood) and inert (vibrissae, feathers) tissues for several seal and marine bird species (Forero & Hobson 2003, Aurioles et al 2006, Bearhop et al 2006, Awkerman et al 2007, Lowther & Goldsworthy 2010. Recently multi-year, individual variation in foraging behaviour has been described using isotope values from serially subsampled adult male Antarctic fur seal vibrissae (Cherel et al 2009).…”

“…Two recent studies have identified inter-individual variation in foraging behaviours of adult female Australian sea lions, which are characterised as benthic predators (Costa & Gales 2003); individuals which forage in shallow, coastal waters (<10 m) and those which forage offshore in waters up to 120 m (Baylis et al 2009, Lowther & Goldsworthy 2010). However, neither study was able to characterise the temporal stability of these 2 behaviour types, though previous short-term deployments (> 30 d) of telemetry devices on adult females suggested foraging site fidelity over the duration of tracking events (S. D. Goldsworthy unpubl.…”