Diet of a specialist in a changing environment: the crabeater seal along the western Antarctic Peninsula

Although crabeater seals Lobodon carcinophaga are among the most abundant consumers of Antarctic krill Euphausia superba, their diet has rarely been studied throughout most of the species' range. Using delta C-13 and delta N-15 values in vibrissae from 53 seals, we examined the trophic ecology of crabeater seals from the western Antarctic Peninsula (wAP) in 2001, 2002 and 2007. We observed a wide variability in individual seal mean delta C-13 values, which ranged from -19.8 parts per thousand to -24.9 parts per thousand, whereas mean delta N-15 value varied from 5.4 parts per thousand to 7.9 parts per thousand. We identified a positive significant effect of seal mass on delta C-13 values, as well as a significant seasonal effect (higher delta C-13 values in austral winter), which likely resulted from changes in the composition of the community of primary producers. delta N-15 values for crabeater seals, on the other hand, were affected by year, with individuals in 2002 having higher delta N-15 values. The median (with range) contribution of Antarctic krill to the diet of crabeater seals, as estimated using the Bayesian mixing model MixSIR, was 87.9% (81.2 to 94.8%). During 2002, krill biomass in the wAP was at one of its lowest levels during the last 2 decades, coinciding with a slight reduction in the importance of krill for the diet of the seals that year, which reached 84.5% (75.1 to 92.4%). Despite the relative plasticity observed in the diet of crabeater seals, it is unknown to what extent, and at what rate, crabeater seals might be able to switch to a more generalized diet, which might impact their fitness, given the ongoing environmental change along the wAP.