Assessing the influence of blubber stratification on the estimation of diet in phocid seals using the MixSIAR model

Blubber stratification can significantly impact diet estimation, as using the inner or outer blubber layer may lead to different results depending on the degree of stratification present. We aimed to quantitatively evaluate the effect of blubber stratification on diet estimation at the population and individual levels in two species of phocid seals: the leopard seal (Hydrurga leptonyx) and the crabeater seal (Lobodon carcinophaga), using the mixing model MixSIAR. In leopard seals, krill contributed most significantly to the diet based on the inner layer (68.2%), while Antarctic silverfish dominated the outer layer (59.2%). For crabeater seals, krill was the primary prey in both layers, but with different contributions (87.1% inner layer vs. 63.4% outer layer). Both species exhibited greater individual specialization according to the outer layer compared to the inner layer, suggesting lower individual variability in the summer than in winter, when it may increase due to reduced prey availability. These findings have implications for trophic studies in marine mammals, as the sampling method may lead to contrasting diet estimates, altering prey contribution percentages and their relative dominance, in particular in generalist species. In specialists with low stratification, however, the need to obtain the inner blubber for accurate results is reduced.