Stable isotope analyses reveal seasonal and inter-individual variation in the foraging ecology of sperm whales

Studying inter-individual variation in foraging by top predators is key for understanding the ecology of their populations, while knowledge of seasonal variability in foraging helps explain temporal changes in habitat use and ecological role. We investigated the inter-individual and seasonal differences in stable isotope ratios of sperm whales Physeter macrocephalus in the temperate foraging ground of the Kaikoura Canyon, New Zealand. Isotope ratios of carbon and nitrogen were measured in 107 samples of sloughed skin from 37 individual males with a wide range of residency patterns and body lengths, sampled over 4 summers and 3 winters. Variability in individual isotope ratios was analysed with generalised additive mixed models. The whales' residency patterns, but not body size, accounted for most heterogeneity of delta C-13 and delta N-15. Specifically, whales that visited Kaikoura occasionally had more diverse and lower isotope ratios than more frequent visitors (by ca. -1% delta C-13 and -2% delta N-15), likely reflecting a range of foraging habitats further offshore and/or south of Kaikoura Canyon. We suggest that these patterns reflect differences in large-scale foraging patterns within the population. In addition, whales sampled in winter had significantly lower values of delta C-13 than whales sampled in summer (by ca. -0.5%), indicating seasonal differences in the use of food resources. Our results provide new insights into foraging patterns of sperm whales, and highlight the value of accounting for individual differences in the ecology of top predators.