Seasonal variation in the use of food resources by sperm whales in a submarine canyon

Abstract

Understanding the food and habitat requirements of apex predators relies on characterising the food web structure and carbon sources that support them. We examined the food web of sperm whales (parāoa, Physeter macrocephalus) foraging in the submarine canyon of Kaikōura (Aotearoa New Zealand) by measuring bulk stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) in sperm whale skin, potential prey and two candidate primary producers. To determine the underlying drivers of variability in bulk ratios, we measured δ13C and δ15N from individual amino acids (AAs) in a subset of whale samples. Mixing models and food web δ13C:δ15N correlations strongly suggest a food web sustained mostly by pelagic production, with a marginal contribution from coastal macroalgae. In contrast to the cephalopod-based diet recorded in most populations, the sperm whales' diet at Kaikōura appeared to be a mixture of squid and demersal fish, which may be related to the extremely high benthic productivity of the canyon. Combined with evidence from studies of habitat use, differences in the whales’ stable isotope values between summer and winter suggested seasonal variability in targeted prey, and ultimately different utilisation of organic carbon sources at the base of the food web. In summer, sperm whales appear to depend mostly on mesopelagic squid, with an increased contribution from demersal fish in winter, consistent with patterns in diving behaviour and spatial distribution. This seasonal shift in food resources suggests that submarine canyons offer a temporally dynamic range of foraging opportunities for this apex predator. Our results provide new insights into the structure of food webs supporting sperm whales, and advance our understanding of the seasonal dynamics in the use of food resources by apex predators in submarine canyons.