Abstract
The submarine canyon of Kaikōura, New Zealand, is a foraging ground for male sperm whales (Physeter macrocephalus). The abundance of whales feeding in this area during summer has almost halved over the last three decades, for reasons that are unknown. The decline may reflect a shift in distribution away from the area, possibly caused by underlying oceanographic or ecological changes. It is therefore important to understand what sustains the whales’ diet and what environmental factors drive their distribution. First, I examined the whales’ food web using stable isotope analyses. Isotope ratios of carbon and nitrogen were measured in samples of sloughed whale skin, potential prey and primary producers. Generalised additive models suggested that occasional visitors to Kaikōura had more diverse and lower isotope ratios than more frequent visitors (by c. -1‰ δ¹³C and -2‰ δ¹⁵N, n = 90), likely reflecting a range of foraging areas further south and/or offshore. The ultimate contribution of pelagic phytoplankton vs coastal macroalgae to the whales’ food web could not be determined precisely, but there was strong support for it being sustained mostly by pelagic production. Whales appeared to feed on a mixture of squid and demersal fish, and differences in the whales’ isotope ratios between summer and winter suggested seasonal variability in diet. Summer food resources were likely to comprise a high proportion of squid. Surveys of foraging whales (n = 334) in conjunction with in situ oceanographic sampling (n = 486) were carried out over three years. I used species-distribution models to investigate the topographic and oceanographic characteristics that determine good foraging habitat. Physical factors that may contribute to aggregating prey seemed particularly important; these included strong thermal stratification in the water column, steep slopes, and slope orientations likely to concentrate food resources through the interaction with local currents. Habitat preferences differed between summer and winter, consistent with stable isotope ratios and previously recorded patterns in diving behaviour, suggesting seasonal fluctuations in targeted prey. An evaluation of the overlap between foraging habitat and the recently established Hikurangi Marine Reserve suggested little meaningful protection for sperm whales from the potential impacts of fishing. The importance of thermal stratification for foraging habitat in summer suggested that whales may be susceptible to changes in oceanographic conditions of the ecosystem. I investigated the long-term variability in whale abundance during summer in relation to environmental conditions, using abundance estimates from 1990–2017 and remotely sensed oceanographic data. Linear models suggested that warmer sea temperatures in winter, and lower inputs of Subantarctic water to the south of Kaikōura in summer, were correlated with fewer whales foraging in the area, explaining 59% of inter-annual variability. These correlations may be mediated by indirect effects on prey availability, potentially contributing to the decline in whale abundance. Precautionary management of anthropogenic activities affecting the whales and their habitat is necessary to increase the resilience of the population to climate-driven changes in the ecosystem. This is important to ensure the persistence of sperm whales at Kaikōura and protect the canyon ecosystem that supports them.