No abstract

Kaikōura, New Zealand, is one of the few places worldwide where sperm whales can be routinely found close to the coast. Although whales are present nearly all year round, no individuals are truly resident. In this study, we analyzed photo‐identification data collected over 27 years to investigate long‐term trends in inshore abundance. We contrasted two analytical approaches: Cormack‐Jolly‐Seber (CJS) mark‐recapture models, and the “Robust Design,” which can incorporate temporary emigration of individuals. CJS models for open populations showed a significant decline in the annual number of individual males, from 89, 95% CI [60, 133] in 1991, to 40, 95% CI [33–49] in 2017. The Robust Design models indicated that the trend was driven by a decline in abundance of whales using Kaikōura during summer, from 54, 95% CI [18, 156] in 1990, to 23, 95% CI [16, 33] in 2017. In contrast, there was no evidence for change in the numbers of whales using Kaikōura during winter. Incorporating temporary emigration had the most support, indicating Robust Design was more appropriate for estimating abundance. The results of Pradel recruitment models suggest that recruitment to the population using the study area during summer declined during the late 1990s/early 2000s, potentially explaining the decline in abundance.

Knowledge of the spatial distribution and habitat preferences of marine top‐predators is essential for monitoring their populations. The summertime abundance of male sperm whales (parāoa, Physeter macrocephalus) at the Kaikōura Canyon (New Zealand) has almost halved over the last three decades, possibly reflecting a shift in distribution away from this foraging ground. To better understand the seasonal drivers of habitat use by sperm whales, we recorded presence and absence locations of foraging whales during acoustic‐visual surveys, in conjunction with water‐column oceanographic sampling. Season‐specific generalized additive models (GAM) indicated that whale distribution was best predicted by seafloor depth, thermal stratification in the water‐column, and slope gradient and orientation. Habitat use differed between summer and winter, consistent with patterns in diving behavior and stable isotope ratios, suggesting seasonal fluctuations in prey targeted by sperm whales. Our results advance understanding of fine‐scale habitat use by sperm whales, with new insights into the ecological drivers of seasonal fluctuations in foraging distribution. Our study demonstrates the importance of quantifying water‐column processes in the study of habitat preferences by deep‐diving predators. We also identify thermal stratification as an oceanographic feature susceptible to climate change, which could contribute to the decline in sperm whale abundance at Kaikōura.