Long‐term decline in abundance of male sperm whales visiting Kaikōura, New Zealand

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.

A piece of the puzzle: analyses of recent strandings and historical records reveal new genetic and ecological insights on New Zealand sperm whales

“…Furthermore, season did not predict the latitudinal location of mass or single strandings. This indicates that sperm whales are present in New Zealand waters year-round, confirming what is known about males off Kaikōura (Somerford et al 2022). There was no record of a male-dominated mass stranding in summer; however, 3 of the 4 mass strandings recorded in summer were of unknown sex, thus making it hard to draw conclusions.…”

“…Our analysis of sperm whale strandings in New Zealand highlights a knowledge gap, as most New Zealand based research on sperm whales has focussed on males off the Kaikōura Canyon (Jaquet et al 2000b, Sagnol et al 2014, Guerra et al 2020b, Somerford et al 2022). However, sperm whales strand over much of the nation's coastline, and a notable proportion of those around the North Island are female (as we showed here, 32% of recorded mass strandings with known type were female-dominated, and 29% of single strandings with known sex were females).…”

Fine‐scale habitat use of foraging sperm whales is driven by seafloor topography and water column structure

“…SDMs that use regression techniques are most accurate when presence and absence data have the same weight and there are at least as many absences as presence points (Barbet‐Massin et al, 2012). This was achieved differently for each season, due to whales being more abundant in winter than summer (Somerford et al, in press). For the summer models, one pseudo‐absence was generated per absence area, resulting in a slightly higher number of pseudo‐absences than presences (overall, a ratio of 1.2:1).…”

“…The population is of socio‐economic importance, as whale watching is the main driver of the local economy of Kaikōura (Curtin, 2003). The number of whales visiting the area during summer has declined over the last three decades, from 54 (95% CI [18, 156]) in 1990, to 23 (95% CI [16, 33]) in 2017 (Somerford et al, in press; van der Linde, 2009). While the reasons for the decline are unknown, this trend may reflect a shift in the whales' distribution, potentially driven by ecological or oceanographic changes to their habitat.…”

Unraveling site fidelity and residency patterns of sperm whales in the insular oceanic waters of Macaronesia

“…Sperm whales, like other mammalian species (e.g., deers, Bose et al, 2017;elephants, Archie et al, 2006), demonstrate female philopatry and male dispersal due to the higher dependency of females on local resources (Greenwood, 1980). Male sperm whales show limited site fidelity to their feeding grounds, with few possible resident individuals (Jaquet et al, 2000;Lettevall et al, 2002;Rødland & Bjørge, 2015;Somerford et al, 2021). On the other hand, females exhibit site fidelity across years in several locations (e.g., Caribbean, Gero et al, 2014;Mediterranean Sea, Drouot-Dulau & Gannier, 2007), which may lead to genetic differentiation of specific populations (Engelhaupt et al, 2009).…”