Population demographics, social structure and interspecific associations of free-ranging long-finned pilot whales (Globicephala melas) in New Zealand.

Abstract

Despite frequently stranding on New Zealand’s beaches, there remains a paucity of information about free-ranging long-finned pilot whales (Globicephala melas edwardii) in these waters. The purpose of this study was to assess the demographics, social structure and interspecific associations of pilot whales frequenting eastern New Zealand waters. Photo-identification images and associated demographic data of pilot whale groups were collected opportunistically from tour operators and research vessels from the Bay of Islands to Kaikoura between January 2003 and July 2019. Group size ranged from 5 – 250 whales (median = 50, IQ = 30 – 80), with neonates and/or calves present in the majority of encounters (79%, n = 64). Pilot whales were most frequently encountered in mixed-species groups (79% n = 64), primarily accompanied by oceanic bottlenose dolphins (Tursiops truncatus). A total of 145 individuals were photo-identified, establishing the New Zealand Long-Finned Pilot Whale Identification Catalogue. The mark rate (i.e. proportion of individuals with long-term or permanent dorsal fin notches and/or nicks; 13.4%) and the re-sight rate (31%) for the sampled population were low, with most (82.2%) re-sights occurring at one study location, the Bay of Islands. Patterns of pilot whale occurrence suggest some degree of seasonal site fidelity, possibly influenced by prey availability or the peak calving season. The low re-sight rate suggests either a large population, high transience and/or large individual home ranges. Social structure analysis was carried out using the program SOCPROG to determine the strength and temporal stability of associations between pilot whales from northeastern New Zealand. Individuals associated randomly and there was no evidence to suggest the existence of preferred long- or short-term dyadic associations, however, there were differences in individual gregariousness. Some strong dyadic associations were evident, with hierarchical cluster and social network analyses supporting the division of the study population into multiple social clusters of between three and eight individuals. Dyadic associations were irregular, with individuals disassociating on two different time scales. This indicates that pilot whales may have a hierarchical society, where small social clusters form groups of casual acquaintances, as seen in populations elsewhere. A similar analysis of social structure investigated the possibility of an interspecific social network based on dyadic associations between oceanic bottlenose dolphins iv and pilot whales encountered off the Bay of Islands. There was no evidence of temporally stable interspecific associations, most likely due to the small sample size. However, repeat associations between individuals of both species on variable temporal scales suggests that there may be an underlying pattern worthy of future investigation. This study revealed the value of opportunistic datasets in understanding the demographics and social behaviours of a species that remains poorly-studied in New Zealand and the southern hemisphere in general. Considering that our knowledge of pilot whales in New Zealand waters is largely based on studies of stranded individuals, this research has revealed important information about the lives of freeranging pilot whales.