Distribution, group characteristics and movements of the critically endangered Maui's dolphin Cephalorhynchus hectori maui

Abstract

Maui's dolphin Cephalorhynchus hectori maui is one of the most endangered subspecies of mammals, and yet its ecology is poorly known, partly because of the difficulty in following individuals throughout their range and over time. Here we combined group sightings (n = 45) from 23 boat surveys and genotype recaptures from biopsy samples (n = 73, 20 microsatellite loci) collected over 2 summers (2010 and 2011) to investigate along-shore distribution, group characteristics and individual movements of Maui's dolphins. We found a clumped distribution extending along 139 km of coastline, with the highest density of dolphins between Manukau Harbour and south of Port Waikato, New Zealand. As recapture events (n = 32) revealed movements throughout most of their range, we suggest that the clumped distribution is driven by patchy food resources and/or social factors rather than by site fidelity. Mean group size was 4.7 (SD = 3.0), with several large aggregations (≥8 dolphins) containing a higher proportion of calves than smaller groups and thus probably acting as nursery groups. Group composition by sex was different in large and small groups, with more adult females in large groups. The majority of small groups contained individuals of both sexes, which contrasts with the pattern of sex segregation described in the South Island, New Zealand. A conservative estimate indicates that the mean along-shore range for Maui's dolphins is at least 35.5 km, suggesting similarity to Hector's dolphins. However, some aspects of Maui's dolphin ecology as described here might have been affected by the recent history of population decline and low abundance.