Abundance, population dynamics and social structure of bottlenose dolphins (Tursiops truncatus) in the Bay of Islands, New Zealand

Abstract

Obtaining estimates of abundance and understanding the demographic factors that cause change in abundance for wildlife populations is an important task for conservation biologists. This is particularly true when the animal under study is highly social, as the loss of individuals may directly impact the population’s social structure. The purpose of this study was to assess the current population status and social structure of bottlenose dolphins (Tursiops truncatus) in the Bay of Islands, New Zealand. Boatbased surveys were conducted to photo-identify individuals and collect demographic data from an independent research vessel between February and December 2012. Group size ranged from 3-28 dolphins (median = 25), groups containing calves were not significantly larger than those without calves, and the population was mainly composed of adults (95%). A total of 56 individuals were photo-identified, ten new to the Bay of Islands photo-identification catalogue in 2012, with an average of 5 sightings per individual. Robust design mark-recapture models were used to estimate abundance, apparent survival and temporary emigration rates from photo-identification data collected in 2009 and in 2012. Apparent survival was estimated at 0.63 (95% CI, 0.53-0.72) and abundance estimates fluctuated from a low of 24 (February 2012: 95% CI: 24-24) to a high of 94 (Demember 2009: 95% CI: 84-105). Temporary emigration patterns were Markovian, which is in contrast to prior research, where temporary emigration patterns were random. A low apparent survival rate indicated that a number of dolphins had permanently migrated from the bay during 2009-2012, suggesting a shift in habitat use during the study period. Small abundance estimates and Markovian emigration pattern indicate that small number of dolphins use the bay regularly, while many others only visit occasionally. Social analysis was carried out using the program SOCPROG to determine the strength and stability of associations between individuals and identify whether associations were sex-specific. Individuals were found to associate in a non-random manner without preference to sex. The population is characterised by two levels of associations: short-term acquaintances and long-term companionships. Long lasting associations were found across sexes. Fine-scale changes in association patterns have occurred, which is probably due to changes in the population size and individual residency patterns. The shift in habitat use and changes in association patterns suggests the bay is a less important part of the range for a number of dolphins. The observed changes in association patterns are most likely a consequence of the decline in population size; a number of social units have been fragmented due to a shift in habitat use.