Abstract
Hector’s dolphins (Cephalorhynchus hectori) are an endangered species endemic to New Zealand. Populations in some areas have been continuously studied for years, but this project is the first to extensively survey Otago’s Hector’s dolphins from Oamaru to Taieri Mouth, fully covering Statistical Fishing Area 24.
Between September 2010 and November 2011, alongshore transects were followed in small boats at a speed between 10-15 knots, approximately 400m from shore. A total of 43 field days were spent travelling 1082 nautical miles while at least two people were actively looking for Hector’s dolphins. When dolphins were seen, they were photographed using “random photography” to maximize the chances of capturing all individuals in a group and establish the mark rate of the population. Only high quality photos were used in analysis, and included photos of 20 recognisable individuals encountered while conducting fieldwork.
Distribution information was collected on all cetaceans encountered, including Hector’s dolphins and dusky dolphins (Lagenorhynchus obscurus). Sightings were plotted using ArcGIS version 10 software, showing preferred areas of use for each species. Hector’s dolphins were only found in two areas: (1) between Taiaroa Head and Cornish Head and (2) in the vicinity of Moeraki Bay. Photo-ID of recognisable Hector’s dolphins suggests no movement of individuals between the two areas. Dusky dolphins were found to be more dispersed along the coastline and in deeper water. Seasonally, Hector’s dolphins were more common inshore in the spring, summer and fall, while dusky dolphins were inshore in the spring and winter.
A Huggins closed capture model was run in the mark-recapture program MARK version 6.1 to estimate abundance of marked Hector’s dolphins between Taiaroa Head and Cornish Head. Scaling this estimate by the mark rate of 36% (CV=12%; CI=28-45%) for Category 1, 2, and 3 individuals, total abundance in this area was found to be 44 individuals (CV=44%, CI=19-100). A population estimate for Moeraki Bay was not possible due to the low number of marked individuals.
Data from this project will be used by the Department of Conservation to assist in developing appropriate conservation measures for Otago’s Hector’s dolphins. In particular, information can now be incorporated into the Hector’s dolphin threat management plan. Furthermore, these data will be useful in developing management for the Project Next Generation proposal by Port Otago, which would increase dredging of the Otago Harbour.