Aerial Surveys for Coastal Dolphins: Abundance of Hector's Dolphins Off the South Island West Coast, New Zealand

Abstract

Line‐transect surveys of abundance depend critically on the estimation of detection probability, which includes corrections for availability and visibility. In this aerial line‐transect survey for Hector's dolphin (Cephalorhynchus hectori) we recorded dive times from a helicopter to estimate the proportion of time that dolphins could be seen at or near the water surface and were therefore “available” to be counted. The proportion of available sightings that were counted on transect lines was estimated by comparing sightings by two independent teams of two observers in the fixed‐wing plane. The survey covered the area between Farewell Spit and Milford Sound, off the west coast of the South Island of New Zealand. Survey effort was stratified according to existing distribution data. A total of 142 separate sightings was made on 1,355 km of trackline. Average availability for fifty Hector's dolphin groups observed from the helicopter (161 dive/surface cycles) was 46.3% (CV = 4.2%). Data from the two independent observer teams indicated that 96.2% (CV = 2.3%) of the dolphin groups that were at the surface and on the trackline were seen. The abundance estimate for the South Island west coast, corrected for visibility and availability, was 5,388 Hector's dolphins (CV = 20.6%). The total population estimate for South Island Hector's dolphins is 7,270 (CV = 16.2%).

Offshore distribution of Hector's dolphins at Banks Peninsula, New Zealand: Is the Banks Peninsula Marine Mammal sanctuary large enough?

“…However, the west coast of the South Island is a stronghold for Hector's dolphins , and lacks similar shelter. A more dispersed distribution in winter is also observed in Hector's dolphin populations off the west coast of the North Island and the west and south coasts of the South Island (Bejder et al 1999;Rayment et al 2003;Slooten et al 2004).…”

“…These data were used for training only, and are not analysed here. For more information on survey methods and design, see Dawson et al (2004) and Slooten et al (2004).…”

“…Placement of transect lines followed design principles set out in Dawson et al (2004) and Slooten et al (2004). Briefly, the Banks Peninsula Marine Mammal Sanctuary was divided into four sections of relatively straight coastline, and a straight baseline was drawn along each section of the coast.…”

Conservation management in the face of uncertainty: effectiveness of four options for managing Hector’s dolphin bycatch

“…A similar method of back-calculation is described in Smith & Polachek (1979), Barlow & Hanan (1995) and Martien et al (1999). The distribution for population size was based on the most recent population estimate for each area (see Slooten et al 2004Slooten et al , 2006a.…”

“…Small populations are found around the South Island (7270 individuals, CV 0.16; Slooten et al 2004) and off the North Island west coast (111 individuals, CV 0.44; Slooten et al 2006a). Since the early 1970s, Hector's dolphins have been caught in commercial gillnet fisheries (Taylor 1992, Baird & Bradford 2000.…”