New abundance estimates suggest Doubtful Sound bottlenose dolphins are declining

Abstract

The bottlenose dolphins (Tursiops sp.) of Doubtful Sound, Fiordland, New Zealand, live at the southern limit of the species' worldwide range. They are subject to impacts from tourism and habitat modification. Photographic resightings of individually distinctive dolphins were gathered from 19 225 dorsal fin photographs taken during systematic surveys of the fiord. Field effort spanned nine seasons from summer 2004/2005 to summer 2006/2007, including 142 days on the water. Both capture-recapture and census approaches were taken in analysis. A total of 71 individuals were observed over the study period (54 adults, sub-adults and calves more than one year old and 17 newborn calves). Births were seasonal, occurring in the Austral summer and autumn, with calf survival (9 survivors of 17 births) lower than observed for this population in the past. Individual dolphins were resighted frequently within and across seasons, confirming the group is resident within the fiord year-round. Fifty-six bottlenose dolphins (CV=1.0%) were resident in Doubtful Sound in summer 2006/2007. Comparison with prior abundance estimates (for adults and sub-adults) indicates an apparent population decline of 34?39% over the past 12 years. This apparent decline suggests that precautionary management of human impacts is vital for the dolphins' long-term survival.