How many dolphins live near a coastal development?

Coastal developments continue to increase in Western Australia (WA) despite limited information on environmental consequences for coastal dolphins. As an example, a gas processing facility was constructed near the town of Onslow in the Pilbara region in 2012-2015. Boat-based surveys to estimate abundance, density and movement patterns of two species of coastal dolphins in waters around the Onslow development site and Thevenard Island began in 2015. Employing POPAN mark-recapture models, the Indo-Pacific bottlenose dolphin (Tursiops aduncus) total abundance estimate corrected for the unmarked proportion of the population was 79 SE ± 24 (CI 43-148) for Onslow. There were insufficient resightings of Australian humpback dolphins (Sousa sahulensis) to model abundance accurately. The densities were similar between both sites for each species; 0.59 bottlenose dolphins per km2in coastal Onslow waters versus 0.83 bottlenose dolphins per km2around Thevenard Island and 0.36 humpback dolphins per km2at Onslow and 0.38 humpback dolphins per km2around Thevenard Island. The density of humpback dolphins, although low, is comparable to estimates across their range. Movements of the two species between the island and coastal waters of the mainland was not detected. The Australian snubfin dolphin was not sighted during these surveys. The study confirms that Indo-Pacific bottlenose and Australian humpback dolphins use the waters near Onslow, however the low re-capture rates suggest that their home ranges are larger than, and only partially overlapping, the 128 km2 area. More intense sampling effort would be required to precisely estimate abundance for humpback dolphins.