The conservation biology of tropical inshore dolphins in north-western Australian waters

Concerns exist over the vulnerability of tropical inshore dolphin populations in waters off northern Australia to anthropogenic impacts, yet a lack of data precludes assessment of their conservation status and the management of threats. Three species occur in shallow, nearshore waters: the Australian snubfin (Orcaella heinsohni), Australian humpback (Sousa sahulensis) and Indo-Pacific bottlenose dolphin (Tursiops aduncus). In this thesis, I provide: i) quantitative data on the abundance and site fidelity of all three species at five sites in north-western Australia; (ii) an examination of population genetic structure in snubfin and humpback dolphins; (iii) a sex-specific investigation of the social structure of one population of snubfin dolphins; and, (iv) an analysis of sex- and geographic-differences in dorsal fin features of humpback dolphins. The abundance of each species was highly variable across the five c. 130 km2 study sites surveyed. While the estimated abundance of most species was ≤ 60 individuals, and fewer than 20 humpback dolphins were identified at each site in any one sampling period, larger estimates of c. 130 snubfin and c. 160 bottlenose dolphins were obtained at two different sites. Several local populations showed evidence of site fidelity, particularly snubfin dolphins. Mitochondrial and microsatellite data revealed significant genetic differentiation of local populations separated by geographic distances of >200 km, suggesting that snubfin and humpback dolphins may exist as metapopulations of small, predominantly isolated population fragments, and should be managed accordingly. Additionally, genetic data revealed the first documented case of hybridisation between a snubfin and a humpback dolphin. I documented pronounced sex-differences in individual sociability within a small population of snubfin dolphins: males formed stronger, longer-lasting associations and were far more gregarious than females. Associations were not correlated to genetic relatedness for either sex. Based on a quantitative analysis of dorsal fin images of a sample of humpback dolphins of known sex from north-western and north-eastern Australia, I revealed that the sex of adult individuals could be distinguished with a high level of accuracy (97%) based on dorsal fin features. Additionally, significant differences in dorsal fin colouration between the two regions suggested some level of population structure. Overall, these results extend the geographic scope of quantitative population data on Australia’s tropical inshore dolphins into the western third of their distribution, and provide valuable data to inform their conservation and management both within this region and throughout northern Australia