Collation and review of sightings and distribution of three coastal dolphin species in waters of the Northern Territory, Australia.

Abstract

On a global scale, the coastal waters of the Northern Territory (NT), Australia, are relatively undisturbed, but the pace and extent of coastal development is increasing. Three species of dolphin occur in these waters: the Australian snubfin Orcaella heinsohni, Indo-Pacific humpback Sousa chinensis and bottlenose Tursiops sp., but their distribution is poorly documented. To provide a broader distributional context and complement recent local-scale population studies (Palmer in press), we review the broader distribution of these coastal dolphins, via the collation of historic and contemporary data from sighting surveys, stranding and museum records, and a community sighting programme. Records spanned 1948 to 2010, with Sousa (44%) the most frequently recorded followed by Orcaella and Tursiops (both 28%). The compiled records indicate that the three species are widely distributed along the NT coast but with some apparent differences in habitat use. All species were recorded within 20 km of a major tidal river; but fewer than 3% of Tursiops records were from within tidal rivers, whereas nearly a quarter of Orcaella and Sousa records were as far as 20 to 50 km upstream. Differences in environmental settings between Orcaella and Sousa were less pronounced, but a lower proportion of Orcaella were recorded within 20 km of a river mouth. There are probable but unquantifiable biases in the record sources, but most records of dolphins were from estuaries, tidal rivers and coastal areas within 20 km of river mouths, and these sites probably represent important habitat for these species. The NT’s remote and relatively pristine waters likely hold significant subpopulations of all three species. The information provided here should aid future research efforts, however; further information on the dolphins’ population size, trend and structure are needed to resolve their conservation status at state and national jurisdictions, inform environmental impact assessments and species management.