Distribution and Population Demographics of Common Dolphins (Delphinus delphis) in the Gulf St. Vincent, South Australia

Within Australian waters, short-beaked common dolphins (Delphinus delphis) are exposed to a variety of human-induced impacts, including aquaculture and fisheries. Nonetheless, the occurrence and distribution of common dolphins within these waters remains unknown. Data detailed herein represent the first report of the occurrence and distribution of common dolphins from Australian waters. The density and relative abundance of common dolphins within Gulf St. Vincent (GSV), South Australia, was examined between September 2005 and May 2008 using systematic boat surveys. During 1,850 km of survey effort, a total of 108 independent groups, involving 564 common dolphins, were observed. Group size ranged from 2 to 21 individuals (mean = 5.26, SD = 3.687), with immature dolphins found in larger group sizes. Adults were the most frequent age class observed in this population (60.3%, n = 340), with neonates and calves observed most frequently between December and April. Sighting frequency was 3 groups/100 km² travelled, with an encounter rate of 16 common dolphins/100 km². The western longitude and southern latitude sections of GSV were used most frequently by this species, with most groups recorded in water depths of 35 to 40 m (mean = 37.2 m, SD = 1.4), and in areas 21 to 31 km from land (mean = 27.4 km, SD = 2.6). Common dolphin density was estimated to be 0.5 dolphins/100 km², with a population estimate of 1,957 dolphins within their preferred habitat (waters deeper than 14 m). Results suggest the GSV is important for this species and that common dolphins use these waters as a nursery area.