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

Abstract

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 km 2 , 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.

Socio‐genetic structure of short‐beaked common dolphins in southern Australia

“…Common dolphin school sizes in GSV were also relatively small, mostly containing ≤10 individuals (Filby et al ., ). This is in contrast to what is typically reported for common dolphins, a species that is considered to aggregate in large schools, especially when inhabiting neritic and pelagic waters.…”

“…Within southern Australian waters, common dolphins are frequently sighted within gulfs and embayments (Mason, 2007;Filby et al, 2010). For example, common dolphins in Gulf St Vincent (GSV), South Australia, utilize the shallower, northern parts of the gulf (Filby et al, 2010).…”

“…Within southern Australian waters, common dolphins are frequently sighted within gulfs and embayments (Mason, 2007;Filby et al, 2010). For example, common dolphins in Gulf St Vincent (GSV), South Australia, utilize the shallower, northern parts of the gulf (Filby et al, 2010). This area has been suggested as an important nursery area for the species as the sheltered gulf waters can provide increased protection from predators (Filby et al, 2010).…”

“…Furthermore, to optimize food intake, specifically on large schools of pelagic fish, common dolphins may benefit most from cooperative foraging in larger schools (Norris & Dohl, 1980;Wells, Irvine & Scott, 1980). Interestingly, in southern Australia, common dolphins have typically been found to associate in small schools (Filby et al, 2010).…”

Long-term responses of Burrunan dolphins ( Tursiops australis ) to swim-with dolphin tourism in Port Phillip Bay, Victoria, Australia: A population at risk

“…These ''risk takers'' are more likely to approach tour vessels, possibly explaining the increase in approach responses to tour vessels from P1 to P2. The increase in approach responses may also be a consequence of bow-riding behaviour, with many delphinid species exhibiting responsive movements towards vessels in order to bow-ride (Filby et al, 2010). However, it is important to recognise that just because these dolphins approach the tour vessels; it does not imply no detriment, long-term consequence (Martinez et al, 2011).…”