Records of Australian humpback dolphins (Sousa sahulensis) from an offshore island group in Western Australia

Abstract

We report records of humpback dolphins at the Montebello Islands (20°26'S, 115°33′E) approximately 80 km from the mainland coast of Western Australia. This island group is a marine protected area (MPA), with approximately half of the area a sanctuary (no take) zone managed solely for nature conservation that excludes activities such as fishing and resource extraction, activities known to impact dolphins elsewhere. The habitat varies but includes fringing coral reef and shallow, sheltered sandy lagoons mostly < 10 m water depth and not exceeding 20 m. Several opportunistic sightings and one targeted five-day survey have resulted in 28 dolphins being catalogued in the MPA through photo-identification of natural marks on the dorsal fin and are considered to be recognisable through either distinctive or subtle marks. The species was initially reported in the MPA in June 2015 and was present in summer, autumn and winter of 2017. Six calves have been observed in the MPA, with one a neonate. Given the recent listing of the species as Vulnerable on the IUCN Redlist, and increasing pressure on near-shore coastal habitat, recognising the presence of this species at offshore islands such as these, and understanding their status and connectivity is a priority for the conservation and management of the species in Western Australia.

Identifying priority habitat for conservation and management of Australian humpback dolphins within a marine protected area

“…Elsewhere in WA, humpback dolphins have been observed some 70 km from the mainland coast at the Montebello Islands Marine Park, but close to the shoreline and in shallow water (i.e. < 10 m 46 ). In the Northern Territory, humpback dolphins occur within 20 km of major tidal rivers, and as far as 50 km upstream 47 , and along the east coast of Queensland, they occur primarily in waters of < 15 m depth 48 – 50 .…”

Isotopic niche overlap between sympatric Australian snubfin and humpback dolphins

“…Humpback dolphins are known to use a wide diversity of habitats associated with coastal waters, including dredged channels, inshore reefs, seagrass flats, and mangroves (Parra, 2006 ; Parra & Cagnazzi, 2016 ). Moreover, humpback dolphins have been sighted more than 50 km from the mainland coast in shallow shelf waters (i.e., <30 m deep) and near offshore islands off Queensland and Western Australia (Corkeron et al, 1997 ; Hanf et al, 2016 ; Parra et al, 2004 ; Raudino et al, 2018 ). Such sightings indicate that this species may use a wider range of different habitats, including intertidal areas around offshore islands.…”

“…Humpback dolphins are known to use a wide diversity of habitats associated with coastal waters, including dredged channels, inshore reefs, seagrass flats, and mangroves (Parra, 2006;Parra & Cagnazzi, 2016). Moreover, humpback dolphins have been sighted Western Australia (Corkeron et al, 1997;Hanf et al, 2016;Parra et al, 2004;Raudino et al, 2018) for humpback dolphins) was also consumed by the other (Parra & Jedensjö, 2014).…”

Aerial abundance estimates for two sympatric dolphin species at a regional scale using distance sampling and density surface modeling

“…Survey effort was modified in those areas by increasing the number of transects and reducing the spacing between transects from 5 to 2.5 km (Figure 1), similar to Slooten et al (2004). Additional habitats where humpback dolphins had been sighted incidentally during aerial surveys for dugong (Hodgson, 2007) or during boat-based surveys for humpback dolphins, were added to the flight plan in 2016 and 2017 and included the Montebello Islands (Raudino et al, 2018b) and Exmouth Gulf (Hunt et al, 2017; Figures 1, 3). In the latter region (19,943 km 2 ), transect lines ran east-west rather than north-south to align with expected gradients in dolphin densities and reduce variance in encounter rates (Buckland et al, 2004;Thomas et al, 2010; Figures 1, 3).…”

“…Limited gene flow detected between survey locations in Western Australia confirms that humpback dolphins are present as small, fragmented populations (Brown et al, 2014(Brown et al, , 2017; Figure 1). Further, preliminary evidence of populations occurring in offshore areas associated with islands (e.g., Montebello Islands) indicates that the extent of the species' distribution has yet to be fully mapped and it is not yet understood whether there is genetic connectivity between these dolphins and those using nearshore coastal waters of the Pilbara (Raudino et al, 2018b). Overall, their low density, uneven distribution (Hanf et al, 2022), and limited genetic connectivity across their range (Brown et al, 2014(Brown et al, , 2017Parra et al, 2018) make humpback dolphins particularly vulnerable to extinction (Parra et al, 2017).…”