Orcaella heinsohni, Beasley, Robertson & Arnold, 2005

Russell A. Mittermeier & Don E. Wilson, 2014, Delphinidae, Handbook of the Mammals of the World – Volume 4 Sea Mammals, Barcelona: Lynx Edicions, pp. 410-526 : 493-494

8.

Australian Snubfin Dolphin

Orcaella heinsohni

French: Orcelle dAustralie / German: Australischer Stupsflossendelfin / Spanish: Delfin de Heinsohn

Taxonomy. Orcaella heinsohni Beasley, Robertson & Arnold, 2005 ,

Horseshoe Bay, Australia.

This recently described species was previously considered to be a population of O. brevirostris . Distinguishing differences in genetics and external and cranial morphology warrant its classification as a separate species. Monotypic.

Distribution. Coastal waters and river mouths on the Sahul Shelf of NW New Guinea (Cenderawasih Bay); S New Guinea including Gulf of Papua, extending S along N Australian coast from Broome (Western Australia State) to Brisbane River (Queensland State).

Descriptive notes. Total length reaches 270 cm (males) and 230 cm (females); weight 114-133 kg. Neonatal length is ¢.100 cm. The Australian Snubfin Dolphin has rounded head with no visible beak, flexible neck (usually with a visible neck crease), small dorsal fin (4-4% oftotal length) with a rounded tip situated just over halfway along the back, and broad (6-6% oftotal length), paddle-shaped flippers. It does not have dorsal groove like the Irrawaddy Dolphin ( O. brevirostris ), which runs from head along back to dorsal fin. Skin of the Australian Snubfin Dolphin has three-toned pigmentation. Dark-gray—to—brown cape covers dorsal area, flanks are paler gray or brown, and ventral body is pale gray to white. There are 11-22 pairs of teeth in upper jaw and 14-19 pairs in lowerjaw.

Habitat. Shallow, brackish, coastal waters and freshwater or estuarine systems, including sea grass beds at river mouths,likely due to their high productivity. In northern Australian waters, Australian Snubfin Dolphins prefer depths of 2:5—-18 m. The majority of sightings have been within 6 km of coastlines. The Australian Snubfin Dolphin and Irrawaddy Dolphin are distributed on the Sahul Shelf of Australia/Papua New Guinea and the Sunda Shelf of South-east Asia, respectively. Deep oceanic waters have separated the shelves, likely since before the Pleistocene ice ages, so the divergence of these two species has probably been driven by geographical segregation. The specific identity of dolphins in northern and north-western Australia was uncertain for a long time, but recent molecular analyses have confirmed that populations occurring in this region are indeed Australian Snubfin Dolphins.

Food and Feeding. The Australian Snubfin Dolphin, like the Irrawaddy Dolphin, is a generalist feeder and forages on a wide variety offish, cephalopods, and crustaceans found in coastal and estuarine waters throughoutits distribution. Prey includes both bottom-dwelling and pelagic species, indicating that Australian Snubfin Dolphins feed throughout the water column. The most commonly documented prey are cardinal fish ( Apogon sp. ), cuttlefish (Sepiasp.), squid ( Uroteuthis sp. ), and toothpony fish (Gaza sp.). Fish from the families Clupeidae , Engraulidae , Apogonidae , Chirocentridae , Anguilidae, Pomadasydae, Sillaginidae , Hemirhampidae, Terapontidae , and Leiognathidae also have been recorded in diets of Australian Snubfin Dolphins. Echolocation clicks are broadband and pulsed, generally above 22 kHz.

Breeding. Very little data is available on the life history of the Australian Snubfin Dolphin. Itis thought thatlife history parameters are very similar to those of the Irrawaddy Dolphin. Gestation likely lasts c.14 months, and young mature to adult size at 4-6 years. Longevity is ¢.30 years. Breeding seasonality of the Australian Snubfin Dolphin is unresolved.

Activity patterns. Little data is currently available on daily activity budgets or diving patterns of the Australian Snubfin Dolphin. It is a cryptic animal, and its surfacing behavioris usually subtle. Individuals surface with a low roll that onlylifts a small margin of their dorsal areas above the water’s surface, and they are generally shy around boats.

Movements, Home range and Social organization. Group sizes are small at 2-6 individuals, but groups of up to 14 individuals have been documented. Social groups have a fluid structure, that overlies more stable associations; individuals have strong, longlasting associations and casual acquaintances. Social vocalization whistles are 1-8 Hz. Interactions have been observed between Australian Snubfin Dolphins and Indo-Pacific Humpback Dolphins ( Sousa chinensis ). These usually involve dominantly aggressive or sexual behavior on the part of the humpback dolphins. There is some evidence for seasonal movements of the Australian Snubfin Dolphin along the northern Australian coast, and populations that use Cleveland Bay in Queensland are non-resident. Otherwise, there are scant data on migration, movement, and residency patterns for the Australian Snubfin Dolphin.

Status and Conservation. CITES Appendix I. Classified as Near Threatened on The IUCN Red List. Abundance of the Australian Snubfin Dolphin is estimated to be less than 10,000 mature individuals, and there are currently no data on population trends. Given its limited distribution and vulnerability to incidental catch, the Near Threatened listing is warranted. More extensive monitoring in the future may provide data that suggest a Vulnerable or Endangered status is more appropriate. Local abundance estimates currently exist for two regions. There are ¢.1000 individuals in the Gulf of Carpentaria in northern Australia, but surveys may have included Irrawaddy Dolphins. There are fewer than 100 Australian Snubfin Dolphins in Cleveland Bay, Queensland. As a near-shore species, the Australian Snubfin Dolphin is particularly vulnerable to human activities. Incidental catch in fisheries is its most serious threat. Gillnets that are set across creek and river mouths to catch barramundi (Lates calcarifer) and threadfin salmon (Polydactylus macrochir) routinely trap Australian Snubfin Dolphins. Starting in the 1960s, individuals were consistently caught in anti-shark nets meant to protect swimmers. These have been replaced with baited drum-lines, reducing take of Australian Snubfin Dolphins to 1-3 ind/year in 1992-1995. Gillnetting is banned or strictly regulated in some regions that became protected by the rezoning of the Great Barrier Reef in 2004, but coastal coverage is minimal and most areas outside the park are poorly regulated. Net-attendance rules and gear modifications have been introduced in northern Australia, but enforcement has been largely inadequate. More recently, habitat loss and degradation due to human population growth have become potential threats within the distribution of the Australian Snubfin Dolphin.

Bibliography. Beasley et al. (2005), Freeland & Bayliss (1989), Hale (1997), Jefferson et al. (2008), Palmer et al. (2011), Parra (2006), Parra & Jedensjo (2009), Parra, Azuma et al. (2002), Parra, Corkeron & Arnold (2011), Parra, Corkeron & Marsh (2006), Reeves, Jefferson et al. (2008b), Robertson & Arnold (2009), Van Parjis et al. (2000).

Figures

On following pages: 3. Peale’s Dolphin (Lagenorhynchus australis); 4. Hourglass Dolphin (Lagenorhynchus crucigen; 5. Pacific White-sided Dolphin (Lagenorhynchus obliquidens); 6. Dusky Dolphin (Lagenorhynchus obscurus); 7. Irrawaddy Dolphin (Orcaella brevirostris); 8. Australian Snubfin Dolphin (Orcaella heinsohni).

Distribution. Coastal waters and river mouths on the Sahul Shelf of NW New Guinea (Cenderawasih Bay); S New Guinea including Gulf of Papua, extending S along N Australian coast from Broome (Western Australia State) to Brisbane River (Queensland State).