Australian snubfin dolphin

April 26, 2011, 7:21 am
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Australian snubfin dolphin. © 2008 Deb Thiele

The Australian snubfin dolphin (Orcaella heinsohni ) is a marine mammal in the family of oceanic dolphins, within of the order of cetaceans. This dolphin was only recently (2005) recognized as a separate species from the Irrawaddy dolphin (Orcaella brevirostris). Like the Irrawaddy, this species has a rounded head with no beak and a flexible neck, causing visible creases behind the head. Although most closely related to the Orca, the Irrawaddy dolphin is similar in body form to the Beluga whale, but darker in colour, with a pale to dark grey back and a light underside. The dorsal fin is small, triangular and rounded, and the flippers are long and broad.

caption Australian Snubfin Dolphin (Orcaella heinsohni). Source: Isabel Beasley

 Conservation Status: 

Scientific Classification

Kingdom: Animalia (Animals)
Phylum:--- Chordata
Class:------ Mammalia (Mammals)
Order:-------- Cetacea (Cetaceans)
Family:-------- Delphinidae   
Genus:----------Orcaella
Species:----------- Orcaella heinsohni (Beasley, Robertson & Arnold, 2005)

Physical Description

Like the Irrawaddy, this species has a high, anteriorly convex forehead which overhangs the mouth. It does not have a beak, and its U-shaped blowhole is to the left of the midline. Unlike the condition in most dolphin species, the blowhole opens toward the front. It resembles the finless porpoise, but unlike that species, has a small, triangular, and bluntly rounded dorsal fin (with a barely concave rear margin) set just behind the midback. The flippers are relatively large (about one-sixth as long as the body) and have a great breadth with a gently curved leading edge. The mouthline is straight, and there may be a visible neck crease. The neck is unusually flexible because only the first two cervical vertebrae are fused. The tail is also quite flexible. It has homodont, narrow, pointed, and peg-like teeth with slightly expanded crowns. The teeth are about 1 cm in length, and tooth counts are 17 to 20 (upper) and 15 to 18 (lower) in each quadrant. The skull is characterized by its globular shape, short rostrum, and broad facial region. The dolphin does not have a cardiac sphincter; the stomach is subdivided into compartments communicating through narrow orifices.

Lifespan

The lifespan of this species is approximately 25 years.

Behaviour

Orcaella spend most of their time feeding, sometimes spitting water while feeding, moreover, they can expel water from the mouth for distances of up to 1.5 metres, apparently to herd fish.

Distribution

Australian snubfin dolphins dwell in shalllow coastal waters the tropics and subtropics of Australia, and potentiallly certain parts of New Guinea (Beasley et al. 2005). In Australia, they occur from Broome, Western Australia, north and east to the Brisbane River, Queensland. The range along the northern Australian coast and New Guinea is poorly documented. (Parra et al. 2002)

Habitat

The IUCN states that the: Australian snubfin dolphins are chiefly found in coastal, shallow waters and are most common in brackish estuaries. They have been observed in the same areas as Indo-Pacific humpback dolphins, which sometimes chase them aggressively. The Austalian snubfin occur most often near river and creek mouths, generally in waters less than 10 metres deep (with a preference in some areas for very shallow waters, less than two metres deep) (Parra et al. 2006b)

Feeding Habits

Australian snubfin dolphins seem to be generalist feeders, consuming a gamut of fish species (including anchovies, sardines, eels, halibut, breams, grunters and other estuarine species). They also prey upon cephalopods (squid, cuttlefish, and octopus), and crustaceans (shrimps and isopods, although the latter may be consumed incidentally). (IUCN: 2008)

Conservation Status

IUCN: "Although the species has been surveyed in only two areas (Cleveland Bay and Gulf of Carpentaria) which, together, comprise less than 20% of the species’ range, the results of those surveys, as well as the preliminary results from a reconnaissance survey of a portion of the Kimberly coast in 2006, indicate that the number of mature individuals is well below 10,000. It is assumed that the surveyed areas are broadly representative of the species’ density across its range. The population may be declining due to bycatch in commercial fishing nets (e.g. gillnets) and anti-shark nets.

 "The available evidence supports the reasoning that there are fewer than 10,000 mature individuals and therefore the species meets the C criterion for Vulnerable in terms of population size. However, data are lacking to substantiate a continuing decline (C2). Similarly, no studies of population structure have been carried out so it is uncertain if either of the C2a subcriteria is met (i.e., whether no subpopulation is larger than 1000 mature, or all mature individuals are in a single subpopulation). Although the species could be listed as Data Deficient, Near Threatened is more appropriate given its limited range, low densities in surveyed areas, and its continuing vulnerability to bycatch. Rigorous, more extensive surveys are needed to support a reassessment of the species; it may then be found to qualify for listing as Vulnerable or possibly even Endangered."

Threats

IUCN "The nearshore occurrence of this species makes it particularly vulnerable to human activities. However, most of its range in northern Australia and New Guinea has not been severely degraded. Substantial numbers of snubfin dolphins have been killed in anti-shark nets set to protect bathers (Paterson 1990). For example, in the Townsville region between 1968-1976, 15 of 24 dolphins known to have been killed were this species (Heinsohn 1979). The mortality rate of snubfin dolphins in anti-shark nets along the Queensland coast declined to an estimated 1.3/year between 1992-1995, coincident with the replacement of most anti-shark nets with baited drumlines (Gribble et al. 1998). In addition to the mortality in anti-shark nets, these dolphins die in inshore gillnets set across creeks, rivers and shallow estuaries primarily for barramundi (Lates calcarifer) and threadfin salmon (Polynemus sheridani) and (Eleutheronema tetradactylum) (Anderson 1995; Hale 1997)."

Further Reading

  • Orcaella heinsohni Beasley, Robertson & Arnold, 2005, Encyclopedia of Life
  • Reeves, R.R., Jefferson, T.A., Karczmarski, L., Laidre, K., O’Corry-Crowe, G., Rojas-Bracho, L., Secchi, E.R., Slooten, E., Smith, B.D., Wang, J.Y. & Zhou, K. 2008. Orcaella heinsohni. In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.4. . Downloaded on 22 February 2011.
  • Beasley, Isabel, Kelly M. Robertson, and Peter Arnold. 2005. Description of a new dolphin, the Australian snubfin dolphin Orcaella heinsohni sp. n. (Cetacea: Delphinidae). Marine Mammal Science, vol. 21, no. 3. 356-400
  • IUCN (2008) Cetacean update of the 2008 IUCN Red List of Threatened Species.
  • Anderson, G. R. V. 1995. Australia: progress report on cetacean research, May 1993-April 1994. Reports of the International Whaling Commission 45: 225-231.
  • Beasley, I., Arnold, P. and Heinsohn, G. 2002. Geographical variation in skull morphology of the Irrawaddy dolphin, Orcaella brevirostris (Owen in Gray, 1866). Raffles Bulletin of Zoology 10: 15-34.
  • Freeland, W. J. and Bayliss, P. 1989. The Irrawaddy dolphin (Orcaella brevirostris) in coastal waters of the Northern Territory, Australia: distribution, abundance and seasonal changes. Mammalia 53: 49-57.
  • Gribble, N. A., Mcpherson, G. and Lane, B. 1998. Effect of the Queensland Shark Control Program on non target species: whale, dugong, turtle and dolphin: a review. Marine and Freshwater Research 49: 645-651.
  • Hale, P. 1997. Conservation of inshore dolphins in Australia. Asian Marine Biology 14: 83-92.
  • Heinsohn, G. E. 1979. Biology of small cetaceans in north Queensland waters. Great Barrier Reef Marine Park Authority, Townsville, Australia. Parra, G., Azuma, C., Preen, A. R., Corkeron, P. J. and Marsh, H. 2002. Distribution of Irrawaddy dolphins, Orcaella brevirostris, in Australian waters. Raffles Bulletin of Zoology 10: 141-154.
  • Parra, G., Corkeron, P. J. and Marsh, H. 2006. Population sizes, site fidelity and residence patterns of Australian snubfin and Indo-Pacific humpback dolphins: Implications for conservation. Biological Conservation 129: 167-180.
  • Parra, G. J. 2005. Behavioural ecology of Irrawaddy, Orcaella brevirostris (Owen in Gray, 1866), and Indo-Pacific humpback dolphins, Sousa chinensis (Osbeck, 1765), in northeast Queensland, Australia: A comparative study. Thesis, James Cook University.
  • Paterson, R. A. 1998. Irrawaddy dolphins Orcaella brevirostris (Owen in Gray) from southern Queensland. Memoirs of the Queensland Museum 42: 554.
  • Stacey, P. J. and Leatherwood, S. 1997. The Irrawaddy dolphin, Orcaella brevirostris: a summary of current knowledge and recommendations for conservation action. Asian Marine Biology 14: 195-214.
  • Perrin, W. (2010). Orcaella heinsohni Beasley, Robertson & Arnold, 2005. In: Perrin, W.F. World Cetacea Database. Accessed through: Perrin, W.F. World Cetacea Database
  • Reeder, DeeAnn M., Kristofer M. Helgen, and Don E. Wilson. 2007. Global Trends and Biases in New Mammal Species Discoveries. Occasional Papers, Museum of Texas Tech University, no. 269. 35
  • CITES (December, 2004)
  • Convention on Migratory Species (December, 2004)
  • Van Parijs, S.M., Parra, G.J. and Corkeron, P.J. (2000) Sounds produced by Australian Irrawaddy dolphins, Orcaella brevirostris. Journal of the Acoustical Society of America, 108 (4): 1938 - 1940.
  • Arnold, Peter W., and George E. Heinsohn. 1996. Phylogenetic status of the Irrawaddy Dolphin Orcaella brevirostris (Owen in Gray): a cladistic analysis. Memoires of the Queensland Museum, vol. 39, part 2. 141-204
  • Nowak, R.M. 1991. Walker's Mammals of the World. Fifth Edition. Volume II. The Johns Hopkins University Press, Baltimore.

 

Glossary

Citation

(2011). Australian snubfin dolphin. Retrieved from http://www.eoearth.org/view/article/51cbf0a37896bb431f6a279e

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