Investigation of eight Bunbury dolphin deaths

November 21, 2009

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Investigation of eight Bunbury dolphin deathsMurdoch University researchers with the South-West Marine Research Program (SWMRP*) are investigating multiple deaths of dolphins over the last two years within the inner waters around the city of Bunbury. These inner waters encompass the Leschenault Estuary and Inlet, the Inner and Outer Harbour, and the Collie and Brunswick Rivers.

The research findings have come to light through ongoing SWMRP-funded research focusing on the wild dolphins of the Bunbury area.

The initiative is a collaboration of community, government, research, and industry partners, including the Department of Environment and Conservation and Dolphin Discovery Centre. It supports a number of projects on the biology of the dolphins and the ecosystem that they inhabit. This research is coordinated by Dr Lars Bejder, Research Leader of the Murdoch University Cetacean Research Unit.

Research findings – inner water mortalities

PhD candidate Holly Smith conducted intensive year-round field observations of bottlenose dolphins in the Bunbury area from March 2007 until November 2009. This study used systematic transect sampling and photo-identification techniques to identify more than 200 individual dolphins that have been sighted three or more times between Peppermint Beach south of Bunbury and Binningup to the north of Bunbury.

Of this larger population of 200 plus dolphins, 16 have been identified as consistent users of the Leschenault Estuary and Inlet, the Inner and Outer Harbour, and the Collie and Brunswick Rivers.

These 16 dolphins are unique in that they range only within the inner waters around Bunbury. Other dolphins have been identified using the inner waters but they also range over a larger area, including open coastal areas outside of estuarine and harbour areas.

Of these 16 dolphins, eight remain alive as of November 2009. Six are confirmed dead with a further two adult female dolphins presumed dead based on knowledge of their sighting histories, ranging patterns, and their social network with other known dolphins. These two individuals were last observed in August 2008 and April 2009, respectively. The number of deaths documented over the last two years among dolphins using the Bunbury inner waters exceeds natural mortality rates of this species.

Carcasses of dolphins were recovered in 2008 (one in April) and four in 2009 (in January, May, August and November, respectively). The sixth confirmed dead dolphin was a young calf unable to survive on its own; it was last observed in September 2008. There appears to be no clear trend in the timing of the deaths.

Post-mortem analyses

The bodies of four dolphins recovered in the Leschenault area between 2008 and 2009 were taken to the Murdoch University School of Veterinary Medicine for post-mortem analyses. The fifth carcass, recovered in November 2009, was too decomposed to carry out a post-mortem.

Post-mortem analyses revealed no clear common cause of death. Three of four dolphins were emaciated (i.e. in poor body condition). Three of four individuals showed signs indicative of either single organ or multi-organ opportunistic bacterial infection, i.e. infection by naturally occurring pathogens following preceding immunosuppression. One dolphin had severe fungal pneumonia as well as opportunistic bacterial invaders and one dolphin had skin lesions. Two juvenile dolphins showed signs of heavy parasitism.

Post-mortem and histopathological analyses did not identify indications of the presence of a virus such as Cetacean Moribillivirus. However, identification of viral pathogens is difficult, and more specific molecular analyses of tissue samples would be required.

Contaminants analyses

Findings from the Swan River have recently raised the possibility that documented contaminants may be having an adverse effect on dolphins. Contaminant analyses of tissues from dolphins from Bunbury inner waters will be conducted shortly.

Comparison with Swan River mortalities

These mortalities show some similarities with the deaths of six dolphins in the Swan-Canning Estuary in 2009. The most obvious similarity is the environment these dolphins inhabited, i.e. both areas are estuarine ecosystems likely to have broadly similar environmental characteristics. Another commonality is post-mortem indications of opportunistic infection by naturally-occurring pathogens, presumably because of preceding immunosuppression.

Conclusion

As in the Swan River, where the last dolphin died on 25 October, the last of these mortalities occurred in November, and it has taken time to summarize the pattern of the mortalities, complete post-mortem and subsequent histopathological analyses, and consult with other pathologists and researchers elsewhere in Australia and internationally.

Further research and funding support is needed to address the factors underlying the mortality events in the Bunbury area. The mortality events demonstrate a clear need to better understand the health and status of marine mammal populations in estuarine and coastal ecosystems in south-western WA.

(Media can quote Dr. Lars Bejder, Murdoch University, on all material contained above. See also http://www.cffr.murdoch.edu.au/mucru/).

*The partners of the South West Marine Research Program consists of: Bemax Cable Sands, Dolphin Discovery Centre, Bunbury Port Authority, City of Bunbury, Cristal Global, Department of Environment and Conservation, Iluka, Millard Marine, Naturaliste Charters, South West Development Commission, WAPRES, Worsley Alumina.

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