The deaths of six dolphins in the Swan Canning Riverpark last year reflect the stressful nature of the environment that they inhabit, according to Murdoch University’s scientists in their technical report to the Swan River Trust.
The 2009 deaths represented a marked increase in mortalities from previously recorded numbers, and occurred in two clusters — three in June 2009 and three in September-October 2009.
Co-author Dr Carly Holyoake, also Murdoch’s Marine Mammal Health Project Coordinator, said post-mortem examinations of four of the dolphins by Murdoch’s multidisciplinary team indicated several factors combined to result in the deaths of the dolphins.
"One dolphin was found to have died following complications associated with fishing line entanglement," Dr Holyoake explained.
"Another dolphin had a fungal infection of the brain and two dolphins died as the result of severe skin lesions initially caused by a virus called cetacean poxvirus (tattoo skin disease)."
Dr Holyoake said the dolphins affected by the virus were the most intriguing.
"It’s the first time that dolphin poxvirus has been confirmed in Western Australia," she said.
"This virus, which affects only dolphins and porpoises, usually only causes mild skin lesions in infected dolphins. It is highly unusual that the skin lesions observed in the two dolphins progressed to such a severe state.
"We suspect that the severity of the skin lesions observed is a result of the dolphins being compromised by a combination of many factors associated with living in an estuarine environment."
The factors likely to have contributed to the dolphin deaths include:
- Environmental factors: eg. salinity and temperature fluctuations
- Infectious disease (eg. poxvirus)
- Human stressors (eg. fishing line entanglements, noise, boat traffic,).
- An overall "stressful" environment combining all of the above, and also changing landscape – with increasing pressures associated with an expanding population.
Director of Murdoch University’s Centre for Fish and Fisheries Research, Professor Neil Loneragan, said these contributing factors are likely to get worse, if no action is taken, given the projected human population growth forecasts for Perth and also for other South West cities, including Mandurah and Bunbury.
"As a whole, community, state and local government, river users, commercial and recreational fishers need to take collective responsibility to help clean up the river," he said.
"While the situation is complex and not fully understood, there are some immediate and tangible actions that can be done to help the situation, for example using biodegradable fishing gear and not discarding fishing gear, reducing run-off and upgrading sewerage systems to help lessen influx of nutrients and contaminants."
Compared to ocean populations of dolphins near the state’s shores, the technical report said the estuarine dolphins experienced greater levels of stress from environmental fluctuations and human activities.
The Murdoch University investigation was conducted by a multidisciplinary team including animal biologist Dr Hugh Finn, veterinary pathologists Nahiid Stephens and Padraig Duignan (University of Melbourne), members of the Murdoch University Centre for Fish and Fisheries Research, Murdoch University Cetacean Research Unit (MUCRU), including research leader Lars Bejder, and Marine Mammal Health Project Coordinator and veterinary epidemiologist Dr Holyoake.
The Murdoch team worked in consultation with Curtin University of Technology, Swan River Trust, Western Australian Department of Conservation and Environment (DEC), Perth Zoo, Western Australian Department of Agriculture and the University of New South Wales.
Photo gallery – Swan River dolphin postmortems
Images taken during the Murdoch University investigation into the mortalities of six bottlenose dolphins in the Swan Canning Riverpark in 2009.
The Murdoch University investigation was conducted by a multidisciplinary team including animal biologist Dr Hugh Finn, veterinary pathologists Dr Nahiid Stephens and Dr Padraig Duignan, members of the Murdoch University Cetacean Research Unit (MUCRU), including research leader Dr Lars Bejder, and Marine Mammal Health Project Coordinator and veterinary epidemiologist Dr Carly Holyoake.