Murdoch University researchers in collaboration with the Swan River Trust, Department of Environment and Conservation and Curtin University are investigating the death of six dolphins in the Swan and Canning rivers this year.
An urgent meeting of researchers and government agencies on November 12 discussed the findings of post-mortem examinations of the dolphins which died between March and October, and ABC TV's Stateline broke the story on November 13.
Murdoch University wildlife biologist Dr Hugh Finn told Stateline that the dead dolphins were likely to have been consistent users of the Swan and Canning rivers and part of a resident community of about 20-25 dolphins.
“The deaths of the Swan River dolphins share many of the same features observed in significant mortalities of dolphins studied elsewhere in the world, including evidence of suppressed immune systems and bacterial, fungal and virus infection,” he said.
“Entanglement in discarded fishing line is also a contributing factor, with one dolphin dying as a result of an infection arising from entanglement around its tail flukes and a second dolphin suffering from entanglement around a pectoral fin and a fish hook lodged in its oesophagus.”
Two of the dead dolphins were females of a reproductive age, and two had severe skin lesions.
Murdoch veterinary pathologist Dr Nahiid Stephens told ABC TV that the post-mortem examinations indicate that immunosuppression was evident in at least four of the dolphin deaths.
“This may be attributable to a range of factors, including viral infection, rapid seasonal changes in water quality, and long-term exposure to contaminants,” Dr Stephens said.
“In comparison to contaminant levels reported in dolphins internationally, concentrations of dieldrin were high in tissues from three dead dolphins that were analysed for the presence of variety of contaminants,” she said.
Dieldrin is a pesticide banned by the Government in 1988. Levels of polychlorinated biphenyls, DDE and Zinc were also high in the dead dolphins.
International research has shown that dolphin deaths of this nature are not unique to the Swan Canning Estuary and have been observed in other locations in Australia and around the world (such as Gippsland on the east coast, and the south-eastern coast of the US).
Researchers are working on a range of further tests to investigate the post-mortem findings, and address whether an as yet unidentified marine mammal pathogen may be present, such as Cetacean Morbillivirus. This virus can cause immunosuppression.
Swan River Trust principal scientist Dr Kerry Trayler told Stateline that the deaths are concerning and further investigation is being undertaken to better understand the causes of the deaths and identify what can be done to reduce rates of injury and mortality in the Swan River dolphins.
The Trust will fund any future post-mortems into the death of dolphins in the Swan and Canning rivers. The urgent need for more research funding is being discussed within and between agencies to protect the remaining dolphin community.
Since the launch of the Trust’s Dolphin Watch program, in collaboration with Murdoch and Curtin universities, it has been possible to monitor the presence of dolphins in the river more closely and community members are encouraged to join the Trust’s River Guardians program and sign up to become a member of Dolphin Watch.
Recreational fishers have an important role to play, as they can reduce the risk of entanglement for dolphins by properly disposing of fishing line and using biodegradable fishing line.