The year that six dolphins died in the Swan River was the worst on record for dolphin and whale strandings in WA, a new paper by the Department of Environment and Conservation has found.
The paper found that an unexplained increase in whale and dolphin strandings happened in 2009, pushing the average number of events from about 24 to close to 100.
The paper, co-authored by senior wildlife officer Doug Coughran, presents data collected by the department from 1981 to 2010 on strandings that include beached, sick, injured or dead whales and dolphins found along WA's 12,900km coastline.
It found that in the 29 years of the study, 1752 whales and dolphins were involved in 732 strandings along the WA coast.
The research, published in the Journal of the Royal Society of WA, found bottlenose dolphins and humpback whales were the most likely of the 34 cetacean species found in WA waters to become stranded.
In 2009, strandings of bottlenose dolphins rose to 36 and humpbacks to 46, well above other years.
Murdoch University cetacean expert Hugh Finn said the 2009 rise corresponded with the discovery of the devastating dolphin morbillivirus in WA. The virus was seen as a contributing factor in the deaths of some of the Swan River dolphins.
What caused the rise in humpback deaths is still being investigated, with research looking at whether the population may be too big for the available food.
The environment department released data yesterday for the official number of dolphin strandings in the Peel Inlet and Harvey Estuary for the eight months from December to July, after claims 11 dolphins had washed up in the area during the period.
Wildlife relocator Alison Dixon alleged at the weekend that the 11 deaths included a mother and calf and young adults. But Mr Coughran said his records showed that only five dolphins had been recorded as dying in the Peel-Harvey estuary in the time frame and the figures included two oceanic spotted dolphins, a species that had never been recorded so far south.
Mandurah had 40 dolphin strandings of individuals or mother and calf pairs between 1981 and 2010 with the spring tides and shallow water being identified as a major cause.Busselton, Augusta and the South West capes region were the most common place for mass strandings of cetaceans.
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