The health of the Swan River has come under serious question after researchers revealed that dolphins that died in the waterway were found to have high levels of the banned chemical dieldrin.

A report into the deaths says the levels of dieldrin are among the highest found in marine mammals anywhere in the world.

Photos of the dead mammals show shocking skins lesions, growths and discolouration.

Of the six dolphins found dead since June, two were in such a state of decomposition that they could not be tested for contaminants.

Three of the dead dolphins were found to have high levels of contaminants including dieldrin, polychlorinated biphenyls, DDE and Zinc. A report by Murdoch University researchers on the deaths released today noted that little was known about the cumulative impact of mixtures of contaminants on the health of marine mammals.

Three of the mammals died in June and another three in September-October.

Lee Bell from the National Toxics Network said the contamination indicated that the dolphins were picking up chemicals that persisted in the environment.

He said up to 20 former landfill sites along the Swan River could be leaching poisons into the waterway and the spike in contaminants likely to result from dredging in the Fremantle Port in January could spell the end to the river’s dolphin population.

“When you get marine mammals already suffering from high levels of contaminants and you add more to the mix it can be the straw that breaks the camel’s back,” Mr Bell said.

He said both humans and dolphins could take up dieldrin though the skin as well as through eating contaminated fish.

Leading marine biologist Jean Paul Orsini backed Mr Bell's calls saying the threat to the dolphins from Fremantle Port's planned dredging program was too great, especially given the already vacarious state of the pod's health.

Shadow Environment Minister Sally Talbot warned swimmers, water sports lovers and people who eat fish and seafood out of the Swan River that their health could be affected.

She called on the Government to hold an urgent inquiry into the deaths of a nearly quarter of the iconic Swan River dolphins in the past five months and the presence of dieldrin in the water.

“This is very distressing news. It’s not just about the six dead dolphins in five months. It’s about the fact that half of them appear to have been killed by dieldrin. Dieldrin is one most poisonous chemicals we have known in this country and that’s why it was banned more than 20 years ago,” Ms Talbot said.

“We have no way of knowing it is a risk to humans. If there is indeed dieldrin in the Swan River, then that’s a very, very serious thing. The impact is not just upon people who like to fish and eat what they catch, but water-skiers, swimmers and anybody using our waterways.”

Ms Talbot added that she wouldn’t swim in the river herself, but it was up to people to decide that for themselves.

Environment Minister Donna Faragher said the Swan River was safe to swim in and there was no health risk from eating river fish.

“The Government has been advised by fisheries and health agencies that there is no direct risk to the public by swimming in, or by consuming any fish caught in the Swan and Canning rivers," Ms Faragher said.

"The deaths share many of the same features observed in dolphins studied elsewhere in the world."

Murdoch University researchers in collaboration with the Swan River Trust, Department of Environment and Conservation and Curtin University have investigating the dolphin deaths.

An urgent meeting of researchers and government agencies this week discussed the findings of post-mortem examinations of the dolphins.

Murdoch University wildlife biologist Dr Hugh Finn said the dead dolphins were likely to have been regular 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 on the ABC’s Stateline program last night.

"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.

A spokeswoman for Murdoch University said researchers were working on a range of further tests to investigate the post-mortem findings and would be looking at whether an as yet unidentified marine mammal pathogen may be present, such as Cetacean Morbillivirus, a virus known to cause immunosuppression.