A microscopic parasite spread by cats has been found to be the main non-fishery cause of deaths for Māui and Hector's dolphin in a new scientific study.

Toxoplasmosis has killed nine Māui and Hector's dolphins recovered in New Zealand since 2007 and is to blame for more deaths than commercial fishing.

The 18-month study was undertaken by Niwa's Dr Jim Roberts, Massey University and Quantafish, leading to a revived understanding of the dolphin's biology.

Seafood New Zealand chief executive Tim Pankhurst said the industry is doing its part to prevent dolphin deaths and welcomes the results of the study.

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"It is heartening to see recognition that commercial fishing is not the most significant threat to the sustainability of the dolphins and with that, the opportunity to prevent further deaths," said Pankhurst.

"The industry remains focused on preventing dolphin deaths.

"No Maui death has been attributed to commercial fishing since 2002 and that is because we have vast areas of ocean closed to trawling and set-netting in dolphin territory."

Māui dolphins are thought to only number 60 alive today, compared to about 15,000 Hector's dolphins.

Both species of dolphin have a preference for cloudy, coastal waters which house red cod and small inshore fish, their main food source.

Unfortunately, these waters have also bought them into contact with other threats, including toxoplasmosis which is spread by cats and is carried down streams and rivers into the ocean, Roberts said.

He said little is known about the parasite but Professor Wendi Roe and colleagues from Massey University has found it has killed native birds including kererū and kiwi.

However, the new study as altered the understanding of how much commercial fisheries weighs into the risk of Māui and Hector's dolphins.

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"Commercial fishery deaths are estimated using data collected by Government fisheries observers, and those estimates are more precise than for non-fishery deaths," he said.

"But if carcasses that wash up on our shoreline are representative of the cause of death in the wider population, the numbers of dolphins dying from toxoplasmosis are likely to be much greater than commercial fishery deaths.

"They may add up to hundreds each year for both the East and West Coast dolphin populations."