A new suspect has emerged in the decline of the critically endangered Maui's dolphin - cats.
A parasite traced to cats has been identified as the primary cause of death in seven of 28 Hector's-type dolphins analysed by Massey University scientist Dr Wendi Roe.
Two of three Maui's, autopsied after they washed up dead on beaches, were found to have died primarily from toxoplasmosis, an infection caused by the toxoplasma parasite.
The Maui's are a sub-species of Hector's dolphins and their adult population was this year estimated at 55. The world's smallest dolphins, they are found only in New Zealand, and are thought to be confined to the west coast of Auckland and Waikato.
Dr Roe, a marine mammal pathologist, says the parasite multiplies in cats and ocysts (eggs) are excreted in the animals' faeces.
The risk of toxoplasmosis is the reason pregnant women are told to avoid cats but the parasite rarely causes serious infection.
It was likely to be getting into waterways and entering the food chain in several ways, she said.
"Cat faeces can be washed off soils into streams or off concrete paths in cities and into stormwater."
Feral cats around estuaries were another likely source.
"It seems that the highest risk areas are those with the most freshwater runoff."
Hector's and Maui's dolphins eat squid and fish such as mullet, kahawai and red cod. They live in shallow coastal waters and forage in harbours and estuaries.
Dr Roe said the dolphins could ingest ocysts either directly from water or through their prey.
Another theory is that pet owners may be contributing by flushing kitten litter down toilets.
"An infected kitten could excrete millions of ocysts in a couple of days. But there are lots of possibilities to look at. We don't yet know enough to suggest how we could go about reducing or eliminating it."
Dr Roe's findings have been seized on by the fishing industry, which claims set net fishing and trawling have been unfairly targeted as threats to Maui's dolphins while diseases and other human influences such as pollution are ignored.
In the 1990s, high levels of organochlorines and DDT found in Hector's dolphins raised concerns that contamination in run-off from farming and industry could be killing dolphins.
But Dr Roe points out her findings are from a skewed sample - "just what's washed up on the beach".
She supports other marine scientists' findings about the impact of fishing on the Hector's populations.
A paper on her findings is due for publication. She found four of the 28 she examined died from being caught in nets. Another was killed by blunt trauma, but there was no way of telling what caused it.
"It's really hard sometimes to work out what they died of but toxoplasma does seem to pop up more than we were suspecting."
A scientific panel estimated fishing was to blame for 95.5 per cent of human-induced mortalities and calculated the mortality rate from disease at less than 1 per cent.