The seafood industry's responsibility for the decline of rare dolphins has been downgraded in new research.
About 15,000 Hector's dolphins (pictured) remain, along with about 60 Maui dolphins. Photo: Gary Webber/ 123rf
Fishing companies have long been accused of hastening the extinction of endangered Maui and Hector's dolphins by catching them in their nets while trawling for fish.
Only about 60 Maui dolphins remain, while there are about 15,000 Hector's dolphins.
Research by NIWA suggests disease is probably a bigger worry and bycatch deaths are just a tenth of what they were earlier thought to be.
"It's likely we underestimated the importance of non-fisheries threats, such as toxoplasmosis which may be killing hundreds of dolphins each year," NIWA marine scientist Jim Roberts said.
NIWA used government-observer fishery data to estimate the number of dolphin deaths through bycatch from commercial fishing.
"This resulted in a more than a 10-fold reduction in the estimates of annual deaths of Maui dolphins each year from fisheries bycatch."
Dr Roberts said the previous, higher figure of bycatch deaths was based on assessments from experts, not from actual catch data.
He said toxoplasmosis, which was spread by cats, was a bigger problem for dolphins. Parasites from cats' faeces were washed down rivers into the sea, where they could survive for a year while passing up the food chain to dolphins.
Methods to limit the scale of this problem were still being researched.
"There is lots of research quite recently looking at vaccines for cats, so they don't shed the cyst," Dr Roberts said.
"We also know from research in the US that regeneration of wetlands can help.
"If you don't have wetlands, you lose the capacity for filtration (of running water) to take out the cyst before it gets into coastal waters."
Dr Roberts said toxoplasmosis killed nine out of 55 post-weaning age Hector's and Maui dolphins for which carcasses were recovered since 2007.
Other causes of death included bacterial and fungal infections, as well as fishery bycatch and predation.
He said Maui dolphins were more at risk from toxoplasmosis than other species, which included kaka, kereru and kiwi.