Toxoplasmosis, a disease caused by a hardy microscopic parasite that is spread by cat faeces and carried in streams and rivers to the ocean, has been found to be the main non-fishery cause of death.
It killed nine 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.
“The numbers of dolphins dying from toxoplasmosis are likely to be much greater than commercial fishery deaths,” NIWA’s Dr Robert said.
“They may add up to hundreds each year for both the east and west coast dolphin populations.”
Maui dolphins are found off the west coast of the North Island and number just over 60.
They are a closely related sub-species of Hector’s dolphins which are much more numerous, with a population of about 15,000.
Seafood NZ welcomed the research and hoped it would lead to more informed debate.
The toxoplasmosis threat has been known for some time – it was identified as far back as 2013 in a Massey University study – but this is the most emphatic finding.
Blaming commercial fishing for Maui dolphin deaths has been the default position of the anti-fishing lobby.
This is despite the lack of evidence, the closure of 6200 square kilometres of coastal waters to set netting and 1700 square kilometres to trawl fishing and almost 100 percent observer coverage in known Maui territory.
There has now been more than 2000 days of observer coverage with no Maui dolphin sighted, let alone captured.
The last Maui death attributed to commercial fishing was in 2002.
The 18-month scientific study has found the dolphins can breed earlier, live longer and are capable of faster population growth than previously thought, which makes them more resilient to human threats.
The experts undertaking the risk assessment used data obtained from aerial surveys of the dolphins’ locations and then related it to the turbidity of waters around New Zealand.
Their main food source – red cod and small inshore fish – prefer turbid (cloudy) water as well .
So does their main predator – sevengill sharks.
Being able to determine where the dolphins go allows scientists to predict much more accurately where commercial fishery deaths could occur and therefore alleviate the risks posed.
The media reaction to what some would see as an inconvenient truth, has been varied.
Disease bigger worry than bycatch for rare dolphins - NIWA, Radio NZ reported.
Rural reporter Eric Frykberg, an old style journo who plays it straight, said the seafood industry’s responsibility for the decline of rare dolphins has been downgraded in new research.
The NZ Herald accurately headlined: Microscopic parasite spread by cats found to be of greater risk to dolphins than commercial fishing.
And the struggling Stuff?
An alarmist, confused report at the weekend blamed the commercial fishing industry for dolphin deaths and included an emotive photo of dead Hector’s dolphins in a net.
This oft repeated image is distressing.
What the caption did not say was that the dolphins were caught in a recreational fishing net at Jacksons Bay some years ago.
A little misleading perhaps?
The same image, supplied by DOC, features in Raewyn Peart’s Dolphins of Aotearoa (p263) where it is correctly captioned.
Stuff is yet to report the NIWA research.