Crayfish numbers in the Hauraki Gulf have plummeted even inside fully-protected marine reserves - and those outside them are likely to be much fewer than official estimates.
That's according to a new University of Auckland study, which suggests the gulf's reserves aren't large enough to protect the treasured species.
"We estimate that inside reserves, crayfish have declined by 59 to 80 per cent in the past 10 to 15 years despite being inside a strict 'no-take' area," said study co-author Dr Nick Shears, from the university's Institute of Marine Science.
"Further, our estimate in the Hauraki Gulf is that the crayfish stock is just 3 to 12 per cent of unfished levels."
That was "significantly" lower than official fishery estimates of 20 per cent, he said.
The study, just published in the journal Conservation Science and Practice, combined long-term crayfish monitoring data from three marine reserves and their surrounding fished coasts from the past five decades.
It includes data from Tawharanui collected by the late Dr Roger Grace, and Department of Conservation monitoring data collected at Leigh, Tawharanui and Hahei marine reserves over the last 20 to 25 years.
While the marine reserves at Leigh, Tawharanui and Hahei continue to support much higher numbers and larger crayfish than surrounding fished waters, monitoring shows declines in crayfish inside the reserves of between 59 to 80 per cent.
This decline coincides with a wider decline in the surrounding CRA 2 fishery, thought to be a result of poor recruitment combined with sustained fishing pressure.
Using data from both fished and reserve areas the paper estimates that crayfish stocks in the region are between three and 12 per cent of unfished levels.
However, poor recruitment on its own couldn't explain the decline.
The study indicated it had been exacerbated by the continued capture of lobster on the offshore boundaries of these relatively small reserves.
Tracking research has shown that crayfish undertake seasonal movements whereby they move off the reef, and out onto sandy habitats where they feed on bivalves.
These movements often take them near to and beyond reserve boundaries where they are likely to be caught.
Extending both the Leigh and Hahei reserves have been proposed as part of the SeaChange process in the Hauraki Gulf Marine Park, and these recommendations are currently under consideration by the Minister of Conservation and Minister for Oceans and Fisheries.
Steps have been taken to start rebuilding crayfish stocks in the Hauraki Gulf through cuts to commercial quota and halving the daily bag limit from six to three for recreational fishers.
But it likely won't be enough, Shears said.
"It's unclear whether these measures are sufficient to result in a recovery in cray numbers," he said.
"Given the growing pressure on the gulf, it's unlikely traditional fishery management approaches will be enough to restore populations and the important ecological role crayfish once played."
Currently, less than 1 per cent of crayfish populations in the Hauraki Gulf are protected in marine reserves and these results highlight the urgent and overdue need to substantially increase the level of marine protection in the Gulf, he said.
This could be achieved through expansion of existing reserves, and new marine protected areas that were well-designed and large enough to offer effective protection.