Even inside fully-protected marine reserves crayfish numbers have plummeted and numbers outside reserves are likely much lower than official estimates, a new University of Auckland study shows.
Researchers, including Dr Nick Shears from the University’s Institute of Marine Science, say existing marine reserves in the Hauraki Gulf are not large enough to protect crayfish.
“We estimate that inside reserves, crayfish have declined by 59-80 per cent in the past 10-15 years despite being inside a strict ‘no-take’ area,” he says.
“Further, our estimate in the Hauraki Gulf is that the crayfish stock is just 3-12 per cent of unfished levels. That’s significantly lower than official fishery estimates of 20 per cent.”
The study 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 20-25 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 59–80 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.
However, poor recruitment on its own doesn’t explain the decline. The study indicates it has 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.
Dr Shears says 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, he says.
“It’s unclear whether these measures are sufficient to result in a recovery in cray numbers. 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, Dr Shears says.
This can be achieved through expansion of existing reserves and implementation of new marine protected areas that are well-designed and large enough to effectively protect these important species and their associated ecosystems.