There are calls for an expansion of Auckland's marine reserves, after a University of Auckland study found crayfish numbers have dropped about 25 per cent from 2014.

LISTEN ABOVE: Chairman of the Hauraki Gulf Forum John Tegidga spoke to Rachel Smalley

In reserves at Leigh and Tawharanui, which are 800m offshore, there's less crayfish than were put in in 1975.

Hauraki Gulf Forum Chairman John Tregidga wants to see the reserves extended to 3km.

"If we make them big enough, maybe we just lock them up to recover. So once they can recover to a certain level, we may be able to get them back up to limited fishing."

He said dwindling crayfish numbers needed to be addressed "rather urgently."

"The real problem here is that these marine reserves look like they're just too small. The research is showing that we need to extend these by about 3kms offshore."