Auckland University operates the Leigh Marine Laboratory, and the Cape Rodney-Okakari Point (Goat Island) Marine Reserve was originally set up for scientific research purposes. It is administered by the Department of Conservation.
Laboratory senior lecturer Nick Shears told Mahurangi Matters the first research into crayfish numbers, when the reserve opened in 1975, showed there were 10 per 500 square metres.
As the 518ha reserve became more effective, that figure grew to around 40.
But due to more intensive fishing on the reserve border, numbers have dropped back to just 10 - wiping out more than 40 years of conservation efforts.
"The reserve boundary extends 800m offshore, but the sand flats where the crayfish feed are beyond this," Dr Shears said.
"The reserve needs to constantly protect its marine life, which isn't happening, and this means we could soon have less crayfish than when the reserve first opened."
Relevant agencies are aware of the issue and a Sea Change Plan, designed to enhance the Hauraki Gulf, proposes to extend the boundary a further 2.2km to 3km.
However, no time frames have been set for when the proposal might come into effect.
In August, the university approached Auckland Conservation Board with its concerns.
Board chairperson Lyn Mayes shares the university's concern.
"The board supports the call by researchers to extend the boundary of the reserve to a minimum of 3km offshore."
The board could not comment on any action it may have taken to try to get the boundary extended.
Rodney MP Mark Mitchell said he would be right behind any proposal to expand the reserve.
"I'm a huge fan of marine reserves and I can't see any downsides to expanding the one at Goat Island," Mr Mitchell said.
"It serves our researchers, is a good learning facility and a great tourist attraction."
Leigh resident and honorary Department of Conservation ranger, Tony Enderby, said he supported a boundary expansion, but had concerns about opposition.
"It's a great idea, but I have doubts it will happen because a lot of fishermen are opposed to the idea," he said.
Minister of Conservation Eugenie Sage said she could not comment on the specific proposal to expand the reserve, but government agencies were working out the best way to progress the Sea Change Plan proposals.
Meanwhile, marine life in local reserves is having to face the additional challenge of sea pollution.
Earlier this month, Goat Island Dive and Snorkel owners Stone and Tine Meharry-Roland found a set net drifting near Tawharanui Marine Reserve full of dead fish.
"People set these nets and then don't monitor them closely. They drift off and catch all sorts of marine life that gets in their way," Mr Meharry-Roland said.
"The nets are often made of nylon so can easily last 30 years in the water."
The pair regularly do clean-ups around the coastline and find a lot of rubbish.
"We pick up everything from tyres to fridges, bikes, crayfish pots and, most commonly, plastic waste."
Goat Island sees around 350,000 visitors each summer. Mr Meharry-Roland said beachgoers and anglers needed to be mindful of rubbish and the damage it did.