Plummeting crayfish numbers in small marine reserves leads to call for more protection in Hauraki Gulf

Saturday, 8 May 2021

The Government has bowed to fishing industry pressure and refused to extend a marine reserve around Campbell Island/Moutere Ihupuku, a subantarctic sanctuary for sea birds and marine mammals.

Plummeting crayfish numbers in small marine reserves in the upper North Island is mainly the result of fishing just outside the reserves, researchers say.

They also say the wider crayfish fishery in which the three reserves are situated, is in much poorer condition than had been thought, and are calling for an expansion of protected areas in the Hauraki Gulf.

The University of Auckland researchers looked at numbers of crayfish, also known as rock lobster, in the 5.2 square kilometre Leigh Marine Reserve, the 3.9sq km Tawharanui reserve and the 8.4sq km Hahei reserve.

They found that in all three reserves, crayfish numbers did increase strongly at first after protection measures were introduced, but declined steeply in the past decade.

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At Leigh, the researchers found crayfish numbers increased from about 11 to nearly 40 per 500sq metres from 1978-1990, but then fell to about eight animals per 500sq m from 1995-2019.

Gannets dive for fish in the Hauraki Gulf.

At Tawharanui, crayfish numbers fell from nearly 17 per 500sq m to fewer than seven from 2009-2019.

At Hahei, crayfish numbers rose from nearly five per 500sq m to more than 15 from 1996-2007, but then fell to about six by 2019.

On unprotected reefs near Leigh and Tawharanui the researchers found only about 2.5 crayfish per 500sq m in 2019, and near Hahei they found only about 1.5 animals.

The best bays by boat in Auckland's Hauraki Gulf.

Crayfish densities inside the marine reserves were less than 41 per cent of peak levels following protection, while densities on nearby fished reefs were 6–15 per cent of peak levels in the reserves, the researchers said in a paper published in Conservation Science and Practice.

Fishing on the boundary of the reserves was the most likely reason for the decline in crayfish numbers inside the reserves, the paper said. The impact of the fishing was worsened by years of low recruitment – the number of larvae that develop into juvenile crayfish.

The researchers estimated that inside the reserves, crayfish numbers had declined by 59-80 per cent during the past 10-15 years, despite the animals being inside strict no-take areas.

They also looked at a measure called the spawning stock biomass – the total mass of breeding-age fish – and estimated that for crayfish in the Hauraki Gulf the measure was just 3-12 per cent of unfished levels, significantly lower than official fishery estimates of 20 per cent.

The findings coincide with a rise in recent years in the amount of effort needed to catch a given amount of crayfish in the CRA2 management area, which extends along the east of the North Island from East Cape to north of Auckland.

Crayfish recruitment had also been consistently low in the CRA2 area since the mid-1990s, the study said.

Rock lobster photographed in the Tapuae Marine Reserve, off New Plymouth.

Oceanic and climate variability may be partly responsible for that trend, although little was known about how those factors affected recruitment, the paper said. In any case, low recruitment alone would not explain the size of the decline in crayfish numbers inside the reserves studied.

The researchers said they were not aware of any evidence of other factors, such as disease or marine heatwaves, that could be driving the decline in crayfish numbers.

While a large marine heatwave happened in New Zealand coastal waters in 2017/2018, there had been limited long‐term warming in northern New Zealand compared to other parts of the southwest Pacific, the study said.

Also, surveys since the heatwave seemed to show total crayfish densities had stabilised, with a small rise in the number of crayfish below legal size.

The low numbers of crayfish in fished areas meant the species could be characterised as ecologically extinct, the study said.

“In other words, lobsters no longer play an important role as predators in these systems; the expansion of urchin barrens provides evidence of this.

“If lobster populations continue to decline in marine reserves, similar ecosystem‐level changes may emerge and the long‐term ecological value of marine reserves may be compromised,” the study said.

There was a clear case to extend further offshore the boundaries of the three marine reserves studied by the researchers.

That would make the reserves big enough to include areas crayfish travelled to at certain times of year. While crayfish spent most of their time on shallow, nearshore reefs, large animals made seasonal movements to feed on shellfish at offshore sandflats and isolated pieces of reef.

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 researcher Dr Nick Shears said those steps were unlikely to be enough.

“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,” Shears, from Auckland university’s Institute of Marine Science, said.

The research highlighted the urgent and overdue need to substantially increase the level of marine protection in the Hauraki Gulf, he said.

As well as extending the existing reserves, that could be done by setting up new marine protected areas that were well-designed and large enough.