Asian paddle crab impacts on estuary ecology

NRC survey of Asian Crabs at Ngunguru. Photo / Michael Cunningham

It is hard to gauge the size of the Asian paddle crab invasion at Ngunguru as they are so aggressive and territorial they won't share a trap they've been caught in with another of their own species.

Even from within the trap, they drive off other crabs coming too close. That means Northland Regional Council (NRC) staff only catch one of the vicious, voracious invaders in each trap.

NRC biosecurity staff sometimes catch none at all during the regular surveying, which might indicate the crab numbers are relatively low.

Even so, Charybdis japonica appears to have made itself at home in the estuary and its presence may be impacting on the local ecology.

There are concerns the crab species' arrival and the demise of the estuary's pipi and cockle beds may be connected. Recent gut content analysis has confirmed the predatory crabs had been eating bivalve shellfish and smaller crustaceans.

The invader was first noticed in the estuary in late 2014 but the species is widespread in the Hauraki Gulf and has been detected in small numbers in Whangarei Harbour and Opua/Waitangi.

"We're unsure how these paddle crabs have ended up in Northland, although the most likely scenario is that they have arrived as an unwanted hitchhiker on a fouled boat hull or in ballast water from other parts of New Zealand," Northland Regional Council biosecurity manager Don McKenzie said.

The Asian paddle crabs - more aggressive than the similar sized native paddle crab - are edible and make good bait but are well-known for being very aggressive and they can do some serious damage with their lethal-looking claws, Mr McKenzie said.

It is illegal to take them away from where they are found.

Steve Rudsdale, District Compliance Manager for Ministry of Primary Industries, said the ban on collecting pipi from the estuary and environs was in place for an indefinite period.

The Asian paddle crabs are out there but not always easy to bag. Biosecurity officer Cameron Bunton with an empty trap. Photo / Michael Cunningham

The mysterious collapse of the population led to the ban being imposed in early January, backing up a rahui placed by Ngunguru Marae in November 2015.

Anyone found breaching the closure order is liable to a conviction and a fine of up to $250,000.

A survey of pipi beds last year found no specimens over 5cm in length, compared with 10 years ago when there were about 50 pipi that size per square metre. The harvest was not thought responsible but there was concern continued gathering may inhibit the recovery.