Unwanted crab invaders now fixture in estuary

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Regional council biosecurity officer Irene Middleton with one of the paddle crabs trapped at Ngunguru.
Regional council biosecurity officer Irene Middleton with one of the paddle crabs trapped at Ngunguru.

An infestation of crabs in the Ngunguru estuary looks set to be permanent and may already be impacting on the local ecology.

The unwanted and aggres-sive Asian paddle crab species (Charybdis japonica) appears to have made itself at home in the estuary, said Don McKenzie, Northland Regional Council biosecurity manager.

More than a dozen crabs have been caught by council staff or members of the public since the invader was first noticed there in late 2014.

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 probably that they have arrived as an unwanted hitchhiker on a fouled boat hull or in ballast water from other parts of New Zealand," Mr McKenzie said.

The crabs are an added threat to the already vulnerable native bivalve molluscs like pipi and cockles, which they feed on.

"Asian paddle crabs are edible, but are well-known for being very aggressive and they can do some serious damage with their lethal-looking claws. They also compete with our native crabs," Mr McKenzie said.

People cannot take live crabs away from where they were found.

"By law, these crabs aren't allowed to be spread around so if anyone catches them, we encourage people to kill them. They're edible, but you can also use them as bait for fishing."

Mr McKenzie said the NRC's biosecurity team had been trialling the best way and times to trap the crabs.

Recent gut content analysis confirmed the crabs had been eating bivalve shellfish and smaller crustaceans.

"We already know these crabs are generalist predators and this is a very preliminary finding, but it may suggest they could already be impacting the local ecology."

Mr McKenzie said it was clear they were well-established and unlikely to be eradicated.

- Northern Advocate

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