Asian paddle crabs already impacting Ngunguru estuary?

8 Jul 2016

An unwanted and aggressive Asian paddle crab species appears to have made itself at home in the Ngunguru estuary, with preliminary examinations of captured specimens showing it may already be impacting on the local ecology.

Don McKenzie, Biosecurity Senior Programme Manager for the Northland Regional Council, says Asian paddle crabs ‘Charybdis japonica’ were discovered in the estuary by a member of the public in late 2014.  Since then more than a dozen have been caught there either by council staff or members of the public.

“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 says what is known about the crabs is that they are an added threat to our already vulnerable 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.”

“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 says council’s Biosecurity Team has been investigating the best way to trap the crabs to try and limit their spread.”

“We’ve been trialling a range of different traps, locations and times and working to establish how often to empty traps.”

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

While the largest crabs in particular had been eating the shellfish, even the smallest crab had some crustacean fragments in its stomach.

“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 says with more than a dozen crabs now caught over a reasonable sized area over the past 18 month, it’s clear they’re well-established and are unlikely to be eradicated.

“This is unfortunate as it’s just another unwanted competitor for our native species, however, people can still make a difference by doing the right thing when they catch these crabs.  If we can get it right, long-term trapping may be a way of at least reducing the impact of these crabs.”

He says people interested in learning more about some of the wide range of weeds and pests threatening Northland can do so via the Northland Regional Council’s website: www.nrc.govt.nz/nasties