Invaders hitching a lift on boat hulls

February 2012 News

A population of an invasive marine crustacean that "hitch-hikes" on the hulls of boats is living quietly in Otago Harbour.

Caprella mutica or skeleton shrimps are spreading rapidly around New Zealand, relying on boat users or drifting algae for transport as they do not swim well, Niwa biosecurity scientist Dr Chris Woods says.

"They readily colonise artificial structures, at times occurring in huge densities on anchored buoys, fish cages, wharves and vessel hulls. We have observed densities up to 180,000 caprellids per sq m."

In Otago Harbour, a small population was found in October 2010 clinging to the underside of the pontoon off Custom House Quay during the harbour's general six-monthly biosecurity check.

The population had not increased in size since its discovery but could be more prevalent as its favourite habitats were not regularly included in the biosecurity checks, he said.

They had previously been detected in Lyttelton Harbour, Port Levy and Pelorus Sound in the Marlborough Sounds after first being found in the Port of Timaru in 2002.

"It will likely spread to most areas of marine human activity throughout New Zealand in the near future."

Boat owners transporting their vessels between different areas needed to think about what uninvited guests they might be taking along for the ride, he said.

"Boat owners are saying to us, 'What are these waving things all over the hulls of our boats?' when they slip their craft and discover the hull alive with movement."

Maintaining a clean and antifouled boat hull was one of the best defences there was against the spread of marine invaders and pests.

It was not known what impact the "invader" would have on New Zealand's marine biodiversity, but overseas studies had shown that it could displace native caprellids and potentially affect food supply to filter-feeding organisms, Dr Woods said.



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