The invasive Mediterranean fanworm is on the move, spreading outside Lyttelton Harbour for the first time.
The introduced underwater pest has scientists concerned, as it can be difficult to stop once it gets out of a contained area.
A gloomy day in Lyttleton and a dark presence below deck.
"Fanworm is a prolific organism which means it spreads widely and quickly and one female fanworm can release 50,000 eggs at one time," Lyttleton Port Company's environmental business partner Kirsty Brennan said.
And it's now spread outside Lyttelton Harbour - where it had been contained since 2008.
"This is a bit alarming that it is now outside the inner harbour," Brennan said.
An Environment Canterbury survey in March found four vessels in Cass and Corsair Bay with the species onboard.
"It can effectively cost the aquaculture industry millions of dollars per year once it gets established on the aquaculture farms," NIWA marine ecologist Dr Chris Woods said.
Dr Woods said it can overcrowd native shellfish species.
"The tubes themselves grow up to 85cm in length," he said. "In terms of density or numbers you get, they can get up to about 1000 per metre square, so a very high-density animal."
And it is hard to get rid of.
"You can remove this by hand - removal from the port environment - but once it spreads into the harbour environment then it's really hard to manage that species," Dr Wood said.
The Mediterranean fanworm is considered one of the three main marine biosecurity threats in the region. But because there is also a native fanworm people shouldn't try to remove them themselves, but should instead report them to authorities
Authorities and local stakeholders are pushing for a national clean hull policy to manage the pest.
"For recreational boat owners, it really does mean keeping up to date with making sure your bottom is clean, 'clean below is good to go' is one of the slogans that they use," Brennan said.
A message for boaties throughout New Zealand as Lyttelton's not the only port harbouring this foreign invader.
