Although there has recently been a lot of focus on our freshwater streams and rivers, the threat to the seas where we do most of our boating has largely gone unnoticed.

The Ministry for Primary Industries says these waters are in danger, primarily because of an invasion of marine organisms.

With an increasing number of these potentially very harmful pests now taking hold here MPI is urging boat owners to play their part in ensuring that they do not spread.

"It is essential that we all work together to stop the spread of these potentially devastating organisms," says John Sanson, MPI's National Co-ordination Manager.

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"Boat owners need to ensure the hulls of the vessels are thoroughly cleaned and properly antifouled at all times, especially before they travel from one part of the country to another."

One of the organisms causing concern is Sabella spallanzanii, better known as the Mediterranean fanworm.

"This worm, which can smother native marine species and is a nuisance to marine farming, is now well-established in the Waitemata Harbour and Hauraki Gulf," says Sanson. "While it is, unfortunately, now unable to be eradicated from those areas, it is vital that we stop it spreading to other areas such as Northland and the Bay of Plenty."

According to MPI, mature female fanworms are able to release more than 50,000 larvae into the water in just one spawning (during the autumn and winter reproductive season). Fanworms can also spread by becoming established in the enclosed wet areas on boats or on dirty vessels and equipment that are then moved to another location.

Sanson says that, although many boat owners believe these foreign organisms mostly arrive in New Zealand in the ballast water of commercial ships, detailed scientific research shows that the majority come in on fouled vessels (both recreational and commercial) - and are then spread in the same way.

"Sadly, once established here, marine pests are extremely difficult to control or eliminate," he says. "They can also have a very negative impact on many of things we really value: fishing, shellfish gathering, aquaculture and even the appearance of our treasured bays and beaches."

Sanson says other prolific pests include Styela clava (also known as the sea squirt or clubbed tunicate), which has infested many New Zealand harbours; and Undaria, a seaweed which has reached the pristine Fiordland marine environment.

Styela is an unsightly knobbly sausage-like organism that is a serious nuisance to aquaculture, fouling mussel lines and competing with mussels for space and food. It is present in a number of regions including Auckland, Northland, Nelson, Canterbury and Otago and is now too well established to eradicate.

On a more positive note, since April 2010, MPI, the Department of Conservation and Environment Southland have joined forces with the community group Fiordland Marine Guardians in an attempt to remove Undaria from a remote area in Breaksea Sound, Fiordland.

"We still have a long way to go to be able to say we have no more Undaria, but the results so far are very promising," says Sanson.

Looking at the impacts of marine pests and the costs of eradication attempts, it is not surprising that local authorities are pleading with boat owners to ensure that their hulls are cleaned and protected, with antifouling in good condition, before they head anywhere for the summer.

"The last thing Northland needs is an infestation of the Mediterranean fanworm or any other exotic introduced pest," says Don McKenzie, Biosecurity Manager with the Northland Regional Council.

"Our marine environment is very important - both for recreational use and also for the marine farming industry. Both tourism and aquaculture are important earners for our region and are vulnerable to the impacts of introduced marine pests."

Recent experience has shown just how easy it is for these kinds of pests to "hitch-hike" their way around the coast on a recreational boat. The Bay of Plenty Regional Council has, this November, found two vessels fouled with Styela sea squirts moored in Tauranga Harbour. These boats had arrived from Auckland and Coromandel with "dirty" bottoms.

"Both owners were very concerned and co-operative but we need boaties to be much more vigilant if we are to keep Tauranga Harbour free from these marine pests," says Warwick Murray, Natural Resource Operations General Manager for the Bay of Plenty Regional Council.

"It's absolutely critical that boaties keep their boat hulls clean and take extra care to remove fouling growth from their vessel responsibly before travelling from one area to another."

Like Sanson, Murray stresses that boat owners need to apply antifouling paint regularly and keep it in good working condition.

"This means it needs to be replaced regularly in line with the manufacturer's recommendation. It also needs to be renewed in places where the hull has been scraped or damaged or if the hull is being persistently fouled.

"Just as importantly, owners need to regularly check and clean their hull bottoms. They should not allow the build-up of fouling beyond a slight slime layer and need to pay particular attention to what are known as "niche areas". These are the prime areas for harbouring pests and are the parts of the hull that stick out or retain water [such as the keel, intakes and outlets, propellers and shafts, rudders and casings]."

Sanson says a simple boat owners' guide to preventing the spread of marine pests is available on line at: http://www.biosecurity.govt.nz/files/enter/ships/clean-boats-brochure.pdf

As well as asking owners to regularly clean their hull bottoms MPI is also urging owners to keep a sharp eye out for anything unusual in the water or on their hulls.

"If anyone sees any marine plants or sea life that looks out of the ordinary, please note its location, collect a sample if possible and contact us as soon as possible on 0800 80 99 66."