The current situation
Among the marine invasives already established in New Zealand are species of seaweed and other algae (from large to microscopic); crabs, barnacles and other crustaceans; coral-like bryozoans; tube worms; sea squirts; and oysters, mussels and other molluscs. Some grow more prolifically in New Zealand than in their place of origin.
By 1998 around 150 species of introduced marine organisms had been identified in New Zealand waters, of which 127 had become established. Not all are considered pests, but by 2002, 16 had become a serious and expensive nuisance. The majority are thought to have arrived on ships’ hulls. The Waitematā Harbour, home to the busy port of Auckland, has 60 such species, the highest concentration in the country.
Swapping algae for whales
Among the earliest marine stowaways to jump ship in New Zealand were marine algae, which became established around whaling stations. They were transported on wooden hulls or in ballast water of whaling vessels.
Changing the ecosystem
Marine invasive species can have a variety of negative effects. Many species are more aggressive or competitive than native counterparts, which they displace. They may cause changes to important features of the habitat, such as kelp forests and sea-grass meadows, and to the functioning of an ecosystem as a whole. There can also be a major impact on economic and recreational activities such as fishing and swimming.
Some invasive species grow so prolifically that they clog up – or foul – every surface. Known as fouling species, they block shallow waterways and economically important facilities such as water intakes and outlets, and they cover boat hulls, wharf piers and aquaculture equipment.
Threats to health and industry
Organisms taken into ballast water can include toxic microalgae or pathogens harmful to people or marine life. The cholera bacterium was spread to parts of the world in this way. In New Zealand, massive numbers of pilchards died in 1995 because of a herpes virus carried in dead pilchards imported as fish bait.
Since its beginning in the 1960s, farming of seafood (aquaculture) has grown to a $280 million industry in 2000, and it is expected to expand further. The economic impact of introduced pests could be considerable because they can crowd the habitat and compete for resources, predate on farmed shellfish, and infect them with pathogens.