• Displace native species
• Change habitats and ecosystems
• Harm human health
• Affect recreational and commercial fishing
• Affect businesses, jobs and the economy

Some can be suitable for harvest and be valuable, such as the Pacific Oyster. Overseas examples show that the impacts can be substantial:

• Zebra mussel (Dreissena polymorpha) - a mussel that forms dense mats, clogging industrial water intakes, discharge pipes etc. Invaded the Great Lakes in the United States. Projected cost of management: $US5 billion over 10 years.
• Comb jelly (Mnemiopsis leidyi) - caused collapse of the fin fishing industry in the Black and Azov Seas. Catch in former USSR countries fell from 250,000 tonnes to 30,000 tonnes.
• Northern Pacific Seastar (Asterias amurensis) - eats bivalves such as mussels. Is spreading along Tasmania's coast from the Derwent estuary.

At least 148 exotic marine species have been introduced to New Zealand. In New Zealand, algal blooms have caused disease in shellfish and possibly health problems in some humans. These blooms may have resulted from organisms brought to New Zealand in ballast water.

How Marine Pests are spread
Marine pests arrive in and move around New Zealand's coastal and marine environment in a number of ways:

• Attached to ships' hulls, flotsam, structures such as oil rigs and animals such as turtles (known as 'fouling')
• Contained in ballast water, which ships carry for stability, then discharge when cargo is unloaded. Ballast water pumped into a ship in one port contains organisms and larvae, even whole fish.
• Transported on fishing or marine farming equipment.
• Contained in aquarium material or introduced deliberately.

Marine pests can compete with native species, upset ecosystem balance and reduce biodiversity.

Marine pests are most likely to arrive in busy ports and marinas, and can spread rapidly into nearby communities or be transported around the coast.

New Zealand receives over 2,500 visits by international vessels each year at over 20 ports. The annual discharge of foreign ballast water in New Zealand is estimated at up to 6 million tonnes.

More information about six marine pests
The following links are to pages describing six marine pests threatening our waters.

• The Mediterranean fanworm
• European shore (or green) crab
• Northern Pacific seastar
• Chinese mitten crab
• Caulerpa
• Asian clam