Story: Marine invaders

Thousands of animals and plants are travelling around the world on ships’ hulls, in their ballast water, or caught up in fishing tackle. Some of them become prolific colonisers in their new habitats.

Story by Christina Troup
Main image: An invasive sea squirt

Story summary

All images & media in this story

What are marine invasive species?

A marine invasive species is a sea animal or plant that is carried, often by ship, from one country to another, and then multiplies so much that it damages the marine environment or other animals or plants.

How do they get to New Zealand?

Shellfish or seaweeds can attach themselves to the outside of boats travelling to New Zealand and then float off in the water.

Also, ships take in ballast water (sea water to help them float at the right level in the sea) and water to cool their engines. When they do this they can suck in tiny creatures, which come out when the ship empties the water at another port.

Plants and animals can get caught up in gear from a fishing boat and be taken to another country’s coast.

Why are these species a problem?

Sea creatures brought to New Zealand may be aggressive and squeeze out native animals and plants. They can also change the environment so it is not so good for native species.

Some grow quickly and cover boat hulls and parts of wharfs, or block up estuaries. In seafood farms they can crowd out or eat the shellfish growing there.

Some diseases can be spread to different parts of the world in ballast water.

Which species are invaders?

  • Some oysters, mussels, crabs and barnacles. Pacific oysters were brought to New Zealand and are now farmed. But they grow in big clumps that gather mud, and their shells break into sharp pieces that wash up on beaches.
  • Some sea squirts. One type smothers the lines that mussels are meant to grow on at seafood farms.
  • Some tubeworms. One kind of tubeworm, which looks like coral, blocked pipes that took water to cool a power station. The station was shut down until the tubeworms were cleaned out.
  • Some seaweeds and algae. A brown seaweed called Undaria is taking over places where a crusty pink seaweed grows. This threatens pāua (a shellfish people eat) because pink seaweed gives out a chemical message that encourages young pāua to settle on the sea floor nearby.

Protecting the coast

Biosecurity officers look for marine invaders and remove them. There are rules about where ships can safely get rid of their cooling and ballast water. Ships are also searched and cleaned to keep them free of pests.

How to cite this page:

Christina Troup, 'Marine invaders', Te Ara - the Encyclopedia of New Zealand, (accessed 22 April 2024)

Story by Christina Troup, published 12 June 2006