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Marine invaders

by  Christina Troup

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.

Reaching New Zealand

Riding the high seas are thousands of hitchhikers, criss-crossing the world courtesy of tankers, cargo ships, cruise ships, leisure craft and other floating hosts. Many are tiny for part of their life cycle, too small to be noticed. Some are firmly attached to hulls, others are caught in hollow spaces or water intakes, while many slosh about in ballast water. Invasive species can also arrive among fishing materials, and some even come as aquarium plants or animals and are released unwittingly.

Hideaways for stowaways

Most ships’ hulls have recesses below the water-line, known as sea chests, which take in sea water for ballast or engine cooling. Pumps draw water in, sucking in small organisms. Live marine creatures found in sea chests have included sponges, sea anemones, hydroids, worms, sea slugs, mussels, oysters, scallops, bryozoans, barnacles, crabs, sea spiders, sea stars, sea urchins, sea squirts and fish.

Introduced and invasive species

Introduced species are living organisms that are brought by humans to an area where they are not native, and where they survive and reproduce themselves. Introduced species that become pests in the new environment are described as invasive.

An isolated coastal ecosystem

Despite being connected to other regions by water, New Zealand’s coastal ecosystems have evolved in isolation, cut off by deep ocean basins, currents, and climate zones such as tropical or polar regions. Some coastal organisms that can overcome these barriers unassisted are found naturally all over the southern hemisphere and the globe. However, many more are unable to propel themselves over long distances or would not survive a slow, drifting journey away from their usual coastal environment.

Contact through trade

Growth in international trade means that New Zealand’s coastal ecosystems are increasingly exposed to contact with the outside world. There were 3,500 international shipping arrivals in New Zealand in 2003, compared with fewer than 1,000 in 1960. With new trading partners, there are links to different coastal regions – each a possible source of new and aggressive pests. In addition, the itineraries of cruise ships and yachts differ from trade routes, which increases the range of places of origin.

Effects on the environment

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.

Fouling species

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.

Invasive marine animals in New Zealand

Encrusting invasives

The bryozoan Watersipora arcuata arrived in the Auckland region in 1955–56 and later spread to the South Island. It was partially displaced by another harmful arrival, Watersipora subtorquata. Both species form encrusting coral-like growths on surfaces between low- and high-tide lines. In 2005 Watersipora arcuata was known at 13 sites around the North Island and Kermadec Islands, and Watersipora subtorquata was at 16 sites, including the South Island.

A tubeworm (Ficopomatus enigmaticus), which causes major fouling in brackish waters, invaded the Whangārei tidal basin in 1967. Its calcareous growths formed encrustations 20 centimetres thick, which locals referred to as ‘coral’, on the hulls of boats and wharf piles. From there it spread to the Tāmaki estuary in Auckland where, in 1980, it caused blockages to the cooling-water intake of Ōtāhuhu Power Station, forcing temporary closures. To grow, this species needs a minimum water temperature of 18°C during summer spawning, and enclosed harbours or estuaries with an inflow of fresh water to lower the salinity.

Other fouling organisms

Ciona intestinalis, a sea squirt from the North Atlantic, was first found in Lyttelton in 1940 and Auckland in 1968. Sea squirts are immobile filter feeders with a sac-like translucent body, which is retractable. Ciona intestinalis grows up to 15 centimetres long and lives from the lower shore to a depth of 500 metres in the ocean. It attaches itself to the sea floor, rocks and artificial surfaces, including metal, concrete and mooring lines. It smothers mussel lines and has been a nuisance on mussel farms in the Marlborough Sounds.

In 2005, the clubbed tunicate Styela clava, a sea squirt originally from Korea, was discovered in the Viaduct Basin in Auckland Harbour, and in Lyttelton Harbour. It grows rapidly and breeds prolifically, spawning every 24 hours. It affects aquaculture by competing with shellfish for food and by smothering lines. This has led to calls from the aquaculture industry for urgent action to prevent it from becoming established.

Introduced shellfish

The Pacific oyster (Crassostrea gigas), which became established in the early 1970s, has brought benefits as well as costs. Faster growing than the native New Zealand rock oyster (Saccostrea glomerata), it quickly replaced that species in oyster farms and in the wild. This has brought higher returns to the aquaculture industry. However, in natural environments Pacific oysters form dense clumps that accumulate mud, changing the character of intertidal rock platforms. Also, their shells break into sharp fragments that transform nearby beaches, making them less appealing for recreation.

A small black Asian mussel (Musculista senhousia) was introduced in the late 1970s, probably in ballast water or by fouled hulls. First recorded at Black Reef near Auckland in 1981, it is now widespread between the Bay of Islands and the Coromandel. Its colonies reach densities of 16,000 per square metre and form mats of up to hundreds of square metres, which accumulate mud and exclude other shellfish such as pipi (Paphies australis) and cockles (Austrovenus stutchburyi). Fortunately, the patches tend to collapse after one or two years, so some of the adverse effects are relatively short lived. However, some recreational beaches in the area have been turned from sand to soft mud.

Two other small exotic bivalves, the file shell Limaria orientalis (first found in New Zealand in 1972) and the semelid bivalve Theora lubrica (probably from Japan) have reached high densities in Waitematā Harbour.

Invasive marine algae and plants in New Zealand

Invasive seaweed

A brown laminarian kelp (Undaria pinnatifida), first found in Wellington Harbour in 1987, probably arrived in ballast water from Japan or Australia. A ‘space invader’, it grows about 1 centimetre a day to a size of 3 metres and can quickly overtake native seaweeds. It is considered a threat to coastal seaweed communities, and particularly to the habitat of pāua (Haliotis iris), a popular seafood, as it suppresses the pink seaweed that pāua depend on. It can cause problems to aquaculture by clogging equipment and machinery and restricting the flow of water that farmed shellfish feed on.

At the reproductive stage the kelp is tiny and can attach itself to boat hulls and be transported to new sites unnoticed. By 1999 it was established at sites on Stewart Island, the east and north coasts of the South Island, the southern North Island, Napier and Gisborne. The Department of Conservation has been hand-weeding individual plants from Bluff and Stewart Island to prevent their spread by vessels to pristine Fiordland and the subantarctic island coasts.

New toxic microalgae

Introduced toxic microalgae or phytoplankton can cause illness in humans and close down aquaculture operations. Harmless enough at low levels, when conditions are right they reproduce explosively, giving rise to blooms. Shellfish feeding on these microalgae absorb the toxins, which in turn can make the people who eat the shellfish extremely ill. These organisms are easily transported over long distances as resting cysts in ballast water.

It can be difficult to determine which microalgae species occur naturally around New Zealand, but one that is thought to have been introduced is Gymnodinium catenatum, discovered in 2000 in Manukau Harbour. Fortunately, it was detected by the Ministry of Health’s regular monitoring programme before poisonings occurred, but it caused the most extensive toxic algal bloom recorded at that date. This closed shellfish gathering and mussel farming for nine months along 1,500 kilometres of coastline. The species survives in cooler water than most other algae that produce toxic blooms in New Zealand, hence it can have a longer season over a larger area.

Estuarine cordgrass

As on land, some harmful marine species were introduced deliberately, with problems only becoming apparent years later as the species spread uncontrollably. Three species of spartina cordgrass, introduced from England several times from the early 1900s, were planted in estuaries and salt-marsh areas around the country.

Spartina anglica has become the most widespread of the three. At the time it was introduced, the importance of estuaries and wetlands was poorly understood, and the intention was to use the silt-trapping role of the plants to help fill in sites for reclamation. By choking estuaries, Spartina species fundamentally alters local ecosystems. It reduces the habitat for wading birds, reduces spawning and feeding grounds for recreational and commercial fish, and exacerbates flooding problems. It also takes over sand and pebble beaches. Similar problems have arisen from invasive Spartina species in other countries.

Preventing future invasions

Marine biosecurity

A number of agencies are involved in the prevention, detection and management of marine invasives. The Biosecurity Act 1993 gave the Ministry of Fisheries tools to control the introduction of exotic organisms into New Zealand’s coastal waters. In 2004 many biosecurity responsibilities were taken over by the Ministry of Agriculture and Forestry’s new authority – Biosecurity New Zealand.

The Ministry for the Environment, the Department of Conservation, the Ministry of Health, the Environmental Risk Management Authority, and regional councils are also involved. These agencies develop policy guidelines, enforce legislation, monitor at-risk areas and control or eradicate pests at a local or national level.


The Ministry of Fisheries has made a list of six of the most unwanted marine pests, which would be far more damaging than those already in residence. These have had a severe impact on environments similar to New Zealand’s. They are:

  • Mediterranean fanworm (Sabella spallanzanii)
  • European shore or green crab (Carcinus maenas)
  • northern Pacific sea star (Asterias amurensis)
  • Chinese mitten crab (Eriocheir sinensis)
  • seaweed caulerpa (Caulerpa taxifolia)
  • Asian clam (Potamocorbula amurensis).

Preventing accidental introductions

There are national and regional laws and guidelines to cover high-risk practices such as the discharge of ballast water and the transfer of organisms on hulls.

Ballast water discharge

To reduce the risk of transferring unwanted organisms, ships coming into New Zealand are required to exchange ballast water taken near land with water free from coastal influences. This must be taken at least 200 nautical miles from land, and in water over 200 metres deep.

Hull fouling

A variety of fouling species are able to attach themselves and grow on ships’ hulls. Unwanted organisms can enter New Zealand waters in this way. In 1995, one Russian super trawler had 90 tonnes of foreign organisms, including mussels, sponges, goose barnacles, seaweed and anemones, removed from its hull in the Devonport dry dock. Most hulls are regularly treated with anti-fouling products, because fouling considerably reduces speed and increases fuel costs. However, wear and tear can reduce the effectiveness.

Fouling organisms can be inadvertently released in a number of ways – during hull cleaning, by becoming dislodged accidentally, or during the reproductive stage. There are guidelines and regulations to reduce these risks.

Monitoring and surveillance

High-risk areas such as major international ports are routinely monitored by marine biologists to identify any new unwanted organisms. The public are also asked to watch for invasives. Poster campaigns and websites are targeted at people involved in marine activities.

Control and research

When pests are identified, the difficulty and cost of control or eradication are weighed against the potential harm from the spread of the organism. In some cases, the only effective way to eradicate a pest from an area is for divers to remove each specimen by hand.

As well as the agencies involved in marine biosecurity, the National Institute for Water and Atmospheric Research, the Cawthron Institute and most New Zealand universities carry out research into all aspects of marine bioinvasions. There is also collaboration with Australian research institutes on issues common to both countries.

External links and sources

More suggestions and sources

How to cite this page: Christina Troup, 'Marine invaders', Te Ara - the Encyclopedia of New Zealand, (accessed 25 May 2024)

Story by Christina Troup, published 12 June 2006