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.
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.