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 Whāngārei 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.
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.