New Zealanders are partial to oysters and two types are harvested locally: dredge oysters and rock oysters. Both have been commercially harvested since the mid-1800s. Dredge oysters, or Bluff oysters (Ostrea chilensis) have not been farmed in New Zealand. Rock oysters (Saccostrea commercialis) are found naturally in the intertidal zone in the north of the North Island, and were the subject of early cultivation experiments. These trials showed that a marketable product could be farmed within 3½ years – half the time it took an oyster to mature when growing naturally on rocks. During the 1960s the commercial farming of native rock oysters spread throughout the harbours and inlets of the northern North Island. Most farmers grew their oysters on cement-coated sticks laid down in racks.
Oyster spat, the free-swimming larval stage of an oyster, were collected during summer on sticks placed at mid-tide level in Mahurangi estuary, near the Hauraki Gulf. In 1970, farmers noticed another oyster that quickly outgrew the rock oysters. This newcomer was the Pacific oyster (Crassostrea gigas), which had probably been introduced into New Zealand waters from a Japanese source in the 1950s. At first, farmers tried to clear it from their collecting sticks, but each year more and more Pacific oyster spat attached itself until it proved futile to try and get rid of it.
Within a few years most commercial growers wisely began cultivating Pacific oysters. They grow three times faster than native rock oysters, produce a consistent quantity of spat, and have a well-established international market. The Pacific oyster turned up in the Marlborough Sounds in 1977 and farming began there in the 1990s. In contrast to the northern North Island method of racks, Marlborough farmers cultivated their oysters on suspended longlines. By 2001, 100 oyster farms were producing 3,500 tonnes of Pacific oysters; 30% was consumed by the domestic market and the rest was exported, with a value of $13.6 million.
Enhancement is the name given to artificial techniques that boost the recruitment or survival of young animals or seaweeds in the wild. In New Zealand, enhancement of scallops has been very successful in Tasman Bay and Golden Bay. New Zealand scallops (Pecten novaezelandiae) are large fan-shaped shellfish with a flat upper shell and a deeply curved lower shell. They live on the seabed of coastal waters at depths of 30 metres or more.
Scallops are cultivated by suspending spat-collecting bags in coastal waters during summer. Thousands of scallop larvae settle out of the plankton onto the fine feathery surface of the bag. The larvae are left to grow to a suitable size before they are collected and released onto known natural scallop beds at densities of about six per square metre of sea floor. There, they grow for one or two years before the bed is harvested by dredging. This technique resurrected the Tasman scallop fishery, which was near collapse in the 1980s, to a level where 747 tonnes were harvested in 2004.