Variously known in New Zealand as Bluff oyster, dredge oyster and flat oyster, or by its Māori name tio, this much-loved delicacy is really the Chilean oyster – Ostrea chilensis.
Found all around the New Zealand coast in small patches to depths of 35 metres or more, dredge oysters grow in extensive beds in the South Island’s Foveaux Strait, Golden Bay and Tasman Bay. The pear-shaped adult is a heavy-shelled bivalve, some 6–10 centimetres long, with a cupped lower shell and a flat upper shell.
A dredge oyster spends its first 18–30 days as a larva within its parent’s body. It is then released into the water and settles a few centimetres away. It immediately cements the lower shell to some surface, often the shell of another oyster.
After about two years the cement bonding breaks down and the oyster lies on the muddy sea floor. It reaches sexual maturity around this time. A dredge oyster may be a hermaphrodite, producing eggs and sperm at the same time, or it may be in a definite male or female phase. It can alternate between each sex.
Dredge oysters can live for eight years or more and reach harvest width of 58 millimetres in four to six years.
Foveaux Strait oyster fishery
Foveaux Strait oyster beds have been dredged since 1863. The first beds were small, and were soon depleted. A large bed discovered in eastern Foveaux Strait was dredged from 1888 until the 1950s. When this proved uneconomic, the focus moved to the centre of the strait. In the early 1930s, the oysters were struck by disease and many died. Other disease outbreaks followed in the early 1960s and between 1986 and 1992. The culprit was Bonamia exitiosus, a parasite that lives inside the oyster’s blood cells.
Fishermen knew that the densest beds of oyster were associated with reefs of material known as mullock. Resembling coral, mullock reefs are built up from the skeletons of assorted bryozoans, shellfish, polychaete worms and sponges. They are common around New Zealand where strong currents flow through shallow channels, and they may stretch for several kilometres.
After 140 years of dredging, most mullock reefs in Foveaux Strait have been destroyed by heavy equipment, and few sites are available for young oysters to settle. The oyster population in the eastern Foveaux Strait has never recovered since the 1950s, and neither have the mullock reefs. Some environmentalists believe the fishery will collapse if destruction of the reefs continues.