acidification alters the flavor of shellfish or other qualities that make them desirable for human consumption. Productivity Productivity of the Pacific oyster can be described as the amount of meat produced in relation to the amount of seed planted on cultch. The productivity of a farm also depends on the interaction of biotic factors, such as mortality, growth, and oyster size, as well as the quality of the seed and the growing technique used (off bottom, bottom, suspended or floating culture). The main causes of mortality in the Pacific oysters are natural mortality (age), predators, disease, environmental conditions (ice, freak winds), competition for space (crowding of cultch), silting (sediment runoff from land), and cluster separation (process of breaking up clusters of oysters into as many individual oysters as possible). Aquaculture in New Zealand In New Zealand, the Pacific oyster was unintentionally introduced in the 1950s, most likely through ballast water and from the hulls of ships. Aquaculture farmers at the time noticed the Pacific oyster outcompeted the endemic species, the Sydney rock oyster (Saccostrea glomerata), which naturally occurs in intertidal areas in the North Island. Early experiments in rock oyster cultivation procedures attached spat to cement-covered sticks and laid them down in racks. The farmers noticed, however, the Pacific oyster outgrew the endemic species in most areas, and constantly was attaching to the rock oyster collection sticks. A few years later, Pacific oysters were the dominant species in the farms, as it grew three times faster than the rock oyster, produced a reliable and constant supply of spat, and had an already established market overseas. In 1977, the Pacific oyster was accidentally introduced to the Marlborough Sounds, and farming began there in the 1990s. Marlborough farmers developed a different method of cultivation in comparison to the North Island method of racks; they instead suspended their oysters on longlines. Production status The Pacific oyster is one of the three main aquaculture species in New Zealand along with king salmon and the greenshell mussels. Pacific oyster aquaculture production has grown from an export value of $11 million in 1986 to $32 million in 2006. In 2006, the 23 Pacific oyster farms throughout New Zealand covered a total of 750 hectares of marine space and produced 2,800 tonnes of product per year. Annual production is now between about 3,300 and 4,000 tonnes. In 2005, the value of New Zealand's Pacific oyster production was