Between 1901 and 1907, three species of salmon were introduced into New Zealand rivers. Quinnat salmon (Oncorhynchus tschawytscha) was the only one that successfully established sea-running populations, becoming the focus of a successful recreational fishery in the glacier-fed rivers of Canterbury and Otago. For years recreational fishers opposed the development of salmon and trout aquaculture in New Zealand. They feared that disease would spread from fish farms into recreational fisheries, and that poaching of wild fish would increase if their sale was allowed. In 1973 the government declared trout farms illegal, but granted permission for salmon farms.
Koi carp (Cyprinus carpio) and tilapia (Oreochromis species) are amongst the most popular fish to be farmed worldwide, but they are unwelcome in New Zealand. They destroy native plant and fish habitats in freshwater systems, and are classed as noxious. It is illegal to breed, sell or release them into the wild.
The first salmon farm was established in 1976 at Waikoropupū Springs, near Tākaka in Golden Bay. Salmon were raised in fresh water, growing to a length of 25 centimetres in two years. The venture was originally set up for ocean ranching, where juvenile salmon are released into the sea in the expectation that some will return as adults. But few did return. The Waikoropupū Springs facility was converted into a hatchery, supplying stock to sea farms. Other ocean-ranching salmon farms were set up in the South Island, but proved uneconomic.
The first sea-cage salmon farm was developed in Big Glory Bay, Stewart Island, in 1983. It was soon followed by farms in the Marlborough Sounds and at Akaroa, Banks Peninsula. These three production areas accounted for 93% of the 8,500 tonnes of salmon produced in 2001. In sea-cage ranching young fish (smolt) are taken from freshwater hatcheries and transferred to net cages about 25 metres across and 15 metres deep, sunk in clean, fast-flowing coastal waters. The salmon remain in the cages all their life, and are fed fishmeal pellets which are high in protein and oil. The fish are harvested at two or three years of age, at weights between 2 and 4 kilograms. Approximately 65% of salmon is exported. Japan is the main market, but smaller quantities go to Australia and Pacific rim countries. Export sales were worth $39 million in 2003.
Farmed salmon are anaesthetised with a herbal extract before being killed by the instant method of brain spiking. The heart continues to beat for a time while the animal is bled from its sliced gills. Because the creature is in a relaxed state when it is killed, a firm, long-keeping flesh is produced.
A unique form of salmon farming developed in the hydroelectric canals of the central South Island in the 1990s. Young salmon were enclosed in net pens in the Ōhau and Tekapo canals, and reared in a similar manner to sea-farmed salmon. The Tekapo site, at 677 metres above sea level, is the highest salmon farm in the world and is fed by fast-flowing cold waters from the Southern Alps.