Shellfish such as oysters and mussels need only a fairly primitive form of husbandry to attain worthwhile yields and have the potential to make a significant contribution to world protein production. Some remarkably high rates of meat production are claimed. For example, annual yields of about 250-300 metric tons of meat per hectare have been quoted for raft culture of mussels around the Spanish coast. Japanese raft culture of oysters achieves 50 tonnes of meat per hectare per year. In the UK both mussels and oysters are widely cultivated (Spencer, 1990).
A variety of culture methods is used around the world. In France, mussels are traditionally grown on upright posts or buchots and also as beds on the seafloor. Both oysters and mussels are commonly grown on hanging ropes suspended beneath rafts. In some areas such as Xiamen harbour in China, the rafts are so densely packed that it is difficult for boats to find their way between them. Culture depends largely on larvae (spat) settling naturally on the substrate provided by the cultivator. In some cases, the spat is collected from settlement areas and then transferred to trays, mesh bags or beds for growing on. In the Irish Sea, experimental spat collectors for scallops have been developed at the Marine Station on the Isle of Man. The idea here is not to grow the scallops to maturity but to seed fishing areas with young scallops.
Molluscs are cultivated not only to provide food, but also for other products such as pearls. Recently experiments have started in Australia, Micronesia and Papua New Guinea, in rearing giant clams (Tridacna), valued both for their meat and their shells. If successful, this may relieve pressure on threatened wild populations. Other cultivated molluscs include cockles, the abalone (Haliotis) and various clams.
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