 Oysters , mussels , clams , scallops and other bivalve species are grown with food materials that occur naturally in their culture environment in the sea and lagoons . One @-@ third of the world ’ s farmed food fish harvested in 2010 was achieved without the use of feed , through the production of bivalves and filter @-@ feeding carps . European flat oysters ( Ostrea edulis ) were first farmed by the Romans in shallow ponds and similar techniques are still in use . Seed oysters are either raised in a hatchery or harvested from the wild . Hatchery production provides some control of the broodstock but remains problematic because disease @-@ resistant strains of this oyster have not yet been developed . Wild spats are harvested either by broadcasting empty mussel shells on the seabed or by the use of long , small @-@ mesh nets filled with mussel shells supported on steel frames . The oyster larvae preferentially settle out on the mussel shells . Juvenile oysters are then grown on in nursery trays and are transferred to open waters when they reach 5 to 6 millimetres ( 0 @.@ 20 to 0 @.@ 24 in ) in length . 
