 Many juveniles are further reared off the seabed in suspended rafts , on floating trays or cemented to ropes . Here they are largely free from bottom @-@ dwelling predators such as starfish and crabs but more labour is required to tend them . They can be harvested by hand when they reach a suitable size . Other juveniles are laid directly on the seabed at the rate of 50 to 100 kilograms ( 110 to 220 lb ) per hectare . They grow on for about two years before being harvested by dredging . Survival rates are low at about 5 % . 
