Experimental deepwater culture of the Sydney rock oyster (Crassostrea commercialis): V. Commercial raft trials

Four large rafts (6.4 × 4.3 m), each costing $ A 543 totally equipped and holding 600 kg of trayed layers of oysters, were sited in New South Wales estuaries during 1977 and 1978. Small, single oysters (spat at 25–29 g whole weight or seconds at 29–40 g) were grown to restaurant (plate) grade (40–67 g).Sixteen crops were harvested, usually after three months growout. None showed evidence of overspatting (attachment of young oysters to the crop). Growth rates (measured as oyster weight increases) ranged from 5 to 30% per month and averaged 19%. In four of the crops mortality was high (12, 22, 22 and 90%) due to parasites, predators or wave action. In the remaining 12 crops natural mortality averaged 6% total for the growout period. The average actual wholesale value of the crops was $ A 422 and this could be raised to an estimated wholesale value of $A 527 per crop by utilising pregrowout grading and harvest date selection. Several husbandry practices are recommended. (1) Grading oysters mechanically before growout to produce more uniform crops. (2) Covering top trays to prevent fish attacks and losses through waves. (3) Deciding optimum harvest date by inspection after three months growth.