Studies on triploid oysters in Australia. 2. Growth, condition index gonad area, and glycogen content of triploid and diploid Pacific oysters, Crassostrea gigas, from oyster leases in Tasmania, Australia

This research was undertaken to evaluate triploid Pacific oysters under Tasmanian environmental conditions using local commercial growout techniques. Diploid Pacific oysters were strip spawned in February 1990, and diploid and meiosis 2 triploid larvae reaued from the same pool of gametes. Cytochalasin B (0.5 mg ^-1 for 20 min) was the triploidy induction stress used. After setting, the oyster spat were held onshore in upwellers and then offshore in intertidal sectionalised trays. From August 1990 they were grown intertidally at three commercial leases in oyster bags within 6 mm mesh baskets and then free in 12 mm baskets. At all sites triploids (76% triploidy) maintained marketable meat condition during the summer - autumn spawning period after attaining commercial size (about 60 g whole oyster weight). At the two sites where diploid oysters spawned they remained in much poorer condition than triploids for at least 4 months post spawning. At two sites oysters reached commercial size, as they approached 2 years of age and at the other site at 3 years of age. Triploid oysters grew faster than diploids as they approached commercial size and were 23.4% larger than diploids on a whole weight basis at the two better sites (age 27 months) and 19.6% larger at the poorest site (age 38 months). At the two better sites mortality of diploid and triploid oysters was negligible during growout. At the third site mortality of triploid and diploid oysters (excluding sampling and whole bags lost through storm damage) averaged 1.0 and 0.3% per month respectively, during growout through April 1993. A greater proportion of the cross sectional area of oyster meat was occupied by gonad in diploids (maximum of 47.7 to 72.6%, depending on site) than in triploids (maximum of 7.7 to 1.9%). Similar maximum glycogen reselves were developed by triploids (15.6 to 21.2 g 100 g ^-1 dry meat, depending on site) and diploids (14.5 to 20.2 g 100 g^-1 dry meat). Triploids maintained these reserves whereas diploids sacrificed most of their glycogen as they approached the spawning season. Triploid Pacific oysters show great promise for overcoming marketing problems caused by poor meat condition of diploid oysters in summer and autumn in Tasmania. However, to exploit this marketing niche, most of the triploids would need to be marketed at about 2 to 2.5 years of age and shell growth rates may have to be repressed through use of elevated growing heights. This should ensure that the oysters do not become too large or that rapid shell growth during the maturation and spawning season does not reduce their condition index advantage over diploids.