Commercialisation of triploid Sydney rock oysters, Saccostrea Glomerata, in New South Wales : growth, survival and meat condition

To aid the commercialisation of triploid technology for Sydney rock oysters (Saccostrea glomerata) in NSW, triploids and diploids were evaluated for growth, mortality and meat condition on commercial oyster farms throughout the state. After 21/2 years, triploids were on average 30.7% heavier and 8.6% larger in shell height than same parent diploids. Mortality of triploids was significantly lower (p 0.05) from that of diploids at 12 of the 13 sites. Average cumulative mortality after 21/2 years across the 13 sites was 28% for diploids compared to only 16% for triploids. Performance of triploids over diploids varied considerably between sites. Wild-caught diploids had lower growth rates and higher mortality than both diploid and triploid hatchery stock. At seven sites where oysters were exposed to the parasite Mikrocytos roughleyi (cause of "winter mortality"), triploid Sydney rock oysters survived the disease better than diploids. Cumulative mortality of diploids during the second winter/spring at these sites was 35% compared to only 12.2% for triploids. Meat condition of diploids and triploids varied between the five sites throughout NSW. Over the final year on leases, ploidy, month and the ploidy*month interaction had a significant effect on meat condition at all sites except for ploidy at the southern, Lake Pambula site. From March to December (autumn to the first month of summer) condition indices of triploids were greater, or not significantly different from those of diploids at all sites. Triploid Sydney rock oysters were susceptible to brown discolouration of the gonad surface. Discolouration occurred in localised areas of the gonad and was not correlated to condition index except for triploids at Lake Pambula. As discolouration was less noticeable during cooler months of the year, it coincided with the generally superior condition of triploids relative to diploids during winter and spring, so that triploids remain a viable winter crop for farmers throughout NSW. After two years, an experiment in Port Stephens showed triploid oysters from two initial size grades were heavier and larger than equivalent size grades of same parent diploids (p <0.05). Initial size grade had a significant effect on final mean whole weight and shell height for both ploidy types (r) < 0.05). There was no significant difference in the final percentage triploidy between small and large grade triploids. A large proportion of diploid/triploid mosaicism was detected in adult triploid oysters. To determine if improvements in growth of a selected oyster line (L2) were additive to the faster growth of triploids, the performance of diploid and triploid selected and control oysters (four oyster lines) was compared. After a grow-out period of 21 months both mean whole weights and shell heights were in the order: L2 triploids > control triploids > L2 diploids > control diploids. A significant (p < 0.05) site* line interaction effect on whole weights and shell heights was detected. Growth improvements from selective breeding and triploidy were found to be additive with L2 triploids being 63% heavier than control diploids after 21 months grow-out. In this experiment, the type of oyster had no effect on final condition index, percent cavity volume, percent shell weight or cumulative mortality. Both diploid and triploid selected oysters had significantly (p < 0.05) higher whole weight to shell height ratios than diploid and triploid control oysters. Triploid Sydney rock oysters were shown to outperform diploids throughout NSW in terms of growth, survival and meat condition. The demand for both diploid and triploid Sydney rock oyster spat is now increasing and the demonstrated ability to combine the growth advantages of triploidy with selective breeding will no doubt increase the demand for hatchery spat further. However, commercial uptake of triploid technology will rely on overcoming the problems of early larval and spat mortality of hatchery reared Sydney rock oysters to ensure continuity of supply to farmers