Selective breeding of Sydney rock oysters for fast growth in Port Stephens and Georges River began in 1990. In 1997 the Georges River section of the project was modified to include breeding for Winter Mortality and QX disease resistance. There are currently no management or other options available to combat QX disease, other than selection for disease resistance. The successful breeding of a disease resistant oyster will not lead to a recovery of the industry in QX infested estuaries, but it is needed as insurance should yet another estuary fall to this disease. Selection for disease resistance to a Winter Mortality like disease, in eastern oysters in the US and Bonamia in flat oysters in France, has been very successful.
Following three generations of selection, time to market has been reduced by 6 months. The faster growth achieved with triploidy are fully additive to those obtained with selective breeding, to that triploids produced from 3rd generation selection lines reach market size 9 months earlier than wild caught oysters. These faster growth rates obtained without any change in meat yield or percentage shell cavity and the survival of the selection lines has been higher than the controls at all locations. The 5th generation Port Stephens breeding lines were produced in February 2001 and established on commercial leases in May 2001.
Mortality of oysters bred for resistance to QX disease for two generations was reduced from 86% for the controls to 64%, for the QX resistant breeding line at the worst affected site (Lime Kiln Bar) after two generations of selection. On the basis of these and other results from overseas it is expected that it is likely to take another four generations or eight years to reduce mortality from 62% to the background level of 10% and produce a fully QX disease resistant oyster. Unfortunately, these QX disease resistant oysters are carriers or hosts for parasites.