Evaluation of the progeny of third-generation Sydney rock oyster Saccostrea glomerata (Gould, 1850) breeding lines for resistance to QX disease Marteilia sydneyi and winter mortality Bonamia roughleyi

A breeding program for Sydney rock oysters Saccostrea glomerata (Gould, 1850) has been selecting oysters for resistance to QX disease (Marteilia sydneyi) and winter mortality (Bonamia roughleyi) for three generations at three sites in Georges River, New South Wales, Australia. The experimental sites are located at the upper, middle and lower reaches of oyster growing areas in the estuary. QX disease mainly occurs in the middle and upper reaches and is most severe at the latter. Winter mortality on the other hand occurs mainly at the lower and middle reaches and is most severe at the former. Progeny of third‐generation Sydney rock oyster breeding lines were evaluated for resistance to both QX disease and winter mortality against a non‐selected control. Line 1, selected for QX disease resistance at the upper estuary site, had excellent resistance to one season of exposure to disease, but suffered high mortality during the second season of exposure. However, these oysters had already reached market size of 50 g whole weight, with low mortality at 2 years of age, before the second episode of QX disease. Line 2 showed good improvement in resistance to both diseases, whereas Line 3, was the most resistant to winter mortality. Selection for resistance to QX disease did not appear to confer resistance to winter mortality and the converse also applied.