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

The Sydney rock oyster breeding programme began in 1990 and initially focused on faster growth and resistance to winter mortality disease ( WM , Bonamia roughleyi ) before including QX disease ( QX , Marteilia sydneyi ) resistance in 1997. Four generations of oysters have now been exposed to these diseases at three sites in the Georges River, NSW , Australia (Lime Kiln Bar, LKB ; Woolooware Bay, WB ; and, Quibray Bay, QB ). Non‐selected control oysters and lines developed at each site (Line 1 – LKB , Line 2 – WB , Line 3 – QB ) were placed at the three sites alongside a new QX ‐resistant line (Line 4) bred from QX survivors in other NSW estuaries. Line 1 oysters grew to a marketable size (>50 g) in two years with minimal losses following QX disease outbreaks: 28% mortality compared with 97% in controls. Losses in Line 3 oysters selected for WM resistance at QB were reduced by more than half (23% versus 52%). Line 2 performed best at WB , but this line showed excellent resistance to QX at LKB and WM at QB , indicating that breeding for resistance to both diseases is effective. However, selection of oysters for QX did not confer resistance to WM and vice versa .