The effect of growing level and growing method on winter mortality, Mikrocytos roughleyi, in diploid and triploid Sydney rock oysters, Saccostrea glomerata

Abstract The effect of growing height on mortality of Sydney rock oysters, Saccostrea glomerata , caused by Australian winter mortality Mikrocytos roughleyi was evaluated on a commercial intertidal growing lease on the Georges River, NSW, Australia. The mortality level of 35.1±8.4% at normal growing height, fell to 9.3±0.9% when oysters were cultured 300-mm higher, and increased to 52.4±0.2% at 300-mm lower in the tidal range. No significant growth penalty was evident at the higher growing level, and no relationship could be discerned between the mortalities at different growing levels and either glycogen content or condition index. The concern among oyster farmers, that M. roughleyi may cause higher mortality levels among single seed (cultchless) diploid and triploid S. glomerata , compared with the same oysters grown on sticks, was not supported in a trial at two intertidal growing levels at Merimbula, NSW. These findings imply that under the conditions tested, considerable protection against M. roughleyi can be gained by raising the growing height over winter in southern NSW growing areas, and that the growth advantages of triploid S. glomerata can be utilized without an increased mortality level caused by this disease. Further trials in other growing areas, and during a severe epizootic of M. roughleyi , are required before these findings can be generalized to southern to mid-NSW.