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

The effect of growing height on mortality of Sydney rock oysters, Saccostre a glomerata, caused by Australian winter mortality Mikrocytos roughleyi was evaluated on a commercial intertidal growing lease on the Georges River, N SW, Australia. The mortality level of 35.1 +/- 8.4% at normal growing heigh t, fell to 9.3 +/- 0.9% when oysters were cultured 300-mm higher, and incre ased to 52.4 +/- 0.2% at 300-mm lower in the tidal range. No significant gr owth penalty was evident at the higher growing level, and no relationship c ould be discerned between the mortalities at different growing levels and e ither glycogen content or condition index. The concern among oyster farmers , that M. roughleyi may cause higher mortality levels among single seed (cu ltch-less) diploid and triploid S. glomerata, compared with the same oyster s grown on sticks, was not supported in a trial at two intertidal growing l evels at Merimbula, NSW. These findings imply that under the conditions tes ted, 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 incre ased mortality level caused by this disease. Further trials in other growin g areas, and during a severe epizootic of M. roughleyi, are required before these findings can be generalized ro southern to mid-NSW. (C) 2000 Elsevie r Science B.V. All rights reserved.