Flow, stocking density and treatment against Polydora spp. Influences on nursery growth and mortality of the oysters Crassostrea virginica and C. ariakensis

Triploid Suminoe oysters, Crassostrea ariakensis, are currently being considered for culture on the mid Atlantic coast of the USA. Despite studies comparing the performance of C. ariakensis with the native oyster, Crassostrea virginica, at the grow-out stage, growth and mortality of C. ariakensis and C. virginica have not been compared during nursery rearing in upwellers. If, as in grow-out bags, triploid C. ariakensis in upwellers grow at significantly faster rates than diploid C. virginica, they may suffer food limitation in intensive nursery systems at flows and densities where diploid C. virginica do not. If a consequence of the rapid growth of C. ariakensis is a thinner shell with greater susceptibility to boring worms, it may require potentially stressful treatment against Polydora spp. These hypotheses were tested in an orthogonal experiment, done in fall 2003, in which triploid C. ariakensis and diploid C. virginica were grown in upwellers with high (11.34×10^-3 m³ s ^-1) or low flow (3.56×10^-3 m³ s ^-1), that were treated or untreated against Polydora spp. (by hypersaline dip and drying) and stocked at high or low densities. In the first 2 weeks of the experiment, during which both species grew rapidly, growth of C. ariakensis was up to seven times greater than C. virginica. This difference was reduced in subsequent weeks, when water temperature began to decrease and rates of growth of each species slowed. Contrary to expectation, neither species displayed sizeable reductions in growth at high as compared to low density. Mortality of C. ariakensis was, however, 1-2% greater at high than low density. C. virginica appeared more susceptible to decreased growth at low flow, although the effect of flow in this experiment was small. Both species suffered greatest mortality when previously treated against Polydora spp. The treatment was, however, effective in reducing infestation, to which C. virginica, contrary to expectation, appeared most susceptible. Thus, it appears that similar conditions are suitable for the culture of triploid C. ariakensis in upwellers as for diploid C. virginica.