Conditioning of eastern oysters in a closed, recirculating system

Techniques were developed for holding and conditioning of eastern oysters, Crassostrea virginica (Gmelin), in a recirculating system. Oysters collected in February from public oyster grounds off the coast of Louisiana were maintained in a recirculating system for 8 wks. For conditioning, water temperature in the system was gradually raised with a heat pump from 14 degree C, and held at 25 degree C for 6 wks. Oysters were fed a diet of algal paste (Isochrysis galbana for the first 6 wks and Chaetocerus calcitrans for the last 2 wks). Water quality, mortality, Perkinsus marinus infection, gonad development, and physiologic condition (dry tissue-to-dry shell ratio, dry tissue-to-wet tissue ratio, digestive diverticula tubule ratio) were monitored. At weeks 7 and 8, the laboratory-held oysters were compared with field controls held at Grand Isle, Louisiana. Water quality in the system remained within target ranges. Mortality was low (18 of 300 oysters stocked) and not associated with P. marinus infection. In the laboratory at week 1, the gonads of all oysters sampled were classified as immature or in early development. By week 5, the gonads of 73% of oysters sampled were classified as mature. Physiologic condition decreased in the laboratory. Field controls reached a higher mean gametic stage and were in better physiologic condition at the end of the 8-wk study. These differences were attributed to differences in nutrition available between the field and laboratory. This study demonstrated that conditioning of Crassostrea virginica is possible in a closed, recirculating system, although improvements in nutrition would be useful.