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 collecte d in February from public oyster grounds off the coast of Louisiana were ma intained in a recirculating system for 8 wks. For conditioning, water tempe rature in the system was gradually raised with a heat pump from 14 degrees C, and held at 25 degrees C for 6 wks. Oysters were fed a diet of algal pas te (Isochrysis galbana for the first 6 wks and Chaetocerus calcitrans for t he last 2 wks). Water quality, mortality, Perkinsus marinus infection, gona d development, and physiologic condition (dry tissue-to-dry shell ratio, dr y tissue-to-wet tissue ratio, digestive diverticula tubule ratio) were moni tored. At weeks 7 and 8, the laboratory-held oysters were compared with fie ld controls held at Grand Isle, Louisiana. Water quality in the system rema ined within target ranges. Mortality was low (18 of 300 oysters stocked) an d not associated with P. marinus infection. In the laboratory at week 1, th e gonads of all oysters sampled were classified as immature or in early dev elopment. By week 5, the gonads of 73% of oysters sampled were classified a s mature. Physiologic condition decreased in the laboratory. Field controls reached a higher mean gametic stage and were in better physiologic conditi on at the end of the 8-wk study. These differences were attributed to diffe rences in nutrition available between the field and laboratory. This study demonstrated that conditioning of Crassostrea virginica is possible in a cl osed, recirculating system, although improvements in nutrition would be use ful.