THE SUITABILITY OF LAND-BASED EVALUATIONS OF CRASSOSTREA-GIGAS (THUNBERG, 1793) AS AN INDICATOR OF PERFORMANCE IN THE FIELD

The introduction of Crassostrea gigas to the mid-Atlantic requires pri or knowledge of their likely ecological response, according to the Int ernational Council for Exploration of the Seas Committee guidelines. W ithout at least an experimental introduction, however, such knowledge is unattainable. Are comparisons of survival: growth, disease resistan ce, etc., conducted in land-based tanks suitable for estimating the pe rformance of C. gigas in the field? In June 1991, equal numbers of spa t from three crosses-MSX-resistant Crassostrea virginica (eastern), C. gigas form Miyagi, and C. gigas form Hiroshima-were split into two re plicates and reared in upwellers for the first summer and in a land-ba sed tank for the second. After the first season, C. virginica had the highest mortality (65, 36, and 13% for eastern, Miyagi, and Hiroshima, respectively) and average spat size was about 30% greater in both C. gigas groups. For the second year, the three crosses were transferred to a 16,000-L tank; two replicates of eastern oyster were also placed in Delaware Bay. Cumulative mortality for the second season (through 1 1/92) was eastern, 60%; Miyagi, 73%; Hiroshima, 93%; and eastern in De laware Bay, 37%. In the tank, Miyagi oysters grew fastest, followed by Hiroshima and eastern; however, eastern oysters grown in the field we re larger than all tank-reared groups. All oysters in the tank were in fested with Polydora websteri, C. gigas heavily and eastern oysters li ghtly; eastern oysters grown in the field were virtually free of infes tation. These data indicate that tank-based comparisons are unlikely t o yield a true measure of performance in the local environment.