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 prior knowledge of their likely ecological response, according to the International Council for Exploration of the Seas Committee guidelines. Without at least an experimental introduction, however, such knowledge is unattainable. Are comparisons of survival: growth, disease resistance, etc., conducted in land-based tanks suitable for estimating the performance of C. gigas in the field? In June 1991, equal numbers of spat from three crosses-MSX-resistant Crassostrea virginica (eastern), C. gigas form Miyagi, and C. gigas form Hiroshima-were split into two replicates and reared in upwellers for the first summer and in a land-based 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 11/92) was eastern, 60%; Miyagi, 73%; Hiroshima, 93%; and eastern in Delaware Bay, 37%. In the tank, Miyagi oysters grew fastest, followed by Hiroshima and eastern; however, eastern oysters grown in the field were larger than all tank-reared groups. All oysters in the tank were infested with Polydora websteri, C. gigas heavily and eastern oysters lightly; eastern oysters grown in the field were virtually free of infestation. These data indicate that tank-based comparisons are unlikely to yield a true measure of performance in the local environment.