Dual disease resistance in a selectively bred eastern oyster, Crassostrea virginica, strain tested in Chesapeake Bay

Selective breeding efforts have yielded oyster strains, Crassostrea virginica, with improved survival and resistance against Haplosporidium nelsoni (MSX); however, because of susceptibility to the oyster pathogen Perkinsus marinus (Dermo), their utility has been limited in areas where the two parasites co-occur. Dual resistance to H. nelsoni and P marinus was achieved through four generations of artificial selection of wild Delaware Bay oyster progeny at a site in the lower York River, Virginia, USA where both diseases are enzootic. During 1993-1995, survival, growth, and disease susceptibility of third generation Delaware Bay (F-3-DEBY) oysters were evaluated at the York River site in comparison to that of similarly selected third generation James River, Virginia oysters (F-3-JR), and first generation Louisiana oysters (F-1-LA), whose parents were naturally selected in the wild for resistance to P marinus. During 1997-1999, the performance of F-4-DEBY was evaluated at three sites in Virginia in comparison to two groups of first generation oysters whose parents originated from Mobjack Bay and Tangier Sound, Virginia where both R. marinus and H. nelsoni are enzootic. In the presence of high infection pressure from both H. nelsoni and P. marinus, the F-3-DEBY stock showed significantly higher survival and growth than either the F-3-JR or F-1-LA strain. After 15 months of deployment, 79% of F-3-DEBY, 11% of F-3-JR and 17% of F-1-LA oysters were market size (greater than or equal to76.2 mm) and cumulative mortality was only 16% in F3-DEBY as compared to 42% in FOR and F-1-LA. At the termination of the study, F-3-DEBY oysters exhibited 22% lower mortality than the F-1-LA stock, which outperformed the F-3-JR stock. Relative performance in respect to disease varied considerably with sample date; however, average H. nelsoni weighted prevalence varied such that F-3-DEBY