In April 2004, triploid native (Crassostrea virginica) and nonnative (Crassostrea ariakensis) oysters were deployed in cages at four sites along a salinity gradient in Chesapeake Bay. In Maryland, the lowest salinity site was located in the Severn River and two low to mid-salinity sites were located in the Choptank and Patuxent Rivers. The highest salinity site was located in the York River in Virginia. Growth, disease acquisition, and mortality were measured in the deployed oysters through August 2006. Although ANOVA revealed that the nonnative oysters were significantly larger at the end of the experiment than the native oysters at all sites, the differences were much greater at the Virginia site (59 mm) than in Maryland waters (9–23 mm). With the exception of C. ariakensis in the Severn River, Perkinsus marinus infected both species at all sites. Prevalences and weighted prevalences in both species remained relatively low throughout the experiment, but native oysters consistently acquired higher prevalences and weighted prevalences than C. ariakensis by August 2006. With the exception of several mortality-inducing events including winter freezing and hypoxic exposure, mortality was generally low in both species. No disease-related mortality was suspected in either species given the low weighted prevalences observed. In the York River, where a substantial natural spatfall occurred in 2004, more native spat were found on C. ariakensis than on C. virginica. To our knowledge, this is the first comparison of triploid C. ariakensis to triploid C. virginica conducted in the field. Because we did not observe substantial disease-related mortality, it is too soon to draw conclusions regarding the disease tolerance of C. ariakensis in the field or its viability as a replacement for the native species.
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