Document Type



Virginia Institute of Marine Science

Publication Date



Journal of Shellfish Research





First Page


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Since 2012, aquacultured eastern oysters Crassostrea virginica have been reported by oyster farmers to display mortality approaching 30%, and in some cases 85%, in areas of the lower Chesapeake Bay, VA. Based on accounts from industry, this mortality has typically affected 1-y-old oysters between May and early July, and has tended to occur in triploid oysters, which represent the vast bulk of production in the area. During this period, samples submitted for pathology have not revealed the presence of major pathogens as a cause. In 2015, to gain deeper insight into this mortality and determine whether specific sites, ploidy condition, or genetic lines were affected, oyster seed commercially produced in early 2014 were obtained from four lines, one diploid (2N DEBY) and three triploid (3N DEBY, 3N hANA, and 3N Northern). These lines were deployed in July 2014 at aquaculture farms at five Chesapeake Bay locations: Locklies Creek and Milford Haven on the western shore, and Pungoteague Creek, Nassawadox Creek, and Cherrystone Creek on the Eastern Shore. During this study, mortality was observed to peak in June at most sites, reaching a mean mortality across all tested lines of 17.0% and a cumulative mortality for the study period of 32.0% at Nassawadox Creek, the site most severely affected by mortality that followed the expected early summer mortality pattern. Interval mortality at all sites decreased to under 5% after June, but cumulative levels for the study period reached from 8.8% to 18.6% even at the sites least affected by mortality. This represents a high level of mortality given the documented absence of material involvement by major oyster pathogens such as Hapolosporidium nelsoni and Perkinsus marinus. Infiltration of gill tissues by hemocytes, observed in up to 33% of individuals at Nassawadox Creek coincident with the increase in mortality, was the only pathology observed. Harmful algal blooms were not associated with the mortality, nor were abnormal temperatures or salinities. There was no clear relationship of mortality to oyster genetic heritage, although there was variability in susceptibility among oyster lines and interactions between lines and specific sites. At some locations and in comparison with diploids, triploid oysters appeared to be more susceptible to mortality. Mortality in triploids was coincident with the timing of peak gametogenic development in diploids. Given the lack of involvement by major pathogens and the possible association of mortality with oyster gametogenesis, future work should seek to better understand the suite of environmental stressors potentially impacting cultured oysters in these systems and their interactions with the physiology and energetics of these animals.




Crassostrea virginica, mollusc health, marine disease, summer mortality, aquaculture-environment interactions, polyploidy