Growth and Mortality of North Carolina-Heritage Oysters, Crassostrea virginica, in North Carolina and in Chesapeake Bay

Abstract

ABSTRACT Populations of the Eastern oyster, Crassostrea virginica, in Chesapeake Bay have been greatly reduced over the last decade by two protozoan parasites. One of these, Perkinsus marinus, has been especially harmful, sometimes killing 70-80% of harvestable oysters in a given area. However, while P. marinus has caused substantial mortalities in the Chesapeake, oysters in states farther south and along the Gulf coast have not experienced such intense effects. This may be due to differences in P. marinus abundance and virulence at various sites or to differences in the tolerance of C. virginica stocks or to a combination of these and other factors. During the period March 1992 to May 1993, growth, intensity of infection with P. marinus, and survival of native North Carolina oysters (NC) held under standard tray culture conditions at several sites in both regions (Wye River, Maryland; Mobjack Bay, Virginia; Pamlico River, North Carolina; and Bogue Banks, North Carolina) were evaluated. At the Chesapeake Bay sites (Wye River and Mobjack Bay) the performance of the NC strain to native Chesapeake Bay oysters (CB) spawned synchronously were compared. Both NC and CB oyster spat were introduced at average lengths of 20-22 mm in March 1992. P. marinus infection reached 100% in oysters held at all high and moderate salinity sites by July 1992, at which time the CB strain held in Chesapeake Bay ceased to grow. Shortly after growth ceased, CB oysters experienced an accelerated mortality that rapidly progressed to 100%. Meanwhile, the native NC strain held at high and moderate salinity sites in both regions also experienced mortality, but at a much reduced rate of only 20-30%. High levels of P. marinus infection were accompanied by continued growth of the NC strain. When mortality did occur in the NC strain, it was associated with a much higher intensity of P. marinus than the infection intensity correlated with death of CB oysters. At the low salinity site, P. marinus infection persisted at low intensity throughout the latter portion of the culture period but was not associated with mortality of either strain. The trends in growth and disease tolerance for the two strains suggest that performance of oysters is related to genetic makeup and to other factors such as acclimatization and virulence off! marinus.