Unusual intensification of Chesapeake Bay oyster diseases during recent drought conditions

Two protozoan parasites of oysters. Haplosporidium nelsoni (MSX) and Perkinsus marinus. have severely limited the harvest of oysters in lower Chesapeake Bay during the past 30 years. The distribution and abundance of both parasites appears to be regulated by salinity and the three consecutive drought years from 1985-1987 have resulted in a dramatic increase in mortality from these diseases in Chesapeake Bay. Oyster mortality from MSX in monitoring trays at VIMS was 77% and 78% for 1986 and 1987 respectively. the highest values ever recorded in 28 years of continuous monitoring. In addition. Perkinsus was at record high levels and. for the first time in history. both diseases invaded and caused serious mortality in the upper James River seed-oyster area. The increased salinity resulting from low runoff over the watershed allowed both diseases to spread deep into the Maryland portion of the Bay in 1987 and cause serious mortality. Bay-wide oyster stocks are now severely depressed and prospects for rapid recovery are poor.