R/ODR-23 A

Description:

Oysters have long been critical to the ecology of the Chesapeake Bay and the base of a valuable fishery. Although they have declined dramatically over the last 20-30 years due to a combination of harvesting and disease, major efforts are presently underway in Maryland by the Department of Natural Resources and the Oyster Recovery Partnership to reestablish oyster populations. Maryland harvests averaged 2.5 million bushels (bu) during much of the 20th century, but decreased to 80 thousand bu by the 1993-94 season. Subsequent harvests climbed above 400 thousand bu as recently as 1998-99, but with projections for the 2002-03 season of 50 thousand bu, and possibly less the following year, populations are clearly not recovering as expected. Recent efforts to increase population size have tried to manage around disease by planting disease-free oysters in lower salinity areas that are less prone to disease pressures. However, after 3 years of drought, former lower-salinity areas are now 4-6 ppt above their normal ranges. Ongoing work funded by the National Sea Grant Oyster Disease Program in Maryland's Patuxent River, has shown that during drought conditions, initial SPF (specific-pathogen-free) oysters grew faster at the two higher-salinity sites, but survival was only 2-3% after 2 years. At the up-river site, survival was only slightly better. Obviously these survival rates are not acceptable for reestablishing oyster populations. To determine if survivorship can be effectively improved by the use of disease-resistant seed oysters, we propose to compare the dynamics of dermo disease transmission in generic (1) and disease-resistant (2) strains of SPF spat planted at three salinity regimes in dermo disease-enzootic waters, and to determine the effects of environmental parameters and resident oyster disease status on disease transmission rates, growth and survival.