Report: Chesapeake oysters gain disease resistance

Fewer oysters in the Chesapeake Bay are dying from the diseases that have devastated the bivalve population in recent decades, leading some to believe they may be developing a natural resistance, according to a new report by the Chesapeake Bay Foundation.

Dermo and MSX, the two parasites that have been killing oysters, still afflict them throughout the bay — but scientists are seeing more oysters surviving, the Annapolis-based environmental group reports.

Citing data from the Maryland Department of Natural Resources, the report says that disease-related oyster mortality in the upper bay declined from 2005 through 2009 to 17 percent a year, down from 29 percent on average from 1985 through 2004.

Both parasites seem to spread and kill more readily in saltier water, as is generally found in Virginia's portion of the bay. Dermo, which is more tolerant of low salinity, is the greater problem in Maryland waters.

A scientist with the Virginia Institute of Marine Science, Ryan Carnegie, has seen marked declines both in disease prevalence and death among oysters in that state. In the York River, for instance, fewer than 5 percent are dying from MSX now, compared with more than 50 percent a decade ago, the report says. In the lower James River, only about 24 percent of oysters were infected with MSX in 2009, down from 80 percent in 1995.

Dermo continues to plague Virginia oysters, but the foundation report says mortality appears to have moderated in the Lynnhaven and Great Wicomico rivers, where oyster reefs have been built up using old shell dredged from the bay floor.

The environmental group says the improved survival of bay oysters supports Maryland's new oyster restoration strategy. State officials are moving to ease commercial harvest pressure on the bivalves by expanding sanctuaries and encouraging watermen to move into private aquaculture, as is already being done in Virginia.

Maryland's Department of Natural Resources is holding four public hearings on proposed regulations to set aside more sanctuaries and open up more of the bay for leasing to private aquaculture. The dates and locations are:

•6 p.m. today, Anne Arundel Community College, Pascal Center for the Performing Arts, 101 College Parkway, Arnold.

•6 p.m. July 13, Leonardtown High School Auditorium, 23995 Point Lookout Road, Leonardtown.

•6 p.m. July 22, Salisbury University, Caruthers Hall Auditorium, 1101 Camden Ave., Salisbury.

•6 p.m. Aug. 5, Chesapeake College, Todd Performing Arts Center, Route 50 and Route 213, Wye Mills.

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