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DNR Releases Preliminary Oyster Mortality Rates For The Upper Bay

Fall Survey finds concentrated pockets of high oyster mortality

Annapolis, Md. (November 9, 2011) - In preliminary findings from the Maryland Department of Natural Resources (DNR) annual Fall Oyster Survey biologists have found concentrated pockets of dead oysters in some areas of the upper Bay, caused by record-high flows of freshwater.

In early November, as part of the Bay-wide survey, biologists collected samples from 15 individual oyster bars north of the Bay Bridge. In the four northernmost bars along the Eastern Shore, oysters suffered a cumulative mortality of 79 percent, with no live oysters on the two northernmost bars. The few live oysters that were found in upper Bay bars were in poor condition — bloated, watery and translucent — and mortalities may continue for some time.

Higher than normal mortality rates were also found along the Western Shore, North of the Bay Bridge. The combined mortality rate for the six bars in the mainstem of the bay between the Magothy River and Man O War shoal at the mouth of the Patapsco Riverwas 74 percent, a 7-fold increase over the 2010 rate of 11 percent. While biologists are still analyzing data from the rest of the Bay, preliminary results indicate low mortality from Sandy Point to points south, and oysters in these areas seemed in prime condition.

Biologists believe the high mortality was caused by the lack of salinity in the upper Bay from March through July, 2011 having set many modern day records for high flow and low salinity. March to May freshwater flows were the highest ever recorded in the Susquehanna River, driving salinity levels to record lows at many Maryland monitoring stations by June. The fact that mortality was highest along the upper western shore, where salinity was lowest, reinforces this hypothesis.

“This is a setback for the upper Bay,” said Mike Naylor, DNR shellfish program director. “This hits a fragile area of the Bay particularly hard. There were very few mature oysters in this area of the Bay to begin with, but it’s very disheartening to see that the remarkable, encouraging 2010 spatset has been lost.”

The loss of young oysters from the widespread 2010 spat set – which was vital to sustaining the fragile upper Bay populations -- is particularly unfortunate. These areas only receive a significant spat set every 10 years or so. Although spat sets in this region usually survive, spat are vulnerable to the spring freshwater flows and this year was no exception. (After the Gulf oil spill, deliberate flooding of the Mississippi delta to keep the oil at bay caused low salinities resulting in massive oyster mortality in Louisiana and Mississippi – in excess of 95 percent mortality in some areas.)

“These types of oyster kills commonly occur when oysters grow near a major source of freshwater,” added Naylor. “Widespread freshwater losses in the Chesapeake Bay have occurred many times over the past century, with severe die-offs in 1909, 1944, 1958, 1972 and 1993.”

While biologists are not completely certain when Maryland’s oysters died, barnacles and other fouling inside some of the dead oysters suggest many died during the March to July freshwater flows. The less-fouled condition of other dead oysters indicates that there could have been an additional die-off after the tropical storms. Sediment from Hurricane Irene and Tropical Storm Lee does not appear to have been a factor, as evidenced by live organisms (including barnacles, mussels, bryozoans, etc.) found in the sampling that would not have been able to attach to the oysters shells or survive.

Photos of various oysters

“Unfortunately, this die-off will have an economic impact to oystermen in the upper Bay, which is an important area for skipjacks and patent tongers,” said DNR Secretary John Griffin. “Our scientists and managers will begin working with stakeholders immediately to explore ways to restore our native oyster to these areas.”

Over the past two years, 30 watermen reported oyster harvests in the high mortality areas of the upper Bay, averaging 8 days harvesting and 102 bushels per person per year. Cumulatively, watermen reported an average annual harvest of 3,047 bushels north of the Bay Bridge in the main stem for 2010 and 2011. At an average price of $40 per bushel (as reported by watermen), the loss would translate into an estimated dockside value of $121,860 each year. The upper Bay harvest is approximately 2 percent of the total Bay-wide harvest.

The approximate acreage of the bars in the upper mainstem of the Bay (from the Bay Bridge north) is or about 1.6 percent of the total acreage of all the mapped bars in the Maryland portion of the Chesapeake.

Oyster maps from 2010 and for 2011 showing a higher rate of mortality in 2011

   November 9, 2011

Contact: Josh Davidsburg
410-260-8002 office I 410-507-7526 cell

The Maryland Department of Natural Resources is the state agency responsible for providing natural and living resource-related services to citizens and visitors. DNR manages nearly one-half million acres of public lands and 17,000 miles of waterways, along with Maryland's forests, fisheries and wildlife for maximum environmental, economic and quality of life benefits. A national leader in land conservation, DNR-managed parks and natural, historic and cultural resources attract 11 million visitors annually. DNR is the lead agency in Maryland's effort to restore the Chesapeake Bay, the state's number one environmental priority. DNR is the lead agency in Maryland's effort to restore the Chesapeake Bay, the state's number one environmental priority. Learn more at