5th Consecutive Year of Good Native-Oyster Survivorship

Oxford, Md. — Maryland Department of Natural Resources fisheries biologists recently concluded the 2008 oyster survey, an annual assessment of the health and population of oyster bars in Maryland’s portion of the Chesapeake Bay. Since 1939, each fall biologists use a dredge to monitor natural oyster bars, seed production and planting areas, dredged and fresh shell plantings and oyster sanctuaries.

“Preliminary results from 2008 indicate that reproduction was poor throughout most of the bay, with the exception of the lower eastern shore areas of Tangier Sound, Honga River, and the Little Choptank River,” said Mitch Tarnowski, DNR fisheries biologist. “In general, mortalities and oyster disease levels appear to be relatively low again this year.”

During the late 1950’s, biologists starting seeing the effects of Dermo (Perkinsus marinus) and MSX (Haplosporidium nelsoni) diseases on the bay’s oyster population. While not harmful to humans, these parasitic organisms infect oysters and have lead to the death of up to 90 percent of oysters in some areas. Both diseases thrive in higher salinities, so mortality is much higher in the lower parts of the Bay.

Typically, disease causes the greatest problems during years with higher than average salinity brought about by lower than normal stream flow. With higher salinity in the bay between 1999 and 2002, MSX, Dermo and oyster mortality was higher than average. The relatively wet years between 2003 and 2006 translated to reduced disease pressure and lower oyster mortality.

As part of the 2008 survey, biologists assessed more than 1,800 oysters collected from 282 oyster bars. The dry summer of 2007 did not result in high disease levels similar to previous dry years. Dermo increased in 2007, but remained below normal in both prevalence and intensity. MSX increased in frequency, but for the fifth straight year observed oyster mortality remained low.

“Oyster mortality in 2006 and 2007 were the two lowest years since the 1980’s. It’s too early to know if this is a trend, but this is a very positive development that we will be monitoring carefully,” said Mike Naylor, Director of DNR’s Shellfish Program.