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Chesapeake Bay Oyster Survey Gives Good News

by Karl Blankenship

Virginia and Maryland last year had the biggest baby oyster boom in more than a decade, providing a sliver of good news for the critical Chesapeake shellfish whose overall population hovers near historic lows.

The annual fall oyster survey found an average of nearly 80 spat, or baby oysters, per bushel, the highest since 1997, when the spatfall index, as the count is called, was 277. The annual fall survey also found low disease mortality for the seventh straight year.

The index is the average number of spat found in each bushel sample collected at 53 oyster bars scattered through Maryland’s portion of the Bay. The fall survey has been conducted annually since 1939.

Unlike 1997, when the index was driven primarily by a very heavy set in Eastern Bay, the 2010 boom was widespread. Eleven bars had the highest or second highest spat counts since 1985.

The heaviest spat counts were in the southern Eastern Shore, where one sample showed 910 spat. But baby oysters were also found in low salinity areas, where spat is found only about once a decade.

The timing couldn’t have been better, biologists say. Last year, in a controversial move, Maryland set aside about a quarter of its high quality oyster bars as protected sanctuaries. Spat in those areas will be protected from harvest.

“Historically, when an area was open to fishing, once you developed a high concentration of mature oysters, they would be quickly discovered and exploited,” said Mike Naylor, director of the Maryland Department of Natural Resources Shellfish Program. “Now you can have high density oyster reefs that will be left alone.”

A good spat set does not guarantee a boost in future populations. The 1997 spat set was followed by a series of poor years, driven in part by a four-year drought from 1999 through 2002 which saw oyster populations plummet even further as drier conditions allowed the diseases MSX and Dermo, which thrive in high salinities, to flourish.

This year’s survey also showed good news for disease. While Dermo was widespread, infection rates were below averages seen over the past two decades, and the range of MSX continued to decrease.

Non-fishing oyster mortality was just 12 percent, as low as it’s been in the past quarter century when the diseases began devastating oyster populations in the state. That was a dramatic drop from the worst year, 2002, when drought conditions resulted in oyster mortality reaching about 60 percent. The survey showed that 2010 was the seventh straight year oyster mortality has been below average observed over the past quarter century.

Biologists hope that means oysters are showing some signs of disease tolerance, or even resistance. “This is exactly what we need to have happen,” Naylor said. “We need to have disease resistance develop if there is any real hope of large scale recovery of oysters in the Bay.”

But biologists won’t know for sure how tolerant oysters are to disease until there is another drought. “That is when you tend to have massive, widespread mortality,” Naylor said. “But droughts are natural, and to some extent, we have to expect that there will be setbacks. We are not going to just see a continual climb in oyster populations. We will move forward and backwards, hopefully lurching in an upward direction.”

Even if positive trends continue, the Bay is, at best, decades from seeing a large-scale recovery in oyster populations. The amount of oyster habitat in the Bay is thought to be less than 10 percent of historic levels.

“We still have dramatically reduced physical oyster habitat,” Naylor said. “The better spat sets we are seeing are only occurring in a very small percentage of the historic habitat.”

Creating large amounts of new habitat hinges on oysters being able to survive long enough, and grow large enough, so oyster shell builds up faster than it is buried by sediment.

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