Weather cited for better-than-average spatfalls in MD, VA
Scientists warn that reproduction success doesn't always lead to more adults in the future
Maryland last year saw the biggest baby oyster boom -or spatfall - in its portion of the Bay in more than a decade, providing a sliver of good news for the critical Chesapeake shellfish whose overall population hovers near historic lows.
Most Virginia rivers also reported better-than-average spatfalls.
But several scientists cautioned that good spat production - though good news - does not necessarily result in a greater number of adults in the future. "You have got to have the spat to have any chance," said Jim Wesson, who oversees oyster restoration programs for the Virginia Marine Resources Commission. But so many small oysters fall victim to predators or succumb to disease, he said, it's difficult to predict how many will be left after two or three years. "You can only read so much into it each year. You have to watch and see what happens."
Biologists said weather was likely the biggest factor leading to the good spat set. Much of the spring was cooler than normal before a rapid warm-up in June. The sudden temperature increase may have spurred many oysters to release sperm and eggs at the same time, causing greatly increased fertilization rates and larvae production.
Oyster spat are larvae that successfully attach to a solid substrate, usually other oyster shells on oyster bar, and begin growing and forming shells.
Maryland biologists reported that their annual fall oyster survey found an average of nearly 80 spat per bushel, the highest since 1997, when the spatfall index was 277. The index is the average number of spat found in each bushel sample collected at 53 oyster bars scattered throughout Maryland's portion of the Bay. The fall survey has been conducted annually since 1939.
Unlike 1997, when the index was driven primarily by the Eastern Bay count, the 2010 boom was widespread. Eleven bars had the highest or second highest spat counts since 1985. The heaviest spat counts were in southern Eastern Shore waters, where one sample showed 910 spat. 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. Oysters in those areas will be safe from harvest as they grow.
"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."
The Maryland survey also showed good news regarding the two diseases that have devastated oyster populations. While Dermo was widespread, infection rates were below the averages seen over the past two decades. Meanwhile, the geographic range of MSX distribution continued to decrease.
Non-fishing oyster mortality was just 12 percent, as low as it has been in the last 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 the average observed over the last quarter century.
Biologists hope that means oysters are developing some 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 a large-scale recovery of oysters in the Bay."
But biologists won't know how tolerant oysters are to disease until there is another drought. The 1997 spat set was followed by a series of poor years, driven in part by a four-year dry stretch from 1999 through 2002 which saw oyster populations plummet even further as drought conditions allowed MSX and Dermo, which thrive in high salinities, to flourish.
Spat sets are typically better in Virginia than Maryland because larvae production is better in its higher salinity water, however, survival is often better in Maryland where disease pressure is less intense and there are fewer predators.
Still, Virginia scientists said that 2010 spat counts were also much better than normal in most areas.
"In a lot of the James River it was the highest that we've seen in the last 20 years," said Melissa Southworth, who oversees the annual surveys for the Virginia Institute of Marine Science. "And the same holds true for the Piankatank and the Rappahannock. Those systems were among the highest that we've seen."
But surveys showed that other areas, such as Mobjack Bay and the Great Wicomico, were "not great."
"I'm not sure why that is," Southwick said. "It is very bizarre to me. Usually the Great Wicomico leads the pack."
Some areas of the river may have been better than others.
David Schulte, a marine biologist with the U.S. Army Corps of Engineers Norfolk District, said his surveys showed good spat sets on high reefs built as part of a Corps restoration project in the Great Wicomico in 2004-05, and that owners of nearby private leased grounds also reported good spat sets. He said low counts in other parts of the river may have been caused by degraded shell condition. "There is a lot of oyster larvae in the water, but they really don't have anything to settle on," he said.
Indeed, he and others say the lack of enough suitable habitat for spat to land on and grow is a major obstacle for oyster recovery in the Bay.
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 reefs are build before they are buried by sediment.
- Category:
Fisheries
Karl Blankenship is editor of the Bay Journal and Executive Director of Chesapeake Media Service. He has served as editor of the Bay Journal since its inception in 1991. .(JavaScript must be enabled to view this email address).
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