Bay Journal

Survey will develop a clearer understanding of Bay oyster population

Data collected will be used to guide fishery management decisions

  • By Karl Blankenship on December 01, 2011
  • Comments are closed for this article.
DNR biologists, l-r, Robert Bussell, Mitch Tarnowski and Carol McCollough sort and measure oysters during the 2011 Oyster Survey aboard the Miss Kay in the Choptank River near La Trappe Creek. A commercial waterman tongs for the tasty bivalves in the background.  (Dave Harp) DNR biologist Chris Judy guides a dredge full of oyster shell and live oysters aboard the Miss Kay during the annual oyster survey. They were sampling in the Choptank River near Cambridge. Biologist Robert Bussell is at right. (Dave Harp) Maryland DNR biologists and a graduate student from University of Delaware (in blue hood) sort oysters by size in the Choptank River near Oxford, MD. Mitch Tarnowski, who led the survey, tallies

For the last century and a half, Chesapeake Bay oysters have yielded huge harvests and inspired debates, disputes and even open combat.

Yet during all that time, scientists and fishery managers never answered the most fundamental question about the Chesapeake's most iconic species: Just how many oysters are in what Native Americans called the Great Shellfish Bay?

Roger Mann, a longtime oyster researcher with the Virginia Institute of Marine Science, calls past efforts to assess the Baywide population "embarrassingly inadequate."

"We spend a lot of time reading about their demise, and endless things are said - some misguided, some not - about potential restoration. Yet it is often done without any single, statistically accurate barometer of the numbers that are out there," he said.

But the situation is about to change.

Sometime next year, Mann and a team of scientists from universities and agencies in Maryland and Virginia hope to complete what would constitute the first Baywide oyster stock assessment. The assessment will provide an overall population estimate, as well as a clearer idea as to whether the bivalves are increasing, decreasing or holding their own.

Further, the scientists hope to be able to break down oyster populations in specific rivers and on specific oyster bars by age; determine their growth rates; assess the level of disease present; and discern whether it is fishing pressure or mortality from disease that most limits the oyster population's increase.

This information is critical for fishery managers, who need it for informed decisions about harvest levels and restoration work. Yet, alone among major Chesapeake species, a Baywide stock assessment has never been done for oysters.

"It may not be very sexy to do this sort of stuff, but it is very basic," Mann said. "And until you get an estimate of what is out there, and how it is changing over time, it is very difficult to make decisions about what's good and what works."

The assessment is being funded by the National Oceanic and Atmospheric Administration's Chesapeake Bay Office, which previously supported blue crab stock assessments that established similar much-needed baseline information and recently led to an overhaul of how that fishery is managed in the Chesapeake.

Oysters were so abundant when European settlers arrived that ships sometimes ran aground on oyster bars that towered far above the Bay floor. By the mid-1800s, the bivalves were supporting massive harvests that surpassed 20 million bushels a year as new technologies allowed Chesapeake oysters to be canned and shipped across the country.

But the resource has often divided Maryland and Virginia. Bitter disputes arose between the states over harvesting methods and they sometimes resulted in outright combat. Maryland even formed an "oyster navy" to combat "oyster pirates" and enforce its laws.

The heavy harvests took a toll on the population and harvests declined in the 1900s. Shortly after the mid-century, two oyster diseases - MSX and Dermo - further hammered the bivalves, sending populations to all-time lows. Today, harvests peak at a few hundred thousand bushels, in good years.

Some estimates place the Baywide oyster population at 1 percent of historic levels, but generally stable. But a recent analysis by scientists at the University of Maryland Center for Environmental Science put the Maryland oyster population at 0.3 percent of historic levels, and continuing to decline - conclusions questioned by some other scientists.

Complicating the picture even more is the role of continued harvest on oyster populations. The UMCES study recommended a moratorium on oyster harvests to help protect the population. But some other scientists and managers contend that harvests have little impact on the population because market size oysters are likely to quickly succumb to disease.

"There is a big dispute among researchers," said Mike Naylor, who heads the Maryland Department of Natural Resources' shellfish program, which is assisting with the assessment. "Some believe disease is the driver for oyster populations in the Bay, and others suggest that fishing-related mortality is more important. Hopefully, the new stock assessment will help inform that discussion."

A good stock assessment, peer-reviewed by independent scientists, could build consensus about the current oyster situation and help guide future decisions.

The biggest obstacle to a solid, shared assessment is the differences in the way that the states collect data.

"It's almost as if Maryland and Virginia didn't agree, on principal, to anything," Mann said. "Even the bushel is a different size."

Indeed, a Virginia oyster bushel is 3,003 cubic inches; a Maryland bushel is 2,800 cubic inches. And the two states use completely different methods to survey oysters and rank oyster disease infections.

Mann and Steve Jordan, the former director of the Cooperative Oxford Laboratory in Maryland, attempted to piece together a stock estimate from disparate sets of information a decade ago. But, Mann said, "each little thing you tried to compare was done slightly differently. And when you added them all up, the cumulative difference was really considerable."

VIMS conducts an annual survey of Virginia oyster bars using metal tongs that grab a cubic meter sample from the bottom. Scientists sort, count and measure each oyster, getting a precise measure for the cubic meter. Random samples are collected from each bar in the state, which ultimately allows scientists to estimate the number of oysters on each bar.

But covering Maryland's more extensive oyster bars with a labor-intensive patent tong survey is not considered feasible. The state has a longstanding dredge survey developed originally to help monitor spat sets. This survey covers a far greater area, but the data is less precise than that developed by patent tongs. The dredge, pulled by a boat, bounces along the bottom; it collects oysters from some places and bounces over oysters in others, and once it's filled, it doesn't collect anything at all.

A big part of the stock assessment effort is developing ways to credibly compare information collected by such vastly different techniques. Scientists hope to modify the Maryland collection process so the oyster dredge will be pulled a set distance for a specific period of time. Information collected from the dredge samples will be adjusted based on oyster samples collected from 43 sentinel sites surveyed using patent tongs.

Adapting the dredge survey data to get more precise oyster counts will be challenging, Mann said, but has been done with other shellfish, including Delaware's oyster fishery. "We really are, from a quantitative point of view, in a very different place in terms of being able to make those comparisons than we were 10 years ago," he said.

Scientists also need to coordinate disease reporting. With Dermo, the severity of disease is ranked on a seven-point scale in Maryland, and a five-point scale in Virginia. The two scales can be standardized without great difficulty, said Ryan Carnegie, an oyster disease researcher at VIMS.

But with MSX, the states describe the infections in entirely different ways, he added. "We see so much more MSX in Virginia than they do in Maryland that they are a lot more impressed by a relatively small number of MSX cells than we are down here. We have to adjust the scales, and how we are characterizing the disease."

The assessment will also collect new information about the status of oyster shell around the Bay. Shells are a critical building block of oyster populations, but work in Virginia indicates that the amount of shell — and therefore potential oyster habitat — is steadily shrinking. The assessment will provide the first Baywide look at the situation.

It will also test new high-tech monitoring techniques, such as the side-scan sonar, which may allow surveys to cover more ground at less cost in the future.

Once the oyster survey and techniques are comparable between the two states, scientists will be able to review records dating back for at least the past couple of decades and piece together a clearer picture of population trends by age, and the impact - if any - of fisheries on oyster stocks Baywide, in individual tributaries and even individual oyster bars.

"If this study shows that fishing-related mortality is far more important than we realized, that argues for greatly restricting the fishing effort to allow populations to rebuild," Naylor said. "If the study suggests that it is true that disease-related mortality is the population driver, then making the fishery smaller wouldn't necessarily have any benefit. So I think this study has the potential to dramatically change the way that Maryland manages its fishery."

But, he said, it could show that the truth is more complicated - that in some places disease is most important in influencing populations, while harvest is most important in other areas.

"I'm personally just curious," Naylor said. "I can see a strong argument for both sides."

Naylor said he has seen oyster bars surrounded by fishing boats that are picked clean of market size oysters in a matter of days. "It's hard to believe that wouldn't have an impact," he said.

"At the same time," he added, "I have been to oyster bars that haven't been fished in decades, and there are still very few oysters there. They haven't come back and people quit fishing them. And when we look, oyster disease is all over. So clearly, disease is a huge driver."

Likewise, Mann said that Virginia data often show tremendous volatility in oyster numbers, even on unfished reefs. "The numbers go all over the place," he said.

Nonetheless, scientists and fishery managers in Virginia have recently begun to use some of the survey data to make future projections of population size, which are used to help guide harvests in some areas. They are also using it to experiment with rotating harvests in a way that ensures old oysters that have survived disease reproduce, without totally closing areas to fishing.

The ultimate hope is that the oyster effort will follow in the path set out by blue crabs. One to two decades ago, there were wildly different views about how - even whether - blue crabs should be managed. But NOAA began funding a Baywide winter dredge survey to help estimate the population. The survey has proven to be a remarkably accurate tool to measure the blue crab population.

Meanwhile, a series of peer-reviewed stock assessments begun in the late 1990s used that data to help provide increasingly precise estimates of the overall health of the stock, ultimately showing that it was being overfished. That led to bi-state efforts to slash catches in recent years that have helped the stock rebuild from near-record lows.

Because NOAA has no management responsibility for oysters, officials hope the project will be viewed as unbiased, and build common ground around a species whose management remains so polarizing that some scientists and managers barely talk to one another.

"We are trying to be objective parties that can support such an assessment and provide it to the management community," said Peyton Robertson, director of NOAA's Bay Office. "It will hopefully be viewed as the reliable source of information on the status of oyster stocks in the Bay. Hopefully, that will provide a solid baseline from which to evaluate the protection and restoration efforts as we proceed."

Indeed, just knowing how many oysters are in the Bay would be a landmark step forward.

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About Karl Blankenship

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).

Read more articles by Karl Blankenship

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