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Suited in yellow rain gear, with hoods pulled up for protection against a stiff fall breeze, a small group of biologists prepared to dig into a mound of oysters piled onto clean, white tables.
Fortunately, they weren’t looking for dinner. The oysters were small, mostly dead—and often coated in black, sulfuric-smelling muck.
The biologists sorted and measured oysters one by one, chanting their findings to a clipboard-wielding recorder.
”Old box, 43.” (A dead oyster, 43 millimeters long.)
”72.” (A live oyster, 72 mm long.)
”Spat 28.” (A 28 mm baby oyster.)
Many samples contained more mud than oysters. Each batch had to be hosed down to wash off the worst of the muck. That’s a sign the oysters had been silted over or sunk into the sediment.
Since the Virginia patent tong survey began in 1993, samples of mud-covered shell have become more common. And it has led some scientists to a sobering conclusion: In many areas, oyster shell habitat is being lost faster than it can be replaced by today’s diminished oyster population.
The observation has given rise to a new term: the shell budget. Or, more accurately, the calcium carbonate budget – the material oyster shells are made from. In this accounting system, most Virginia tributaries are running deficits.
”That’s one of the biggest findings that has come out of 18 years of survey work,” said Jim Wesson, who oversees oyster restoration programs for the Virginia Marine Resources Commission. ”The shell budget is controlling everything.”
Recent papers using data from the Virginia survey show that outside of a couple of small bars on the James River, most oyster bars are in a downward spiral of shell loss that is difficult to reverse.
Oysters are one of the few species that build their own habitat. Living oyster larvae pull calcium carbonate out of the water and build shells that keep getting bigger as long as they live. Those shells create the solid surface that future oyster larvae need to land on and grow. As shell disappears, there are fewer places for baby oysters, or spat, to land, meaning there are fewer oysters to produce more shell.
”Once you start down this path, it reinforces and accelerates, and that is truly problematic,” said Roger Mann, a longtime oyster researcher at the Virginia Institute of Marine Science who has written several papers dealing with the shell budget in recent years.
The situation in Maryland is unknown – the state uses a different survey technique that can’t be used to calculate the shell balance. But officials there are concerned as well, and this year Mann and Maryland Department of Natural Resources biologists hope to begin a survey to start calculating a shell budget for the states’ oyster grounds.
The loss of oyster shell – already in short supply for restoration projects – also means fewer habitats not only for oysters, but the host of animals and plants that call an oyster reef home.
”Anything that lives around hard substrates or requires it for part of its life cycle is therefore going into a degraded system,” Mann said.
In addition, when algae and other Bay life die, they sink to the bottom and decompose, a process that makes sediment more acidic. Calcium from the natural decay of oyster shells helps to neutralize that acidity — much like antacids in the stomach. But with less shell, there is less ability to neutralize that acidity around reefs.
”Even if you are not interested in oysters, you should be worried that you don't have as much buffering capacity in surface sediments,” Mann said. ”It all comes down to oyster shells, and the fact that we don't have enough of them.”
The first oyster larvae to wash into the newly formed Bay around 10,000 years ago likely attached to a rock. Larvae then began growing, and over thousands of years built reef communities that became the dominant feature in the Chesapeake. Oysters on those reefs could live a decade or more, and reach a foot in length.
After European settlement, reefs underwent heavy harvest pressure. But the impact of that habitat loss was partly offset by good oyster reproduction. That equation changed in recent decades when oyster diseases, MSX and Dermo, killed many oysters before they reached market size. The situation reinforced the long-held view of management agencies that, since oysters were going to die, it was all right to harvest them – neglecting the fact that continued harvest further reduced shell habitat.
The smaller populations were less able to keep up with increased rates of siltation, further reducing the amount of suitable habitat for new oysters.
As old oysters with thick shells disappear because of disease and harvest, maintenance of the shell budget in many areas hinges on the annual production of large numbers of small oysters. That generally doesn't happen. In many areas, reefs cannot grow vertically fast enough to keep from sinking into, or being buried by, sediment.
As a result, the downward spiral reinforces itself. Less habitat means, even when oyster reproduction is good, there are fewer places for spat to land and grow, thus less opportunity to push the calcium carbonate balance to the positive side of the ledger sheet.
Shells are lost to a number of factors: They naturally dissolve in water; harvest removes them; oyster drills, barnacles, sponges and other organisms break them down; sedimentation buries them; and predators literally take a bite out of the shell budget.
Maryland oysters are generally subject to less predation and fewer fouling organisms, and in many places disease pressure is less severe. But spat sets in Maryland are poorer, on average, than those in Virginia, making it difficult to replace old oysters lost to harvest and disease, said Roger Newell, a longtime oyster researcher at the University of Maryland Center for Environmental Science.
The situation will grow worse. Newell and other scientists recently reported that climate change is causing the Bay to gradually become more acidic, leading to reduced oyster shell growth.
Right now, it’s unknown exactly how far out of balance the Bay’s shell budget is, but Mann said the deficit would be ”a very large number, and very scary.” Breaking the downward spiral is difficult, but some caution against assuming that the shell balance is already tipped in an unwinnable equation.
”Those things are sobering and need to be paid attention to, but I don't think any of it concludes that we cannot have a positive shell balance,” said Mark Luckenbach, a VIMS scientist who has studied oyster reef ecology and reef restoration.
Restoration projects often have taken place at marginal sites. Instead, Luckenbach said, to increase production, sites should be selected where oysters have the least mortality from predators, where sedimentation rates are lowest, and where fertilization rates are highest. Even then, he said, successful restoration may mean continued maintenance, rather than building a project and letting it try to survive on its own.
Scientists say the worsening oyster shell situation will make all restoration projects more challenging.
For decades, Maryland maintained a shell balance on many reefs by replenishing them annually with dredged fossil shells, but those shell supplies are nearly exhausted and the programs halted. Alternate substrates such as concrete reef balls or clean debris from demolition projects can help. But none work as well as oyster shell, and they tend to be expensive.
”There is only one way that you can get enough substrate that makes much sense,” Luckenbach said, ”and that is for a living oyster to pull the calcium out of the water column and build the shells.”
For many places, that day seems a long way off. On the survey boat the biologists found fewer than seven adult oysters per cubic meter scooped from the bottom – a sharp drop from the previous couple of years. Ominously, there were even fewer spat than adults. ”That,” Mann concluded, ”was the most pathetic morning I’ve ever spent on this boat.”
This article credits Bay Journal News
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