Most of us think of oysters on the half-shell, in a milky stew or breaded and fried. But oysters are much more. “Oysters are too valuable to the overall ecological balance of the Bay to think of them only as food,” chides Roger Newell, a researcher at the University of Maryland’s Center for Environmental Science at Horn Point.
Oysters are filter-feeders that help keep Bay waters clear. In addition, their reefs, which once rose high from the Bay bottom, provided important shelter for young fish, food for older fish and solid structures for numerous other filter-feeders, such as clams and mussels.
Because of their importance, restoring the oyster population is often considered a key part of the Bay cleanup effort. Across the region, researchers, citizens, governments and environmental organizations are diving into a multitude of projects, hopeful that more oysters will filter the Bay’s waters and grace our tables in the future.
But returning vast oyster populations to the Chesapeake Bay will require the protection of this valuable natural resource and their habitat, plus the wise culture and harvest of this delicacy. Restoring the oyster population is a complex task. It requires more than just rearing large numbers in hatcheries and releasing them — a strategy successful with many fish. It also requires full-scale habitat re-creation efforts, mainly in the form of reef reconstruction. The task is further complicated by two parasitic diseases, MSX and Dermo, which can decimate oyster populations.
Habitat restoration is critical because, while oysters are flexible creatures that can tolerate many of the Bay’s swings in temperature, salinity, dissolved oxygen and clarity, they’re not as flexible about where they attach. Young oysters require hard substrate to latch onto and begin growing, and they seem to live, grow and reproduce best on oyster shell. Structures that rise off the bottom, such as natural oyster reefs, offer the added advantage of keeping oysters away from sediments that can settle on and smother bottom-dwellers.
These three-dimensional reefs are good for more than just oysters. They once supported complex communities of barnacles, mussels, clams, crabs, worms and fish. Many of these animals, like their oyster host, require hard surfaces for attachment. Many others simply use the nooks and crannies of the reef to hide from predators or to scavenge for food.
Oysters can play an important part in the Bay cleanup effort because they filter plankton and other particles out of the water column, then deposit ”pseudofeces” for hungry bottom scavengers. This filtering, combined with the actions of other filter-feeders, such as barnacles and clams, helps to remove nutrients and sediments from the Bay.
Newell estimates that the Bay’s oyster populations were once so vast they could filter the entire Chesapeake in a few days during the summer. Today’s considerably diminished populations — only about 1 percent of historic levels — may require a full year to accomplish the same task.
Their incredible ability to filter water, combined with the number of species that depend on oyster reefs for survival, make oysters one of the Bay’s most ecologically valuable species.
To reap those benefits, states and private groups are taking a variety of steps to explore oyster biology and diseases, and to restore reef and shell habitat. Restoring the resource and the fishery requires long-term thinking and it’s receiving lots of thought. Oyster recovery efforts are engaging diverse groups in some rather unusual collaborations. Resource managers are staking out protected areas while juggling commercial fishery and aquaculture concerns. Citizens will soon be oyster gardening with scientifically bred, more disease-tolerant oysters. Watermen are forming alliances with scientists and environmentalists.
As Don Meritt, a researcher and aquaculture specialist with University of Maryland Center for Environmental Sciences notes, “We’ve all agreed to work together, but we’re still experimenting with what works.”
Past and Present Problems
The oyster industry was once Chesapeake Bay’s most lucrative fishery. Overfishing, habitat destruction and disease devastated Chesapeake Bay oyster populations. The Baywide commercial catch has plummeted from 35 million to 40 million pounds a year in the mid-1950s to less than 2 million pounds a year in the late 1990s.
An industry that once employed hundreds of watermen, workers in dozens of shucking houses, as well as boat builders, seafood distributors and restaurateurs, has been reduced to a handful of watermen and aquaculturists and limited business opportunities.
Early residents of the Bay hand-tonged for oysters. Hand tongs probably caused little damage to oyster reef structures, because their harvest was limited to 6 feet below the surface and their small size limited takes. Oyster dredges, legalized in 1865, scraped the bottom at much greater depths than hand tongs.
Hydraulic-powered tongs, introduced by 1950, made the harvest even more efficient by removing chunks of oyster-filled reef a little at a time. Dredging and hydraulic-tonging kept oystering economically viable, but they chipped away at ancient oyster shell reefs, eventually reducing them to flat layers of shell on the bottom.
The oyster’s problems were compounded by two parasites, first noticed in the 1950s, which have altered oyster population structure and devastated the commercial industry. MSX (Haplosporidum nelsoni) was probably introduced to the Chesapeake Bay. Little is known about this protozoan parasite, except that it kills very young oysters in high-salinity waters.
Meanwhile, Dermo (Perkinsus marinus), probably a natural oyster parasite in Atlantic estuaries, appears to be moving north. Early oyster operations moved oyster seed between regions, spreading Dermo. Like MSX, Dermo is most destructive in high-salinity waters, killing oysters at around 2 years of age, just before they reach harvestable size.
While disease can cause high mortality in oysters, VIMS director of the Eastern Shore Laboratory, Mark Luckenbach, says, “Diseases are not a huge barrier to restoration. A severe disease event may kill up to 80 percent of 2-year-old oysters, but the summer before, 60 percent of 1-year-old oysters may have been eaten by crabs. Oysters have a naturally high mortality in every year-class.”
He notes, though, that diseases are a big problem for the fishery, because they kill oysters before they reach harvestable size. At the same time, large oysters that survive the diseases are especially important both to the restoration of the oyster population and the fishery.
First, those large oysters have survived disease infestations and may be able to pass that disease-tolerant trait onto their offspring. Second, the number of eggs produced by female oysters increases exponentially as shell length increases.
A few large oysters can produce substantially more eggs than a large number of small oysters — as many as 20 million in a single spawning — and the closer those large oysters are located to each other, the greater their chances for success. In many areas of the Bay, oyster densities are low and large oysters are scattered, reducing their ability to reproduce.
What's Being Done
Protecting the Resource
The big-picture strategy to restore oyster populations is basic: restore reef habitat, protect habitat and breeding stock and seek out and propagate disease-tolerant oysters. Of course, the difficulty lies with the details. Maryland, Virginia and the Potomac River are taking very different roads to restoration.
To help protect the resource, the Chesapeake Bay Program directs Maryland and Virginia to designate 5,000 acres each, and another 1,000 acres in the Potomac River, as “Oyster Restoration Areas.” Oyster harvests are to be restricted in these areas, so the ecological value of the resource can be enhanced. More than 27,000 acres were designated between 1995 and 1998, far surpassing the Bay Program's goal and more than 200 acres of oyster habitat have been constructed in Maryland and Virginia.
New goals being considered for the “Chesapeake Bay Program 2000 and Beyond” agreement — to be signed by the Chesapeake Bay Executive Council next year — include increasing the number of reefs built, focusing on multispecies management and developing fisheries management plans with recommendations for managing sustainable fisheries.
Further guidance recently came from the scientific community. In January 1999, oyster experts from Maryland, Virginia and North Carolina gathered for a workshop chaired by VIMS director of Oyster Disease Research, Eugene Burreson, to develop recommendations for oyster recovery. These scientific experts agreed that managing oyster populations should enhance their ecological value so that a sustainable fishery can exist without support from public funds.
The resulting consensus document, “Chesapeake Bay Oyster Restoration” (Chesapeake Research Consortium, June 1999), details two essential components for successfully restoring oysters: three-dimensional reefs and permanent reef sanctuaries.
The combination of these two components is absolutely necessary, the scientists agreed. Three-dimensional reefs, rising above the bottom, are important for the successful reproduction and protection of young oysters from predators. Three-dimensional reefs also provide homes and food for numerous creatures, including many recreationally and commercially important fish and shellfish species. Permanent reef sanctuaries that prohibit oystering would give oysters a chance to grow large, if they survive disease, and to remain in close proximity to each other. Large, disease-resistant, successfully reproducing oysters have the potential to populate areas beyond sanctuary boundaries, with the long-term capability of supporting a sustainable oyster fishery.
A moratorium on oyster fishing was considered several years ago, but was not viewed as a good strategy for oysters. A moratorium on harvesting striped bass has greatly aided the recovery of this sportfish, but because oysters are not mobile, alternative strategies are available. With oysters, some areas need to be closed to fishing to aid recovery, while it is still possible to sustainably harvest other areas. In addition, a moratorium would seriously damage the marketing of oysters shucked on Chesapeake Bay, with possible widespread economic consequences.
Oyster diseases devastated the Virginia oyster populations during drought years in the 1980s. The high salinity in Virginia waters allows diseases to persist and proliferate, challenging oyster recovery efforts.
Jim Wesson, chief of Conservation Replenishment at the Virginia Marine Resources Commission and chair of the Chesapeake Bay Program’s Aquatic Reef Habitat Work-group, sums up Virginia’s oyster recovery strategy: “We have to build on strong oysters. We need a large percentage of survivors [from disease] out there. A small number of big survivors is much more valuable than a large number of small oysters.”
To encourage natural reproduction, Virginia, with assistance and funding from the Chesapeake Bay Foundation, bought large oysters harvested by commercial watermen from Tangier and Pocomoke Sounds during the 1996-1998 seasons. These large “survivors” were placed on sanctuary reefs. Survivors of disease are presumably a bit more tolerant of the oyster diseases MSX and Dermo and, it is hoped, these survivors will pass their disease-tolerant traits on to future generations.
Virginia’s strategy appears to be working. Monitoring of constructed reefs in Wicomico River showed that large oysters placed on reefs in winter 1996 spawned the following year; spat sets were up 20,000– 26,000 percent between 1996 and 1997.
Of course, the summer of 1997 had ideal environmental conditions for spawning oysters. Summer 1998 was an El Nino year with lots of rain and, although conditions weren’t good for spawning oysters, spat set was still 20-30 times higher on Wicomico River reefs than reefs without the big survivors.
Virginia is taking the long-term ecological approach recommended by the Chesapeake Bay Oyster Restoration scientists by constructing reefs, protecting them and encouraging natural reproduction. Twenty “brood stock” sanctuaries have been designated around these created reefs.
Three-dimensional reefs are constructed by placing mounds of shell in intertidal areas where conditions are excellent for growing oysters. Oyster shell makes the very best oyster reef habitat. Shell for both Virginia and Maryland restoration programs is dredged from upper Bay regions that no longer support oysters because salinities are too low. Shell is a limited resource, making it expensive, and environmental concerns about sedimentation and nutrient-release during dredging remain.
Virginia is exploring alternative materials, such as coal ash pellets and surf clam shell. Research shows that oysters still do best on oyster shell, but alternative materials might work as a base, which is then covered with oyster shell. Recreational finfish fishing, but not oystering, is allowed in sanctuaries.
Virginia is also encouraging lease-holders surrounding sanctuaries to put down shell. Nearly a third of Virginia’s oyster grounds are leased to private individuals, with approximately 100,000 acres of private leases and 240,000 acres open to the public. Areas abutting sanctuaries are benefiting from the enhanced spat sets spawned by the large survivors, and lease-holders with good bottom conditions should reap the benefits with larger harvests.
Researchers are continuing to explore the best spacing of sanctuaries, so that the brood stock will help populate adjoining, open-to-oystering areas with strong oysters. Meanwhile, the multispecies benefits of oyster reef sanctuaries are already appearing — they are attracting so many finfish that Virginia reef sanctuaries have become fishing hotspots.
Maryland’s oyster grounds, by tradition and now law, are mostly public grounds. Only about 10,000 acres are leased to private individuals; approximately 285,000 acres of oyster grounds are public. The state is working to restore the oyster resource to maintain its public oystering tradition and to improve the Bay ecologically.
Maryland’s 1993 Maryland Oyster Roundtable Action Plan — developed from a series of meetings that included watermen, environmentalists, scientists and others — recommends enhancing both the ecological and economic benefits of oysters over a seven-year period via the creation and rehabilitation of oyster bars, the construction of seed bars, and the planting of hatchery-produced spat. A nonprofit organization, called the Oyster Recovery Partnership, was formed to coordinate efforts with local communities and to implement projects.
Oyster diseases are normally only a problem during dry years, when salinity is high in Maryland tributaries, so the state tries to “manage around the disease” by concentrating habitat restoration in salinity zones less favorable for MSX and Dermo and by restricting the movement of diseased oyster seed into those areas.
Maryland currently produces “certified seed” — tiny baby oysters with no detectable disease — at three, state-owned hatcheries. Between 10 million and 50 million hatchery-produced, generally disease-free, spat are stocked on sanctuaries located in zones where diseases are least likely to be severe.
To support the fishery, another 200 million to 1 billion spat are produced in Maryland’s natural seed program, which replenishes public oyster grounds. A $300 surcharge on licenses and a $1/bushel tax on harvested oysters helps to fund the replenishment program and some of the placement of shell in public-access waters.
State officials say studies have shown little benefit in Maryland waters from three-dimensional shell piles, which require using a large amount of the limited shell supply. Therefore, Maryland’s habitat restoration program focuses on building oyster “bars” by spreading a thin (approximately 6-inch) layer of oyster shell on the Bay bottom in areas that already contain a hard surface. Maryland concentrates this shell-spreading on bottom areas with natural hills. This takes advantage of natural bottom topography, where hills may help keep sediments from settling on oysters. Maryland officials claim that 10 acres of hilly bottom can be restored by spreading the same amount of shell that it takes to build one acre of three-dimensional piles.
Because so much of the Bay’s bottom contains oyster shell buried by sediments, “bottom cultivation” may offer an alternative to spreading expensive shell. With bottom cultivation, areas known to contain hard shell bottom could be dredged with special equipment to raise the old shell above the sediments. Bottom cultivation is in the early stages of exploration.
By the beginning of next year, Maryland should have 12 sanctuaries in place. The first few Maryland sanctuaries were already closed to oyster fishing because of sewage plant discharge. According to Chris Judy, director of the Maryland Department of Natural Resources’ Shellfish Division, these sanctuaries will remain closed to oyster fishing even if water quality improves and will, it is hoped, support brood stock oysters capable of populating nearby areas with oyster spat.
Several new sanctuaries are in the works, including one each in Dorchester, Somerset and St. Mary’s counties, where power dredging has been recently approved by the legislature to help boost local fisheries and as an attempt to clean buried shell during harvesting. To compensate for the loss of oysters removed by power dredging, these sanctuaries are required by law to be at least 100 acres in size. Such brood sanctuaries could produce spat for the much larger surrounding area, but there is at present no data confirming this in Maryland.
The Potomac River Fisheries Commission is not actively restoring reef habitat, but they have begun designating sanctuaries in the river. By law, all of the Potomac’s oyster grounds are public. The PRFC has an industry-oriented oyster seed repletion program that places fresh shell and generally disease-free seed on mid-river sections that have some chance of a natural spat set.
Because disease remains a serious problem at the mouth of the river, two sanctuaries have been designated — one in a low-salinity area and another mid-river, where moderate salinity prevails. Sanctuaries are located on oyster bars that commercial oystering operations tend to ignore because of low oyster densities and difficult access.
PRFC Executive Secretary A.C. Carpenter, says the Potomac strategy relies on Mother Nature doing her job. If conditions are right and diseases are low, oysters should naturally settle, grow and reproduce in protected sanctuaries. Over a long period of time, the lack of reef-destroying harvest techniques and the accumulation of larger oysters should allow naturally reproducing oysters living on protected bars to eventually rebuild their three-dimensional reefs.
One harvest management strategy that started several years ago, was then abandoned, and is now being reconsidered is “slot limits.” With slot limits, 2.5–4-inch oysters could be harvested. This size window allows the harvest of oysters just before Dermo kills them, but requires oyster harvesters to return oysters larger than 4 inches to the reef. Slot limits preserve larger, possibly disease-tolerant oysters for future reproduction.
Although slot limits appear to be a sensible way to save large oysters, Jim Wesson, of the VMRC, notes that removing, then replacing, large oysters disrupts oyster density. Large oysters become scattered and reproductive success may decline because success is linked to maintaining oysters in close proximity to each other.
Will Efforts Succeed?
The oyster restoration strategies of Maryland, Virginia and the Potomac River are so different it’s hard to believe they share the same Bay.
None of the experts interviewed would venture to guess how long it would take before self-sustaining oyster populations are filtering the Bay. Many questioned whether spreading a thin layer of shell, rather than constructing three-dimensional reefs, will succeed. Problems with siltation and, possibly, greater vulnerability to disease might keep bottom-dwelling oysters small and scattered.
The scientific consensus noted that historical evidence suggests that oyster reefs were a “dominant feature” of the Bay when colonists arrived, and are a critical component to restoring the oyster. “The lack of reef structures results in sub-optimal habitat for oyster growth and survival,” the scientists wrote. “Three-dimensional reefs are critical for reproductive success, predator protection and, of course, for the habitat they provide for other estuarine fauna.”
Still, three-dimensional habitat creation is expensive, requiring hundreds of tons of shell and heavy equipment. The risk exists that a reef could be destroyed by high-energy flow or waves, if reefs aren’t carefully placed. Since public funds for restoration are always limited, most oyster experts say efforts should focus on achieving the greatest ecological benefits.
Only time will show whether the designation of permanent sanctuaries will be enough to allow oysters to grow, survive disease, reproduce and rebuild their own reefs. And, just how much protected acreage is needed is yet unknown. The scientific consensus report recommended that 10 percent of the historic oyster bar acreage be set aside, and researchers are trying to identify a more exact number for protected acreage.
One thing is certain, “natural” reef restoration and self-sustaining oyster populations can occur only if sanctuaries are designated in areas where oysters can thrive; if those sanctuaries are patrolled and protected from poaching; and if permanent, indeed, means forever.