Disease and poor reproduction teamed up to produce bad news for oysters in both Maryland and Virginia.
Officials in both states reported poor “spat sets” — a measurement of reproduction — in almost all areas of the Chesapeake and its tributaries during 2001.
Meanwhile, three consecutive years of lower-than-normal freshwater flows into the Bay have allowed diseases to reach further upstream, infecting oysters in areas usually safe from the deadly parasites MSX and Dermo.
“Over the past couple of years, areas that used to be pretty much mortality-free have lost a lot of oysters,” said Roy Scott, a biologist with the Maryland Department of Natural Resources. “You used to have pretty significant areas of good survivorship, but the last couple of years, they have turned pretty sour.”
Disease pressure has been intense because both MSX and Dermo thrive in the high-salinity conditions that result during dry periods. Maryland relies on periodic high freshwater flows to push the diseases down the Bay, allowing some oysters to survive.
In Virginia, by contrast, salinities are always high in most areas. Therefore, its disease news was the same as always — bad. “Disease is just everywhere,” said Jim Wesson, who oversees oyster restoration efforts for the Virginia Marine Resources Commission. “It’s just a constant that we deal with.”
The main difference in 2001, he said, was that dry conditions allowed diseases to infect and kill oysters further up the James River, where salinities are usually lower. “The standing stock in the James River has declined this year, and that was almost all due to disease mortality,” Wesson said.
The toll taken by disease was not replaced by new reproduction. Both states reported poorer-than-normal spat sets — measurements of oysters larvae, or spat, which successfully latch onto oyster bars or other solid substrate to begin growing.
Maryland’s oyster spat index — the average number of spat counted in half-bushel samples taken at 53 oyster bars — was 16, well below the long-term average of about 50.
“It’s below average, but not disastrous,” said Steve Jordan, head of the Sarbanes Cooperative Oxford Laboratory, a joint operation of the Maryland Department of Natural Resources and the National Oceanic and Atmospheric Administration. “We had some fairly good sets in some areas.”
Jordan said the long-term index was skewed by a handful of very high years. In reality, he said, although the 2001 index was below average, it remained significantly better than several recent years, such as the 6 recorded in 2000, and the 3.5 in 1998.
And, Scott said, the survey did find a few bright spots. Some oyster bars in Tangier Sound and the St. Mary’s River had counts in the 100–150 range.
As a result of several years of poor reproduction and intense disease, Jordan predicted that this year’s oyster harvest would be less than 170,000 bushels in Maryland, down from about 347,000 bushels last year, which was down from 423,000 bushels in 2000.
Virginia’s oyster harvest in 2000 was about 20,000 bushels. A similar harvest was expected for 2001.
And, Wesson said, Virginia spat sets “were not good at all.” Usually, spat sets improve with high salinities, but that was not the case this year.
But in keeping with what has been seen in recent years, Wesson said large spat sets were found on new oyster reefs constructed as restoration projects in the lower Rappahannock, Back and Elizabeth rivers.
“That is the pattern we’ve seen,” he said. “When we put a new reef out, we get a real good set on it initially.” The problem, Wesson said, is after the initial spat set, “it drops back down to background levels.”
Part of the idea behind reefs is that they provide a three-dimensional surface to catch oyster larvae as they float in the water. Once the oysters latch onto the reef, they are supposed to begin growing and provide a solid surface for future spat to set on.
But after the initial spat sets on the reefs, many of the small oysters are ultimately killed by disease or predation.
Instead of covering the reefs with a “living veneer” to catch future spat sets, Wesson said the old shells on the reefs become home to boring sponges and other organisms that foul the reef’s surface, making it less suitable for spat settlement. Essentially, Wesson said, the quality of the reef surface degrades before oysters can dominate the structure.
“The theory of it all is that the oyster takes over once you build it,” he said. “But that is not the reality of it.”
Although the state is planning a new set of oyster reef projects for 2002, Wesson and others in Virginia are increasingly pessimistic about the ability to revive the native oyster population in the state’s high-salinity water. Such a recovery — if it happens — may take decades, he said.
As a result, many in the state have increasingly advocated the use of nonnative oysters in aquaculture, and perhaps even an outright introduction in the Bay.
In Maryland, though, officials remain optimistic that — aided by periodic high freshwater flows to flush their part of the Bay — they can manage around the diseases to revive the oyster population.
Maryland has been setting aside sanctuary areas, mainly in upstream areas where diseases will periodically be flushed out by high flows, and stocking those areas with hatchery-reared oyster larvae.
Hatchery operations in Maryland have been gearing up to eventually produce hundreds of millions of oysters annually to stock in sanctuaries and nearby areas. Without hatchery operations, Jordan said the oyster population in Maryland would spiral downward.
“The long-term trend of the population should be approximately stable based on what we are doing right now, and that depends a lot on the hatchery input that is going on,” he said. The escalated hatchery production expected in the next few years, combined with the establishment of sanctuaries where oysters will be protected from harvest “has a potential to really make a difference” for the population, Jordan said.
The hope is that over time, the stocked oysters — left unharvested — will become more tolerant of disease and reproduce, with their offspring gradually expanding into new areas.
Some oysters in both states have shown signs of increased disease resistance, though not immunity, over the years. But no one knows how long it would take for disease-tolerant oysters to dominate the Chesapeake.
Oysters were once the Bay’s most important species, both ecologically and economically. In the early 1900s, they were the most valuable species for watermen, as harvests from the Bay surpassed all other parts of the nation combined.
But overharvesting, combined with worsening water quality and the arrival in recent decades of two diseases — MSX and dermo — have combined to devastate the oyster population. Although the diseases pose no threats to humans, they can kill huge numbers of oysters.
The huge oyster populations that once filled the Bay played an important role in removing algae and particles from the water. Some scientists estimate that oysters were once able to filter all of the Chesapeake’s water in a matter of days. Today’s stock, depleted from overfishing and disease, takes nearly a year to do the same job.
As a result, some believe that efforts to clean the Chesapeake will fall short unless stocks of oysters and other filter feeders are rebuilt. Besides filtering pollutants, oyster bars and reefs provide important habitat for many types of fish and other aquatic species.
Because of their critical roles, the Chesapeake 2000 agreement calls for achieving a tenfold increase in oyster populations by 2010. By some estimates, today’s oyster population is only about 1 percent of historic levels.
To help meet that goal, federal agencies are planning to spend about $50 million over the next decade, and Maryland has pledged another $25 million in support. A variety of nonprofit organizations are also helping.