Alien oysters landed on the Bay bottom last fall as part of what scientists say is the most realistic study so far to determine how the native oyster species fares against a potential foreign rival.
The 30-month study, under way in both Virginia and Maryland, will attempt to determine how sterile Asian oysters, Crassostrea ariakensis and sterile native species, C. virginica, endure when placed in real-world growing conditions.
Past aquaculture tests have shown that C. ariakensis, a native of China, grows rapidly when protected in mesh bags and placed on trays that are suspended high in the water. They also resist the diseases that have plagued the native oyster.
The new study will test their endurance in bottom conditions, where the oysters will be exposed to predators, siltation, poor water quality and have to compete for space with other oysters—just as would happen if breeding populations were introduced into the Bay, as has been proposed by Maryland and Virginia.
“I certainly expect much more realistic growth and survival rates for ariakensis out of this than I do from the aquaculture trials,” said Mark Luckenbach, a research professor with the Virginia Institute of Marine Science. “It’s going to be lower growth rates, and lower survival.”
Such a finding would not be stunning, he added. The native species also grows more poorly on the bottom than when suspended in the water.
The findings will be important. Many watermen and the seafood industry support an introduction of the foreign oyster into the Bay as populations of the native species are near an all-time low because of disease, historic overharvesting and loss of habitat. The Army Corps of Engineers and the states are developing an Environmental Impact Statement that examines the potential of introducing the nonnative oyster into the Bay.
Part of that assessment relies on the development of models that would predict how the oysters would grow and survive in the Bay. But C. ariakensis is a poorly studied oyster, so scientists cannot say with confidence whether the advantages it shows in aquaculture tests would exist under conditions encountered by oysters released into the wild. Some laboratory experiments, for example, suggest they are more vulnerable to predation than the native oyster.
And laboratory studies keep revealing new surprises. When water circulation was accidentally turned off to tanks containing C. ariakensis and C. virginica oysters over a weekend last year, Ken Paynter, a University of Maryland oyster scientist—who is heading the new study with Luckenbach—was surprised to find a large number of dead Asian oysters.
Follow-up studies showed that when the Asian oysters were placed in jars, they soon opened their shells and began filtering water—and using up oxygen. On average, they were dead in a little more than three days.
C. virginica, by contrast, would close up and nearly become dormant. Their average time to death was well over two weeks. “Somehow, it sensed it was in a foreign environment or a hostile environment and just stayed closed,” Paynter said.
In some areas of the Bay, low oxygen conditions have taken a heavy toll on native oysters in recent years, including this summer, when oysters suffocated in restoration projects in Virginia. The laboratory findings suggest C. ariakensis may perform even worse.
“If there were periods of anoxia or hypoxia, virginica would probably be able to survive weeks, whereas ariakensis will be able to survive days,” Paynter said. “That’s one reason that it is important to get these bottom experiments done.”
Many scientists and regulatory agencies have been reluctant to place nonnative oysters on the bottom because of concerns that some would be lost. Because the technique used to render them sterile is not 100 percent effective, such an experiment could lead to an accidental introduction into the Bay. Nonetheless, many have concluded there is no other way to address key issues about C. ariakensis.
That led Luckenbach and Paynter to design a project with extensive safeguards that won support from regulators, as well as funding from Maryland and Virginia agencies, the Keith Campbell Foundation for the Environment and the National Oceanic and Atmospheric Administration’s Chesapeake Bay Office.
“It really is a huge question of how ariakensis is going to perform in a much more stressful environment,” said Jamie King, an oyster biologist with NOAA’s Bay Office. “There was no other way that we could answer these kinds of questions about ariakensis’ performance in experiencing true Chesapeake Bay bottom conditions, because it is not something we can emulate in the lab.”
To prevent escapees, the scientists designed cages that were 10-by-10 feet by 2 feet high. The cages are made of galvanized steel pipe, covered with chain link and screwed into the ground. A finer mesh screen covers the bottom and part of the sides to prevent oysters from spilling out. Each cage weighs hundreds of pounds and required cranes to move into place.
Eight cages went into the water in late October at four locations in the Severn and Patuxent rivers in Maryland, and the York and Machipongo rivers in Virginia. Each cage had 25, 2-by-2-foot trays filled with oyster shells placed inside.
Some trays have C. ariakensis oysters, others C. virginica. Some have both species, and some none at all. Seven times between now and the end of 2007, three trays will be removed from each cage and replaced with a tray of clean shell.
The removal of trays will not only allow scientists to examine how the oysters fare over time, but will reduce the number of oysters in the wild, so if any of the C. ariakensis oysters are not sterile, the odds of an oyster capable of reproduction finding a mate will be greatly reduced.
But the heavy cages are difficult to open and work with, and most can be accessed only by divers using scuba gear. “It’s the project from hell,” Paynter quipped.
It has also annoyed some watermen, who found one oyster bar in the Patuxent was placed off-limits to protect the research site. “It wasn’t our intention to take away a valuable piece of the watermen’s oyster bar,” Paynter said. “But this is really pretty important research and we needed to have a good, representative, natural oyster bar on which to do the work and make a really good comparison.”
The researchers believe the payoff for the inconvenience will be valuable insights.
The chain-link fence around the cages is designed so small predators, such as mud crabs, will be able to move in and out of the cages, giving scientists a chance to see if predation on C. ariakensis is greater in the wild, as has been seen in lab tests.
The experiment will allow the scientists to see how the oysters fare when grown separately, and when they go head-to-head in the same trays, which could prove especially important. Oysters latch onto solid substrates to grow on when placed on the bottom. In laboratory experiments, C. virginica outcompetes C. ariakensis for space at early life stages—literally pushing them aside or growing over the top.
But even if C. ariakensis is heavily preyed up, and initially loses the space competition with C. virginica, they still may prove superior in the end if C. virginica oysters suffer from disease.
As a result, while some clues may emerge early, the scientists say it will likely take the full two-and-a-half years to allow the full picture to develop. “We might make a mistake if we rush to judgement based on whatever the early patterns are,” Luckenbach said. “Things might be adding up in favor of virginica for quite some time until disease hits, and that might change the pattern.”
But even disease dynamics may change in the wild. While C. ariakensis has proved remarkably disease-resistant in tests so far, no one is sure what will happen on the Bay bottom. In more stressful conditions, it may become more susceptible to the parasites that have decimated native oysters.
It’s also possible that if nearby native oysters become heavily infected, they will become, in effect, parasite repositories that cause C. ariakensis to become infected at a higher rate.
Conversely, it’s possible that because C. ariakensis filters water more rapidly than C. virginica, they will remove parasites from the water, reducing infection levels in the native species. “Virginica could be spared because ariakensis is loading up on all the parasites,” Paynter said.
The researchers also hope to get a clue as to whether C. ariakensis oysters build reefs like C. virginica, which provide important habitat for a multitude of species. Scientists have seen little evidence of C. ariakensis creating reefs in its native habitats in China. But if they do exhibit reef-building characteristics, their rapid growth may allow them to develop the structures more rapidly than the native oyster.
The study locations also span a range of salinities and other conditions, so results may vary from site to site. There are fewer predators and less disease pressure in low-salinity areas, but both species in previous experiments have also grown more slowly at low salinities.
“I would be surprised if every location has the same relative performance between virginica and ariakensis,” Luckenbach said.
While the study will provide insights that could be gained no other way, the researchers caution that it has limitations. The oysters are sterile, and therefore will grow faster than oysters capable of reproduction because they put less energy into creating eggs and sperm. Also, the four sites selected do not come close to covering the full range of conditions found throughout the Bay.
And, perhaps, most significantly, Mother Nature may weigh heavily upon the results. Variables such as temperature and the amount of rainfall could affect the degree to which the oysters are challenged by disease and low oxygen conditions.
“The potential for having some interesting, and potentially important differences between the two species to arise is there,” Paynter said. “Whether or not they actually occur is up to nature. It’s not the type of controlled experiment that you can conduct at your lab bench.”