Chesapeake Bay Journal: VIMS scientist urges expanded testing of ‘triploid’ oysters

VIMS scientist urges expanded testing of ‘triploid’ oysters

By Karl Blankenship

A Virginia scientist is suggesting that foreign oysters be freed from their bondage, but that they should continue to be deprived of a sex life.

It’s not a twisted animal rights message. Rather, the concept being promoted by Stan Allen, an oyster researcher with the Virginia Institute of Marine Science, is something of a middle ground in the debate over the future of the fast-growing foreign oyster, Crassostrea ariakensis, in the Chesapeake Bay.

With various sides polarized over the issue of whether reproducing populations of the oyster should be placed in the Bay, Allen suggests an expanded role for sterile oysters—both for research, and commercial harvest.

This fall, 800,000 “triploid” oysters—bred to have three sets of chromosomes instead of two, thereby rendering them sterile—were placed in the Bay as part of a tightly controlled experiment to see whether they can be profitably grown.

Conditions placed on the project require that the oysters be contained in mesh bags off the Bay bottom and other measures aimed at preventing any accidental release. Those safeguards, though, may add to the cost of rearing the oysters, and minimize the potential to learn about how the nonnative species might affect the Chesapeake ecosystem.

That, is equivalent to “straight-jacketed” aquaculture, said Allen, who developed the technique being used to create triploid C. ariakensis oysters.

Instead, he suggested, oysters might be placed on the ground—or on oyster reefs—in carefully selected creeks or embayments where the animals could be carefully controlled and recovered.

“There are many interesting questions that could be asked both academically and commercially by doing some open water plantings, on a carefully controlled scale at first, but not containerized,” Allen said.

The pressure is on to learn more about the oysters, and quickly.

Maryland and Virginia have indicated that they would like to make a decision about introducing a breeding population in less than two years. Scientists have said it could take up to five years to answer some of the key ecological questions concerning the potential of C. ariakensis if it were introduced into the Bay.

Placing sterile oysters directly in the water would allow scientists to observe how C. ariakensis interacts with other species, and under more realistic conditions than laboratory work would allow.

Further, Allen said, if the triploids could be economically reared and harvested, it may reduce the demand for the quick introduction of reproducing oysters.

But placing triploids on the ground, where some might not be recovered, would increase the risk of an accidental introduction over containerized aquaculture where all oysters can be removed from the water, Allen acknowledged.

Because the process for rendering the oysters sterile is not perfect, some fraction of those left behind may be able to reproduce. But, Allen said, it poses less of a risk than the alternative — a full-scale introduction.

“It would seem to me that the risk from triploid aquaculture, less bridled than it is now, is still orders of magnitude less risky for causing an environmental problem than an actual introduction,” Allen said. “But it is not without a risk. So it depends how risk-averse the community would be to allowing triploid aquaculture.”

Frances Porter, executive director of the Virginia Seafood Council, which is sponsoring the current in-water experiments with triploid oysters, said her organization would be “pretty excited” about about such a project, though it has not made such a proposal.

“I know that is being talked about,” she said. “The seafood council itself has not at this point asked for a project of that nature. We certainly wouldn’t be opposed to that.”

Allen recently proposed stepped-up, triploid-based aquaculture and research in a congressional hearing, where it attracted some interest.

“I think we’ve got to consider everything with open eyes,” said Bill Goldsborough, senior scientist with the Chesapeake Bay Foundation.

Don Boesch, president of the University of Maryland’s Center for Environmental Science, said an expanded triploid aquaculture “has a lot of potential,” although he noted it would likely benefit Virginia, which has a longer history of aquaculture, than Maryland.

Others were more skeptical. Larry Simns, president of the Maryland Waterman’s Association, expressed doubt that enough triploid C. ariakensis could be produced soon enough to keep the dwindling number of oyster harvesters in business.

“I’m not going to live long enough to see the triploid have enough production out there to keep them [watermen] around,” he said at the Congressional hearing.

When the National Academy of Sciences reviewed the C. ariakensis issue in the Bay, it made the “controlled aquaculture” of triploid oysters its preferred short-term management option, saying it had a relatively low risk and could allow scientists to learn more about the poorly studied species.

It did not define “controlled aquaculture,” though. Sometimes the report refers to “containerized” aquaculture, but in other times it suggests placement of triploids on reefs.

“I think there is a lot of scope in that interpretation for doing some really creative things,” Allen said. “I would like to see us go there because of the need to answer some of those questions fairly quickly, and the problem of not being able to use [reproductive] diploids.”

The idea offers a way to examine many key questions about C. ariakensis that cannot easily be answered in laboratory experiments, such as how it interacts with the myriad of species found in the Chesapeake.

“It would be fascinating to have them out there for a period of time so they would become part of the community,” Allen said.

A major question about C. ariakensis is whether it forms the types of reefs created by the native species, which provide important habitat for a host of other species. If it doesn’t, it might be of less ecological value in the Bay.

While aquaculture experiments show that C. ariakensis clearly grows faster than the native oyster when grown in containers high in the water column, it’s less clear whether they would grow much faster on the bottom, where habitat quality is often poorer.

Some laboratory experiments suggest that early life stages of the native C. virginica outcompete similar stages of C. ariakensis, but it’s not known how the two species would interact in the wild.

In addition, the placement of oysters on the bottom could yield information about whether they could be harvested using the dredging and tonging techniques long used in the Bay.

“You could quantify the suitability of ariakensis to be harvested in the first place,” Allen said. “They are generally recognized to have lighter shells than virginica. Would they be able to withstand the rigor of harvesting tools?”

Placing hatchery-reared oysters on the ground and allowing harvest is an aquaculture technique used in much of the world, and could allow greater harvests at lower costs—something that would benefit seafood packing houses, which currently import most of their oysters from the Gulf Coast.

Nor is the use of triploids for such a venture unprecedented. About 40 percent of the West Coast oyster harvest comes from triploid oysters placed on the bottom for aquaculture, although it’s done to enhance growth—triploids grow faster than normal diploids because they don’t reproduce—rather than to control their spread. Triploids can also be reared in hatcheries for about the same cost as diploid oysters, Allen said.

By comparison, some believe that oysters from containerized aquaculture, because of the added expense, would likely be restricted to the half-shell market, which attracts premium prices but has limited demand.

Triploid aquaculture could also provide financial relief to watermen more quickly than the introduction of diploids, Allen said. Triploids, after all, would be placed in the water with the intention of removing them all. By contrast, any effort to introduce breeding oysters would have to restrict harvest, likely for years, to allow the population to grow.

But the tradeoff for the information and potential economic benefit, Allen said, is accepting more risk. The process for creating sterile triploids is not 100 percent effective. Roughly one of every 1,000 oysters would be capable or reproducing.

Another concern is that, if left in the water long enough, a small fraction of the oysters would gradually revert back to diploid oysters, with the normal two sets of chromosomes, and might be able to reproduce.

“The reverted ones we have examined have not shown signs that they are capable of normal reproduction,” Allen said. “But I admit that we should do a lot more work on that. So that is kind of an open question as to how reproductive they become.”

Even if the oysters were harvested, Allen said some “lost souls” would likely be left behind and—if they were diploids or reverted triploids—they could potentially breed. But he rejects what he calls the “H-bomb” view of risk, in which any any accidental reproduction becomes “the big one” which leads to an irreversible introduction and an cataclysmic ecosystem change.

The actual number of reproductive oysters left behind would likely be small, he said, and successful reproduction would be hampered by some of the same barriers that limit the native oysters, such as water quality, loss of suitable substrate and sedimentation. If any reproduction took place, the population might be contained and eradicated, especially if sites are carefully chosen to limit spread, Allen said.

The big question, he said, is what level of risk people are willing to accept. “It’s not the zero-risk scenario that everyone is looking for,” Allen said. “So is it reasonable to go that far, or not?”