Introducing…the Asian Oyster

As native oysters decline, officials seek to restore fishery with disease-resistant species

Hoping to save the economically important Chesapeake Bay oyster farming industry, Maryland and Virginia have proposed replacing declining native eastern oysters with an introduced species: Asian oysters. Larger, faster-growing, and more disease-resistant than the native species, Asian oysters sound too good to be true—and indeed they might be.

Researchers in both states are raising sterile Asian oysters (Crassostrea ariakensis) and placing them in parts of the bay in highly controlled experiments, to find out how they grow, compete with native oysters, and whether they avoid disease and predation.

That, along with long-standing pollution concerns about the bay itself, spurred Wolf-Dieter Busch, to file a petition asking the National Marine Fisheries Service (NMFS) to designate the eastern oyster (Crassostrea virginica) as a federally protected threatened or endangered species. That triggered the formation of a team of fisheries experts to assess the status of the eastern oyster species and, if necessary, propose ways to protect them.

Fears about a takeover

Busch, owner of Environmental Initiatives Advisory Services in Maryland, believes that introducing Asian oysters to Chesapeake Bay is unwise because “it will cause stress on the remaining eastern oysters through competition for limited habitats,” and because it is not clear that the Asian oyster, once released, will remain sterile. Introducing Asian oysters, Busch says, “may also result in hybridization between the species. And how long would it take for the Asian oyster to be transported out of the Chesapeake to other eastern oyster habitat sites along the Atlantic and Gulf coasts?” Some scientists agree with that assessment.

“Oyster larvae spend two to three weeks drifting on currents before they settle,” said William Walton, shellfish and aquaculture researcher with Woods Hole Sea Grant and Cape Cod Cooperative Extension. “What are the implications of two states making that decision potentially for an entire coastal range?”

The once legendary abundance of eastern oysters fell precipitously in the late 19th through the 20th centuries, and Atlantic coast harvests of wild oysters are now 2 percent of historic high levels. Within Chesapeake Bay, harvests are less than 0.2 percent of historic levels.

Generations of overharvesting have driven down native populations. On top of this, the oyster has been dealt severe blows by two introduced protozoan diseases, known as Dermo and MSX, which kill nearly all eastern oysters in saltier areas of the Chesapeake before they grow to harvestable size. Because of disease, attempts to restore commercially viable populations of native oysters in the Chesapeake have been mostly unsuccessful.

Cleaning the water as they eat

As wild oysters have steadily disappeared, oyster harvesters have increasingly turned to aquaculture for a reliable yield. Oyster farming is now a successful part of all shellfish aquaculture from Maine to the Gulf of Mexico.

In Massachusetts, “the income from oysters is about 50 percent of all aquaculture,” Walton said. “Oysters command a higher price than clams, and they are flexible to being grown in different areas. They can be grown on various bottoms or suspended in the water, shallow or deep, and in bays.”

In Chesapeake Bay, culturing native oysters—by interested citizens and commercial growers—is also an essential part of future plans to maintain the species and clean the bay’s water, since oysters are filter-feeding animals that remove large amounts of algae from the water. However, disease kills farmed oysters as well as wild, and that is why there is strong interest in bringing in Asian oysters for aquaculture.

“Growers will select for disease-resistant [native] animals and breed them,” Walton said, “but that takes longer.”

Asian oysters could also contribute substantially to cleaning the bay, if enough of the sterile ones can be grown in hatcheries and controlled once they’re in the environment. But “introductions of exotic species have generally caused more problems than benefits,” Busch warned, “and usually cannot be reversed.”

Researchers are also investigating whether genetic differences they see in oysters are enough to consider the Chesapeake region’s native oysters a distinct “subspecies” that could then be regulated by NMFS separately from other regions. But research to determine this takes time, and by law, NMFS had to rule on Busch’s petition by January 2006. On Oct. 13, Busch withdrew his petition. NMFS now has the option to continue the eastern oyster species status review without deadlines.