Parasite puts Asian oysters at risk

Experts call for further research before introducing them into the bay

November 27, 2006|By Rona Kobell | Rona Kobell,Sun reporter

Researchers have concluded that Asian oysters are susceptible to a parasite that could wipe them out if they were ever planted in the Chesapeake Bay, raising new concerns about a proposal to use the foreign species to revive the region's struggling seafood industry.

The research found that Asian oysters experienced "almost total mortality" when exposed to the parasite Bonamia from the earliest stages of life, said Ryan Carnegie, a scientist at the Virginia Institute of Marine Science, where the study is being done.

Upon taking office four years ago, the Ehrlich administration said it planned to introduce Asian oysters into the bay to help filter the increasingly polluted water and to give struggling watermen a crop to harvest. Diseases and overharvesting have all but destroyed the native oyster populations in the bay.

But researchers and officials in neighboring states criticized the plan, saying Maryland was moving too fast on an endeavor that could jeopardize the health of the ecosystem. Environmentalists were alarmed when Gov. Robert L. Ehrlich Jr. wrote a letter to the head of the U.S. Environmental Protection Agency asking him to have government scientists stop calling for additional research. But the state eventually agreed to postpone action while waiting for the results of more studies.

Bonamia, which attacks an oyster's blood and slowly kills it, is not known to exist in the Chesapeake Bay. And the native oyster isn't susceptible to the parasite. But ever since a 2003 experiment showed that about 60 percent of the Asian oysters planted in North Carolina's Bogue Sound were infected with a New Zealand strain of Bonamia, scientists in the Chesapeake region have been trying to determine where the parasite comes from and how it grows.

"It's more of a problem in younger oysters. It would almost seem to be a juvenile oyster disease," Carnegie said. "But the reality is that in most of the size ranges, we are still seeing a serious impact by this parasite."

It was a different Asian oyster that introduced the MSX parasite into the Delaware Bay in the 1960s. The species didn't survive here, but MSX did, and it spread throughout the Chesapeake Bay to infect millions of native oysters.

Scientists, for the most part, aren't worried about bringing in a similar deadly strain of Bonamia that would kill what is left of the natives. But they are concerned that the parasite would make introduction of Asian oysters a poor investment.

"If, in five years, we had a very active aquaculture industry, then you could have enough hosts for the disease to take hold," said Roger Newell, an oyster biologist and the University of Maryland's Center for Environmental Science. "You could all get geared up for this wonderful, oyster-industry-saving species, and then it nails you."

Rick Abbruzzese, a spokesman for Gov.-elect Martin O'Malley, said the new administration will watch the continuing research and will work closely with other states as it decides the next step.

"We would defer to the best possible science that is conclusive," Abbruzzese said. "I'm not sure we're there yet."

Tom O'Connell, assistant fisheries director at the Maryland Department of Natural Resources, said the Bonamia findings are a continuing concern, particularly in the high-salinity areas of the lower bay where the parasite has been shown to thrive. But, he added, Bonamia hasn't turned up in any of the Asian oysters that have been in the Chesapeake Bay in controlled experiments over the past 10 years. While the parasite is something to watch, O'Connell said, so far it's not alarming enough to derail an introduction.

"Regardless of how long you study something, there's always going to be some uncertainty," he said. "But there have been no show-stoppers."

Researchers will meet next month to update each other on their progress. The U.S. Army Corps of Engineers, which is coordinating the work, expects to issue a draft environmental impact statement by next spring.

In addition to the Bonamia susceptibility, researchers have found another major problem with the Asian oyster now being studied, Crassostrea ariakensis. It spawns at the same time as the native oyster, causing a reduction in breeding for both species. Newell, who has conducted some of those studies, said that could be catastrophic if ariakensis migrated to areas like Florida and Louisiana, where the native oyster is relatively abundant.

"We are 100 percent certain that they neutralize each other's gametes," Newell said. "That is the single most adverse consequence you can find in an introduction."

Newell said the research into the Asian oyster may yield restoration techniques that will help bolster native populations. And while he thinks the Asian oyster has benefits for the Chesapeake Bay, he says the risks to other states make it unlikely that it will be introduced.

"What [the Maryland Department of Natural Resources] proposed was a very bold strategy. No one can fault them for that," he said. "But at the end of the day, the neighboring states are very concerned. And there will be strong political pressure for them not to do it."

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