Chesapeake oyster revitalization could bring viruses

By: Ann Wang

Posted: 10/9/08

The Chesapeake Bay is both a crucial part of Maryland's economy and an incredibly fragile ecosystem. New research from Hopkins suggests that efforts to revitalize oyster harvesting in the Bay could have unintended consequences for both the economy and the ecosystem.

At their peak in the late 19th century, the oyster beds in Chesapeake Bay yielded 15 million bushels, or 154 million pounds of oysters each year, accounting for up to half of the nation's harvest.

The oysters collectively filtered enough water to fill the entire Chesapeake Bay - about 19 trillion gallons - once a week.

The main species in the bay is Crassostrea virginica, known alternatively as the Eastern or American oyster. A combination of overharvesting, habitat degradation and parasitic disease has devastated the Chesapeake oyster population.

Today's oyster population has been decimated: It is at just one percent of the peak levels of a century ago, and harvests have likewise declined. For several years, officials have been looking for a way to reinvigorate the oyster populations in the Bay. One long-debated plan has been to bring an Asian species of oyster to the Bay - specifically, Crassostrea ariakensis, or the Suminoe oyster - in the hopes that it will fare better than its western relative.

Many scientists are curious about the impact the new species would have on the Chesapeake Bay ecology, but a group of scientists at the Bloomberg School of Public Health were the first to study how C. ariakensis might change human interactions with the Bay.

A research team led by Kellogg Schwab discovered that C. ariakensis is significantly more likely than C. virginica to harbor viruses that can cause gastroenteritis in people who eat raw shellfish. Their findings appear in the September issue of the journal Applied Environmental Microbiology.

The team studied the two oyster species and five different viruses over a month, alternating days when the oysters were exposed to the viruses and days when the oysters were kept in clean water, giving them a chance to flush out the accumulated viruses from their tissues.

They found that C. ariakensis had accumulated three of the five viruses - human norovirus, marine norovirus and hepatitis A virus - at greater concentrations than had C. virginica.

C. ariakensis, an oyster that is native to Chinese and Japanese shores, is bigger than C. virginica, tolerant of a wider range of saltwater conditions and resistant to the parasites that plague the Eastern oyster.

Should C. ariakensis be deliberately introduced to the Chesapeake, it would take over much of C. virginica's former habitat and eventually be harvested for human consumption. Its increased likelihood to harbor viruses that are dangerous to humans is a matter of concern.

No final decision has been made on whether this new species will be brought to the Bay.

This method of introducing new species to an ecosystem is not new. Sailors have done it unintentionally for years, carrying species from one port to another in ballast water or clinging to the hulls of their ships.

Other regions have introduced new oyster species to their coasts in hopes of rebuilding their beds, with mixed success. C. virginica has in fact been brought from the Chesapeake to several regions on the West Coast, where it is better adapted than the local species.

Oyster beds are not only an important part of the coastal economy, but a vital part of the Chesapeake Bay ecosystem. A grown oyster can filter up to 60 gallons of water a day as it feeds, which helps keep the water clear of algae.

Oyster beds also provide surfaces for hundreds of other animals and plants to feed, hide and live. The effects of replacing a native species with a foreign species are not always predictable in such a complex ecosystem.

This year, Maryland has planted 485 million juvenile C. virginica oysters in the Bay in the hopes that they will mature into harvestable oyster beds. There is also an ongoing effort to breed a type of C. virginica that is more resistant to disease.

It is clear that no one step or one plan is sufficient to restore the Chesapeake's oyster beds; Despite many efforts, the oyster population has continued to decline dramatically in recent years. But these findings suggest that caution is in order before introducing a foreign oyster species.