Chesapeake Drought Could Mean Dry Spell for Oysters

This summer’s drought has stunted fields of corn, wilted flower beds and turned lawns a sickly brownish-green. It may also be bad news for the health of Chesapeake Bay oysters.

Two diseases that have ravaged the oyster population, especially in the lower bay, thrive in salty water and could strike with a new vengeance.

But the news is not all bad. Scientists say oyster reproduction could benefit from lower rainfall, as could overall water quality.

“It’s likely that with the higher salinity, the diseases will be more prevalent,” said Carin Bisland, ecosystem coordinator for the Environmental Protection Agency’s Chesapeake Bay Program office in Annapolis.

“But with high salinity, you get higher spat set,” she said.

Spat are the baby oysters that attach to shells and other hard surfaces during spawning season.

Bisland said Virginia traditionally gets a better spat set than Maryland because water in the lower bay is saltier than upper bay water.

If the spat set is better and diseases are worse, it becomes a balancing act whether lower rainfall is better or worse overall for the oyster population, Bisland said.

The U.S. Geological Survey reports the total flow of water in the Chesapeake Bay this year is about 16% below the long-term average. In July, 16.2 billion gallons of water a day flowed into the bay, about 30% below the normal July flow.

Oyster stocks in the bay have plummeted in recent years, and the main culprits seem to be two diseases, MSX and dermo. Both kill oysters but do not have any known effects on humans.

The commercial oyster catch has dropped dramatically in the last two decades. In 1974, watermen reported a catch of 15.7 million pounds. That fell to 7.5 million pounds by 1986 and 1.2 million pounds last year.

When salinity levels drop during years of heavy rainfall, both diseases retreat down the bay to saltier waters. They virtually disappear from Maryland waters during years of exceptionally heavy rainfall, but reappear when the flow of fresh water into the bay drops and salinity levels climb.

Scientists say effects of low rainfall on water quality are not certain.

Ed Temple, habitat-restoration coordinator for the Chesapeake Bay Foundation, said patterns of rainfall are so cyclical that a year of low or high rainfall probably will have little or no effect on water quality.

When rainfall is less than normal, it might reduce the flow of toxic chemicals, nitrogen and phosphorus into the bay, Temple said.

Bisland said lower rainfall also means less sediment washing into the bay. “That could translate to clearer water and to bay grasses doing better,” she said.