Mutant Microbe Blamed in Maryland Fish Kill

A virulent microbe that normally exists in a harmless state in the Chesapeake Bay mutated into a toxic form, killing menhaden and other fish species last week on the Eastern Shore’s Pocomoke River, Maryland officials confirmed Thursday.

Scientists are now searching for other possible contributing factors in the four-day kill that left thousands of fish dead and prompted officials to close the river to swimming and fishing from Aug. 7 to Wednesday.

“A piece of the puzzle now fits,” Department of Natural Resources Secretary John R. Griffin said at a news conference in Annapolis. “We continue to work diligently on identifying other agents that may have been acting” on the fish.

The one-cell predator Pfiesteria piscicida emerged from the mud flats of the lower Pocomoke River and emitted powerful neurotoxins that stunned and then slaughtered the fish.

Pfiesteria thrives in waters polluted with nutrients, such as nitrogen and phosphorous, which are found in many fertilizers and often wash off farm fields and homeowners’ lawns. Sewage treatment plants are also important sources of nutrients.

State workers are examining farming practices near the Pocomoke River and investigating whether agricultural chemicals and natural fertilizers are running from fields into the river, helping Pfiesteria to mutate, Griffin said.

Pfiesteria causes lesions in fish and at high levels can kill them. Its effect on humans who eat afflicted fish or come in contact with the microbe is unclear.