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In order to address boater concerns regarding this source of fish kills and fish lesions in 1998, the BoatU.S. Clean Water Trust developed the following flyer and distributed through Chesapeake Bay region BoatU.S. stores.  What is it?

Pfiesteria piscicida is a microorganism that belongs to a group of algae known as "dinoflagellates". It occurs naturally in the Chesapeake Bay (has for thousands of years) in at least 24 known life stages. All but four stages are harmless. Under certain conditions, populations may suddenly increase (bloom) and change into one of the four toxic life stages. The Pfiesteria cells then emit toxins that stun fish and dissolve their scales and skin, enabling them to feed on the fish and cause deep ulcers (lesions). Pfiesteria has been linked to fish lesions and fish kills in coastal waters from Maryland to North Carolina.

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What are the "certain conditions" that cause Pfiesteria to turn toxic?

Causes are not yet fully understood. Scientists believe that these factors may help trigger the shift to its toxic form:

  • Presence of a lot of fish.
  • Warm (80 degrees and over), brackish, and poorly flushed water.
  • High levels of the nutrients nitrogen and phosphorus.

Where have Pfiesteria-related fish kills occurred in the Bay?

In 1996 and 1997, fish lesions and fish kills in King's Creek and the Pocomoke and Chicamacomico Rivers on the lower Eastern Shore were attributed to Pfiesteria. The most recent 1998 outbreak was in the Wicomico River, also in Maryland's lower Eastern Shore.

Pfiesteria is only one possible cause of fish kills and fish lesions. Sudden changes in salinity or water temperature, a chemical or sewage spill, or other stressful conditions can also increase the potential for fish skin infection, lesions, and fish kills not related to Pfiesteria.

Can I get sick from Pfiesteria?

There have been reports linking human illness (like memory loss and dizziness) to Pfiesteria exposure. Protect yourself by observing closures and other guidelines of state natural resources agencies and health departments. For human health questions relating to Pfiesteria in Maryland, call 1-888-258-8989. For other Chesapeake Bay states, use the same contact numbers listed below.

What should I do if I see a fish kill or large numbers of fish with lesions?

  • Avoid contact with the water.
  • Use caution when handling fish with lesions. Wear gloves or release fish without touching by using a dehooking tool.
  • Leave area immediately.
  • Contact the appropriate agency to report your observations:
  • Maryland: 1-888-584-3110
  • Virginia: 1-888-238-6154
  • Delaware: 1-800-922-9283
  • Pennsylvania: 814-359-5147
Resources for additional information:

Maryland Department of Natural Resources
Virginia Institute of Marine Science
Chesapeake Bay Foundation


Alliance for the Chesapeake Bay
Chesapeake Regional Information Service:
1-800-662-CRIS or ACB@ari.net