Field Notes
U.S. Fish & Wildlife Service
Field Notes Entry   
Catfish Tumors in the South, Severn, Rhode, and Choptank Rivers: Survey 2004-2008
Northeast Region, November 1, 2011
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Bullhead catfish with tumor
Bullhead catfish with tumor - Photo Credit: Fred Pinkney, USFWS

The brown bullhead catfish (Ameiurus nebulosus), which lives in rivers, lakes, and pond, feeds on worms, insect larvae, and small crustaceans living in the mud. These fish often develop liver and skin tumors from exposure to cancer-causing chemicals that accumulate in sediments.


Since 1992, the U.S. Fish and Wildlife Service, Chesapeake Bay Field Office (CBFO) has surveyed brown bullheads with tumors in Chesapeake Bay tributaries. In 2005, the CBFO found that 53% surveyed from the South River had visible skin tumors. Twenty percent of these fish also had liver tumors. Scientists consider areas to be highly contaminated when skin tumors rates are above about 12% and liver tumor rates are above about 5%.

The 2005 study raised many questions which we have addressed by surveying the South, Severn, and Rhode Rivers on the western shore and the Choptank River on the eastern shore of the Bay. Both the South and Severn watersheds have developed rapidly over the past 30 years. The Rhode watershed remains less developed and the Choptank watershed is still largely agricultural.

In four years of sampling, skin tumor percentages have remained high in the South River, ranging from 19% to 58%. Liver tumors never approached 20% again; the rates were 0% to 6%. The Severn River had a wide range in skin tumors (52%, 10%, and 2%). Liver tumors were low: 0–2%. Our only collection in the Rhode River had 6% skin tumors and 6% liver tumors. In two collections in 2008, the Choptank had 0% and 2% skin tumors and 0% and 5% liver tumors.

Exposure to cancer causing chemicals in the sediment, such as polynuclear aromatic hydrocarbons (PAHs), is a major cause of liver tumors. These chemicals, found in gasoline, oil, coal, car exhaust, and road particles, run off roads into our rivers. In several studies, we found that higher total PAH concentrations in sediments (> 15 parts per million) often translated into higher percentages of bullhead liver tumors. There is less certainty of the skin tumor causes.

In laboratory studies, PAHs and another class of chemicals, alkylating agents, have caused liver and skin tumors. But in the environment, a consistent relationship between sediment contamination and skin tumors has not been observed.

The difference in tumor rates between these various rivers is unknown. Investigation is continuing. CBFO is collaborating with the U.S. Geological Survey, National Fish Health Research Laboratory to see if a virus may be involved. The immune status of South River is also being looked into to determine why they frequently develop these skin tumors.

For information contact:
Fred Pinkney

Contact Info: Kathryn Reshetiloff, 410-573-4582, kathryn_reshetiloff@fws.gov
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