Title - Schoolyard Habitat

Male fish, with immature female eggs.

Yes…that’s right.

Last year, researchers with the U.S. Geological Survey found male fish with immature female eggs, right here in the Potomac River.

What’s in the water?

These fish, called intersex fish, may occur as a result of chemicals in the water that mimic or antagonize hormone levels. Known as endocrine disruptors, these substances can interfere with an organism’s normal hormone functions. Endocrine disruption has the potential to compromise proper development, leading to reproductive, behavioral, immune system and neurological problems, as well as the development of cancer.

Endocrine disruptor compounds can enter a waterway from sewage outfalls, industrial and municipal pollution, and agricultural runoff. Endocrine disruptors may be entering the Potomac River watershed through synthetic estrogen, such as those in birth control pills and hormone replacement therapy, or from those hormones associated with livestock operations.

Current Research

Federal agencies are now investigating the effects of organic pollutants and endocrine disruptors on hormone activity in fish in the Potomac River watershed. The U.S. Fish & Wildlife Service and our partners at U.S. Geological Survey, Maryland Department of Natural Resources and D.C. Department of the Environment (formerly the Department of Health), collected water samples and fish at various sites in the Potomac River watershed.

Smallmouth and largemouth bass were selected as the target fish because they are sensitive to pollution and frequently display the physical symptoms of intersex. Biologists measured concentrations of endocrine disruptor chemicals in the water and sampled bass from the Monocacy River and Conococheague Creek in Washington County, Maryland, and the Potomac River in Washington, D.C.


Eighty to 100 percent of the male smallmouth and 30-35 percent of the largemouth bass exhibited intersexbass collected at all sites exhibited intersex.  Both sexes at all sites had relatively low gonadosomatic indexes (GSI), meaning their gonads were especially light-weight relative to body weight.

Current Status

Further research to determine the extent, causes, and population impacts of intersex and other forms of endocrine disruption is needed.

Coordination across State lines (e.g., West Virginia, Virginia, Maryland, Pennsylvania and District of Columbia) and a comprehensive watershed evaluation are essential components of a long-term plan to address endocrine disruptors in the Potomac River.

For more information, please contact Chris Guy at (410)573-4529.

Students find joy in environmental stewardship

Scientists with the U.S. Geological Survey process small mouth bass to determine if endocrine disruptors have affected sex tissue

Completed Schoolyard Habitat project

U.S. Fish & Wildlife Service biologist Pete McGowan collects water samples in the Potomac River watershed to measure organic pollutants and endocrine disruptors in the water


Getting down and dirty

Volunteers and staff with state and federal government agencies electro-shock and catch fish in the Potomac River watershed.

USFWS photos

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