© FLICKR/VELO STEVEFISHY WATERS: Some male bass have been growing eggs, which researchers attribute to the presence of estrogen in the rivers.
Exposure to estrogen puts fish at greater risk of disease and premature death, according to a new federal study.

The U.S. Geological Survey study showed that estrogen exposure reduces a fish's ability to produce proteins that help it ward off disease and pointed to a possible link between the occurrence of intersex fish and recent fish kills in the Potomac and Shenandoah rivers.

The report, published in the current issue of Fish & Shellfish Immunology, adds to a growing body of research pointing to problems with estrogen in the nation's waterways.

Other research has found evidence of estrogen exposure in freshwater and some marine fish populations. In a previous report, USGS scientists found widespread occurrences of fish in the Potomac and Shenandoah rivers with "intersex" characteristics -- male fish carrying immature female egg cells in their testes. Other scientists observed similar problems in fish in Southern California and in labs in Canada and the United States.

Scientists have not targeted the source of estrogen, but many suspect it stems from certain pollutants and drugs in waterways.

For the new research, USGS scientists injected largemouth bass with estrogen in laboratory tests. They discovered that the fish produced lower levels of hepcidins, proteins that regulate iron and may be a first line of defense against disease-causing bacteria, fungi and viruses.

Largemouth bass usually produce two kinds of these sickness-shielding proteins. After being exposed to estrogen, they reduced production of one type of the protein, and exposure to estrogen and bacteria completely blocked the production of the other. The researchers say this could explain why the Potomac and Shenandoah rivers seem to have the co-occurence of intersex fish, fish lesions and fish kills.

"The fact that estrogen blocked production of hepcidins in fish exposed to bacteria gives more weight to the theory that estrogen or estrogen-mimicking chemicals could be making fish more susceptible to diseases," said Laura Robertson, who led the research.

Male fish with the capability to develop immature eggs inside their sex organs were first found in a West Virginia stream in 2003, raising fears that there were endocrine disruptors in the water that scientists were not finding in repeated water tests.

In a 2006 study, the USGS scientists found widespread endocrine disruption among smallmouth and largemouth bass in the Potomac River and its tributaries across Maryland and Virginia. Tests on smallmouth bass in the Shenandoah River in Virginia and in the Monocacy River in Maryland -- both of which feed the Potomac -- concluded that more than 80 percent of all the male bass were growing eggs.

Environmental groups have asked U.S. EPA to ban chemicals used in many household detergents that are linked to endocrine disruption and gender changes in fish. One chemical, nonylphenol, imitates estrogen. The Sierra Club and other environmental groups want EPA to use a provision of the Toxic Substances Control Act to regulate individual substances.