Something fishy is happening in the headwaters of the Potomac River. Scientists have discovered that some male bass are producing eggs—a decidedly female reproductive function. In June 2002 reports appeared of fish die-offs in the South Branch of the Potomac River. The West Virginia Division of Natural Resources asked U.S. Geological Survey (USGS) scientists to examine fish health in the watershed near the town of Moorefield, about three hours' drive from Washington, D.C.

Anglers were also reporting fish with lesions. USGS scientists determined that some of the lesions indicated exposure to bacteria and other contaminants. The findings have perplexed the government scientists, who suspect a little-understood class of emerging contaminants. The contaminants include natural hormones excreted by humans and livestock as well as hormone-mimicking synthetic chemicals. The chemicals appear to confuse the endocrine systems of fish, essentially fooling males into producing female cells.

Endocrine disruptors work like biological disinformation campaigns. Sometimes mimicking natural hormones like estrogen, they alter other hormone concentrations. The disruptors can either prevent or weaken the normal cell-signaling process.

National Geographic