Male Fish Producing Eggs in Potomac River

William Cocke
for National Geographic News
November 3, 2004

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 following year, the USGS conducted a more intensive assessment with a statistically significant number of fish, this time looking for internal damage. That's when they discovered a so-called intersex condition—where one sex exhibits both testicular and ovarian tissue.

"It was not something we were really looking for," said Vicki Blazer, a fish pathologist with the USGS's Leetown Science Center in Kearneysville, West Virginia.

Some 42 percent of male smallmouth bass surveyed showed signs of intersex development. A second sampling this spring produced an even higher rate—79 percent showed sexual abnormalities.

Mysterious Sex Changes

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.

David O. Norris, a professor in the University of Colorado's Department of Integrative Physiology, has specialized in environmental endocrinology for over 35 years. He is leading an ongoing research project looking into hormone pollution in three rivers in the Denver area.

"We're looking at the fish above and below where sewage treatment plant effluents are being added into the rivers," he said. "The best data we have are on Boulder Creek in terms of numbers of individuals. In all three cases we found reproductive abnormalities in fish downstream from where the effluent is."

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