Some species of male fish are acquiring female sexual characteristics at unusually high frequencies in the Potomac River and its tributaries, prompting concerns about pollutants that might be causing the problem.

In some Potomac tributaries, including the Shenandoah River in Virginia, nearly all of the male smallmouth bass caught in a survey last year by the U.S. Geological Survey were so-called “intersex fish,” producing immature eggs in their testes.

In the Potomac itself, seven of 13 largemouth bass exhibited female characteristics, including three that were producing eggs.

Intersex fish were discovered in the Potomac rivershed in 2003 and have also been found in other parts of the country. But the frequency found by the Geological Survey is much higher than what has been found elsewhere, said fish pathologist Vicki Blazer.

Female fish caught in the survey did not develop any unusual sex traits, although fish of both sexes exhibited lesions and other problems related to pollution, said Blazer, who coordinated the survey.

Smallmouth bass appear to be more susceptible to intersex development than largemouth bass, Blazer said.

Although the frequency of intersex fish discovered was surprisingly high, Blazer cautioned that the sample size was relatively small, with about 10 male and 10 female fish taken from each of eight different locations in Maryland, Virginia and the District of Columbia. Researchers were reluctant to remove large numbers of bass from the rivers because of conservation concerns, she said.

It is not exactly clear what is causing the changes, although it is likely a combination of pollutants. Certain chemicals and pesticides are believed to stimulate estrogen production. Also, estrogen from birth control pills and human waste can make its way from sewage treatment plants to the waterways.

Blazer said researchers are still waiting on data that would help them determine the water quality at the time the fish were caught, but preliminary data taken from the Potomac found a variety of chemical pollutants.

Gerald LeBlanc, an environmental toxicologist at North Carolina State University and an expert in the field, agreed that the high percentages of intersex fish found in the Potomac survey were surprising. He said it is not uncommon for such fish to be found in other parts of the country, but at lesser frequencies.

“It’s an emerging issue,” he said.

The EPA has been studying the issue of so-called “endocrine disrupters” since 1996, but at present, does not issue guidelines to water treatment plants for allowable levels of estrogenic compounds.

EPA spokesman Dale Kemery said research in the field is particularly complicated because of the difficulties in determining how various compounds interact.

Jeanne Bailey, a spokeswoman for Fairfax Water, said the findings are a concern. The water authority, which draws from the Potomac and Occoquan rivers to provide service to roughly 1.5 million people, is working with the USGS and other agencies to facilitate research and develop ways to improve water treatment to eliminate potentially harmful compounds.

The water treatments used by Fairfax Water, including ozone and activated charcoal, have been shown to reduce levels of estrogenic compounds, she said.

While the water authority is concerned about estrogenic compounds and all pollutants, Bailey cautioned against drawing dire conclusions about the impact on human health based how fish are affected.

“Fish are a great indicator of the health of our waters, but they are not a great indicator of what may translate to humans,” she said.