A form of intersex fish, which have male and female traits, is found more frequently in areas with more farming and human population density, according to a new study by the U.S. Geological Survey.

The so-called intersex fish have been found in U.S. waters over the past decade, including the Potomac River watershed, the southern Great Lakes and the Southern California coast. The cause isn't fully understood, but researchers suspect waste-water and farm runoff polluted with chemicals that stimulate estrogen production.

.S. Geological Survey researchers found the frequency of male smallmouth bass with immature female eggs in their testes was highest where farming is most intense and where human population density is highest. The study also found the prevalence of the form of intersex known as testicular oocytes was greatest just before and during the spring spawning season.

The results were published in the current edition of the Journal of Aquatic Animal Health.

Scientist Vicki Blazer, who led the study, said smallmouth bass were collected from the Shenandoah River, the South Branch of the Potomac and the Potomac River basin.

"On the Shenandoah, rates of intersex were highest, ranging from 80-100 percent intersex," he said.

In the Potomac basin, 75 percent of fish had testicular oocytes in the most heavily farmed and populated areas, dropping to 14 percent to 35 percent in less farmed and developed sites.

In the South Branch, percentages ranged from 47 percent to 77 percent, increasing along with farming and population, the researchers said.

Last year, the Sierra Club asked the Environmental Protection Agency to ban the use of certain toxic chemical compounds in industrial and household detergents because the ingredients are thought to cause male fish to develop female characteristics.

The Sierra Club also asked the agency to bar the use of the products in areas where waste-water treatment plants are not equipped to remove nonylphenol ethoxylates, or NPEs.

The compounds, derived from petroleum, are used mainly in detergents but also in paper manufacturing and flame retardants.

NPEs are more tightly restricted in Canada and Europe than in the United States, which issued water-quality limits for the key ingredient, nonylphenol, or NP, in December 2005.

Detergent manufacturers Procter & Gamble and Unilever have substituted other chemicals in their products. Wal-Mart is seeking to phase NPEs out of its stores by rewarding companies that find alternatives.