Largemouth bass, part of the black bass family, are among the fish most affected by feminization of males. (iStockphoto.com)

Government scientists figure that one out of five male black bass in American river basins have egg cells growing inside their sexual organs, a sign of how widespread fish feminizing has become.

The findings come from the U.S. Geological Survey in its first comprehensive examination of intersex fish in America, a problem linked to women's birth control pills and other hormone treatments that seep into rivers. Sporadic reports of feminized fish have been reported for a few years.

The agency looked at past data from nine river basins, which cover about two-thirds of the United States — and found that about six per cent of the nearly 1,500 male fish had a bit of female in them. The study looked at 16 different species, with most not affected.

The fish most feminized are two of the most sought-after freshwater sport fish: the largemouth and smallmouth, which are part of the black bass family. Those two species also were the most examined, with nearly 500 black bass tallied.

"It's widespread," said USGS biologist Jo Ellen Hinck. She is the lead author of the study, published online this month in Aquatic Toxicology. She said 44 per cent of the sites where black bass were tested had at least one male with egg cells growing inside.

Past studies have linked the problem to endocrine-disrupting hormones, such as estrogen from women's medicines. While the fish still can reproduce, studies have shown they do not reproduce as well, Hinck said.

Intersex fish also are seen as a general warning about what some experts see as a wider problem of endocrine disruptors in the environment.

The egg cells growing in the male fish's gonads can be seen only with a microscope after the fish has been caught and dissected.

The study used data from 1995 to 2004, when the government stopped funding the research. The only river basin examined that did not show any problems was Alaska's Yukon River Basin.

The Southeast, especially the Pee Dee River Basin in North and South Carolina, had the highest rates of feminization. In Bucksport, S.C., 10 of 11 largemouth bass examined were intersex. In parts of the Mississippi River in Minnesota and the Yampa River in Colorado, 70 per cent of the smallmouth bass had female signs.

Hinck said black bass seem to be more prone to the problem, but researchers do not know why. She also found one common carp that was female with bits of male testes growing inside.