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Scientists say they have conducted the biggest survey ever in U.S. rivers and streams of gender-bending in fish. They found that a large percentage of male bass have acquired feminine characteristics.

NPR's Christopher Joyce explains.

CHRISTOPHER JOYCE: Jo Ellen Hinck of the U.S. Geological Survey spent nine years catching fish to see if there was anything strange about their gender. Gender changes in male fish are not new, but its cause has puzzled scientists.

Ms. JO ELLEN HINCK (Biologist, United States Geological Survey): Many of the other studies have been isolated to specific drainages or geographic regions. So our work is really the first to show the widespread occurrence of intersex in fish across the United States.

JOYCE: Intersex means a male fish having immature egg cells growing in its testes. Hinck's team tested fish from 111 different sites and found the problem was worst in large and smallmouth bass, a popular species for recreational fishing.

Ms. HINCK: Thirty-three percent of the smallmouth bass that we examined across our whole study were found to have intersex, and 18 percent of male largemouth bass were found to have intersex.

JOYCE: In regions of the southeast, 70 to 90 percent of the fish were found to be intersex. Only in Alaska's Yukon River were fish completely free of the condition. Hinck says her results, described in the journal Aquatic Toxicology, surprised her.

Ms. HINCK: When we're up with the majority of fish showing this condition, we don't feel that that's normal, and we do need to answer those questions on what is the cause.

JOYCE: And that's hard to nail down. Scientists have wondered whether it's something in nature, like changes in water temperature or acidity or something genetic about some kinds of fish. But experiments in laboratories point to a certain class of chemicals, especially a group loosely called estrogenic compounds. These mimic the behavior of natural sex hormones, estrogens, in the body.

Ms. HINCK: Pharmaceuticals have been implicated, other personal care products, compounds that are released from wastewater treatment plants or agricultural runoff, urban runoff.

JOYCE: Chemicals in birth control pills, for example, are suspects as well as an ingredient in plastics.

David Norris is an endocrinologist at the University of Colorado who's been looking back in time to see when all this started.

Dr. DAVID NORRIS (Endocrinology, University of Colorado): We have an analysis of museum specimens from 50 and 100 years ago. And if you examine them, we find that there's no evidence of any intersex.

JOYCE: Norris says there's some evidence that this gender-bending may hamper the ability of fish to reproduce, but there's no evidence that these fish are unsafe to eat. As for humans, he says that in the quantities measured in waterways, these chemicals apparently are not causing harm. Nonetheless, he says he'd like to see fewer of them in the environment.

Mr. NORRIS: I think we have to consider reducing, as much as we can, the use of products that have these kinds of chemicals in them.

JOYCE: That could be pretty difficult. Researchers keep turning up more of these sex-altering compounds. Paige Novak is an environmental engineer at the University of Minnesota.

Professor PAIGE NOVAK (Environmental Engineering, University of Minnesota): In my opinion, there is not a cause. There are probably a lot of different causes in different areas. There are probably a huge number of chemicals, some of which we haven't even thought of yet, that could cause some of these problems.

JOYCE: For example, Novak has just discovered that factories that make biodiesel fuel produce them.

Ms. NOVAK: What we found is industries that process soy, they release large amounts of these compounds called phytoestrogens from plants.

JOYCE: Laboratory studies show that plant estrogens have the same effects on fish that synthetic versions do. They even make fish less aggressive. Novak says that's no reason to limit biodiesel production. But like other scientists studying this phenomenon, she says these chemicals probably should be reduced in waterways.

Christopher Joyce, NPR News.

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