Intersex Fish Numbers Swell in U.S. Rivers

Sept. 15, 2009 -- Male fish with female body parts have been showing up in our nation's rivers for a while now, but a new study found a surprising number of mixed-up fish.

From the Mississippi to the Rio Grande, from the Appalachia to the Colorado, researchers found large numbers of river fish with egg cells in their testes, particularly in two species: smallmouth and largemouth bass.

At some sites, more than 70 percent of males from these species were intersex, a condition that has been linked to lowered sperm production, trouble reproducing and other negative health consequences.

Scientists don't yet know what's causing the problem or whether intersex conditions are becoming more common in these bass. For now, the study is simply the first one to take a broad geographic look at how common intersex fish are in the United States.

"The occurrence was more widespread than we anticipated," said Jo Ellen Hinck, a biologist with the U.S. Geological Survey in Columbia, Mo. "When you have the majority of fish at a site showing up intersex, that's worrisome. We think that's enough reason to try to find out what's the cause of this and if it has implications for ecosystem health."

Hinck's study began in 1995 as an attempt to monitor fish health in response to "legacy" chemicals -- such as DDT, PCB's, pesticides and mercury. These contaminants get into rivers, where they linger long after they've been banned.