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.
For nearly ten years, Hinck and colleagues collected 16 species of fish from 111 sites in nine major river basins around the U.S.
After analyzing fish carcasses for both chemicals and related health conditions, the researchers found that male smallmouth bass and male largemouth bass had female parts in close to half of the sites and in all but one of the basins sampled.
The Yukon River was the only basin that appeared to be free of intersex fish, the scientists reported in the journal Aquatic Toxicology. In the southeastern United States, on the other hand, the condition was extremely common, particularly in largemouth bass.
In the Pee Dee River at Buckport, S.C., for example, 91 percent of male largemouth bass had female parts, along with 60 percent of males in the Apalachicola River at Blountstown, Fla., and 50 percent in the Savannah River at two sites in Georgia.