Male largemouth bass, pictured above, and smallmouth bass with female anatomy are showing up in river basins across the United States. At some sites, as much as 91 percent of the population of local, male largemouth bass were intersex.iStockPhoto
Male fish with female body parts have been showing up in our nation's rivers for a while now, but a new study that found a surprising number of mixed-up fish may shed new light on ecosystem health.
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.
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.
More than 65 percent of male smallmouth bass were intersex in parts of Minnesota, Idaho and Colarado.
No patterns turned up to finger particular chemicals or environmental conditions that might cause the high rates in some places. It's also not clear why bass might be especially prone to intersex conditions.
In fact, most intersex research has focused on neither bass nor legacy chemicals, said Alan Vajda, an environmental toxicologist at the University of Colorado, Denver. Instead of answering questions, he said, the new study raises a host of new ones.
"This study reminds us that we're dealing both with chemicals that have been introduced into the environment over the decades," Vajda said, "And we're continuing to produce new chemicals despite their thoroughly documented ability to affect our brains, our reproduction and our metabolism."
Fish aren't the only animals whose hormonal systems appear to be going haywire, he added. There is growing evidence for similar conditions in birds, mammals and even people. Studies like this one emphasize the need to do something about it.
"I"m glad there's some renewed spotlight" on the topic of intersex fish, Vajda said. "We've known about this stuff for over 20 years, and there's still very little done policy-wise to address these issues."