13 of the Most Gender-Bent U.S. Rivers

The nation's fish -- particularly two species popular with anglers -- are facing a widespread and ill-defined threat that is blending their genders. In the most comprehensive study to date -- of 16 species in nine river systems over nine years -- government scientists have for the first time documented the surprising pervasiveness of so-called intersex fish.

Across all rivers studied, one-third of male smallmouth bass, and one-fifth of largemouth bass, had female characteristics (most commonly, immature eggs in their testes), and for the first time, researchers documented intersex channel catfish. Overall, one-third of the river locations tested showed the presence of intersex fish -- including rivers thought to be free of synthetic hormones and other contaminants known as endocrine disrupting chemicals. That last fact confounded scientists, who did not set out to diagnose what is causing blended-gender fish, but who have in the past been able to link the condition to contaminants like pesticides, PCBs, heavy metals, household compounds such as laundry detergent and shampoo, and many pharmaceuticals. The Yampa River, sampled at Lay, Colo., for instance, showed no obvious sources of contamination, and yet 70% of male smallmouth bass there were intersex. (The Yukon River basin was the only one in which researchers did not find at least one intersex fish.)

"This study adds a lot to our knowledge of this phenomena, but we still don’t know why certain species seem more prone to this condition or exactly what is causing it," said Jo Ellen Hinck, the lead author of the U.S. Geological Survey study. "In fact, the causes for intersex may vary by location, and we suspect it will be unlikely that a single human activity or kind of contaminant will explain intersex in all species or regions."

She added: "We know that endocrine-active compounds have been associated with intersex in fish, but we lack information on which fish species are most sensitive to such compounds, the way that these compounds interact to cause intersex, and the importance of environmental factors. Proper diagnosis of this condition in wild fish is essential because if the primary causes are compounds that disrupt the endocrine system, then the widespread occurrence of intersex in fish would be a critical environmental concern."

Some of the most* gender-bent U.S. rivers include:

1. Pee Dee River at Bucksport, S.C.: 91% intersex male largemouth bass
2. Mississippi River at Lake City, Minn.: 73% intersex male smallmouth bass
3. Yampa River at Lay, Colo.: 70% intersex male smallmouth bass
4. Columbia River at Warrendale, Ore.: 67% intersex male smallmouth bass
5. Apalachicola River at Blountstown, Fla.: 60% intersex male largemouth bass
6. Savanna River at Port Wentworth and Sylvania, Ga.: 50% intersex male largemouth bass
7. Rio Grande at Brownsville Texas: 50% intersex male largemouth bass
8. Salmon River at Riggins, Idaho: 43% intersex male smallmouth bass
9. Colorado River at Imperial Dam, Ariz.: 40% intersex male largemouth bass
10. Gila River at Hayden, Ariz: 40% intersex male largemouth bass
11. Chattahoochee River at Omaha, Ga.: 30% intersex male largemouth bass
12. Flint River at Albany, Ga.: 30% intersex male largemouth bass
13. Mobile River, Alabama: up to 25% intersex male largemouth bass

* The highest percentage is listed for rivers sampled at multiple locations.

COMMENTS ()