Rising number of hermaphrodite fish in U.S. Rivers

Studies conducted by USGS scientists show how intersex in smallmouth and largemouth bass seem to be widespread across nine U.S. river basins.

Researchers studied 16 different fish species across the United States between 1995 to 2004. A total of 6 percent of the nearly 1,500 male fish studied showed high levels of sexual defects. The most common occurrences of intersex were among bass with a third of all male smallmouth bass and a fifth of male largemouth bass having female body parts. A fewer number of female fish were also found to have male characteristics. This is the first comprehensive study on hermaphroditism among fish species. The mixed-up male fish were carrying immature female eggs cells in their testes. These small female organs growing in the male fish's gonads can only be seen with a microscope. Studies were conducted along Apalachicola, Colorado, Columbia, Mobile, Mississippi, Pee Dee, Rio Grande, Savannah, and Yukon River basins. The occurrence of feminized fish was as high as 70 percent in some parts along Pee Dee, Colorado and Mississippi river basins. The Yukon river in Alaska was the only basin where researchers did not find at least one fish with defects. The scientists aren’t exactly sure about the cause behind this defect. USGS biologist Jo Ellen Hinck who was part of the study said.

This study reveals the prevalence of intersex is more widespread than anyone anticipated.Past studies have linked the problem to endocrine-disrupting hormones, such as estrogen from women's medicines. While the fish can still reproduce, studies have shown they don't reproduce as well.