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Intersex fish, including bass in Mississippi, more widespread than thought

Government biologists in a national study found that nearly three out of four male bass sampled at Lake City, Minn., have egg cells growing inside their sexual organs.

Smallmouth bass in the Mississippi River are sexually confused. And who can blame them?

Government biologists, in the first major national study of the sexual organs of fish, found that nearly three out of four male bass sampled at Lake City, Minn., have egg cells growing inside their sexual organs.

The study, which covered nine U.S. river basins, found widespread “feminizing” of male fish, a problem connected to genetic or environmental factors in previous research. For example, hormones such as women’s birth-control pills may get flushed down the toilet and end up in the watershed and affect the fish, as can other pollutants.

The Mississippi at Lake Pepin had the highest concentration of intersex (hemaphrodism) smallmouth of the 111 sites sampled at 73 percent. Nationally, 33 percent of the smallmouth sampled had male and female parts.

“We can say this is more widespread than we thought,” Jo Ellen Hinck, the lead author of the study, told me. She’s a biologist with the U.S. Geologic Survey in Columbia, Mo. “This just suggests that it is a problem in the United States,” she said.

Part of survey done at Little Falls
A small part of the survey was done at the Mississippi River at Little Falls, Minn., where one of seven male smallmouth had eggs in it.

Nationally, 16 species were looked at, and about 6 percent of the nearly 1,500 male fish had some female in them. Bass, both largemouth and smallmouth, had the highest percentage of intersex occurrence — one-fifth of the largemouth showed signs of hemaphrodism.

“We still don’t know the cause,” Hinck said. “Research will continue on the genetic and environmental factors.”

The surveys, which also found some female fish with male parts, were conducted between 1995 and 2004. The study was published this month in the journal Aquatic Toxicology.

Can still reproduce
Hinck said the intersex fish can still reproduce but not as well as normal fish. The only basin that did not show intersex fish was the Yukon River in Alaska. The rivers in the Pee Dee basin in the southeastern U.S. showed the highest feminization rates.

The other sites were in the Columbia, Colorado, Rio Grande, Savannah, Apalachicola, Mobile basins.

The researchers also documented intersex channel catfish for the first time.

Some of the compounds that can affect the endocrine system include pesticides, heavy metals, household products such as shampoos and laundry detergents, and various pharmaceuticals. Hinck said the results varied by location so it was impossible to pinpoint a single cause or condition.