updated 1/23/2007 9:20:48 PM ET 2007-01-24T02:20:48

Scientists studying intersex fish in the Potomac River basin have found the abnormality for the first time in redbreast sunfish, the third species affected by the mysterious phenomenon, a federal fish pathologist said Tuesday.

Intersex fish possess both male and female characteristics. For example, some male fish have been found with immature eggs in their testes.

The phenomenon was previously documented in smallmouth and largemouth bass in the Potomac River and some of its tributaries in Maryland, Pennsylvania, Virginia and West Virginia.

Vicki Blazer of the U.S. Geological Survey said she verified the abnormality in sunfish last week while preparing for her talk Tuesday at the start of a three-day conference on fish kills in the six-state Chesapeake Bay watershed. The conference was sponsored by the Annapolis-based Chesapeake Bay Foundation.

"We do see it in some of the redbreast sunfish, although I have no idea what the incidence will be," Blazer said.

The incidence of intersex in male smallmouth bass has been as high as 100 percent in some sample areas studied by Blazer and her colleagues. Largemouth bass have had a lesser incidence of intersex, Blazer said.

Blazer said that whatever is causing the abnormality, which was first detected in the watershed in 2003, may also be behind a number of large-scale fish kills across the region since 2002. In one such event in 2005, 80 percent of the smallmouth bass and redbreast sunfish in the South Fork of the Shenandoah River developed lesions and died. A similar kill occurred in 2004 on the river's North Fork. The state of Virginia has formed a Shenandoah River Fish Kill Task Force to investigate the kills.

Blazer said intersex and fish kills may be related because many of the killed fish appear to have had suppressed immune systems. There is increasing evidence that immune cells and disease resistance are affected by contaminants including chemical compounds that stimulate estrogen production, Blazer said.

Another factor in intersex development could be increased aquatic levels of natural and synthetic hormones, such as those in birth-control pills, which often aren't removed by wastewater treatment plants, Blazer said. Poultry manure, which is used as fertilizer in agricultural fields, is also high in steroids identical to human female and male hormones, although no cause-and-effect evidence linking poultry manure to intersex fish exists, researchers said.

"My guess is, is it's a mixture of things, but that's still what we have to figure out," Blazer said.

Charles Pouksih, environmental program manager for the Maryland Department of the Environment, said excessive nutrients from human waste and fertilizers are largely to blame for fish kills in the Chesapeake Bay watershed.

"We're operating in the dark, but maybe not as much in the dark as we were 10 years ago," he said.

The Chesapeake Bay watershed spans nearly 65,000 square miles in Delaware, Maryland, New York, Pennsylvania, Virginia, West Virginia and the District of Columbia.

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