The more waters that scientists investigate, the more intersex bass they find. Latest discovery is in the Des Plaines River, about 125 miles downstream from here.
In dissecting 51 male largemouth bass, researchers from the Illinois Natural History Survey (INHS) found that 21 had grown oocytes, or female eggs, in their testicular tissue.
“Long-term surveys conducted by the INHS in this region have shown big increases in largemouth bass over the past 40 years since the implementation of the Clean Water Act,” said fisheries biologist Mark Fritts. “It's a dichotomy here because we're seeing a population that has increased dramatically, but we're also seeing this potential problem rising.”
But this part of the river is far from pollution free. Treated sewage from Chicago flows into this area from the Chicago sanitary and Ship Canal. In a 2016 water quality report, the Illinois Protection Agency found 12 out of the 14 segments of the river tested were impaired by contaminants such a fecal bacteria and toxic industrial chemicals.
Pollution seems to be the common thread in other discoveries of intersex bass, both from municipal sewage and agricultural runoff. Specifics are elusive, except for the belief that chemicals acting as "endocrine disruptors" are causing the mutations. They distort functions that regulate hormones and the reproductive system.
"This is an emerging field of research. We're kind of on the tip of the iceberg," Fritts said. "There are still a lot more questions than answers."
Starting in 2003, scientists with the U.S. Geological Survey (USGS) found male smallmouth and largemouth bass with immature eggs in several areas of the Potomac River. Then they noted intersex smallmouth and white suckers at 16 sites in the Delaware, Ohio, and Susquehanna Rivers in Pennsylvania. At one site near Hershey, Pa., 100 percent of male bass sampled had eggs.
Of these findings, USGS scientist Vicki Blazer said, "We keep seeing a correlation with the percent of agriculture in the watershed where we conduct a study."
Additionally, studies conducted from 1995 to 2004, revealed intersex bass in the Apalachicola, Savannah, and Pee Dee River basins of the Southeast.
Just last year, meanwhile, two federal agencies found significant numbers of male bass were intersex in waters of or near National Wildlife Refuges in the Northeast. Eighty-five percent of male smallmouths and 27 percent of male largemouths tested positive, according to USGS and the U.S. Fish and Wildlife Service.