At first she was surprised. Then she was disturbed. Each time a batch of male fish with eggs in their testes shows up in the Chesapeake Bay watershed, Vicki Blazer's eyebrows arch a little bit higher.
In the latest study, smallmouth bass and white sucker fish captured at 16 sites in the Delaware, Ohio and Susquehanna rivers in Pennsylvania had crossed over into a category called intersex, an organism with two genders.
"I did not expect to find it quite as widespread," said Blazer, a US Geological Survey biologist. Since 2003, scientists have discovered male smallmouth and largemouth bass with immature eggs in several areas of the Potomac River, including near the Blue Plains Advanced Wastewater Treatment Plant.
The previous studies detected abnormal levels of compounds from chemicals such as herbicides and veterinary pharmaceuticals from farms, and from waste spewed by sewer overflows near smallmouth-bass nesting areas.
Those endocrine-disrupting chemicals throw off functions that regulate hormones and the reproductive system. In the newest findings, at one site in the Susquehanna near Hershey, Pennsylvania, 100 per cent of male smallmouth bass had eggs.
After the first intersex bass were found in the Potomac, the US Fish and Wildlife Service made a remarkable discovery at Blue Plains: "We found female germ cells in the testes of 82 per cent to 100 per cent of the male smallmouth bass and in 23 per cent of the male largemouth bass," the agency said.
It is a problem that extends beyond the Chesapeake Bay region that includes the District, Virginia, Maryland, Delaware, West Virginia and Pennsylvania. Intersex bass were found in the Columbia, Colorado and Mississippi river basins in 2009.
Scientists have yet to identify a chemical responsible for causing male fish to become part female.
In urban areas, estrogen products are flushed down drains, contaminating water. In rural areas, animal hormones in manure are spread on fields and washed into water by rain. "I think it's a complex mixture of chemicals," said Blazer, whose study appeared in Environmental Monitoring and Assessment.