Bryan Brooks has spent a lot of time wading in Pecan Creek, a small Denton stream, searching for mutant fish. For some time, Brooks and his colleagues from the University of North Texas were observing strange things in North Texas fish-males turning into females, for example-but were unable to blame them on traditional water pollutants like metals. The environmental toxicologists thought the mutations might have something to do with other compounds like pharmaceuticals that were showing up in freshwater streams.

Over time, they collected a bunch of fish and tested their flesh in the lab. Sure enough, they found fluoxetine (Prozac) and sertraline (Zoloft) and their human metabolites in every catfish, crappie and bluegill they tested. It was the first time researchers had proved that these human drugs were showing up in wild fish.

Brooks (now at Baylor University) is part of a growing legion of scientists and regulators studying “emerging contaminants,” a loose definition of chemicals that include prescription and over-the-counter drugs, flame retardants, animal hormones, pesticides, plasticizers and cosmetics, to name a few. Many of these unregulated contaminants pass through wastewater treatment plants and end up in streams, exposing fish and other aquatic life to an exotic chemical cocktail.