“Everything we see says that these populations have done nothing but increase,” he said in a telephone interview.

In August, schools of tilapia began showing up in the Monongahela River, to the delight of anglers and the consternation of the Pennsylvania Fish and Boat Commission, which tried unsuccessfully to net and destroy them. How tilapia wound up in the Monongahela is somewhat of a mystery, although a local fish market sells them, and federal wildlife officials say the live release of exotic species  in religious rituals or by immigrants wanting to establish familiar food sources  is a growing problem.

“There’s a lot of anecdotal information that this goes on,” said Greg Conover, the chairman of the Asian Carp Workgroup for the United States Fish and Wildlife Service. “There’s an effort to reach consumers with brochures in Chinese and other languages, but the cultural barriers are there, and more outreach is needed if we are to change behavior.”

Mainstream and ethnic markets sell exotic species, from mud eels to green crabs, according to Walter Courtenay, a retired Florida Atlantic University biology professor who works as a volunteer fishery research biologist with the United States Geological Survey.

“We’ve seen silver carp in live markets in Ontario that show gillnet scars from having been captured in the wild,” he said. “So we know they’re being transported beyond where they were caught, which was probably in the Mississippi Basin.”

Chinese mitten crabs  an Asian delicacy outlawed 20 years ago because of their impact on riverbanks in the San Francisco Bay watershed  are still surfacing in live markets in New York and were recently discovered in Baltimore Harbor.

Purveyors are subject to federal regulations for importation and interstate trade, as well as widely varying state laws about possession and use.