This tilapia was caught in the Monongahela River. The Pennsylvania Fish and Boat Commission has no idea how the exotic fish has found its way into Western Pennsylvania waters. Click photo for larger image.

Tilapia -- both a fish-farming boon and a piscine pest -- appears to have made its way into the Monongahela River.

It's not the first time the African species, introduced in the South to control invasive weeds and now ubiquitous table fare, has surfaced in Pennsylvania. Almost 20 years ago, tilapia caused a mild panic on the Susquehanna River when escapees from a fish farm at Brunner Island near Harrisburg found refuge in the warm water discharge of a power plant.

"We took unprecedented steps to get rid of them," said Bob Lorantas, the Pennsylvania Fish and Boat Commission's chief warm water biologist. "We got the discharge temporarily shut off until they died."

It hasn't gotten to that point on the Monongahela and Rick Lorson, the Fish Commission biologist in charge of Pittsburgh's rivers, said he is considering trying to net and then euthanize the tilapia this week, before their numbers get out of hand.

"Tilapia breed like gangbusters -- often and prolifically -- and they grow quickly," he said. "They're an invasive problem down South and the last thing we want in our waterways."

Pittsburgh Zoo and Aquarium biologists also identified the Monongahela River tilapia last week after the zoo was contacted by John Pinigis of Swisshelm Park who has been watching and catching them for the past two weeks near Braddock.

"There are usually at least 10 of them, all schooled up and then pairing off," said Pinigis, who caught them on redworms and chunks of algae he scraped from rocks.

Tilapia are omnivorous, with forward feeding mouths that let them forage throughout the water column.

"The zoo people told me they would be eating hippopotamus dung back in Africa," said Pinigis, who has caught and kept three, up to 12 inches, so far. "That might be why they're hanging around the sewage outflow."

Although Pinigis put the tilapia in his backyard pond -- an enclosed system where there is no danger of escape -- Lorson removed two for research and has identified them, tentatively, as Mozambique tilapia.

How they wound up in a Western Pennsylvania river in the first place is anyone's guess. Though they are farmed the world over, in Pennsylvania -- following the Susquehanna River incident -- tilapia can be raised only in enclosed aquaculture facilities, such as the one near Harrisburg that supplies Wholey's in the Strip District with live tilapia for a thriving niche clientele.

"We sell about 1,000 pounds of live tilapia a week," said company president Jim Wholey, who has the fish swimming in a big tank. "We started the tank just for show but it's become really big. We have an Asian trade whose cultural practice is, for every fish you eat, you release one. They go home with two bags -- dressed fish in one and live fish in the other."

Evidence of ritual releases of fish -- a practice University of Pittsburgh Chinese culture professor Xin Min Liu said has ancient Daoist origins -- is largely anecdotal, perhaps part urban legend. But growing immigrant populations in many major cities have driven up the demand for live tilapia and other exotics being supplied by farms through out the country, where even species such as big head carp have replaced traditional, soil-based crops. The American Tilapia Association said its growers produce 20 million pounds of live tilapia a year for a primarily U.S. Asian market, because those consumers prefer eating fish soon after they are killed. Experts say evolving ethnic communities and a switch from agri- to aqua-culture are posing new threats to natural resources.

"It's a thorny problem since aquaculture facilities are on low-lying rivers prone to flooding and can't be completely bio-secure," said Phil Moy, an invasive species expert with the University of Wisconsin. "And you've got these cross-cultural hurdles to overcome, where some people like their food to be wiggling when they buy it so they can release it for good luck.

"There needs to be a reckoning between the big business of fish farms and conservation."

As a case in point, he said, snakeheads raised in a Crofton, Md., pond by an Asian grocer got loose a few years ago and now are breeding in the Potomac River. Around the same time, big head and silver carp escaped from a flooded fish farm on the Mississippi and now live in the Illinois River. A $9 million electric gate is being built to try to keep them out of the Great Lakes.

"It's probably only a matter of time before all of those species make it to Pennsylvania," said Fish Commission spokesman Dan Tredinnick. "They're the nastiest exotics and they're banned here. Snakeheads and big head carp are a bigger threat than tilapia, although that's an exotic we don't want in the wild either."

Some of Pennsylvania's most coveted gamefish, including brown and rainbow trout, were considered exotics before the term was coined. They were imported from Europe more than a century ago. So, too, was common carp, although it fizzled as a food fish and has gained only grudging acceptance by all but a small following of specialty anglers. Its cousin, the grass carp, is an unwanted exotic, which parts of Canada on the Great Lakes are trying to ban for sale as a live food fish.

A Pennsylvania man killed a 54-pound grass carp with a bow and arrow in Erie's Presque Isle Bay this spring and tried to claim a state record, although its size, while bigger than any common carp caught here, is small by grass carp standards.

Today, it is illegal to put any non-native fish in Pennsylvania waters because they could introduce disease, out-compete established species for food and upset the existing biodiversity. Until recently, unauthorized plantings have usually been pacu dumped by a home aquarist and doomed to die by summer's end. But the threat of invasive species has environmentalists and regulatory agencies scrambling to address a whole new set of concerns.

The Massachusetts Institute of Technology's SeaGrant program has produced a brochure "Into The Pan Not Into the Wild" in five languages, including Chinese and Korean, to discourage the ceremonial release of exotic food fish.

"We know the practice goes on, but getting anyone to admit it is difficult," said marine biologist Judith Pederson, a Johnstown native who lives in Wooster, Mass., and runs the program. "There are language and cultural barriers and a wariness in the Asian community about people from the outside telling them that what they're doing is wrong."