The Pennsylvania Fish and Boat Commission took an electro-fishing excursion on the Monongahela River Wednesday near Braddock but failed to capture a school of tilapia that had showed itself on the river in recent weeks.

"Conditions were poor but the fish may have moved on," said commission biologist Rick Lorson, whose staff confirmed the presence of the species a week earlier.

But even though tilapia are unwanted in the wild in Pennsylvania, they are the least of the state's problems when it comes to invasive exotic species.

Angler and boater travel, foreign freighters, fish farms and changing cultural practices have created a host of unwelcome fish, aquatic plants -- such as the Cabomba now undermining Conneaut Lake -- and pathogens such as largemouth bass virus and whirling disease.

Although invasives from overseas are typically the most troubling, any fish that doesn't belong in a particular waterway -- even if it is native to another part of the state -- is considered a nuisance, according to John Arway, the fish commission's environmental services chief and a member of the governor's invasive species council.

"A good example is flathead catfish, which began showing up in the Susquehanna a few years ago," he said. "They're native to the Ohio River drainage, but a potential problem in other systems where they could out-compete other catfish for forage and habitat. We don't really know but we'd rather not have to find out."

The Governor's council is hoping to have a plan in place by the end of this year for managing the broad spectrum of nuisance animals and plants.

"Will we ever be able to control exotics 100 percent? No," Arway said. "But it's not a losing battle, either. Plans have always been reactive, not proactive. What we'd like is to try to get in front of some of this."

Some species already have a foothold, such as the round gobies and zebra mussels that pervade Lake Erie. Gobies not only pose a threat to spawning bass but now are being looked at as a possible vector for the botulism virus plaguing parts of the lake because they feed in low oxygen levels and could activate the toxin, while zebra and quagga mussels have made the lake clearer, but not cleaner.

"The zebras have turned Erie into a benthic, or bottom-oriented, fish community by pulling nutrients from algae in the rest of the water column and putting production on the bottom," said fishery biologist Michael Goehle, the northeast regional aquatic nuisance species coordinator for the U.S. Fish and Wildlife Service in Buffalo, N.Y. "That can create all kinds of consequences, which is something you don't want when you're managing a $4.5 million sport fishery."

Erie's connection to Lake Ontario and seafaring ships that can introduce invasives through their ballast water make it especially vulnerable to nuisance fish. But most lakes, rivers and streams in Pennsylvania are also at risk of unwanted species and diseases, said Arway. Largemouth bass virus has been documented in lakes such as Joseph Foster Sayers in Centre County, and spring viremia is suspected in carp die-offs on the Allegheny River.

Measures for controlling the introduction and spread of invasives are mostly voluntary, asking anglers to clean their boats and trailers and disinfect their livewells from one waterway to the next.

"Everyone has livewells these days, so now we have largemouth bass virus in Pennsylvania and surrounding states," said Bob Lorantas, the commission's chief warm water biologist. A livewell or bait bucket may be how rusty crayfish, which are indigenous to the Ohio River, got transported to the Susquehanna, where they could disrupt the ecosystem and grow too big to be kept in check by predator fish, Lorantas said.

Anglers are urged to kill any of the 11 species the commission has identified as most threatening, including rusty crayfish; zebra and quagga mussels; round and tubenose gobies; European rudd; ruffe; snakehead; and black, bighead and silver carp. Some are so new, though, that most fishermen couldn't identify them.

The commission says anglers also can help control spread by not moving bass or other sportfish to new waters.

"We know people move black bass to their favorite lakes," Lorantas said, "and they may ask, what's the harm? Well, in the process of moving one fish you might be moving other organisms as well, like zebra mussel larvae, which are invisible."

The more sensational invasives, like the silver carp captured on video jumping into boats in the Mississippi drainage, haven't arrived in Pennsylvania yet, and federal officials are trying to keep them out of the Great Lakes with a $9 million gate constructed on the Illinois River. The unrelated bighead carp, which can grow to 100 pounds by eating 40 percent of their body weight in plankton every day, are living in Philadelphia's Lake Edgewood. But Goehle said less publicized invasives, such as the European ruffe, can be just as ominous. If the ruffe spreads to Lake Erie from the upper Great Lakes, he said, it could devastate yellow perch, since the two species share the same habitat and forage.

"Until an invasive becomes part of an ecosystem, we don't really know how much damage they can do," Goehle said. "Much of the fear out there is fear of the unknown."