The numbers of blue catfish in some Bay tributaries are nearly unfathomable and, with some individuals reaching weights of more than 100 pounds, they've become a lure for trophy anglers from across the nation.

But their large populations and big sizes have drawn a warning from a Bay Program committee that in December adopted a policy warning that the introduced species "could be posing a threat to native species in all major Chesapeake Bay river systems in Virginia and Maryland."

The Executive Committee of the Sustainable Fisheries Goal Implementation Team, which includes state and federal fishery officials from around the Bay, warned in its new policy that blue catfish, along with the related flathead catfish, could threaten efforts to restore native fish such as shad and river herring.

The policy says "the potential risk posed by blue catfish and flathead catfish on native species warrants action to examine potential measures to reduce densities and limit range expansion, and to evaluate possible negative ecological impacts."

It pledges to coordinate research efforts to better estimate catfish populations, their impacts on other species, and actions that could control populations while supporting recreational fisheries. Other work will include examining contaminant buildups in the fish, and building public awareness among anglers to prevent further introductions.

"We are not simply standing by to see what might happen with these species of invasive catfish," said Peyton Robertson, director of the National Oceanic and Atmospheric Administration's Chesapeake Bay Office, and chair of the goal implementation team. "We are committing to both further study, and also to identify some management measures and implement them."

NOAA's Bay Office is funding a number of research projects related to blue catfish and the flathead catfish.

The policy is dramatically reworked from early versions circulated more than a year earlier that had called for efforts to eliminate or dramatically reduce blue catfish numbers. That stirred the ire of many blue catfish fans who flooded officials with comments.

The Virginia Department of Game and Inland Fisheries, which oversees management of blue catfish in that state, also objected to the language. "Basically, we were trying to make sure that the policy was grounded in science," said Bob Greenlee, a fisheries biologist with the department. "There were a lot of statements and sort-of gut feelings."

It's unclear what can be done about the catfish, which seem well-adapted to the Bay environment. Since they were introduced into the James River in 1974, their populations exploded in low-salinity portions of all of Virginia's tidal tributaries as well as the Potomac. In some areas, scientists say blue catfish account for as much as 75 percent of all fish biomass.

"We are not going to remove the species from the system at this point," Greenlee said. "There isn't a mechanism by which we can reduce their abundance."

But some hold out hope that a commercial fishery might be established to help control the populations, or that methods could be developed to help keep them out of the Bay tributaries that are important breeding areas for shad and river herring.

"There are places that don't have blue catfish yet and may have sensitive resources which can be protected," said Greg Garman, a scientist with Virginia Commonwealth University.

But control methods may be needed quickly. Maryland has found the nonnative catfish in the Patuxent, Nanticoke, Choptank and Upper Bay in recent years. Maryland surveys conducted this winter suggest those populations are growing.