The results could help state and federal officials determine the source and extent of the Potomac's population of northern snakeheads, an invasive species feared for its ability to crowd out and devour other fish.
"It's going to take a while," Orrell said of the tests. "All we can say is results are pending."
Biologists from Maryland, Virginia and the U.S. Fish and Wildlife Service continued searching for snakeheads this week along an eight-mile stretch of the Potomac and its tributaries near Fort Belvoir, Va., where they have been found.
For the past two weeks, they've been using gill nets and seines. They also have used boats equipped with electrofishing gear, which sends a charge into the water that stuns fish and brings them to the surface.
But officials say they can't possibly survey all the areas where snakeheads might be lurking. "You've got a lot of areas and only so much sampling gear you can use," said Steven Minkkinen, a project leader for the U.S. Fish and Wildlife Service.
The scientists have posted notices on fishing piers and Web sites asking anglers to kill and report snakeheads they find.
The Potomac fish, found within weeks of each other, were all about 2 years old and roughly the same size, 12 to 14 inches. So far, biologists say, there is no way to know if the fish are part of an established population or were recently dumped into the river.
"It's kind of a mystery as to why we're finding what we're finding," said Gary Martel, director of inland fisheries for the Virginia Department of Game and Inland Fisheries. "We just don't have enough information."
Martel said the fish could turn out to be a permanent fixture. He noted that many species now in the river are nonnative - including bass, carp and catfish.
"The water's the right temperature, the habitat is right for them. There's no reason they won't establish a population," he said.
The northern snakehead is a voracious Asian predator, nicknamed "Frankenfish," thanks to its sharp teeth, aggressiveness, and ability to live out of the water and wriggle short distances on land with its fins. Imported for sale at Asian food markets before federal officials cut off the flow two years ago, snakeheads are kept as pets - often until they grow too large and their owners dump them in local waterways.
"Our understanding is, it's not an aquarium fish," said Steve Early, an assistant director of fisheries for the Maryland Department of Natural Resources.
The snakehead scare started here two years ago, when officials poisoned three small ponds in Crofton after a pair that had been dumped by their owner was found.
The first snakehead found in the Potomac was caught by a fisherman May 7 in Little Hunting Creek near Mount Vernon. The most recent catch was June 4 in Dogue Creek in Fairfax County, Va.
Florida officials say that concerns about snakeheads may be overblown. A similar species discovered in a Broward County canal system four years ago has had little effect on the large-mouth bass, sunfish and blue gill populations, they say.
"We wish they weren't here, don't get me wrong. But there's been no serious impact on our native species," said Kelly Gestring, a fisheries biologist for the Florida Fish and Wildlife Conservation Commission. "I don't think anyone should be alarmed."
DNR officials ask anyone who catches a snakehead to call them at 410-260-8320.