Potomac Fever

"Like an Applebee's over there," Chaconas said.

"And he has no mind of us."

Splash. The snakehead was still eating.

"Why," Radoski pleaded, "don't you bite the purple worm?"

FROM THE SURFACE, THE LOWER POTOMAC RIVER STILL LOOKS STUNNINGLY PRISTINE. Outside of the stretch from the Key Bridge to the Woodrow Wilson Bridge, where it is lined with monuments, parkways and close-clipped grass, the river runs mainly through canyons of tall trees. Downstream of Washington, it's not hard to stand on the bank and imagine John Smith sailing by in 1608, mapping the Virginia wilderness.

From under the water, though, the Potomac looks like Casablanca, 1941. It's full of refugees, colonists and would-be conquerors, all of them on the make. Many of the creatures from John Smith's time -- migrating shad, prehistoric-looking sturgeon, huge reefs of American oysters -- are now barely tolerated guests in their native ecosystem.

People, of course, imported the invaders in spasms of let's-see-what-happens and what-could-it-hurt. Largemouth bass, native to other sections of the country, were stocked in the river for sport fishing in the mid-1800s. Ditto for blue catfish, a Mississippi River native, in 1905. For years, it seems, state wildlife agencies were run with the mentality of a 12-year-old boy: " Wouldn't it be cool if we . . .?" Even piranhas have been spotted in the Potomac, presumably dumped from somebody's aquarium, though they don't seem to survive mid-Atlantic winters.

Perhaps the best way to examine the Potomac's mismatched menagerie -- short of draining the river -- is to zap the water with 10 amperes of electrical charge, then wait a few seconds to see what rises to the surface. This is called "electrofishing": a less lethal version of the old dynamite-in-the-water trick and strictly off-limits to regular anglers. It's allowed, though, for scientists trying to survey the river's inhabitants. So, early one spring morning, two Virginia Tech fish researchers, Ryan Saylor and Nick Lapointe, set off into Pohick Bay, a Potomac inlet near Lorton, in a boat with a generator in the stern and two sets of wires dangling off the bow.

They steered into the shallows, where fish couldn't dive deep to escape. Then the researchers turned on the juice. The generator rumbled. After a few seconds, something splashed on the right side of the boat: a writhing long-nosed gar, which resembles a needle-nosed alligator. It looked otherworldly, but gars belong in the Potomac. Then the real aliens appeared. Catatonic bass with their white bellies showing. Fat, scaly carp, which tipped up rigidly and then sank like torpedoed battleships.

Then a splash got everybody's attention. On the boat's left side, a monstrous green fish -- the size of a limbless dachshund -- catapulted out of the bay. Saylor jabbed a long-handled net into the water and came up yelling.

"That is a PIG!" he shouted to Lapointe, who was steering the boat. Saylor looked back at the creature suspended in the mesh of his net. Upon closer inspection, it looked like a two-foot section of boa constrictor with fins. "That is a big fish, dude!"

It was an eight-pound, nearly 27-inch specimen of Channa argus, a predator native to East Asia. This fish was first discovered in the Potomac in May 2004. By that time, the northern snakehead needed no introduction in the Washington area. This was Frankenfish.