On the Hunt for Giant Blue Predators in the Potomac River

As a rising sun burned mist from the Potomac River, and seagulls wheeled overhead, a group of scientists launched a strange-looking research vessel in pursuit of giant, exotic predators.

The biologists with the Maryland Department of Natural Resources biologists were on the hunt for blue catfish. Native to the Mississippi River, blue catfish owe their name to their steely blue skin color. They have long whiskers, wide mouths, milky bellies, and can grow up to 140 pounds and four and a half feet long. That's at least twice the size of any native Chesapeake catfish.
 
Mary Groves, a southern regional fisheries manager with the state agency, piloted an electrofishing boat on an expedition to look for the blue giants near Fort Washington, about four miles south of Washington, DC. The motorboat was 18 feet long and sported what looked like a pair of metal arms dangling oversized egg beaters from the bow into the gray-green water.
 
“This puts a pulsed DC current into the water -- just enough to cause the blue catfish to be temporarily stunned, and rise up to the top of the water so we can net them,” Groves explained, motoring past an osprey guarding a nest atop a channel marker.  “Blue catfish are an invasive species. They don’t belong here. But over the last 10 or 15 years the population has exploded. And so we are trying to learn as much as we can about the fish, so we can come up with a management plan for them.”

The Virginia Division of Inland Fish and Game Fisheries in 1975 began introducing blue catfish into Chesapeake Bay tributaries as a game fish, according to a Smithsonian Environmental Research Center database of invasive species.  But ecologists now regret the move, because the monster-sized catfish have voracious appetites for native fish species.
 
Among the prey of blue cats appear to be American shad, an historically important but declining fish species that the federal and state governments are trying to protect and restore.

As Groves and her colleagues cruised along the wooded shores of the Potomac River, high atop a bluff, beneath a stately tree, perched Mount Vernon, home of the most famous shad fisherman: George Washington.
 
An alarm started beeped on the boat, warning passengers that electricity was surging into the water.
 
“Blue catfish, in particular, are very, very sensitive to electrical impulses,” Groves said, staring intently at the screen of a sonar device. “Because of this sensitivity, we can actually go in to 50 or 60 feet of water and electrofish and keep the current down very, very low.”
 
Suddenly, hundreds of blue catfish rose to the surface all around the boat. They swam frantically in circles, their fins breaking the surface.  Others floundered drunkenly on their sides before sobering up and swimming back down into the darkness.

The scientists swooped in with long-handled nets and scooped up about a dozen of the fish. They
plopped the blue cats into a large metal tank that sloshed on the deck.
 
"We’re not having any problem at all finding them,” Groves said, watching the tank fill up with fleshy torpedoes.  “I haven’t seen anything other than blue catfish. When blue catfish come in, a lot of your other catfish species seem to decline.”
 
The researchers planned to later slice open the stomachs of the blue catfish to examine what they are eating. In their studies, the Maryland fisheries scientists are being helped by biologists from Virginia and the Smithsonian Environmental Research Center.  The scientists are using DNA analysis and other techniques to precisely characterize the catfish diet.

If the diet of blue cats includes large numbers of American shad, or river herring (another fish species protected because it is in decline), fisheries managers from the region would feel more urgency to create a management system to reduce blue catfish populations.
 
But even if blue catfish pose a problem, what could possibly slow them down, now that they’re everywhere and multiplying? It would be both impossible and probably dangerous to try to shock or poison them all from the region’s rivers.
 
“One possibility is to create a commercial and recreational fishery for them, and reduce them by targeting human predation on this species,” said Dr. Tuck Hines, Director of the Smithsonian Environmental Research Center.

As it turns out, many fishermen and women find this invasive species delicious.  The same is true with Asian snakeheads, another non-native species whose populations have exploded recently in the Potomac River. 
 
The best weapon against exotic predators may turn out to be not an electrofishing boat -- but a fork and knife.

By Tom Pelton

Chesapeake Bay Foundation

(Photo at top from Maryland Department of Natural Resources website of fisherman Shawn Wetzel of Pennsylvania holding record blue catfish he caught earlier this year near Fort Washington on the Potomac River.  Other photos by Tom Pelton)

 

 

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