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Spring 2013

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

April 30, 2013

Northern Snakehead, Channa argus

The stubborn life of a snakehead

The ‘lazy’ fish may not be harming the Potomac as much as feared

Snakeheads are no ordinary fish. They can live for days out of the water. They're elongated, with a small, flat head and extensive top and bottom fins. The mucus they exude would put a slug to shame.

At least two years before the discovery of a northern snakehead in a Crofton pond in 2002, the exotic, predatory fish from East Asia was already colonizing the Potomac River, says John Odenkirk, a Virginia Department of Game and Inland Fisheries biologist.

Today, "they can be found anywhere" in the tidal Potomac, he says, "but they prefer shallow, sluggish backwater with a lot of vegetation."

That's an apt description of Little Hunting Creek, a small tributary of the Potomac just upstream from Mount Vernon. At that snakehead hotbed, biologists with VDGIF are conducting surveys to find out just how many of the fish live there. That will give them a baseline for future population studies of a fish labeled by many as an exotic invasive species.

To count the slippery intruder, the state agency uses a mild electrical shock from an electrofishing boat, or stunboat, to temporarily anesthetize fish. That may put the snakeheads to sleep, but it spawns adventure for the biologists.

Consider this scene from a recent survey: A snakehead lunges onto shore to escape the voltage that has dazed a dozen lesser fish. Odenkirk vaults from the boat's bow railing onto a high dock in an effort to capture the fish, which is flip-flopping among bleached driftwood and last year's reeds. The eight-foot pole supporting Odenkirk's dip net is just long enough to reach the fish before it can flop back into the water.

The snake, as Odenkirk and his two colleagues call it, is the first of 30 caught that day. The biologists haul them in at a rate of about 10 per hour, measuring, tagging and releasing the animals in an effort to understand the biology, spread and ecological effects of the fish.

Low tide drains most of the water from the creek, exposing broad mud flats covered with an emerging waterlily known as spatterdock. The fish are concentrated in a central channel — an opportune time and place for a survey.

Each species responds differently to the shock: Gizzard shad surface with their silvery sides up; eels leap and wriggle; catfish bob to the top, as do largemouth bass. But snakeheads are unique.

"They expel all the air from their swim bladder," Odenkirk says, "and sink like stone."

A watery cloud of sediment and bubbles reveals a snakehead's reaction to the electrical current. The biologists thrust their nets where experience tells them a snake is. They sometimes retrieve a net full of mud, but most of the time they hit their mark.

Odenkirk nets an 11-pound fish and gently drops it into the bottom of the boat, where it lies quietly alongside other snakeheads, waiting for a thread-like orange plastic tag to be anchored in the muscle beside their dorsal fins.

Snakeheads are survivors. One fish netted by Odenkirk's team seems to be thriving even after having its gills cut earlier by an angler.

"They are obligatory air breathers," says biologist Steve Owens. After surfacing to take a gulp of air, they draw it across a strange-looking brachial chamber at the

back of their throat. The breathing organ is probably there "because the fish evolved in stagnant waters with very low dissolved oxygen," he says.

When water levels drop dangerously low, snakeheads are prepared. The thick mucus covering their skin helps them avoid desiccation. "They are the slimiest fish you've ever seen in your life," says Odenkirk.

Several snakeheads tagged during the survey were healing from wounds delivered by birds — a price they pay for living close to the surface. "They take a licking and keep on swimming," Odenkirk says. They are "the ultimate bad-ass fish."

But one thing snakeheads don't do — contrary to some news reports — is crawl overland. "Freakish Fish Causes Fear in Md.; Carnivore Moves on Land," cried a Washington Post headline in 2002. The story quoted one Maryland state biologist as saying, "It can get up and walk."

A walking fish? "That's one of the worst misnomers," Odenkirk says, recalling that after the misconception spread, "people called the office, afraid for their dogs."

The fish's pointy teeth are also overhyped — and apparently underused. Fish found in snakehead stomachs are always intact, Odenkirk says. "They don't bite or tear their food; they inhale it."

More than 25 percent of a local snakehead's diet consists of banded killifish, a small, short-lived fish of shallow fresh and brackish waters, but more than a dozen other species have been found in Potomac snakehead bellies. "They will eat anything that swims past their head," Odenkirk says. "Snakeheads are a lazy fish," which is why its increasingly popular meat is white and mild.

Lazy, maybe, but no pushover when it comes to snagging them with a hook and line for recreational fishing. "They hit like a freight train," says Mike Isel, the boat's pilot. "They're a bulldog," adds Odenkirk.

The Potomac is now a go-to place for snakehead, luring fishermen "even from out of the country," Odenkirk says.

Licensed Virginia anglers are allowed to possess as many as 20 snakeheads a day, but they must kill the fish when they catch it and report their catch to VDGIF. Unlike Maryland, Virginia doesn't allow commercial sales of the fish. A market for snakehead fillets might encourage people to illegally introduce snakeheads into

Northern Snakehead life cycle, Channa argus

Counting snakeheads in the Potomac

In a marked change from past efforts, the Virginia Department of Game and Inland Fisheries is working with similar agencies in Maryland and D.C. to monitor four different tributaries and count — not kill — snakeheads.
Video by AJ Chavar

other Virginia waters, which could thwart VDGIF's efforts to contain them.

The stunboat kicks up evidence of other Asian invasions: fat carp and huge goldfish. Snakeheads are the latest of at least 30 nonnative fish introduced to the tidal Potomac in the past 160 years, including bluegill, rock bass, channel catfish, blue catfish and largemouth bass.

Odenkirk doesn't think that snakeheads have made any significant impact on the Potomac's ecosystem, but it may be a few more years before biologists can say with certainty how snakeheads fit into the river's not-so-natural waters.

But so far, snakeheads aren't gobbling up every living thing in sight — unless it's small and swims near their lazy heads.


Banded killifish


Northern snakeheads can grow to more than 40 inches and 18 pounds.


Northern Snakehead
Channa argus

Biologists tag the fish
  if it is longer
      than 7 inches.

The snakehead year

A female lays a mass of
tens of thousands of eggs in a
slight clearing at the top of a hydrilla patch. Both sexes guard the eggs and hatched larvae.

Another name for the northern snakehead: Mudfish

"Snakes" spend time near the water's surface, making them vulnerable to bird attacks. In
summer, fish can grow
almost a millimeter a day.

Breeding tapers off

swim in schools
of thousands
until fall. If born
early enough in the
season, a fish can
begin breeding the
following year.

As water temperatures drop,
the fish buries itself in mud to
hibernate for the winter months.

Breeding peaks


exceeds 50
degrees, the fish
emerges from hibernation.

How an electrofishing boat works

The stunboat

Fisheries biologists use electrical current to temporarily anesthetize fish so that they can be netted, tagged, weighed and measured.

Flat-bottom metal boat

Outboard motor



Measuring board



Insulated pole

Dip net




Unlike most fish, snakeheads respond to electrofishing by expelling air from their swim bladder and sinking. However, their reaction creates a telltale cloud of sediment and bubbles.

Fish remain stunned for about a minute as their air-filled swim bladders cause them to float to the surface, where biologists net them.

The current affects fish down to about six feet within a three-foot radius around the droppers.

To complete the electrical circuit, the pilot must activate a switch by pressing a pedal while the net handler at the bow stands on a mat that activates a second switch. If either releases his switch, the current stops.

The current feeds through cables to two droppers, each with six wires that deliver about seven amps of current at 120 pulses per second to the water surrounding the boat.

. . . which a converter turns into a direct current.

A gas-powered
generator produces
an alternating current
of 5,000 watts . . .

Source: Virginia Department of Game and Inland Fisheries