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The Harlem Meer, in the northeastern corner of Central Park, has long been a refuge for city people looking to indulge their inner Huck Finns, whiling away a lazy summer afternoon fishing for bass, yellow perch and black crappies.

But late last week, signs started popping up around the lake notifying anglers of the arrival of an intruder. The dreaded northern snakehead, a fierce predator common in the rivers and lakes of Asia but considered an invasive species in American waters, had been spotted.

The warning to anglers was clear: If you catch this fish, do not release it. Contact the authorities immediately. It does not belong and could radically alter the local fish population.

The snakehead is a relentless and efficient predator that devours just about everything in its path — fish, frogs, crayfish, beetles and aquatic insects. And it does not meet death easily; it is able to survive under ice or live on land for days in damp conditions. It has been called Fishzilla.

“I would describe them as the freshwater fish equivalent of a tank,” said Ron P. Swegman, a fly-fishing expert and author whose writings about fishing in Central Park include an essay, “Bright Fish, Big City.”

“They are heavily armed,” he said, “strong, and can cover almost any territory, aquatic and — at least for short periods — on land.”

Anytime something truly wild makes an appearance in this city built by man, it attracts attention. Give the creature a torpedo-shaped body that can grow to more than three-feet long, a jaw that stretches back well beyond its eyes and a reputation for being both a voracious eater and prodigious breeder and you can set off, well, a feeding frenzy of curiosity.

Mr. Swegman said that the local fishing community was obsessed with the fish, with regulars even wagering on who would be the first to catch a snakehead.

When he was fishing with a friend on the lake Friday, just as the signs started to appear, the friend claimed to have hooked one, reeling it in close enough to see, before it escaped. “That is a true story,” Mr. Swegman said.

On Tuesday night, the New York State Department of Environmental Conservation will begin a survey of the lake. They will try to verify the sightings, determine how many snakeheads there might be and gauge the threat it poses to other wildlife.

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State wildlife officials will use an electro-fishing boat,which releases electric current just beneath the vessel to temporarily stun nearby fish. The fish can then be scooped up in nets and examined on shore.

The results of the survey will not be finished until later in the week, said Melissa Cohen, a regional fisheries manager for the department.

“We got a call few months ago that an angler might have caught one,” she said, but that report was unverified. More recent reports prompted park officials to put up the signs.

If snakeheads have established themselves in the lake, she said, someone probably released them there, perhaps hoping to create a population for later fishing.

The northern snakehead is common in the lakes and streams of China, Korea and Russia but they are not native in American waters. The threat posed by the fish should not be underestimated, wildlife officials said.

The possession, sale and transport of live snakeheads was prohibited by federal law in 2002.

However, they remain a persistent presence in Chinese fish markets across the city, officials said. For many, the fish is prized not only as a meaty, savory ingredient in stew, but for its supposed healing properties.

After the seizure of 353 live snakeheads at Kennedy International Airport on the eve of the 2010 Chinese New Year, an investigation led to the arrest of a local wholesaler in 2011 who illegally imported thousands of snakeheads and sold them from a shop in Brooklyn.

Ms. Cohen said a single snakehead turned up in the Harlem Meer in 2008 and that the fish has recently established a presence in Meadow Lake in Queens.

It remains unclear why the snakeheads in Queens have not decimated the other fish population, but Ms. Cohen speculated that it could have to do with the relatively high salinity of the water. The Meer has a much lower salinity level and therefore could be more conducive to the snakehead.

Since there is not much likelihood that the fish will migrate out of the lake, Ms. Cohen said, if snakeheads are found they will be monitored rather than eradicated.

That is a less aggressive approach than authorities took in 2008, when they found the snakehead in Ridgebury Lake and Catlin Creek in upstate New York. Authorities, fearing that the fish could migrate into the waters of the Hudson River and wreak havoc in the ecosystem across the state, used an aquatic pesticide to kill the snakeheads.

Still, the snakeheads in the Harlem Meer threaten many other fish in what has become one of the city’s most popular fishing holes since it was renovated in the 1990s.

“It has a very wild profile,” Mr. Swegman said. “There are reed beds, lots of curves, nooks and crannies. It is not like you are fishing a swimming pool, it actually has the profile of a really natural lake.”

And the signs warning of the snakehead lent the Meer a touch more of the wild.

On Tuesday, many of the regulars had a tale to tell about a snakefish spotting. And while some hoped to be the first to catch one, others preferred to keep their distance.

Dwayne Coleman, 53, said he preferred more docile fish like bluegills, bass and carp.

“I don’t want to see them!” he said, as he tossed a sunfish back in the lake. “Scared of ’em.”

If he lands one of the sharp-toothed invaders?

He smiled.

“I’m going to run,” he said.

Vivian Yee contributed reporting.