Wanted: Dead or alive -- voracious giant koi

WINCHESTER -- Like any good fish tale, this one involves nicknames, a legend, and a contest of wills.

The fish, in this case, is known alternatively as Flash, Moby Dick, and Darth Vader. He is said to be 2 feet long, 5 or 6 pounds, and fearless. A wily, orange-colored refugee allegedly from a backyard pond.

To some passersby at Winter Pond, he is exotic, romantic, even adorable. But to his nemesis, Susan Fennelly, he is nothing short of a menace, a goldfish gone nuclear. And she is quite blunt as to her intention: She wants him dead.

''Attention Fishermen: Can you catch 'em?" challenges a sign she's posted at the edge of the water. If you can: ''Keep, don't throw back in!"

''Truthfully, he's kind of scary," said Fennelly, a member of Friends of Winter Pond. ''You get used to this peaceful pond then you see this thing moving around and it has this sort of Loch Ness Monster effect and you think, 'My God! What is that?' "

That is a giant koi, Japanese for carp. Cyprinus carpo in Latin. Aquatic pig here at home. Imported from Japan and sold as decorative fish for tony backyard ponds, koi come in brilliant oranges, yellows, whites, and reds. In Japan they are considered sacred. In America they're status symbols that can cost tens of thousands of dollars and induce ooohs and aaaahs from knowing collectors.

But fish royalty or no, koi are also gluttons. Omnivores known to eat half their weight in a day. And while the thousands of backyard ponds that dot New England built especially for them may be luxurious, many koi quickly outgrow their homes.

Faced with tossing their prized pets into the trash or risk clogging up their toilets, well-to-do koi collecting suburbanites have taken to dropping them into nearby public waterways and this is where the problems start. The koi, attractive as they are, stand accused of ecological mayhem.

Locally, scientists have found koi ''in hundreds of ponds and lakes throughout the state," said James Lee, an ichthyologist at Harvard and Cambridge College who grew up on Winter Pond and knows firsthand of Moby Dick. They've been pulled out of Winter Pond, Spy Pond in Arlington, the Brookline Reservoir, Horn Pond in Woburn, Lake Cochituate in Natick, Walden Pond in Concord, and the Charles, Merrimack, and Concord rivers;one world-record specimen was pulled from Jamaica Pond in Boston.

Koi are the pigs of the fish world, said Lee. They eat anything and everything, root around the bottom of ponds, ripping up vegetation, muddying the water and snuffing out nesting habitats of indigenous fish.

''They are not good house guests," opined Lee.

''They can wreak havoc," said Fennelly, who has been on her quest to kill Moby Dick for months. ''They don't belong here."

Tim Czech, vice president of the Pioneer Valley Water Garden and Koi Club, which belongs to the Associated Koi Clubs of America, said if you have a koi that is too big, you should give it to someone with a larger pond or try to give it back to a fish store. The stores can often resell them and most are happy to take them back. Czech also recommended contacting a member of the AKCA to have them take your unwanted koi. ''There are always people willing to take koi," he said.

When koi are dumped into ponds, most are too conspicuous and coddled to escape the native predators -- pickerel, bass, pike -- for long. But the most clever among them survive and can soon outgrow any natural enemies. If a male and a female are dumped in the same pond, their offspring could destroy the natural habitat.

Pamela Schofield, an ichthyologist with the United States Geological Survey and the author of a recently published report about the nonindigenous carp, said it's difficult to do studies on the negative effect of a fish, but it could ''cause big problems."

In New Zealand, where they have been an issue for years, koi are considered ''noxious fish." Government eradication programs are in place to poison them. At the grass-roots level, New Zealanders are known to shoot them with bows and arrows.

In the Midwest the koi's cousin, the jumping Asian carp, have biologists quaking.

About 30 years ago, catfish farmers in the South imported the Asian carp as a way to control algae in catfish tanks. When flood waters rose about a decade ago, the carp escaped into the Mississippi. They have been moving up the river at a rate of about 35 miles a year ever since. Along the way, they are devouring everything in their path and upsetting the food supply for indigenous fish.

Fearful they will eventually spread into Lake Michigan and overrun the waterways, the Army Corps of Engineers is building a $9 million electric fence across the largest gateway to the lake. It is set to be completed in October.

The Moby Dick lolling around Winter Pond doesn't jump, but it still has some area environmentalists pretty worked up.

''This is a beautiful small pond and every little change in the ecosystem can have a dramatic effect," said Jonathan Lang, whose property abuts the water. ''You learn that the pond is a living, breathing thing."

What makes Moby Dick and other koi particularly frustrating is that they can be difficult to catch. Unlike most fish, they have a range of hearing comparable to humans. When domesticated, owners can call them and they will come to the water's edge to be fed. But once they are forced to survive in the wild, they become easily spooked. Talkative fishermen don't stand a chance.

Just how big do they get? In April 2004, Roy Leyva, who was then Massachusetts angler of the year, caught a 20-pounder in Jamaica Pond. It looked like a goldfish that swallowed a bowling ball. At the time it was caught, the fish was a world record, according to the National Freshwater Fishing Hall of Fame in Hayward, Wis.

''It fought like hell," Leyva said. ''Took like 20 minutes to bring it in."

Leyva watched a man dump three koi out of a gray 55-gallon drum in 1990. A few years later, he caught one and gave it to his mother, who kept it in a pond behind her Hyde Park home. When that koi died he went back and caught the second, which had grown to 31 inches long and 25 inches around.

''I had to bear hug it to get on shore," he said.

Leyva said he has caught koi in dozens of ponds in the area, including a 17-pounder in the Charles River.

And the big 20-pounder that broke the world record? It was too big too keep, so he let it go. ''It's somewhere in the Neponset River," he said. ''It's probably even bigger now."

Douglas Belkin can be reached at dbelkin@globe.com.