Ahab can have the great white whale, the local newshounds can go lusting after the great white sharks, but give Jack Gasnick a great white catfish any old day.

Like the 24-incher he found ragged and dead a few years back trapped in its own little Waterloo - caught in a four-inch water pipe and stemming the flow to an entire building. It is just one of the many fish he has serendipitously found in the wake of such minor disasters. A plumbing supplies salesman and inveterate writer of tall tales, jingles ("'Go on the green, and never in between' was one of my biggest") and letters to the editor of The New York Times, Jack Gasnick takes great pleasure in the well-stock waterways of New York. He claims to be one of the few Practitioners of a little-known sport - fishing "via faucet, hydrant and water main."

While they may not be literally leaping from the faucets into the trying pans, according to Gasnick all manner of fish are doing their daily business in the tap water you have only faintly suspected of being a carcenogenic. Little killies are out there cleaning their gills, carp are using it as fish eye wash. They're all out there making minnows in the stuff you were about to make coffee with.

"Please believe me, it doesn't happen everywhere in the city. But it's common in places. Up in the cold-water flats of East Harlem, killifish would come right out of the faucet. Today though, the fixtures are too small. Now fish just keep growing until some plumber finds them clogging a pipe, or they are released in a flood."

Gasnick has friends in the field. Plumbers and members of the department of water supply he has baited with the promise of anonymity ("It wouldn't look good if the boss thought they were taking off from work to call me") use the telephone line to let him know when a lively leak has sprung.

If it isn't too busy in his family's East Side plumbing and hardware supply shop where he has worked 40 years, he'll grab the net he keeps hanging from the ceiling, don a pair of rubber boots and strike out "for a good laugh."

"I netted many a hooded dace, perch, minnow or killi in the floods and have found them trapped in pipes. I've found trout flopping around in the gutter over on 57th Street and pickerel in a flooded basement on East 52nd Street. The last thing I caught was an eel," he says, adding that they no longer tickle his fancy.

"They're so common that I won't even go if it's eels. You find them in the sewers as well, they can survive anywhere - even in the mud, some people say. I understand the Italians eat them - a Christmas fish," he adds with a slight shudder.

What might be one man's quagmire is Gasnick's aquarium. Such was the case of the 48-inch water main that broke more than a year ago on a midtown street, tying up traffic for two days. Gasnick entertained himself with the pursuit of "perch and pickerel and other good-sized fish." His only regret is that an icthyologist wasn't there to share the sun.

New York is host to an endangered species of which Gasnick at the age of 59 is a member; the working class intellectual who, despite the lack of a formal education, has managed to cultivate his mind and his callouses. Twelve hours a day, five days a week he works behind the counter of the family store. At noon and at four each day he takes a break to visit his widowed mother around the corner. Despite a recent cataract operation, she cooks for "her boy," Jack, both lunch and dinner. He has left the old neighborhood, having moved with his wife to Forest Hills, but writes about it.

"I've had abuut 12 long articles published in the Daily News Sunday magazine about old New York. When the papers printed my Christmas stories (recounting his childhood experiences) people wrote in to say that they thought they were second only to that one about that little girl Virginia. I can't stand television, and I don't like to go to the movies, so writing is a hobby for me. I write letters to the editor of the New York Times about odd things. That's what gets published - I stay away from political opinions."

One of the several letters that has appeared on the Times' opinion page involves Gasnick's interpretation of God's little half acre - his purchasing vacant odd lots in the city and maintaining them as urban wildlife refuges.

He waxed pastoral with: "If you spot a man off the Queens Midtown Expressway around 78th Street, filling up a bushel with lopsided apples, that's me. If you see a chap in an old brown suit roaming along Morningstar Road, in Staten Island identifying herbs, flowers and ferns and scaring rabbits out of bushes, that's also me."

Then there's the fiction.

"I also write fillers for Reader's Digest, parables, fables," he says, showing a clip of his "Fables for a Serious Drinker." "I think I'm quite a good fablest. The only magazine I can't seem to crack is the New Yorker."

He is an expert on urban sport fishing, a field he could unequivocably call his own. Meticulous records document the what, when and where of each scaly find, a journal is kept and some of the more solid evidence ("they can get pretty ragged in those traps") has been mounted for the trophy wall of his summer home in Monroe, N.Y. Hisresearch so far has led him to conclude that species will vary from neighborhood to neighborhood not unlike socioeconomic groups and/or restaurants.

"In East Harlem killifish are most common, and on the West Side it's minnows," he says in authoritative tones.

Gasnick has maintained an omnivorous appetite for knoweldge and facts without the aid of a formal education. He speaks authoritatively on the urban ecosystem, and as a meticulous scholar he has documented the what, where and when of his findings in a personal journal. When he can't identify a catch, "I take it over to the fish market around the corner and Kenny tells me what it is."

But in his backlog of fish stories there is no room for the one that got away. "When they're really big ones they get stuck - you find them dead," he says indicating with a shrug a stronger interest in empirical data.

As for the alligator-in-the-sewer that periodically raises its head in print, that's a "bull story. The only thing you'll get in a sewer is a rat.

"The biggest fish I ever found was that 24-inch catfish. The typical size is seven to eight inches, though you get a lot of smaller ones, goldfish and little bait fish. Those are all pretty common; they travel in schools and if you find one, you'll find another. The real kicker is finding that oddball spotted one - a sunfish, or a trout. Those are the rarest because they have to make the longest journey. They come all the way down from the mountains, upstate. Those sunfish are ferocious little fellows."

At least one of the latter has been stuffed and mounted for his collection which includes a pickerel his taxidermist ingeniously shelllacked for posterity. "They're such thin little fishers, he decided that would be better than trying to clean and stuff it. I though it would smell, but he put on about five coats and it's just fine."

The fish grow to an adult stage by feeding off the tiny plankton and "what I call your insect life that is found in the water. You can see it in your bathtub or floating in a glass of water."

"His claims are absolutely false. We have the purest water in the United States," says Marvin Cohen, director of public affairs for New York's department of environmental protection. "You might find an eel in the sewer system but nothing in the fresh water. That's tested every day. We have never had any such complaints, either. Occasionally someone will call up about a slight metallic taste in the water, but that is usually traceable to the building's pipes - not ours."

The department employs sophisticated electronic tracking systems, according to Cohen, and the water is tested daily to ensure purity. For those pranksters who might attempt to dump their own supply of wildlife into the system, "we have guards posted at the purification plant. And no fish could survive the water pressure in the pipes anyway."

Gasnick and Cohen agree on one point: "We have the purest water in the country. If fish can survive in it so can we," says Gasnick. "The water pressure doesn't hurt them, it's stagnant water they can't survive in."

Pressed on this point, Cohen admits, "Well, a trace of dead fish might get through, but everything is screened at the reservoir and the electronic devices would identify it if it existed - plankton or fish."

What if Mr. Gasnick were to bring a fish into Cohen's office and place it before him?

"Habeas corpus delecti." It is unclear whether Cohen is answering from a prepared statement or a menu.

"The whole thing sounds fishy to me," he says, retreating behind that old red herring close to teh heart of many bureaucrats - bad jokes.

As for Gasnick, he'd be happy just to catch a bass before hanging up his net. The only hook - he'd prefer making his last catch in someone else's troubled waters.