It was a big fish, even as striped bass go. But it didn't look big enough to weigh the 18 pounds it registered on the scale. So the judges of the Chesapeake Bay fishing contest put a sharp knife to the bass' belly.

Out came a handful of lead sinkers.

The only nice thing about that fish story is that someone got caught. The worst part of it is that it sounds so familiar. Stories about cheating, whether poaching while hunting or illegally changing a sail during a regatta, are common enough to give the great outdoors a bad odor at times.

Maybe it is unrealistic to expect outdoorsmen to be any more ethical than the general population. But that has always been their reputation. And there is now a growing clamor to restore that flattering presumption.

"The day has passed when ethics could be treated as an unwritten contract between a man and his conscience," says Jack Lorenz, the executive director of the Izaak Walton League of America. "The time has come when we must assume our share of responsibility for the code of our hunting and fishing group and the ethical behavior of our friends."

The Walton League, a 60-year-old conservation group, has spent more than $100,000 in the last three years on outdoor ethics programs. Last week the League received a $150,000 grant to continue its work, which includes a national newsletter, hunter education programs and a planned international conference.

Alone, that effort would barely make a dent in the problem. But during the last year, the movement has begun to gain momentum. There have been articles or editorials calling for ethical reform in every major outdoor magazine.

"A hunting license is neither a legal right nor a shopping list to the great outdoors' meat barn. It is a privilege and a sizable responsibility. Yet we may lose this privilege, and the growth and dignity inherent in outdoor recreation, unless we do what we can to stop this ethical decay," wrote George Reiger this year in Field and Stream.

Sports Afield editorialized, "There are a lot of people out there knocking our sport . . . We must police our own sport, or we are in danger of having others try to do it for us."

Because hunters and anglers have been honest enough to engage in self-criticism, they have been the focus for much of the public blame. But bad behavior exists in any sport.

A recent story in Canada's Wildlife Review titled "Who Will Watch the Birdwatchers" told of a bird sanctuary that had been desecrated by a new breed of birder. Fences and gates were destroyed, nesting birds were harassed to make them fly and no trespassing signs were ignored.

Recently a timber company in Tennessee closed three huge chunks of property to campers because of excessive littering, wood theft and vandalism. Similar situations have led to the loss of recreation areas in half a dozen other states this year.

Blood sports like hunting, however, need to maintain stricter codes of conduct. When a camper or canoeist acts like a slob, our esthetics are offended. An idiot with a loaded gun can leave us dead. And with more hunters crowding into less hunting space each year, the potential for that kind of mayhem increases.

Most of the accidents are the result of stupidity. But many can be blamed on warped ethics. Hunters who have no standards beyond killing game, are more likely to shoot at sounds and movement than ones with at least a basic sporting code.

Mandatory hunter education courses are one logical answer to the problem. Statistics show states with such programs suffer fewer firearm fatalities than states that do not.

Some states have recently begun programs to persuade hunters to turn in others for game violations. As an added incentive, the tipsters are rewarded with cash. New Mexico's "Operation Game Thief" has become a prototype. The program began in 1977, one year after the Department of Game and Fish estimated that 34,000 deer were taken in that state by poachers. That was 14,000 more than killed legally during the hunting season. Since then the project has resulted in more than 400 convictions.

A few years ago a survey was made of nonhunters by the National Shooting Sports Foundation. Results were discouraging in that a majority of nonhunters believed hunters lacked knowledge about wildlife, mistreated animals and behaved badly. Those results were made more acceptable by the finding it was hunters, not the sport of hunting, provoking the worst response.

Now the people who care most about the sport, are calling loudest for ethical reform. Even if that means jeopardizing a friendship.

"Whenever we remain silent about slob behavior in others," says Lorenz, "we become slobs ourselves."