When the U.S. Coast Guard stepped up patrols in the Gulf of Mexico three years ago, it was on the lookout for illegal drugs.

What it found was illegal shrimp.

The "smugglers" are commercial fisherman who contend that the shrimp they poach from Mexico's tantalizingly close and well-stocked waters are all that's kept their fleets afloat through three years of poor catches, low prices and stiffening competition from imports.

The Coast Guard has been sympathetic but unyielding, and Congress is backing it up. "Our position is that as long as there's a law -- and Congress hasn't rescinded it despite intense lobbying by the shrimpers -- we're going to enforce the law," said Lt. Robert Wilson of the 8th Coast Guard District in New Orleans.

The Coast Guard is not the only arm of the federal government reaching far offshore to get involved with the lowly shrimp swimming in Mexico's waters and the Americans trying to catch them. The shrimpers also deal with the National Marine Fisheries Service, the office of the U.S. trade representative and a law that originally applied to endangered species.

Nevertheless, shrimp poaching seems to be increasing. Last year a record 400 shrimpers operating out of this port were cited for illegal catches, nearly double the number cited in 1983. In December, a handful of violators whose cases were the first to be heard by an administrative law judge of the Commerce Department were fined the maximum $10,000 apiece.

The shrimpers are livid.

"Most of the fishermen can accept the role of Mexico enforcing its boundaries; they treat that as a risk of doing business," said Ralph Rayburn, director of the Texas Shrimpers Association. "What really has them going through the roof is when their own government is out there enforcing the boundaries for Mexico."

His members add angrily that the federal government is doing a better job of enforcement than is Mexico. "If you cross into Mexican waters, you're taking a 50-50 chance of getting caught by the Coast Guard and a one in a hundred chance of getting caught by the Mexicans," said David Eymard, who owns a fleet of eight shrimp boats here.

Mexico's enforcement has stiffened recently. More than 120 U.S. shrimping vessels have been seized by Mexican gunboats in the past two years and taken under armed guard to Tampico, Mexico, 200 miles south of here, where their operators have been detained, fined and had their catches confiscated.

"It takes them two weeks just to burp down there," said Joe Manuel, a Port Isabel shrimp captain who lost two weeks in Tampico, shrimp worth $6,000, $3,500 in nets and gear and $5,000 in fines when he was caught shrimping 15 miles south of the line last fall.

Despite these multiple risks, poaching continues. And small wonder: Catches on the Mexican side of the boundary that bisects the Gulf of Mexico 10 miles south of here are often three or four times as large as catches on the Texas side. Moreover, in the winter they're the only catches available. The waters off the Texas coast are too cold for shrimp in the winter.

Eymand, who is president of the Texas Shrimpers Association, says he instructs his boat captains to observe the boundary, but he seems bitter about it. "The Coast Guard is literally tying me to the docks for five months a year," he said. "My season's dead till April."

He said he understands why some shrimpers break the law.

"You got a fisherman out there at sea; he doesn't see a line. He's been fishing in a favorite spot in what they now call Mexican waters since he was 12, and maybe he's 40 now, and he says, 'Geez, this is ridiculous. There's all that shrimp two hours' steaming away and I can catch five times as much down there as I can up here. My family is starving back at home. I've got boat notes I can't make.' You've been away from land for maybe two weeks, and you start saying to yourself, 'Hey, why not?' "

Until 1976, when Mexico extended its territorial claim to 200 miles offshore, it was legal for Americans to shrimp off the Mexican coast. Until 1980, when Mexicans started token enforcement of the line, it was safe. But not until after 1981 -- when Congress amended the Lacey Act to make it illegal to bring into this country shrimp seized illegally in another country -- did it become a high-risk proposition, shrimpers say.

"Our position has been that the Lacey Act was intended to cover endangered species and was never meant for shrimp," Rayburn said. "But Congress hasn't gone along, and I must say I don't see much hope at the moment. I sense sympathy is waning in Congress because there continue to be violations."

Congress normally might be more sympathetic to a hard-pressed domestic industry, but its posture is complicated by the United States' having asserted its own 200-mile fishing zone -- and having incurred the wrath of much of the rest of the world for doing so.

Morever, the shrimpers' cause was not helped when, during the early days of enforcement, the local office of the National Marine Fisheries Service, which handles the shrimp cases after the Coast Guard makes the citations, was hit with vandalism and death threats.

"The shrimp industry is fighting for its economic survival," said Jack Brawner, director of the southern region of the NMFS. "We realize that. But we can't overlook the fact that the law is being violated. We also note that the violations are almost all occurring out of Port Isabel. Shrimpers further north are somehow managing to cope."

Rep. Solomon P. Ortiz (D-Tex.), who represents this port city on the southermost tip of Texas, and Rep. John B. Breaux (D-La.) are seeking a shrimp treaty with Mexico.

Breaux noted that Mexico is the major exporter of shrimp to this country and said he would like the U.S. trade representative to use the threat of tariffs (Mexican shrimp currently are imported duty-free) to pressure the Mexican government into an agreement that would allow U.S. fishermen limited rights in Mexican waters. "They have more shrimp down there than they can harvest," he said. "That's the pity." He said he is not optimistic.

Meanwhile, the shrimpers are angry. The Port Isabel shrimp fleet is the largest in the country; it grosses $175 million annually. But in the past five years, it has shrunk by 25 percent to 450 boats.

"The government is trying to put us out of business," said Harris Lasseigne. "We have a speed limit of 55 miles an hour in this country, but we don't have a state trooper out there with a loaded gun ready to give everyone who drives 58 miles an hour the maximum penalty. That's what they're doing to us."

He said he, too, instructs his boat captains not to cross the line. "I'm not naive enough to think none of them goes south," he said.

Shrimp captains and their two-man crews are paid by what they catch. Manuel, 55, says he earned $30,000 in 1981 and $13,000 last year. "Times have been bad," he said. "If you want to eat, you go south."

U.S. shrimpers consider the catch in Mexican waters to be American-bred. According to many marine biologists, currents in the gulf sweep Texas-spawned shrimp south into Mexican waters.

"The thing of it is, it's really our shrimp," Eymard said.

The NMFS is conducting a $200,000 study to see if Eymard is correct about the migratory patterns of shrimp. But even if he is, said Brawner, "there's no law that says that makes them our shrimp."