All biologists meant to do by injecting electronic bugs into the bellies of thousands of Potomac River bass a few years ago was track fish.

Instead, they reeled in a surprise catch -- what law enforcement authorities are calling Maryland's largest fish-poaching case.

By emitting radio signals that identified the fish as Potomac River natives, the bugs became key evidence in a 22-month investigation that began with an alert Canadian customs inspector and wound through Ohio and Michigan to Charles County.

A federal indictment alleges that for three years, four commercial watermen netted tens of thousands of largemouth bass, a protected species that underpins a booming sport fishing industry at Washington's doorstep. The fish were sold live under false documents, primarily to Asian restaurant owners for use in tanks from which customers choose their dinners, authorities allege.

Two of the accused poachers deny the charges and the other two declined to comment. All are Southern Maryland residents.

The case has drawn vivid reactions from the bass fishing industry, which has blossomed into a multimillion-dollar business in recent years as cleanup measures have turned the formerly dirty, moribund Potomac River into one of the best bass fisheries in the country.

"They're stealing the results of our hard work. It's theft," said Ken Penrod, a full-time fishing guide for 14 years. He noted with satisfaction, "It was the little snoopy bug that got them."

That snoopy bug is formally known as a PIT tag, for Passive Integrated Transponder. Each is about one-third of an inch long and about the same diameter as the tubes that hold ink inside a disposable pen. Once placed in a fish, the bugs cannot be seen.

Each tag emits a signal when activated by a pistol-gripped scanning device attached to a small battery-powered box. Workers can scan a fish quickly and, if a PIT tag is within, know when and where it was tagged and released or when it was last scanned.

In 1990, biologists wanted to investigate what happened to fish that were caught, weighed and released at bass tournaments. This was no petty concern, with more than 100 major tournaments held each year in the Potomac. Events can draw hundreds of participants who stay as long as a week.

State workers using large syringes injected tags into the gut cavities of 3,361 largemouth bass. Subsequent scans, routinely conducted at tournaments, helped allay fears that the contests concentrated fish around release areas or cleared bass out of heavily fished pockets of the river.

Yet another scan was conducted under circumstances that biologists found surprising: under a search warrant, in the company of state Natural Resources Police at commercial fish ponds being used by Dennis P. Woodruff -- one of the four men named in the 10-count federal indictment.

Authorities say they found bass from the river in Woodruff's ponds.

"We could take the wand and determine when and where they were released," said Mel Beaven, a state regional fisheries biologist, "and they certainly weren't released in his ponds."

Woodruff, 47, of Bryans Road in Charles County, is accused of buying fish hauled from the river by the three other men, then shipping the bass onward under cover of his aquaculture permit, which allows him to raise and sell fish. He denied the charges through his attorney, C. Thomas Brown, of Baltimore.

Also accused are Alfred B. Grinder, 42, and Walter I. Maddox, 61, both of Marbury in Charles County; and Robert T. Brown Sr., 44, of Avenue in St. Mary's County. Maddox and Brown declined to comment; Grinder's attorney, Steven A. Allen, of Baltimore, said his client denies the charges.

The four, who are to be arraigned tomorrow, are charged under a federal law that prohibits selling protected wildlife bass across state lines and carries penalties of up to five years' imprisonment. Authorities say that between 1990 and 1993, the foursome shipped more than 40,000 pounds of wild bass worth more than $150,000.

The scale of the alleged offense startled some in wildlife and fishing circles.

"It's to my knowledge the biggest fish case the state of Maryland's ever had," said Natural Resources Police Cpl. Michael Burnham, a 20-year department veteran who spent much of the last two years investigating the case.

The alleged method also shocked many anglers: Authorities say the men used several types of nets to capture the game fish. In the bass industry, a strong conservationist ethic has taken hold, and anglers return fish to the river far more often than they take them home.

"You can literally wipe out a good segment of the spawning males with one haul of the haul seine," a type of net mentioned in the indictment, said Bob Lunsford, state director of freshwater fisheries.

The first nibble in the case came in April 1993, when Maryland authorities received a letter from Michigan wildlife officials questioning a shipment from their jurisdiction to Toronto of 3,000 pounds of live largemouth bass. A copy of Woodruff's aquaculture permit accompanied the shipment, court documents said.

Canadian authorities already had contacted police in Michigan, reporting that a customs officer in Niagara Falls had noticed the shipment contained dark bass in mixed sizes; bass raised in ponds usually are similar in size and are lightly colored.

In interviews, some experts doubted that it was possible to tell hatchery and wild bass apart solely on the basis of color. But Beaven, who works at a state fish hatchery, said largemouth bass are "extremely cannibalistic" and must be segregated by size to prevent the large from eating the small -- a condition that makes it unlikely that pond-raised fish would be of the varied sizes reported in court documents.

Burnham went on to investigate Woodruff's property and uncovered a paper trail of payments and invoices -- some misrepresenting largemouth bass as carp -- that stretched through a wholesale company in Ohio and on to Michigan and Ontario, according to court documents. When the indictment was handed up in Baltimore on Feb. 15, it alleged sales in New York, Georgia and Ontario.

Anglers, commercial fishermen and fisheries officials all said they doubted that the case points toward a large-scale poaching problem, although small-scale poaching is believed to be chronic. Many agreed, however, that the case underscores how thinly the Natural Resources Police are spread. They have five small boats to patrol the Potomac from the Woodrow Wilson Bridge to the Chesapeake Bay, typically with only one officer per boat.

"You're looking at the entire marine police force for Prince George's County," said resources police Cpl. Leonard Sciukas one recent day as he piloted his Boston whaler on a chilled and deserted river. "When I go home, that's it. I do my eight hours, and the other 16 hours are open."

That makes large-scale poaching a possibility, Burnham said.

Local officials are watching the court case closely as it unfolds.

"Bass fishing is our biggest tourist attraction," said Joanne Roland, an official with Charles County, where an 800 telephone number draws 600 inquiries a month about fishing. "You can see how devastating this would be if this activity continued -- taking all our fish."

Like many connected to the river, her reaction fell somewhere between indignation and outrage: "I feel certain if any bass fishermen had seen them, their nets would have been cut." CAPTION: The tracing devices, shown here next to a penny, are about one-third of an inch long. They work by emitting radio signals. CAPTION: Maryland fisheries biologist Mel Beaven, left, scans a largemouth bass held by biologist Tim Groves. The fish contains a tracking device.