The tiny snail darter, the fish that blocked Tennessee's mighty Tellico Dam for three years, has been found alive and well living 80 miles below the dam in a creek where it had never been seen before, the Fish and Wildlife Service announced yesterday.

Biologists said they did not know where the 14 darters, all babies, in the lower end of the South Chickamauga Creek had come from.

Until 1976, the fish, an inedible species of perch, was found only in the Little Tennesse River. Environmentalists had sought to halt the Tellico Dam because the $145 million project would turn that river into a lake, destroying the darter's last known spawning ground.

Then, in 1976, biologists transplanted 710 of the fish to the Hiwassee River, a tributary that enters the Little Tenneessee 10 miles downstream from the dam, The dam's floodgates were subsequently closed, and unitl this week those 710 and their offspring were the only known survivors.

The darter is thriving on the Hiswassee, but if it also lives on the Chickamauga, another 70 miles downstream, that could mean that the little fish always was capable of taking care of itself.

The entire flap over Tellico involved environmentalists' worry that the snail darter would never reestablish itself naturally if the dam were allowed to flood its original spawning grounds.

"We knew there were darters downstream from Tellico when it was closed," said Richard E. Green, information officer at the Tennessee Valley Authority. "The biologists were hoping those below the dam would find somewhere else to spawn, because they sure couldn't go north."

The U.S. Supreme Court ruled in June 1978 that the TVA had to stop work on the dam to save the snail darter, which was protected under the Endangered Species Act. But the little fish had become a test of strength between environmentalists and the forces of growth and development, and howls of outrage from developers moved Congress to exempt the Tellico from the act 15 months later.

The dam was completed and the river closed a year ago. A lake has now formed over 20,000 acres of what had been prime farmland worth perhaps $40 million.

Green said the Chickamauga Creek, which flows into the Little Tennessee River near Chattanooga, had been found bereft of snail darters during TVA's exhaustive check of the area before the dam was closed. "Nobody really knows where they came from," he said. There is one dam along the 70-mile stretch between the new darters and the transplant site on the Hiwassee, but locks allow free passage of fish along with boats, Green said.

If the fishes' parents came from the Hiwassee, however, it would mean that the darters born in one place decided to spawn in another, an event considered unlikely. Biologists disagree on whether the baby fish could have made the trip downstream through the lock from the Hiwassee to the Chickamauga.

Finding the darters in the Chickamauga may mean that Mother Nature has taken care of things, guiding the fish to a new bedroom when the old one was locked. It also may mean only that some of the more adventurous darters picked out for them on the Hiwassee, or that somebody took a bucket of fish and dumped it in the Chickamauga.

Whichever, it will take a few years to learn if the contrary fish has moved permanently or may die out anyway, in spite of everyone's best efforts.