Many Endangered Species Showing Signs of a Comeback : Conservation: The bald eagle, American alligator and California gray whale all offer evidence that determined rescue efforts can pay off. But whooping cranes seem immune from help.

Under a steady snow, wildlife scientist Tom Serfass prepared to give river otter No. 16 his freedom along the swollen Youghiogheny River.

Serfass and his helpers carried a cage down the steep bank, stopped in a patch of weeds 10 feet from the stream and opened the wire door.

No sooner did they turn around than a 3-foot ribbon of dark fur loped for the water and disappeared. Otter No. 16 had become another tile in the coast-to-coast mosaic of restored species.

Serfass has released 15 other river otters in Pennsylvania, at a cost of about $1,500 each, and officials in Maryland have released 23. They hope the wild creature will thrive now that it’s protected from trappers.

“The animal just has a right to be,” Serfass said.

Since the first settlers, people have plundered America’s bounty, pushing hundreds of animals to near-extinction.

Many Americans are more careful now. They think before they develop land. They curb hunters and trappers. They are improving wildlife habitat. They even act as midwives.

At great expense, each state can tell of a hard-won success.

Among the outstanding comebacks is the bald eagle, once diminished by hunting and the now-banned pesticide DDT, which weakened its eggshells. It was placed on the federal Endangered Species List in 1974, and by 1992, the number of nesting pairs in 45 states quadrupled to 3,014.

The DDT ban allowed the peregrine falcon population to begin to mend, too. There used to be only 60 nesting pairs in the United States. In 1991, there were 700, although several hatch their young atop city skyscrapers for lack of natural cliffs.

Sometimes just outlawing the slaughter of an animal has been enough to help rejuvenate the species. That’s largely what saved alligators in the South, beavers in New York state, gray whales off California and wolves in Michigan.

The American alligator was swiftly disappearing from Louisiana swamps and turned into shoes and luggage when the state began shielding it from hunters in the 1950s and ‘60s. It became so plentiful over the next several decades that it began showing up in front yards, prompting the state to allow a 30-day gator-hunting season.

“It took us a year and a half to prove to the government that we had an excess of alligators,” said Johnnie Tarver, a state wildlife administrator.

By the turn of the century, trapping had decimated beavers in New York state, said Pete Nye, who is in charge of endangered species there. Then the state received a seed population from Yellowstone National Park and banned beaver trapping.

“Now we have a tremendous population to the point they’re becoming nuisance animals,” Nye said.

The California gray whale last year became the first marine mammal to be removed from the federal endangered list. The Mexican government is credited with saving it by guarding its calving and winter home off the Baja peninsula.

More gray whales exist now than in the mid-1880s when the whaling industry peaked, the National Marine Fisheries Service says.

It seemed wolves were gone forever from Michigan, thanks to decades of shooting, poisoning and trapping. Moreover, a small project to reintroduce four wolves in 1974 ended in disaster. A car hit and killed one, a deer hunter accidentally shot another, a third was shot after it was caught in a trap, and the fourth died from either trapping or shooting.

But 19 years later, with a hunting ban firmly in place, wolves from neighboring Wisconsin are immigrating naturally.

“Let them do their thing, and they’ll come back on their own,” said Tom Weise, endangered species coordinator for Michigan.

These and other successes aren’t enough for Robin Smith, executive director of In Defense of Endangered Species of Columbus, Ohio. Many species are becoming scarcer and are headed for extinction, he said.

The Tecopa pupfish in California and the seaside dusky sparrow in Florida are believed extinct.

Four thousand more species have been nominated for--but not placed on--the list of endangered or threatened species, Smith said. The list has 300 entries.

Saving them may be in humanity’s self-interest, Nye said, because more than half of all medicines come from plants and animals.

Sometimes protecting a species seems to conflict with human needs.

Take the northern spotted owl, an endangered species living in the old-growth forests of the Pacific Northwest. It needs the forests to survive; thousands of logging jobs depend on cutting the forests.

Hundreds of square miles of timberland have been set aside as owl habitat, according to Chip Murray, attorney for the American Forest and Paper Assn. in Washington, D.C. As a result, the area’s harvest from public land in 1992 amounted to one-eighth the harvest of 1989.

In Utah and California, drought and human encroachment are altering the terrain of the Mojave Desert tortoise. Elden Hughes, chairman of the Sierra Club’s California Desert Committee, estimates perhaps 300,000 of the slow-breeding tortoises remain, about one-tenth the population of 20 to 30 years ago.

More roads bring more cars that squash more tortoises. A less obvious threat is landfills.

“Dumps encourage ravens, and some ravens think that little tortoises are just hors d’oeuvres,” Hughes said.

He can’t release tortoises bred in captivity into the desert because they carry diseases picked up under human care.

Other animals have resisted the most heroic rescue efforts.

Among them is the whooping crane, one of the first blighted species to gain national attention. They numbered only 21 or 22 in 1941, and today there are only 237, despite dedicated work.

One of the two flocks consists of birds hatched by sandhill cranes under human supervision. The flock isn’t reproducing, apparently because its members think they’re sandhills and seek sandhills as mates, a doomed liaison.

If fire or flood destroyed the other flock, the species could disappear.

The river otter isn’t on the U.S. list of endangered or threatened species, but it has been absent from many Pennsylvania streams for a century because of hunting and water pollution. The state says they’re at risk.

No. 16 was trapped in Upstate New York, where otters are more plentiful. Now he must make sense of his new home, with the help of Serfass, a doctoral candidate at Penn State University.

Two students and a handful of state employees waited to see if the otter would weather the experience.

A distant roar announced a freight train, which suddenly thundered down the opposite bank.

Fleeing the noise, the otter left the river and headed toward Dennis Smith, a Game Commission officer. Smith was ready. He shouted and waved his hat at the otter to scare it back again.

If the otter should leave the river, he could perish searching for another stream. This otter took the hint and vanished again into the water. Serfass paced the bank for an hour to make sure he stayed there.

The test of whether No. 16 will survive and propagate had begun.