WHIRLIGIG LAKE, ONTARIO -- A rare variety of the brook trout known to inhabit only three lakes in the world before it was driven to the edge of extinction by acid rain has been reintroduced into its original habitat because of the prescience of a now-retired Canadian biologist a generation ago.

About 1,200 aurora trout, named for their brilliant red coloration when spawning, were loaded onto a seaplane in North Bay, Ontario, last month for the hour-long flight here and released into three small lakes in the remote, uninhabited wilderness of Lady Evelyn-Smoothwater Park at the highest elevation of the province.

Officials of a cooperative fisheries project involving the Ontario Ministry of Natural Resources and Laurentian University in the nearby mining city of Sudbury, said that acidity levels caused by airborne pollution from the United States and nickel-smelting plants in Sudbury have diminished enough to give the young trout a chance to survive in their original habitat.

The officials said that while acid rain from the United States remains a serious problem, sulfur-dioxide emissions from smelting plants in Sudbury have been decreased by pollution controls. Authorities last year dumped 21 tons of limestone powder into the lakes to reduce acidity.

According to Edward Snucins, project biologist for the fisheries cooperative, surveys showed as recently as 1988 that all the aurora trout in Whirligig, White Pine and Wilderness lakes had disappeared and that the lakes had joined the 17,000 bodies of water in Canada regarded as effectively "dead" because of acid rain. It was one of the few confirmed cases in which acid rain had effectively wiped out a variety of fish, Snucins said.

However, in the brood tanks of the ministry's hatchery at Hill's Lake Fish Culture Station, in Kirkland Lake and in several sanctuary lakes maintained by the ministry, conservationists found aurora that had been bred and maintained ever since ministry biologist Paul Graf sensed the danger to them in 1957 -- long before acid rain was perceived as an environmental threat.

Graf, who worked at Hill's Lake, took six female aurora trout out of White Pine and Whirligig lakes 33 years ago, carefully incubated 5,000 of their eggs at the hatchery and carefully maintained the resulting fry because he believed it would be tragic if the rare fish became extinct.

Now retired, the 63-year-old Graf said in a telephone interview from his lakeside home at Englehart, about 90 miles northeast of here, that at one point in the 1960s, ministry officials ordered him to throw out his brood stock of aurora trout to make space. He said he had to fight to save them.

"They told me to get rid of all that old stuff in the hatchery. I was close to being fired because of it," Graf said. The biologist, who immigrated to Canada from Switzerland, said he had never heard the term "acid rain" then but was familiar with acidity caused by air pollution in post-war Western Europe as a result of his work there as a fish culturist.

He said that when he came to Ontario, his boss at Hill's Lake, Jerry Coyne, who is also retired, ordered him to test the acidity levels at a number of lakes. Using a relatively primitive system, Graf found acid readings considerably greater than the level regarded as the threshold for maintaining brook trout and its cousin, the aurora.

Graf then took his six aurora to the hatchery.

"No one would say anything publicly then. The smelters were very big and powerful, and acid rain wasn't a name then. But we just kept the brood stock anyway, even after they told us to get rid of it," Graf said.

Provincial authorities said the acid concentration in the three lakes intensified in the mid-1980s, giving meaning to reports by fishermen that the aurora trout was no longer to be found.

George Duckworth, the ministry's regional biologist and chairman of the aurora trout project committee, said the aurora in the existing brood stocks, including those released last month, are descendants of the eggs collected by Graf.

Chuck McCrudden, another ministry biologist, said this may raise questions about the genetic fitness of the aurora released here, because they do not stem from a naturally thriving population and may be starting out in their original habitat in worse genetic condition than those removed by Graf.

Last month, the biologists carefully loaded the 1,200 aurora trout -- all 2 to 5 years old -- onto a ministry seaplane and flew them to the 178,000-acre Lady Evelyn-Smoothwater Park. Because Whirligig, which is named for its abundance of a water beetle on which trout thrive, is too small for a seaplane, the officials had to land on a nearby lake and carry the fish here in buckets.

John Gunn, head of the cooperative fisheries unit, then dumped them into small holding pens to acclimate them to their new conditions and afterward released them into the lake.

"It was quite a stressful trip. We'll have to monitor them for a few days," Gunn said. He said radio signal tags will be attached to some of the larger fish.

With beetles swarming over the lake's surface -- "a sure sign the lake is dead, otherwise they wouldn't proliferate like this," Gunn said -- the team of conservationists watched proudly as the young aurora swam away from the holding pens. "It's a nice feeling," Gunn said. "Let's hope it works."

Graf, who spends his time fishing and relaxing these days, said he had a nice feeling when told about the aurora project. "It's an accomplishment for me, Jerry Coyne and the hatchery staff. I'm happy." he said.