For years the United States and Canada have been tinkering with the balance and supply of various fish species in the Great Lakes in an effort to turn back the clock.
They want to recapture the time in the 1940s and earlier when lake trout flourished and there were no invaders from the salt waters of the Atlantic.
Fishery experts have scored a stunning victory in reducing the population of the invaders - largely the eel-like sea lamprey - but they are puzzled as to why they have not been more successful in restoring lake trout to former supply levels. Each year the lakes are restocked with more than 5 million fingerling trout from federal hatcheries in Michigan, but very few of the trout have reproduced.
In large part it was the arrival of the sea lamprey that prompted the US and Canada to act. The lamprey moved in from the Welland Canal and destroyed much of the lake trout population by the 1940s. The two nations launched the Great Lakes Fishery Commission in the mid-1950s to try to find some way to control the lamprey. In testing thousands of chemicals, researchers found two that would destroy lamprey larvae but not harm other fish. Starting in Lake Superior, fishery experts planted the chemicals wherever the lamprey spawned.
''It's been a super success story,'' says Ray Argyle of the US Fish and Wildlife Service's Great Lakes Fishery Laboratory in Ann Arbor, Mich. He estimates that the number of sea lamprey in the lakes is about 20 percent of what it once was.
But Mr. Argyle, who monitors restocking efforts for the agency, admits that the lake trout restocking venture has been, by comparison, ''a little disappointing.'' The aim has been to restock the fish in numbers high enough so that natural reproduction will lead to a self-sustaining population. But only the few native trout left in Lake Superior (where fishery experts first gained control over the lamprey) and a tiny fraction of the new trout have spawned offspring. The restocking has been going on over the past 20 years.
Environmentalists say contaminants are largely responsible, particularly in Lake Michigan where trout reproduction has been almost nil.
''Research within the last five years has shown that lake trout fry survive less well in Lake Michigan water than in that of the other lakes,'' says former Great Lakes Basin Commission chairman Lee Botts. ''That's significant because Lake Michigan has more different toxics than any of the other Great Lakes.''
Most scientists agree that pollutants play a role in the failure of the trout to reproduce. But few view it as the whole answer. Other factors, according to Niles Kevern, chairman of the Fish and Wildlife Department at Michigan State University, include the difficulty of controlling overfishing of the trout, the high mortality rate of the species, and the length of time (seven to nine years) for the trout to reach the age of reproductive maturity. He also notes it was only well after restocking had begun that scientists discovered that the trout return for spawning to the place they were stocked. Unaware, fish stockers had often dumped the fish into the lakes in shallow areas instead of over deep reefs with crevices where eggs could be protected.
Aiming still at a self-sustaining trout population, fishery experts are experimenting with development of a hardier strain (by cross-breeding) and will raise the annual restocking effort by more than 50 percent with the help of a new hatchery in Iron River, Wis., within the next year.
A number of Great Lakes states have been trying individually to boost tourism and sports fishing by also restocking the lakes with nonnative salmon. The effort is generally rated a success in more ways that one. The salmon, which tend to swim in shallower waters than the lake trout and are generally viewed as compatible with them, have fed steadily for years on alewives, small herring that came in from the Atlantic with the lamprey. No one living around Lake Michigan shores will forget the smell that lingered on after many alewives died in 1967, presumably caused by a change in water temperature. Indeed, the alewife population has now dwindled to such an extent that experts sometimes joke that alewives may have to be restocked as salmon feed.