It's a mystery that United States and Canadian fishery experts are determined to solve. Lake trout, almost extinct in the Great Lakes only three decades ago, are once again swimming there by the millions under a massive restocking program. But they are not yet reproducing in numbers fishery experts had expected.
The debate over reasons for this lack of a self-sustaining population continues with some experts citing toxic pollution of the lakes as a key factor. Others, however, say hatchery-raised trout are missing vital early cues that would lead them to return to the best spawning grounds.
Some argue that the number of fish ready for spawning at particular periods has simply been insufficient for a fair test. They point out that fish catches -- both legal and illegal -- have been unusually high.
``Everybody has a pet argument. I think the jury's still out,'' says Clayton Edwards, a fisheries biologist with the International Joint Commission, the US-Canadian group that strives for, and monitors, water-quality improvements in the Great Lakes.
The return of the once-native lake trout to the world's largest freshwater lake system has, from start to finish, been due to a carefully drafted strategy.
The Great Lakes Fishery Commission, which teams US and Canadian experts in fishery research programs, was founded in 1955 with the specific goal of finding ways to control the parasitic sea lamprey.
This eel-like invader from the Atlantic Ocean had been attacking and killing lake trout to the point where the fish had almost disappeared from the lakes. After testing thousands of chemicals, US and Canadian experts finally singled out a lampricide, which would not harm other fish, but will kill lamprey larvae in the stream tributaries where they reproduce. So effective has the program been that lamprey population levels have been cut by about 90 percent.
Also, since 1958 some 125 million hatchery-raised lake trout fry have been stocked in the Great Lakes. The expectation was that after six to eight years the planted trout would return to some of the system's best spawning sites to reproduce.
They have not. And the success of such reproduction efforts, better in Lakes Superior and Huron than elsewhere, is still ``agonizingly slow,'' according to Carlos M. Fetterolf Jr., executive secretary of the Great Lakes Fishery Commission.
A growing number of biologists agree that just where and when trout are stocked in the lakes can be vital. Scientists recently have been photographing and mapping once successful spawning reefs with an eye to introducing trout fry there.
In one experiment a few autumns ago Wisconsin scientists placed thousands of trout eggs in layered crates on a historically successful spawning reef in western Lake Superior. By spring a high percentage of the eggs had hatched.
Scientists are now waiting to see if the fish will return as mature trout to spawn. ``If it works, we've won the game,'' says Clayton Edwards of the International Joint Commission.
Randy Eshenroder, senior scientist with the Great Lakes Fishery Commission, says another hope is that the lakes can be stocked with more lake trout to make up both for the overfishing and the current ``inefficiency'' of the trout in reproducing. ``We're hoping to make up in quantity for the loss in quality,'' he says.
A number of other efforts to revitalize Great Lakes fish supplies have been notably successful. Significant strides have been made in the fight against pollution. Several of the lakes are reporting a comeback of both chubs and perch. And the stocking of Pacific salmon in the lakes over the last three decades is considered a strong success from an economic cost-benefit standpoint. ``Salmon fishing is probably the best in the world now right here on the Great Lakes,'' says Dr. Eshenroder.
Important experiments also continue here at the Ann Arbor-based Great Lakes Fishery Laboratory of the US Fish and Wildlife Service. Trout fry of various strains, lengths, and ages swim in fiber-glass tanks here while researchers test everything from their hormone levels to the degree to which various toxic contaminants in fish food are transmitted to trout eggs.
Despite the progress achieved, researchers say they still face a mammoth job in fighting both pollution, much of which now comes from the atmosphere, and in keeping a clamp on the spread of the sea lamprey. The latter job continues to cost the Great Lakes Fishery Commission about $7 million a year.
``It's like keeping your hand or foot on a spring -- if you take it away, the population would just go right back up again,'' says the Fishery Commission's Aarne Lampsa.
Indeed, the lamprey larvae are very hard to control in deep streams and large bodies of water such as the St. Mary's River. Researchers are experimenting with other control methods such as the release of sterile males into such spawning areas to reduce the number of fertile eggs.
Still, IJC fisheries biologist Clayton Edwards says he thinks the worst of the bad news about the lake system is now past. ``I think we've turned the corner and that the next 10 years are going to be the most exciting of all for the Great Lakes.''
Second of two articles on the Great Lakes.