ABOARD R/V SEWARD JOHNSON (AP) _ The first few days of an expedition to explore the depths of Lake Huron by submarine already have borne fruit, including discovery of what may be a prime spot for lake trout spawning.

''We're continuing to unlock the secrets of nature here,'' said John Krezoski, an assistant research scientist at the University of Wisconsin- Milwaukee who is on the expedition.

Krezoski and eight other scientists on the first leg of the expedition are studying sediment and lake trout spawning beds. Their main vehicle is the Johnson Sea-Link II, a four-person submarine with a bubble-shaped transparent front compartment that allows two of its four passenger a clear view of the lake bottom.

They are the first to study the bottom of Lake Huron by manned submarine.

For the next month, they will be conducting biological, chemical and geological research in lakes Huron and Superior, where several of the team members conducted a similar, first-of-its-kind expedition last summer.

Lake Superior is the only one of the Great Lakes where lake trout still spawn regularly, scientists say. Lake trout in the other four lakes are mostly stocked and there has been little success in natural reproduction there.

The lake trout in Lake Huron were destroyed in recent decades by sea lamprey and excessive commercial fishing. The lampreys have since been brought under control with a selective chemical spraying program.

But scientists diving in the submarine at a site called Six Fathom Bank - about five miles from the United States-Canadian border of Lake Huron - say conditions there, especially large numbers of porous rocks, may be ideal for spawning.

''If this site turns out as it appears to be, it could be a key step up the ladder in re-establishing the multimillion-dollar lake trout industry,'' said Richard Cooper of the National Undersea Research Program at the University of Connecticut, the science director of the expedition.

''It has a very direct benefit in terms of commercial fishery, sports fishery and recreation,'' he said.

A large lake trout population also could restore a more natural balance because the fish would keep the alewife population under control, Kerzoski said.

Next week, the scientists will head to Lake Superior and continue some of the research they began last year, including the study of sculpin, a small scaleless fish. They also will study the toxic pollutant dioxin and the sea lamprey.

The Sea-Link II, owned and operated by the Harbor Branch Oceanographic Institution, was used earlier this year in the Atlantic Ocean to search for the remains of the space shuttle Challenger. The research vessel Seward Johnson is its support ship.

The expediton is sponsored by the National Oceanic and Atmospheric Administration through the undersea research project. The program is being coordinated by Michigan State University.

Twenty-four scientists and researchers will participate in this expedition, including a representative from the Federal Great Lakes Fisheries Laboratory and the universities of Michigan, Wisconsin, Minnesota and Connecticut.