MILWAUKEE — Fifty years ago, there were no salmon swimming in the cold, deep, watery environs of Lake Michigan.

The first salmon were released in the lake in 1966 in an effort to control another non-native species — an exploding alewife population that found its way into the Great Lakes basins through the St. Lawrence Seaway and other ocean-connected routes.

And the rest, as they say, is history.

According to Brad Eggold, a fisheries supervisor with the Wisconsin Department of Natural Resources, the stocked salmon — cohos and the Chinook or king salmon — grew fast and fat on the large forage base, quickly attracting the interest of sport anglers.

“Nobody really realized just how big salmon and trout fishing would become on Lake Michigan,” Eggold said.

More than four decades later, sport fishing for cold water species has become an economic mainstay for coastal Wisconsin, Michigan and Illinois, spawning an extensive charter boat industry along with a support systems of hotels, eateries and other tourist attractions

While precise numbers are hard to come by for Wisconsin alone, one study put the economic impact of sport fishing across the entire Great Lake basin at more than four billion dollars annually.

Eggold said that maintaining the Lake Michigan’s cold water fishery traditionally has come through stocking efforts with an eye to balancing the numbers of fish stocked to the available forage base.

 “The states have tried to get a handle of what the available forage base is and just how many fish are out there,” he said.

Alewife populations, which at their peak populations would pile three feet-deep on shorelines during massive die-offs, have since been reined in by hungry salmon and trout populations.

But too many fish and not enough forage can mean slow growth and susceptibility to diseases, something that happened 20 years ago on the lake severely reducing game fish numbers.

Eggold said that stocking efforts in recent years have been reduced to reflect the smaller forage base. But interestingly, biologists at the same time have detected evidence of natural salmon reproduction in some Lake Michigan streams, mainly on the Michigan side.

Eggold said that since 2006, a study has been underway to determine the extent of natural reproduction.

“What we’re finding is that more than 50 percent of the year-old salmon being caught by Wisconsin anglers are the result of natural reproduction,” Eggold said.

Salmon can migrate thousands of miles in the ocean, so migrating to the Wisconsin side of Lake Michigan is no tough feat.

“Lake Michigan is dinky by comparison,” he said.

Whether the reproduction is the result of improving water quality is uncertain. And while the added natural reproduction might seem like a good thing, Eggold says it does create some uncertainty as to how many fish should be stocked.

“If we stock 4 million fish, just how many other fish are out there as the result of natural reproduction?” he said.

In the late 1980s, fish exceeding 30 pounds were not uncommon and Eggold concedes that nowadays, as the result of a smaller forage base, the largest fish now push the 20-pound mark.

Nevertheless, the fishing remains excellent. In 1987, a record 400,000 salmon were harvested by Wisconsin anglers; that record was broken in 2007 when anglers caught 431,000 fish.

Ironically, the future of Lake Michigan’s salmon fishery along with those of other Great Lakes could be threatened by another non-native species — the Asian carp.

Earlier this month, Wisconsin, along with Minnesota, Michigan, Ohio and Pennsylvania, joined in a lawsuit in federal court to force the U.S. Army Corps of Engineers to close down the Chicago Sanitary and Ship Canal, the sole gateway from the carp-infested Illinois River to Lake Michigan.

Presently, all that is keeping the carp from freely moving into the lake is an electronic barrier that biologists fear can easily be breached.  

Conceding that it is a “sensitive subject,” Eggold deferred when asked to speculate about what impact the carp might have on Lake Michigan.

“Intuitively, it can’t be good,” he said, noting that is particularly true when two critical parameters — the ability to reproduce and being in an environment where there are no know predators — are present.

“Once an invasive species gets in, it can be extremely difficult if not impossible to get out,” he said.