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More ominously, some scientists think the decline also might be linked to the mounting number of invasive species colonizing the Great Lakes via ballast water tanks of ocean freighters. A new species invades the Great Lakes, on average, about every eight months. Among the most damaging are the zebra mussel and the slightly larger quagga mussel, both natives of the Caspian Sea region. The worry is those prolific filter feeders are stripping away the bottom of the lakes' food chain, which directly or indirectly affects every fish species.

Johnson believes the alewife decline is tied to a combination of all of the above. The alewives aren't finding enough to eat, which makes them prone to winter-time die offs. At the same time, the alewives that do survive are being consumed by a ballooning number of wild chinook and their hatchery-raised cousins, millions of which are planted in Lake Michigan alone annually.

"You can almost bank on it being a collusion of factors," he said.

Biologist Chuck Madenjian, who conducted the alewife survey, figures the drop is tied to an abundance of chinook. Historically, most all the chinook in the lakes were hatchery-raised and then planted by humans, so biologists had a way of knowing how many fish were out there. But an apparently growing number of chinook mysteriously have begun to reproduce on their own in the last decade, leaving biologists with no clue how many fish are now in the system.

"I think a substantial increase in the number of predators would help explain that big drop," Madenjian said.

Chinook surveys, meanwhile, show that their average size has been shrinking, bolstering worries that the big fish are not finding enough little alewives to eat; Johnson said last summer he netted some chinook that had stomachs that had apparently shriveled from lack of food.

"Their stomachs were almost gone," he said. "They weren't capable of feeding anymore."

Leaving Lake Huron

He said the situation on Lake Huron is so bad that the chinook are leaving the lake in search of food.

He points to a survey that showed just more than 4% of the chinook planted in Lake Huron in 1996, which were eventually caught, were caught in Lake Michigan. Today that figure has grown to 29%.

"We're quite sure they're just searching for prey fish, and they're finding them in Lake Michigan and not in Lake Huron," Johnson said.

Wisconsin biologists worry that it might only be a matter of time until Lake Michigan is in the same fix.

"We see Lake Huron as being a couple of steps ahead of us, in terms of declining (alewife) abundance and reduced fitness or robustness of the salmon. So we wonder if Lake Huron isn't our future," said Bill Horns, Great Lakes fisheries coordinator for the Wisconsin Department of Natural Resources.

Horns believes the drop in alewives is likely tied to too many salmon, and that means changes could be in store for how Lake Michigan is managed. The first casualty could be the stocking levels of chinook, the king of the food chain and favored target for many anglers.

The various agencies that have a hand in managing Lake Michigan annually plant about 4 million chinook, about half the million chinook planted during the peak chinook planting year in 1989, but still perhaps more than the lake can now support.

"There are a lot of warnings out there that give us the idea that we really should look at reducing stocking," said Paul Peeters, a biologist with the Wisconsin Department of Natural Resources.

What's not an option is stocking alewives, which would compete with native species.

Fishery experts from around the region were scheduled to gather in Benton Harbor, Mich., on Saturday to discuss the problem. Wisconsin officials are hoping they can get the other Lake Michigan stocking states of Illinois, Indiana and Michigan to eventually agree on a plan to cut the number of chinook plantings.

"We want to do it in conjunction with other states," Horns said. "This conference is the place, time and opportunity to initiate that conversation."

It is a conversation many anglers did not want to have but are now realizing is inevitable.

Last year, a group of biologists charged with making chinook stocking recommendations on Lake Huron considered proposing a one- year halt in stocking due to the crash in alewife.

Biologists backed off that proposal after fishing groups bristled. But now it is some influential fishermen who are talking about throttling Lake Huron's stocking program.

Just leave it alone'

Dan Thomas, president of the Great Lakes Sport Fishing Council, said the gap in the food chain caused by the loss of alewives is an opportunity "to allow Mother Nature to take its course and fill that niche at her speed and with her species of fish."

"Just leave it alone," he said of a lake that has been so heavily manipulated by fish stocking for the past few decades. "Mother Nature has a way of rebounding. Our best efforts can never equal what nature can do in the scheme of things."

Chinook were first planted in the Great Lakes in the 1960s to eat the alewives, which had overwhelmed the lakes' food chain and squeezed aside many native species, including perch, lake trout, whitefish and chubs. The saltwater natives also had a history of dying off by the millions and washing up and rotting in thigh-high piles on Lake Michigan beaches.

In an act of desperation to reduce the alewife numbers, the chinook were brought in along with smaller coho salmon from hatcheries in the Pacific Northwest. Their first job was to eat the reviled alewives; their second job was to create a fun fishing experience for anglers.

The experiment was a wild success on both accounts.

But today, with alewives more than under control, biologists fear they are losing control of their ability to maintain the predator- prey balance that has made the lakes such a popular fishing destination.

A big change in the last decade is that a significant percentage of chinook have begun to reproduce in the wild, something the stocking pioneers never intended to happen.

Some estimate as many as 80% of Lake Huron's chinook reproduce in the wild, mostly in the pristine, cold streams of Ontario. Perhaps 50% of Lake Michigan's chinook population is also reproducing on its own. That wild reproduction means nobody knows exactly how many are out in the lakes.

"It's hard to balance a predator-prey relationship when you don't know how many predators are there," Johnson said. "Everything we thought we were doing with stocking seems to have changed, and we do have to reconsider why we stock."

Wisconsin DNR's Peeters added: "Mother Nature is producing a lot of salmon, so if we want to hold the line on salmon in the lake, we've got to look at what we're putting in it."

Populations go up and down

Pumping too many chinook into the system can have drastic effects. In the late 1980s, tens of thousands of dead chinook washed ashore in Lake Michigan, the victims of a disease many biologists tied to a lack of nutrition. The states backed off on the stocking, and Wisconsin took measures to protect the alewives by prohibiting their harvest by commercial fisherman, who had been netting them and selling them for pet food.

Chinook numbers rebounded.

Stocking was again reduced in the late 1990s when alewife numbers began to decline. Biologists at the time argued that the chinook fishing would actually improve if stocking numbers better matched the available prey base.

"Those cuts did take place, and that fishery did rebound, and we did have some of the best fishing ever," angler Thomas said.

Thomas is confident the lakes will rebound again, but biologists say predicting how the lakes will change has become increasingly difficult in just the past five years, thanks to the invasive mussels and foreign fish like the round goby.

Another Lake Michigan study, for example, shows that diporeia, a tiny shrimp-like organism that many small fish in the lake depended upon, is disappearing. Diporeia once covered the bottom of Lake Michigan at a density of up to 20,000 per square meter. Today, in vast expanses of the lake, they have disappeared. Their decline is linked to the arrival of foreign mussels.

It is just another example of how the rules of life in the Great Lake are changing, and there is an increasing likelihood that humans will not be able to change them back.

"No one likes change, and this kind of change no one was prepared for," Johnson said. "This probably wouldn't have happened if zebra mussels and quagga mussels and gobies had not locked up a lot of the (available food) in the lakes."