DULUTH, Minn. (AP) - Nearly 900 loons and probably more died while migrating south across Lake Michigan last fall, and scientists suspect invasive species may be to blame.

With the iconic birds of the North Country beginning their migration back from the Atlantic and Gulf coasts in less than a month, Minnesota Public Radio reported Monday that scientists with the U.S. Geological Survey think a complex interplay of invasive species may be the cause of the mass die-offs.

The researchers suspect invasive zebra and quagga mussels create ideal conditions in Lake Michigan for the bacteria that produces botulism toxin. The mussels filter the water so it’s incredibly clear, allowing an algae called cladophora to grow in huge amounts. Storms churn up the algae, which settle to the lake bottom and rot. That creates an environment without any oxygen, an ideal home for bacteria that produce botulism. The toxin is ingested by tiny worms and freshwater shrimp, which are eaten by fish, including the invasive round goby, which are then eaten by diving birds - including loons.

“What happens is they can’t move their muscles, and, eventually, they usually die because they can’t breathe or they can’t hold their head up out of the water,” said Stephen Riley, a fisheries biologist with the U.S. Geological Survey in Ann Arbor, Mich.

Scientists to figure out a way to break a link in that chain before it can kill more loons.

Lynette Grimes saw the problem last October as she was hiking toward Lake Michigan at Sleeping Bear Dunes National Lakeshore, outside Traverse City, Mich., where nearly 600 loons washed ashore. She and her husband worked until sunset burying them in 3-foot-deep trenches.

“The beach was just pockmarked with birds everywhere you looked,” Grimes said. “This one little peninsula had over 100 dead birds.”

Kevin Kenow, a USGS wildlife biologist in LaCrosse, Wis., tracks loons with radio transmitters. His work has shown that some Minnesota loons spend nearly a month on Lake Michigan fattening up before their long flight south.

“They’re diving up to 40, 45 meters in some of these areas,” Kenow said, “and the pattern of dives suggests that they aren’t stopping in the water column anywhere, but they’re continuing all the way down to the bottom, feeding on the bottom substrates and then returning to the surface.”

It’s at the lake bottom where scientists believe fish such as the round gobies pick up botulism before they’re eaten by loons.

Before last fall, it had been five years since the last large botulism outbreak. National Park Service ecologist Brenda Lafrancois of Ashland, Wis., said the outbreaks seem to be tied not just to invasive species but also to warmer weather.

So far the outbreaks don’t seem to have affected Minnesota’s loon population, said Carrol Henderson, Nongame Wildlife Program supervisor at the Minnesota Department of Natural Resources. Henderson said the population appears stable at more than 10,000 adult loons, so it’s still unclear how many of the loons dying in Lake Michigan spend their summers in Minnesota.

Last October, Damon McCormick, a wildlife biologist with Common Coast Research and Conservation in Houghton, Mich., found 300 dead loons in just a seven-mile stretch of Lake Michigan beach near the Upper Peninsula town of Gulliver.

“If the die-off continues, to any extent like it has, then I think it’s a genuine concern for the long term viability of loons,” McCormick said.