BY KATE MILEWSKI
TRAVERSE CITY —
A botulism outbreak blamed for killing hundreds of loons and other waterfowl on Lake Michigan may be on the wane, but experts don't yet know if the epidemic has run its course.
By Monday, the remains of more than 900 birds had been located along the shoreline of Sleeping Bear Dunes National Lakeshore, 378 of which were loons, said Dan Ray, a biological technician and the lakeshore's coordinator for the avian botulism monitoring program.
This week's count of dead birds has been much lower than last week's tally, though it may not tell the whole story, Ray said Wednesday.
"The good news is, it has gone down markedly," he said. "A lot of that has to do with wind direction and weather condition. If we did have a bird that was dead, it would blow away from the lakeshore rather than into our beach. It doesn't mean they're not there, though it appears the bulk of the mortality has dropped drastically."
Experts believe botulism tied to invasive mussels is responsible for bird deaths. Outbreaks have been common in recent years, but concerns mounted because numbers this year are higher than normal.
"Last year, we only had about 30 loons," Ray said. "This is by far the worst year we've had here since 2007, beyond that."
Dead birds likely will continue to wash ashore in northwestern lower Michigan into November. But much will depend on the weather and its impact on tainted mussels and fish.
"Really cold weather triggers migration; that drives birds on their migration route," said Mark Breederland, a Michigan Sea Grant extension educator. "The birds are here in October."
Officials continue to search for clues in what caused so many bird deaths this year.
"We know a little bit about the food web," Breederland said. "I do think there was some weather-related events in the lake, like sustained winds churning up some of the bottom, making it potentially available in the food web. We see bigger die-offs after larger storm events. There seems to be a correlation that we observed, but we haven't gotten it figured out."
Other factors — from higher water temperatures and low lake levels — could play a role in bird mortality.
"We're still in the midst of what's going on here this season," Ray said. "It's easier to sum up once we reach the end of the year in December (and) fine tune our assumptions."