Dead lake sturgeon have been turning up along the shore of Lake Michigan. Seven sturgeon carcasses have been found since mid-July on beaches in the Sleeping Bear Dunes National Lakeshore.
Vincent Cavalieri is a wildlife biologist with the National Park Service. He says to have this many dead sturgeon in such a short period of time is incredibly unusual.
Cavalieri says the die-off could be due to botulism. Botulism is an illness caused by a toxin produced by bacteria. The sturgeon carcasses were too decomposed to test, but some gull carcasses had also been found near the same time as the sturgeons.
“We sent one of those gulls to the national health lab in Madison, and we’re kind of waiting on the results of that testing, just to confirm if botulism is present or not.” he says.
Ed Baker is the research station manager at the Marquette Fisheries Research Station for the Michigan Department of Natural Resources.
He says the botulism could potentially be related to an invasive species, zebra mussels, which have drastically altered the Great Lakes ecosystem since their introduction.
“It’s a bit of a hypothesis, but it’s suspected that the changes that are taking place around zebra mussels and real dense zebra mussel infestations is allowing the Botulinum bacterium to become more abundant than it would otherwise," says Baker. "Then it gets passed up the food chain. We think round gobies are somehow involved. Whether it’s round gobies ingesting the toxin, and the bird-eating fish as well as sturgeon ingesting the gobies, and poisoning them, or some other mechanism. But it does appear to be related to the zebra mussel invasion.”
Gobies are another invasive species in the Great Lakes, and sturgeon have been known to eat them.
Another potential reason for the die-off could be a sudden change in temperature.
“We’ve had some pretty warm water temperatures in Lake Michigan this summer, especially before a bit of a cool down last week. Those warm temperatures can impact oxygen levels in the lake. If they’re low enough, they can potentially cause mortalities for sturgeon. So that’s another possibility," Cavalieri says, adding, “It could be a combination of the two.”
Sturgeon are considered threatened in the state of Michigan. Baker estimates that current sturgeon populations in the Great Lakes are less than 1% of what they’ve been historically.
Cavalieri says the sturgeon deaths are cause for some concern because of the sturgeon’s threatened status.
“It’s somewhat of a concern, the numbers, at least so far, aren’t so big that we’re panicking or anything," says Cavalieri. "But for a rare species, a long-lived species like sturgeon, we don’t like to see any kind of mortality like that, given that we’re trying to restore them across the basin to some semblance of their former numbers. You don’t want to see another threat that could potentially check that recovery.”
He says male sturgeon can live to more than 50 years of age, and females can reach 100 or even older. Their longevity also makes repopulating the Great Lakes slow-going: sturgeon aren’t ready to reproduce until they’re 15-20 years old.