Invasive snail blamed for annual Mississippi River bird kill

Dead waterfowl litter an island


Nov. 16, on the Mississippi River near Ferryville in this U.S. Fish and Wildlife Service photo. A parasite carried by an exotic snail is blamed for the death of up to 1,000 coot and scaup this fall in what has become an annual event during the birds’ migration.

An invasive snail is being blamed for killing hundreds of waterfowl on the Upper Mississippi River this fall.

Field workers have found almost 1,000 dead coot and lesser scaup washed up on the shores near Genoa since early October, according to the U.S. Fish and Wildlife Service. The birds are believed to be the victims of an intestinal parasite found in faucet snails, which the birds eat during stopovers on their fall migration.

Die-offs have become an annual event during the past 15 years, since the arrival of the faucet snail. Native to Europe, the snails were introduced to the Great Lakes in the late 1900s and have since made their way into inland waterways. Faucet snails were first discovered in Lake Onalaska in the early 2000s and are now prevalent on the river between La Crescent and McGregor, Iowa.

“They basically came in and basically out-competed native snails,” said Roger Haro, associate dean for the College of Science and Health at the University of Wisconsin-La Crosse. “They’ve been around for a while but they never caused a detectable problem with waterfowl.”

While the snails provide a food source for waterfowl, they carry a parasite known as trematodes that can infect the birds and cause them to die within three to eight days, according to the Fish and Wildlife Service.

Though trematodes can affect up to 19 species of waterfowl, Haro said they are most harmful to coot and scaup.

The outbreaks are a concern because about 40 percent of all North American waterfowl follow the Mississippi River flyway during their annual migration, stopping to feed as they make their way south each fall and back north in the spring.

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Haro and other scientists at UW-L have been studying the snails to better understand their behaviors and the effects of temperature variations on their growth.

There are no reported health risks from handling or consuming waterfowl infected by trematodes, according to the National Wildlife Health Center, but sick birds can have secondary infections that cause their intestines to leak into the body cavity. Hunters are advised to wear gloves if handling sick birds.

Haro said the infected birds do not appear to be a threat to other species, instead providing an abundant food source for bald eagles and vultures.

In some years the Fish and Wildlife Service collects carcasses, but with cold temperatures forecast in the coming days officials decided the birds would not create a significant nuisance this year, said Hallie Rasmussen, visitors services director for the Upper Mississippi River National Fish and Wildlife Refuge.

“It’s kind of a jolting thing for people to see all these dead birds,” Haro said.

Faucet snails were first discovered in Lake Onalaska in the early 2000s and are now prevalent on the river between La Crescent and McGregor, Iowa.

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