A parasite has killed thousands of ducks on Lake Winnibigoshish in northern Minnesota, and could kill many more before the fall migration is over.
"We picked up 1,000 dead scaup [also known as bluebills] on Saturday," said Steve Cordts, Minnesota Department of Natural Resources waterfowl specialist.
He saw many other scaup still alive but unable to fly, or to fly far. "You could boat right up to them," Cordts said. Perhaps 3,000 ducks, mostly scaup, and some coots have died in the past week.
"I'm sure there's more dead birds," Cordts said Tuesday.
The ducks apparently are dying from trematodes, a tiny 1-millimeter intestinal parasite or fluke that has infected snails in the lake. Scaup -- a duck that dives below water to feed -- eat the snails, then are infected.
"They essentially bleed to death," Cordts said.
The parasite was confirmed in scaup and coots sent to the National Wildlife Health Center in Madison, Wis.
Similar die-offs caused by trematodes have occurred spring and fall since 2002 on the Mississippi River near La Crosse, Wis., killing about 40,000 ducks and coots since then. Die-offs again are occurring there this fall.
Cordts said he's not sure how the trematodes made it to Winnie. They apparently have infected a snail called the banded mystery snail, which was first found on Winnie about eight years ago. They are infecting the faucet snail on the Mississippi.
Officials aren't sure how many ducks might eventually die on Winnie, or what impact, if any, it will have on the scaup population. But Cordts is concerned that the snails and parasites might spread to other Minnesota waters.
Other duck species also could eat the snails and become infected, he said.
There is concern because the continental scaup population has been declining since 1984 and hit an all-time low last year at about 3 million. Hunters annually kill about 300,000. Minnesota hunters killed about 20,000 last year.
Lake Winnibigoshish is a major scaup resting area during migration. "We could have 20,000 scaup show up on Winnie right now," Cordts said. "If that happens, they'd pretty much all be at risk."
Cordts plans to check the lake again today, but he won't collect any more dead ducks. Instead, carcasses will be left to decompose or be eaten by scavengers. The parasite apparently is not a threat to other species, including humans, but Cordts said hunters shouldn't eat sick waterfowl.