An annual fall canoeing and duck hunting trip that Canoeing.com founder Tim Eaton and his ornithologist son Muir made earlier this month turned sad and puzzling when the pair paddled into the carnage of a die-off of thousands of scaup on Lake Winnibigoshish in north-central Minnesota.

"Hunting Lake Winnibigoshish and paddling the Mississippi River in the fall has been a tradition for Muir and me for more than 15 years,” Eaton said. “In that time and before November 3, 2007 we have never had a sad day or an experience of this proportion.”

The waterfowl deaths the pair encountered, numbered now at 6,000, made local news and left waterfowl hunters, bird enthusiasts, and wildlife biologists concerned about the beleaguered birds, also known as bluebills. The Winnibigoshish kill, like previous events down-river near La Crosse, Wisconsin, has been attributed to trematodes, or flukes, a small intestinal parasite that the birds contract from ingesting infected snails.

“We pulled to shore east of the river delta on Winnibigoshish near a small adjoining pool we call Harry Lake,” the elder Eaton said. "Not ten feet from the canoe we found a dead drake scaup on shore. Someone must have shot it, was our initial reaction. Not the case, the bird was big with not a mark on it."

"Puzzled we walked a little further finding another drake, then another, then another,” he continued. "The dog was sniffing out more dead ducks in the grass three or four feet from the shoreline. In 10 minutes we had a pile of 36 dead ducks. We left another dozen headless ducks in the weeds; predators were obviously having a feast.”

Even Muir Eaton, who studies birds professionally was moved by the encounter.

"I had that strange feeling you have when you know something is really wrong, but you have absolutely no frame of reference for understanding why,” he said. "Your mind churns through every possible reason you can think of, none of them really making sense of what you are seeing. Of course, these feelings quickly gave way to concern, and a feeling of needing to do something.”

Later, after the Eatons gathered 30-some ducks to show authorities, they met Minnesota DNR and USDA officials coming off the water after having, themselves, collected roughly a thousand dead scaup.

"They told us about the parasites and the mystery snail; Muir knew right away about the parasites and told me how the snail acts as the host. And, how birds are affected if attacked by the parasites,” Tim Eaton said.

At Winnibigoshish, the DNR says, trematodes appear to have infected a snail called the banded mystery snail, a native of the southern U.S. which was discovered in the lake in the last eight years. The similar die-offs of scaup, beinning in 2002, downstream on the Mississippi more than 280 miles from Winnibigoshish, have been tracked to the same parasite but residing in faucet snails, a European exotic.

The banded mystery snail, a popular aquarium snail, is thought to have spread due to aquarium releases beginning early in the last century in the Northeast. The faucet snail is thought to have made its passage to North America in the late 1800s either in the packageing of cargo or in the ballast of timber ships plying the Great Lakes.

Scaup, which migrate from the boreal forests of Canada to the Gulf of Mexico coast each fall, stop over at spots like Lake Winnibigoshish in the Mississippi headwaters and the pools of the Upper Mississippi to feed and refuel for the remainder of their flight south.

In previous cases of waterfowl kills caused by trematodes, the Minnesota DNR says, ducks usually died three to eight days after ingesting a lethal dose of the parasites. Because these birds on Winnibigoshish appear to be dying within one or two days, the snails may be either extremely abundant or be carrying very high levels of the trematode.

The scaup die-offs along the Mississippi migratory flyway are taking place in the context of a more generalized, long-term population decline for the species since the late 1970s. Research is still ongoing to get to the bottom of the decline, but likeliest culprits include the decreased quality and quantity of food resources in migratory stopover areas, the bio-accumulation of contaminants, and climate and habitat changes in the birds boreal forest breeding areas – all of which may be affecting female survival or reproduction success.

One scenario suggests that healthy scaup leave their springtime, upper Midwestern stopover sites undernourished due to competition from minnows for their favored local food source, amphipods – commonly referred to as scuds or fairy shrimp.

Experts are unsure of the population-wide significance of the recent die-offs, or whether they are a particularly new phenomenon. Scaup, including the greater scaup and lesser scaup species, whose population exceeded 6 million in the 1970s, numbered an all-time low of about 3 million birds last year. Hunters annually kill about 300,000 birds; Minnesota hunters killed about 20,000 last year. Recent research on scaup parasites found, however, that the levels and species of parasites present in wintering scaup were similar to those historically.

"The reality is, this is a very small number of individuals dying on a continental population scale,” Muir Eaton allowed. "Hunters can harvest around 300,000 scaup a year, which has arguably no impact on long term population numbers, so it is hard to make a case that this has a noticeable impact on the scaup population. At the same time, there are many unanswered questions about why the scaup population has been steadily declining over the last decade.”

Study continues in the effort to find the sources of population decline in scaup – and a solution to it – as well as in hope of understanding the outbreaks of trematode kills.

"I would like to know more about the history of this parasite,” Muir Eaton said. "What is its distribution? What types of snail can act as a host? What is the tolerance, by scaup, for infection? What are the other physiological impacts on an individual besides death? Is there any way to attempt to control the population of the snail host, to try and minimize the impact of migrating birds?”

No matter the significance of the die-off on the scaup population, the kill has left a lasting impression on the Eatons.

"Even though, as a biologist/ornithologist, I understand that disease and death are really a normal part of existence for everything,” Muir Eaton said, "to witness, first-hand, such a poignant example of wide-spread infection and mortality was a very moving experience. "It is going to be something I remember, and probably will recount every hunting season, for the rest of my life.”