Scaup and Coot Die-offs in Northern Minnesota


Large scale waterbird losses due to trematodes were first reported in Minnesota on the Upper Mississippi River National Fish and Wildlife Refuge in 2002.  Since 2007, reports of bird losses and discovery of the trematodes and their essential host, the invasive faucet snail, have occurred at several other locations in northern Minnesota.

Location of scaup die-off

The largest losses of waterbirds (several thousand scaup and coots) in northern Minnesota have occurred at Winnibigoshish, Bowstring, and Round lakes during the fall and spring migration periods.  These lakes are located roughly between the towns of Bemidji, Northome and Deer River.  The largest die-off occurred in the fall of 2007 when 6,000-7,000 lesser scaup and 200 coots died in Lake Winnibigoshish.  That year there were unconfirmed reports of dead scaup being found on Bowstring Lake, located 6 miles northeast of Lake Winnibigoshish. 

These three lakes have been monitored by DNR wildlife staff during the spring and fall migration periods to document any additional die-offs.  In 2008, an estimated 2,000 scaup and 200 coots died on Lake Winnibigoshish.  In 2009, an estimated 200 scaup and 200 coots died on Lake Winnibigoshish and 200 scaup died on Bowstring Lake.  In 2010, an estimated 1,200 scaup died on Bowstring Lake and <100 sick scaup were observed on Round Lake.  While a few sick and a few dead birds were observed on Lake Winnibigoshish, few scaup were observed at Winnibigoshish in the fall of 2010.  In the spring of 2011, hundreds of scaup were sick and many likely died at Lake Winnibigoshish during spring migration.  During the fall of 2011, a few hundred scaup died on Winnibigoshish, a few hundred sick and several dead scaup were observed at Bowstring Lake, and a few sick and a few dead scaup were found on Round Lake.  In spring 2012, several hundred sick birds were documented on Lake Winnibigoshish.  During the fall of 2012, approximately 25 sick and >20 dead scaup were reported on Winnibigoshish, >100 scaup were suspected of being sick on Bowstring, and <100 scaup were sick on Round Lake.   Changes in numbers of birds dying may be more reflective of reductions in bird numbers on these areas rather than a change in susceptibility of the birds to the trematodes. 

The largest die-offs of lesser scaup on Lake Winnibigoshish appear to have been along the west shore from where the Mississippi River flows into the lake to the Third River Flowage.  Most of the dead coots on Winnie have been found in Rabbit Flowage and around Mallard Point at the mouth of the Third River Flowage.

Lesser Scaup and American coots are the most common species lost in these die-offs in north-central Minnesota.   We have recorded losses of some greater scaup and a few other species including redhead, white-winged scoter, mallards and ring-necked duck.  This reflects feeding habits (scaup tend to feed more on snails) and distribution of waterbirds and snails in northern Minnesota, as losses of numbers of other waterfowl species have been reported elsewhere.

Wildlife staff have searched for the invasive faucet snail, the initial host for the three species of trematodes that kill the waterfowl, since the summer of 2008.  Faucet snails were immediately found in Lake Winnibigoshish and were well distributed throughout the lake.  The presence of faucet snails in Bowstring Lake was verified in the fall of 2012.  Both lakes are designated as infested waters.  The search for faucet snails continues in Round Lake but their presence has not been verified.

The Cause

Dead scaup on the shoreline

The scaup and coot deaths were caused by small trematodes (Sphaeridiotrema spp., Cyathocotyle bushiensis, and Leyogonimus polyoon) that develop in the bird intestinal tracts.    

These trematodes have a complex life history and require two intermediate hosts for the parasites to develop.   The invasive faucet snail, Bythinia tentaculata is the only known 1st intermediate host of the parasites in the Upper Midwest.  The second intermediate host of Sphaeridiotrema spp. and C. bushiensis is a snail, whereas, an aquatic insect is the second intermediate host for L. polyoon.  When waterbirds consume the second intermediate host, the trematodes attach to the intestinal wall and feed on blood of the birds. Heavily infected birds appear lethargic and have difficulty diving and flying before eventually dying due to blood loss.

What is the risk to humans?

Why does the DNR not clean up all the birds or take other actions to stop the die-off?

What can you do?


In addition to monitoring losses, two  research projects are underway in northern Minnesota to better understand how these parasites interact with both the snail and avian hosts.

DNR Wetland Wildlife Populations and Research Group

Dr. Charlotte Roy began a study in the fall of 2010 to examine distribution and prevalence of trematodes within the faucet snail relative to habitat conditions (water depth, substrate, temperature, etc.) and diving duck distribution.  This research has been conducted with sampling in the spring, summer, and fall and will be completed in fall 2013.  In addition to sampling at Winnibigoshish, Round, and Bowstring lakes, other sampling sites were added to the study as the faucet snail was detected in new locations.  These faucet snail-infested sites include Upper and Lower Twin lakes and the Shell River in Hubbard and Wadena counties, First and Second Crow Wing Lake and the Crow Wing River in Hubbard County, and five ponds on the White Earth Nation in Becker County. 

For more information on this research see report beginning on Page 139:

Minnesota State University, Mankato

Holly Bloom, a MSU-Mankato graduate student, is conducting research on waterbirds collected at Lake Winnibigoshish. The birds are being examined for the presence and abundance of gastrointestinal parasites, particularly Sphaeridiotrema pseudoglobulus and Cyathocotyle bushiensis. This research is being conducted to determine which waterbird species are naturally capable of harboring these parasites and to assess their potential to transport these parasites to new sites. Ten species of waterbirds, including dabbling ducks, diving ducks, and coots, are being collected and examined to determine their levels of parasitism. Preliminary collections were made in the fall of 2012 (60 birds) and subsequent collections will be made in the spring of 2013. This work should better our understanding of the transmission dynamics of S. pseudoglobulus and C. bushiensis parasites between lakes harboring faucet snails (Bithynia tentaculata).

For more information

DNR Invasive Species Program Information:

Upper Mississippi River National Wildlife and Fish Refuge Die-Off

USGS National Wildlife Health Center Parasitic Diseases

Verminous Hemorrhagic Ulcerative Enteritis Fact Sheet (Michigan DNR)

DNR Contacts

Faucet snails
Faucet snails found on Lake Winnibigoshish in August, 2008.

dead scaup
Dead scaup on Lake Winnibigoshish, November 4, 2008.

DNR Enforcement Aircraft on low-level search for scaup.