INVESTIGATION OF WATERBIRD DIE-OFFS ON THE UPPER MISSISSIPPI RIVER NATIONAL 
WILDLIFE AND FISH REFUGE

Jennifer Sauer1, Dr. Rebecca Cole2, James Nissen3 and 4Ed Britton

1U.S. Geological Survey, Upper Midwest Environmental Sciences Center, 2630 
Fanta Reed Road, La Crosse, Wisconsin
2U.S. Geological Survey, National Wildlife Health Center, 6006 Schroeder Rd, 
Madison, Wisconsin
3U.S. Fish and Wildlife Service, Upper Mississippi River National Wildlife 
and Fish Refuge, 555 Lester Avenue, La Crosse, Wisconsin
4U.S. Fish and Wildlife Service, Upper Mississippi River National Wildlife 
and Fish Refuge, 7071 Riverview Road, Thomson, Illinois

Since 2002, there have been major waterbird mortality events every spring and 
fall in Lake Onalaska (Navigation Pool 7 of the Upper Mississippi River) 
located near La Crosse, Wisconsin.  This area is part of the Upper 
Mississippi River National Wildlife and Fish Refuge (Refuge) and lies within 
the Mississippi Flyway, through which an estimated 40 percent of the 
continent's waterfowl migrate.  Recently, bird mortality has spread south to 
Pools 8, 9, and 11 on the Refuge—about 125 miles downstream from Pool 7.  
Total bird mortality on the Refuge is estimated at 32,000–40,000 birds, 
primarily lesser scaup (Aythya affinis) and American coots (Fulica 
americana).  Three trematodes (Sphaeridiotrema globulus, Cyathocotyle 
bushiensis, and Leyogonimus polyoon) that use the faucet snail (Bithynia 
tentaculata) as an intermediate host were found to infect and kill the 
waterbirds.  The faucet snail was introduced into the United States from 
Europe in the late 1800s and first recorded on the Upper Mississippi River in 
2002.  Because the Refuge is a major spring and fall stop-over area for 
waterfowl in the Mississippi Flyway, concerns were raised that the snail and 
trematodes may be spreading to other waterfowl stop-over areas on the river.  
Sampling for faucet snails was conducted in 2005–2007 in select areas in 
Pools 4–9 (excluding Pool 5a which is located between Pools 5 and 6), 11, and 
13.  Faucet snails were found in all the sampled pools.  To our knowledge, 
these are the first records of faucet snails and associated trematodes beyond 
those found in Pool 7, Lake Onalaska.  Waterbird die-offs are becoming a 
Refuge-wide problem.  Information obtained through research and monitoring, 
including the identification of the origin of infections in snails and birds 
and the role various environmental factors have on this process, should help 
guide managers to develop effective mitigation and control measures.

Keywords:  Bithynia tentaculata, scaup, coots, Upper Mississippi River, 
Sphaeridiotrema globulus, Cyathocotyle bushiensis, and Leyogonimus polyoon