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The mission of the U.S. Fish and Wildlife Service's Endangered Species program is conserving and restoring threatened and endangered species and their ecosystems.




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The Midwest Region includes Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio and Wisconsin. Find a location near you.


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Mussels of the St. Croix River


This endangered winged mapleleaf mussel was tagged and then placed back in the St. Croix River. Biologists gain population and other information by tagging and then recapturing mussels at a later date.

Photo by USFWS; Phil Delphey

The St. Croix River watershed is the premier mussel watershed of the Upper Mississippi River, and one of the premier mussel watersheds of the world.


Forty-one mussel species live in the St. Croix River ("Dan Hornbach's Lab" Macalester College, 04 Feb. 2013). Of these, Wisconsin and Minnesota have listed 15 and 20 as state-endangered or state-threatened species, respectively. Five species of mussels that occur in the St. Croix River are also listed as endangered under the Federal Endangered Species Act: Higgins eye, sheepnose, snuffbox, spectaclecase and winged mapleleaf.


North America is the world's great mussel continent; the richest species aggregations found in any watersheds worldwide are found in North America. North America's best mussel watersheds have species numbers in the upper 30s. Thus, with its 41 species, the St. Croix is among the world's greatest mussel watersheds.


The uncommon richness of mussel species in the St. Croix parallels the uncommon richness of the flora and fauna of the watershed as a whole. The watershed has many uncommon Midwestern species, including some species that were once endangered or threatened and have now recovered – including gray wolves, bald eagles, and peregrine falcons – and some that are still endangered, such as the Karner blue butterfly.  One expert characterized the St. Croix as a sanctuary containing the best-preserved (least human-impacted) remnant of pre-settlement natural communities in the Upper Mississippi drainage, and the very best preserved pre-settlement aquatic community in the Upper Mississippi drainage.


The biological diversity of the St. Croix River watershed may be due in large part to the relative lack of intensive agricultural.  In contrast, the Minnesota River historically supported a mussel fauna very similar to the St. Croix River, but its watershed supports widespread and intensive agriculture that has greatly modified runoff patterns and river habitats.  It now supports only low densities of a few mussel species. 


In a sense, the St. Croix watershed is an island of near-pre-settlement conditions surrounded by the more developed and altered North America of the Twentieth Century. Local popular sentiment, increasing involvement and commitment by state and Federal conservation and other agencies, and the Federal Wild and Scenic River designation of the Namekagon-St. Croix reflect the watershed's value and offer practical vehicles for protecting the watershed from what may be disastrous decimation of its mussel fauna should zebra mussels successfully invade.


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