Distribution, Age, and Growth of Redhorse in the Upper St. Croix River

Redhorse (Moxostoma) are ubiquitous in warmwater rivers and streams in the upper Midwestern USA, but vary in their sensitivity to human disturbances, including dams, and inputs of nutrients and sediment.  In Wisconsin, two of five redhorse species native to the upper St. Croix River system are listed as state threatened species, including the river redhorse (M. carinatum)  and greater redhorse (M. valenciennesi).  We collected redhorse to determine relative abundance and age structure.  River redhorse were not collected above dams on the upper St. Croix, Namekagon, and Yellow Rivers, and were rare at sites below the dams.  No individuals younger than age-III were found.  Greater redhorse were uncommon at most sites but locally abundant in some reaches during spring, including reaches of the Namekagon River and Yellow River above dams.  The greater redhorse was found in higher abundance than expected and appears secure in this system.  Both greater and river redhorse reached relatively large sizes.  Aging based on otoliths and opercula suggested that their large adult size compared to shorthead (M. macrolepidotum) and golden redhorse (M. erythrurum) was the result of faster growth rates and greater longevity.  Aging based on fin rays underestimated ages of all five redhorse species examined.