An Investigation of Spinal Deformity of Trout (Salmo sp.) in the Brule River,Wisconsin

An unusually high incidence of spinal deformity in migratory trout from the Brule River, Wisconsin, was investigated to determine its probable cause and extent. Incidence in rainbow trout (Salmo gairdneri) and brown trout (Salmo trutta) was determined by external examination and x-ray. The total incidence of deformity in x-rayed and externally examined lake-run rainbow trout was 50 and 26 percent, respectively. No vertebral anomalies were found by x-ray in 75 Brule River juvenile rainbow trout which had not yet emigrated to Lake Superior, nor in 25 lake-run rainbow trout from the French River, Minnesota. Examination of 25 lake-run brown trout from the Brule River showed external deformity (8 percent) and internal vertebral abnormalities (16 percent). The absence of spinal deformity in juvenile rainbow trout from the Brule River and in lake-run trout from the French River suggest that the electric lamprey control weir on the Brule was affecting fish which migrated downstream. The lower incidence of injury in brown trout reflects the difference in the time of their migration which was not coincident with weir operation. It is apparent that the electric lamprey weir has had a greater total impact on recruitment of trout to Lake Superior than previously thought.