Replacement of Brook Trout by Brown Trout over 15 Years in a Minnesota Stream: Production and Abundance

Abstract

The trout population in Valley Creek, Minnesota, changed over 15 years from virtually 100% brook trout Salvelinus fontinalis in 1965 to predominantly brown trout Salmo trutta, with some brook trout and rainbow trout Salmo gairdneri remaining. Trout densities were 6,618. hectare−1 in spring 1965 (all brook trout), and 3,430 . hectare−1 in spring 1980 (70% brown, 15% brook, and 15% rainbow trout). Initial standing stock in spring 1965 was 184 kg˙hectare−1(wet weight) of brook trout; in spring 1980, brown trout standing stock was 123 kg˙hectare−1(75%), brook trout 22 kg˙hectare−1˙(13%), and rainbow 19 kg˙hectare−1(12%), for a total of 164 kg˙hectare−1. Annual production in 1965 was 61 kg˙hectare−1 by brook trout (low owing to floods in 1965); annual production in 1979 (spring 1979 to spring 1980) was 132 kg˙hectare−1(70%) by brown, 25 kg˙hectare−1(13%) by brook, and 33 kg˙hectare−1(17%) by rainbow trout, for a total of 190 kg˙hectare−1. Mean annual precipitation, greater fluctuation in annual precipitation, notable single-day rainfall events, and occurrences of floods, erosion, and siltation all increased in 1965–1979 relative to the previous 10 years. These changes apparently were the cause of observed weak year classes of trout, decreases in invertebrate food production, and loss of cover for small trout. It is postulated that innate factors in the behavior of brook and brown trout, in interaction with the habitat perturbations, may have resulted in the replacement of brook by brown trout.