Novel Fish Communities: Native and Non-Native Species Trends in Two Run-of-the-River Reservoirs, Clark Fork River, Montana

Abstract

Long-term (1955–2011) trends in fish community structure were investigated in Noxon and Cabinet Gorge Reservoirs, Montana, based on the analysis of gillnet catches since the construction of the reservoirs. Results of gillnetting show significant declines in several forage species in absolute and relative abundance since the mid-1990s. Catches have declined for forage species as a group and for individual species such as non-native yellow perch Perca flavescens, native suckers Catostomus spp., and native peamouth Mylocheilus caurinus. In contrast, the aggregate catches of the non-native predators increased greatly as a percentage of the total catch since the 1990s, including increases in catches of walleye Sander vitreus, and northern pike Esox lucius. As a result of the non-native species increases, species diversity has, overall, trended upward whereas species dominance based on the three most abundant species has decreased, as previously abundant forage species have been reduced and greater evenness has resulted. Overall, the fish community in the reservoirs has shifted from a natural, seasonally coldwater riverine system (pre-impoundment) to a modified, coldwater-managed (i.e., salmonid stocked) system in the 1950s through the 1970s to a coolwater system dominated by forage species in the 1980s and early to mid-1990s, and, most recently, to a coolwater system more strongly structured by non-native piscivores. This novel (i.e., native-non-native) community poses some difficult issues for managers on which species to proactively manage for. Additional effort is needed to assess benefits and costs of proactively managing for non-natives as part of successful, comprehensive management plans.