Evaluating the effectiveness of non-native brown trout suppression to improve native white-spotted charr stocking

The densities of existing fish populations in streams may affect salmonid stocking programs. Non-native brown trout (Salmo trutta) now occur in rivers in Japan that are managed via stocking programs to restore recreational fisheries for native white-spotted charr (Salvelinus leucomaenis), but how they affect charr stocking programs needs to be better understood. We investigate how suppressing populations of brown trout affects the density and biomass of stocked charr population sizes and biomasses. We report no significant change in charr abundance or biomass after stocking into areas in which brown trout are abundant. However, following suppression of brown trout, charr significantly increased in numbers and biomass, and their settlement rates rose in excess of 10 times. We suggest that brown trout outcompete charr for habitat and food resources, and that stocking charr in streams in which brown trout populations exist is unlikely to be effective in the management and restoration of recreational fisheries, but that stocking them into streams in which brown trout have been suppressed is more likely to be effective.