Brook Trout (Salvelinus fontinalis) as affected by interspecific competition with non-native Brown Trout (Salmo trutta) following a stream fish community manipulation in the Wisconsin Driftless Area.

Abstract

Brook and brown trout are the two dominant trout species that inhabit Wisconsin Driftless Area streams. Brook Trout are native within this area, whereas Brown Trout were introduced in the late 1800s. There are widespread concerns about Brown Trout negatively impacting the populations of Brook Trout. This study aimed to investigate the effects of interspecific competition in two streams before and after modifying the fish community in one of the streams. Approximately 20,000 adults/YOY Brown Trout were removed from the treatment stream during the study period. Following Brown Trout removal, Brook Trout proportion of CPUE and relative biomass increased by 275% and 289%, respectively. Additionally, Brook Trout population size structure increased, while Brown Trout populations decreased, thus alleviating interspecific competition. Stable isotope analysis of the food web within the treatment and reference streams revealed minimal differences. In both streams, a two-source mixing model predicted that Slimy Sculpin and YOY brook and brown trout were the primary food source for both trout species. Analyses of stream water temperatures, habitat, and macroinvertebrates showed no changes. Interspecific competition, where Brown Trout holds the advantage, potentially contributes to Brook Trout's population and size structure reduction within many Wisconsin Area streams.