Brook Trout population response to Brown Trout removal by electrofishing in a Wisconsin Driftless Area stream

Abstract

Populations of Brook Trout Salvelinus fontinalis have declined across their native range in North America due to a combination of habitat loss, exploitation, and introductions of nonnative salmonids. Brown Trout Salmo trutta have been widely introduced into streams across Brook Trout's native range and likely displace Brook Trout from suitable habitat. We evaluated the population response of Brook Trout to Brown Trout removal in Maple Dale Creek, a Wisconsin Driftless Area stream, relative to a nearby control stream with similar habitat and sympatric Brook Trout and Brown Trout. Variation in mean July stream temperature (11.6–16.4°C) among survey sites also allowed us to examine whether summer stream temperature was related to the population response of Brook Trout to Brown Trout removal.Between 2019 and 2023, we completed 33.7 km of single‐pass electrofishing during 56 site visits and removed 20,495 Brown Trout from 7.1 km of Maple Dale Creek upstream of an existing fish passage barrier. Concurrently, annual salmonid population estimates were completed on Maple Dale Creek and a nearby reference stream.By 2023, biomass and density of Brown Trout age 1 and older (age 1+) had decreased to 1% or less of 2019 levels. Concurrently, age‐1+ Brook Trout biomass increased by a factor of 5.5 and density by a factor of 9.7. In contrast, age‐1+ Brook Trout density and biomass in our control stream remained low and relatively unchanged. Brook Trout population response to Brown Trout removal was positive across all four sites but exhibited a positive correlation to mean July stream temperature.Our results add to the evidence that Brown Trout displace Brook Trout populations from stream habitat, highlight the utility of electrofishing removal of Brown Trout in restoring Brook Trout populations where downstream fish passage barriers are present, and suggest that Brown Trout suppression of Brook Trout is positively related to summer stream temperature.