No evidence of sustained recovery of native trout in response to angling suppression of invasive Brook Trout

Abstract

AbstractObjectiveNonnative fish invasions have had widespread impacts on freshwater ecosystems, including effects on native fish biodiversity and persistence. Brook Trout Salvelinus fontinalis were first introduced into the Elbow River watershed (Alberta, Canada) in the 1940s. They have since become established in Quirk Creek, and they dominated the fish community by the mid‐1990s, raising concern about the native populations of Westslope Cutthroat Trout Oncorhynchus clarkii lewisi and Bull Trout S. confluentus.MethodsA targeted angling program was operated from 1998 to 2015, along with limited electrofishing removals, to suppress the Brook Trout population. We used 25 years of fish monitoring data from 1978 to 2020 to evaluate the program's effectiveness for reducing the Brook Trout population and the program's consequences for native trout.ResultDensities of Brook Trout larger than 150 mm declined after the onset of the suppression project, and the decline was attributed to removals through angling. However, Brook Trout recruitment remained comparable to presuppression levels. Westslope Cutthroat Trout recruitment increased during and after Brook Trout suppression. Densities of Westslope Cutthroat Trout larger than 150 mm increased during the suppression period but did not reach density goals targeted for recovery of the species. Bull Trout remained at very low densities throughout the suppression project.ConclusionThe lack of native trout recovery during the suppression project was hypothesized to result from (1) incidental release mortality of native trout, (2) Brook Trout suppression that was insufficient to prompt an effective response in native trout populations, or (3) a combination of these factors. Continued low densities of Brook Trout larger than 150 mm and native trout after the end of the suppression project (when harvest and incidental release mortality were alleviated) may point to some other factor impacting the recovery of trout larger than 150 mm, particularly Westslope Cutthroat Trout, since recruitment was at its highest during this period. Overall, angling was not considered an effective method for promoting native trout recovery, and other techniques should be pursued depending on management goals.