When Eradication is not an Option: Modeling Strategies for Electrofishing Suppression of Nonnative Brook Trout to Foster Persistence of Sympatric Native Cutthroat Trout in Small Streams

Abstract

Subspecies of inland cutthroat trout Oncorhynchus clarkii occupy a fraction of their historic ranges, and displacement by nonnative brook trout Salvelinus fontinalis is among the greatest threats to existing populations in small, headwater streams. Electrofishing is often used to suppress brook trout and enhance cutthroat trout populations, but these efforts are labor intensive and costly. To help managers more effectively plan and implement brook trout control programs, we used survival estimates from a field experiment to construct matrix population models for both species and linked the models by making the vital rates of young cutthroat trout a function of brook trout density to represent the effect of invasion. We then explored the response of cutthroat trout populations (growth rate [λ] and probability of persistence for 50 years) to brook trout suppression across various levels of electrofishing frequency, sampling intensity, capture efficiency, and brook trout immigration rate. Cutthroat trout (λ = 1.10) and brook trout (λ = 1.20) both had moderate to high values of λ when biotic interactions were weak or absent, but the cutthroat trout λ declined markedly (λ = 0.61) when biotic interactions with brook trout were strong. Model results supported the hypothesis that rapid displacement of cutthroat trout is facilitated by the brook trout's inherent demographic advantage and biotic effects on young cutthroat trout. Where annual brook trout suppression was not possible, the temporal distribution of suppression events within a systematic control design strongly influenced the response of cutthroat trout. The absolute response and cost effectiveness of suppression efforts were influenced particularly by the brook trout immigration rate and also by removal efficiency. Multiple consecutive years of suppression (≥3 years) benefited cutthroat trout, but to be effective such suppression should not be interrupted for more than two consecutive years once initiated. Eradication would be ideal, but the maintenance control of brook trout and the management of cutthroat trout in sympatry with brook trout will probably remain among the viable management options for the foreseeable future.