Long-term effects of rotenone on the assemblage of aquatic benthic macroinvertebrates at a local level of disturbance

Abstract

The piscicide rotenone is an important and widely used tool for the conservation of native fishes, but comprehensive evaluations of aquatic benthic macroinvertebrate community response to rotenone exposure are lacking. Therefore, this study investigates the effect of rotenone on aquatic benthic macroinvertebrate (BMI) communities within the context of a watershed-scale restoration effort for westslope cutthroat trout (Oncorhynchus clarkia lewisi) conducted between 2007 and 2017. Long term pre- and post-treatment sampling in outlet streams of 13 alpine lakes quantified the response and recovery of BMI exposed to rotenone for periods ranging from weeks to a few months depending on the lake volume and environmental factors that influence the natural neutralization of rotenone. Spatial and temporal quantitative sampling was conducted at the lake outlets for 1 to 6 years before treatment and 2 to 8 years after treatment to capture the variability in density and diversity of the BMI assemblage. Analysis of changes to Ephemeroptera, Plecoptera, Trichoptera (EPT) density and diversity were used as a suitable representative for assessing effects to the entire BMI assemblage. Analysis of variance in EPT density and location shifts in variance in EPT diversity were used to test for significant changes to the EPT assemblage. Significant short-term reductions to EPT density were found 1 year post-treatment at 3 of the 13 lakes, 2 years post-treatment at 1 lake, and no significant reductions found at lakes with 3 or more years of post-treatment monitoring. Significant negative shifts in EPT diversity were found at 2 lakes with only 2 years of post-treatment monitoring, and no significant negative shifts in EPT diversity were found at lakes with 3 or more years of post-treatment monitoring. In all of the study sites, EPT density and diversity returned to baseline conditions within 3 years after treatment, demonstrating resiliency of these taxonomic communities to rotenone exposure.