Rotenone

The paucity of conclusive, scientifically defensible results for piscicide effects on non-target organisms such as macroinvertebrates complicates fisheries management. Therefore, to provide comprehensive recommendations regarding the effects of piscicides on macroinvertebrates, the Bureau of Land Management/Utah State University National Aquatic Monitoring Center collaborated with the U.S. Forest Service and Utah Division of Wildlife Resources to systematically study a basin-wide rotenone treatment within the Middle Fork Sheep Creek Watershed. Specifically, we assessed the effects of rotenone concentrations on zooplankton in five treatment lakes and benthic macroinvertebrates in six treatment streams using a before-after-control-impact study design to answer the following questions:

1. Are there short-term or long-term effects of rotenone treatments on zooplankton and macroinvertebrate assemblages and are there additive or multiplicative effects of successive treatments?
2. Do stream and lake assemblages respond differently?
3. Are some taxa more susceptible to rotenone than others?
4. Does rotenone concentration and duration explain variability in responses?

Rotenone concentrations (1 – 250 ppb) and durations (>39 days in lakes and > 10 days in streams) exceeded targeted applications outlined in both the rotenone treatment plan and best-management practices. Additionally, concentrations were highly variable among years, lakes, and streams despite intentions to treat at similar levels. For example, rotenone persisted 29 days longer in treated lakes in 2013 than it did in 2012. This could be due to many factors including the difficulty of applying basin-wide treatments, but rotenone mixing and persistence was also likely dependent on environmental variables such as temperature, which affects rotenone degradation and lake mixing. Rotenone concentrations and persistence was also effected by system type, with lakes having higher rotenone concentrations and durations than streams.

Benthic macroinvertebrate and zooplankton assemblages exhibited significant responses to the successive, multi-basin treatments, although responses varied among systems and analyzed indicators. For example, we observed significant reductions in total stream macroinvertebrate richness (3 taxa on average). Additionally, assemblage composition changed in both streams and lakes, with long-term reductions (2 years post-treatment) in Ephemeroptera, Plecoptera, and Trichoptera (EPT) taxa in streams and Daphnia in lakes. In contrast, effects of rotenone treatments on total macroinvertebrate and zooplankton densities were minimal or only short-term (<1 month). Past studies have also found little effects on whole community metrics such as total density, but have found large effects on specific taxa such as EPT and Cladocerans. Responses of individual taxa in this study also largely agree with available toxicity literature, but rotenone concentrations largely did not explain variability in responses among streams or lakes.

Observed assemblage shifts have the potential to affect ecosystem function for both streams and lakes. Reductions in EPT could reduce preferred food resources for drift-feeding fishes in streams such as cutthroat trout and reductions in the diversity of the food base could reduce system resistance and resilience to disturbance. Reductions in lake Daphnia likely led to a competitive release of Rotifers. The increase in Rotifers in turn could reduce rates of algal grazing, lead to algal blooms, and affect nutrient cycling or water clarity.

Lakes appeared to recover quicker than streams possibly due to faster life cycles of zooplankton compared to benthic macroinvertebrate and also possibly due to more refugia in lakes than streams. Additionally, recovery in streams may have been slower due to the basin-wide treatment, which likely removed drifting macroinvertebrates as a source of colonists, and recovery was likely more dependent on trans-basin dispersal. Recovery of streams and lakes may have been delayed by successive rotenone treatments, but the treatments did not appear to have additive or multiplicative effects on zooplankton or macroinvertebrate assemblages.

Managing treatment timing, applied concentrations and durations, and extent of area treated are the best ways for managers to mitigate non-target impacts. To minimize long-term impacts in future studies, we recommend the following: 1) Applying rotenone concentrations between 25-50 ppb as suggested by Finlayson et al. 2010a; 2) Making sure all personnel are properly trained and application of rotenone is as accurate to the treatment plan as possible; 3) Measuring applied rotenone concentrations and relevant environmental characteristics that effect dispersal and persistence such as flow and temperature; 4) Keeping some fishless, headwater reaches untreated to provide a source for colonists in streams. Lastly, our ability to predict rotenone impacts on streams and lakes remain limited with variable responses among systems, taxa, years, and applied concentrations. This uncertainty is particularly concerning as treatments are scaled up to multiple basins with sensitive taxa such as EPT. Therefore, we recommend conducting more research on the physical habitat influences on rotenone persistence and macroinvertebrate recovery and the long-term impacts of rotenone on ecosystem functions.