Treatment of Baker Lake to Restore Bonneville Cutthroat Trout

This article was originally published in The Midden – Great Basin National Park: Vol. 21, No. 2, Winter 2021.

The Backcountry Horsemen of Nevada - High Desert Chapter were instrumental in helping get gear to and from Baker Lake.

NPS Photo by Joseph Danielson

By Joseph Danielson, Biological Science Technician

In August and September of 2021, Great Basin National Park resource management staff collaborated with the Nevada Department of Wildlife (NDOW) to conduct a rotenone treatment in Baker Lake to remove the non-native Brook Trout and Lahontan Cutthroat Trout (LCT) that were introduced decades ago. The treatment was conducted as part of a larger initiative for the conservation of Bonneville Cutthroat Trout in their native range by establishing new conservation populations in Johnson and Baker Lakes. If successful, this project may increase the species resilience against the impacts of global climate change.

This year’s project started in the early summer with snorkel surveys in Baker Lake for LCT spawning behavior. Cold water and high elevation make using dry suits for extended periods of time challenging. Neverthess, Park staff made several excursions to the lake and observed instances of aggression, signs of fighting, and redd (nest) building, all of which are indicators of spawning.

Observing established redds or redd building was a primary goal during these spawning surveys because cutthroat trout typically bury their eggs in redds in cold, flowing streams, not in lakes. We think that since LCT have found a way to spawn in a lake with no flowing water then Bonneville Cutthroat Trout should find spawning success as well.

Getting ready for a snorkeling survey at Baker Lake

NPS Photo by Joseph Danielson

The water level at Baker Lake varies throughout the year with the highest lake volume occurring in the spring following snow melt and the lowest levels in the late summer and fall, depending on seasonal precipitation and the persistence of the previous year’s snowpack. Calculating Baker Lake’s volume at the time of the treatment was a necessary step to determine the amount of piscicide needed to effectively treat the lake.

One week before the treatment, Park staff conducted bathymetric surveys of the three distinct water bodies that make up Baker Lake. During these surveys, staff made a 5-meter grid of the lake and then accurately measured the depth of the lake to determine the lake’s total volume and the volume of water in each distinct section of the lake. This allowed staff to apply a consistent concentration of rotenone throughout all sections of the lake.

During the same week, 25 Brook Trout and LCT were collected via angling to perform a bioassay. The fish were placed in buckets of waterwith graduated levels of rotenone mixed from 0 to 2 parts per million of product. The fish were monitored for eight hours, and observations were recorded at set time intervals to see how they reacted to varying levels of rotenone. This told us what the minimum effective concentration of rotenone was, the total amount of rotenone needed to treat the lake, and gave us a baseline to determine how much rotenone was still in the lake water after the treatment.

Once this preliminary work was completed, the Backcountry Horsemen of Nevada - High Desert Chapter volunteered horses, mules, and personnel to haul hundreds of pounds of gear up to Baker Lake before the treatment and back down afterwards. Without their help it would have taken many days and people to hike everything needed to the lake.

On September 1, 2021 Park and NDOW staff began the multifaceted rotenone treatment. The lake’s low water level and talus substrate meant that there was a large amount of interstitial water that would not be reached by diffusion of rotenone from the main body of the lake. This area was treated first on foot with pump sprayers. To treat the main body of the lake, we used an inflatable Zodiac boat with a trolling motor to distribute rotenone evenly throughout the lake.

Following the treatment, we surveyed for fish carcasses and counted 301 fish. These fish, along with the 25 used during the bioassay bring the total to 326, but there were likely more fish that were missed during counts. Approximately 13% of fish observed were LCT.

Although rotenone decays rapidly in the environment and a low concentration was used during the treatment, we returned the following week to monitor the rotenone level in the lake. To check rotenone levels, Park staff hiked Baker Lake water four miles down to the nearest fish in Baker Creek to conduct bioassays. Fish tested during these follow-up bioassays exhibited signs that corresponded to non-lethal levels of rotenone, which indicated that the rotenone was breaking down as expected.

Staff performed bathymetry surveys to make calculations for the rotenone treatment.

NPS Photo by Joseph Danielson

Rotenone does not affect fish eggs, and because Brook Trout and LCT spawn at different times of year, it is possible that eggs could have been fertilized and buried before the treatment. Park staff will conduct validation surveys in 2022 with gill nets and minnow traps to look for surviving fish. Another rotenone treatment will be done in 2022 due to the possibility of fertilized eggs hatching after the 2021 treatment. Bonneville Cutthroat Trout are scheduled for release into Baker Lake in 2023.