Treating Baker Lake to Restore Bonneville Cutthroat Trout

First published in The Midden*, Great Basin National Park's newsletter, Vol. 21, No. 2, Winter 2021. By Joseph Danielson, biological science technician.*

The Back Country Horsemen of Nevada, High Desert Chapter, were vital in getting gear to and from Baker Lake. NPS photo by Joseph Danielson.

In August and September 2021, Great Basin National Park's resource management staff and the Nevada Department of Wildlife treated Baker Lake with the fish poison rotenone to remove the non-native brook trout and Lahontan cutthroat trout stocked decades ago. It is part of a larger effort to conserve Bonneville cutthroat trout in their native range by creating new populations in Johnson and Baker lakes — which, if it works, could make the species more resilient to global climate change.

Early summer: watching for spawning

Staff began with snorkel surveys for Lahontan cutthroat spawning. Cold water and high elevation make long spells in dry suits hard, but over several trips they saw aggression, signs of fighting and redd (nest) building — all signs of spawning.

Redds mattered most: cutthroat trout usually bury their eggs in cold, flowing streams, not lakes. If Lahontan cutthroat can spawn in a lake with no flowing water, Bonneville cutthroat should be able to as well.

Getting ready for a snorkel survey at Baker Lake. NPS photo by Joseph Danielson.

Measuring the lake

Baker Lake's level changes through the year — highest in spring after snowmelt, lowest in late summer and autumn, depending on rain and snow — so its volume at treatment time had to be calculated to know how much rotenone to use.

• Bathymetry. A week beforehand, staff surveyed the lake's three distinct water bodies on a 5-metre grid, measuring depth precisely to work out the volume of each, so the same concentration of rotenone could be applied throughout.
• Bioassay. The same week, they caught 25 brook and Lahontan cutthroat trout by angling and held them for eight hours in buckets with rotenone from 0 to 2 parts per million, recording their reactions at set intervals. That gave the minimum effective dose, the total needed for the lake, and a baseline for checking how much rotenone remained afterwards.

The Back Country Horsemen of Nevada, High Desert Chapter, volunteered horses, mules and people to pack hundreds of pounds of gear up to the lake and back — without them it would have taken many days and many people on foot.

Staff surveying the lake bed to calculate the treatment. NPS photo by Joseph Danielson.

The treatment

On September 1, 2021, park and state staff began:

1. The lake was low, and its talus bed held a lot of water between the rocks that rotenone from the open lake would not reach, so staff treated that first on foot with pump sprayers.
2. For the main lake, they used an inflatable Zodiac with a trolling motor to spread rotenone evenly.

Afterwards they counted 301 dead fish — with the 25 from the bioassay, 326, and probably more that were missed. About 13 percent were Lahontan cutthroat.

Checking afterwards

Rotenone breaks down quickly, and only a low concentration was used, but staff returned a week later to check. They carried lake water four miles down to the nearest fish, in Baker Creek, for bioassays; the fish showed only non-lethal effects, meaning the rotenone was breaking down as expected.

What comes next

Rotenone does not kill fish eggs, and because brook and Lahontan cutthroat trout spawn at different times, some eggs may have been fertilised and buried before the treatment. So the park will:

• survey with gill nets and minnow traps in 2022 for surviving fish;
• treat the lake again in 2022;
• release Bonneville cutthroat trout into Baker Lake in 2023.

Sources

• National Park Service, Great Basin National Park, "Treatment of Baker Lake to Restore Bonneville Cutthroat Trout," The Midden, Vol. 21, No. 2, Winter 2021. https://www.nps.gov/articles/000/treatment-of-baker-lake-to-restore-bonneville-cutthroat-trout.htm
• Rewritten in hubnx's own words.