Rotenone has been used for centuries by indigenous peoples of Southeast Asia and South America to catch fish for human consumption. Several plants in the legume (pea) family contain rotenone. The plants are crushed and introduced into water. The rotenone interferes with cellular respiration so that affected fish rise to the surface to gulp air, where they are easily caught.

The potential harmful effects of rotenone on other aquatic species have been well tested, and it has been used for the management of fish populations in North America since the 1930s. In Montana, over 20 lakes and streams in the South Fork of the Flathead River have been treated with rotenone and successfully restocked with westslope cutthroat trout by Montana Fish, Wildlife and Parks (MFWP). Monitoring by MFWP has indicated that the aquatic invertebrate communities quickly recovered from any harmful effects. In addition, non-native Yellowstone cutthroat trout were eradicated successfully with rotenone from Evangeline and Camas Lakes on the west side of Glacier National Park.

Tens of millions of non-native trout were introduced into Glacier National Park between 1910 and 1970. This was done in spite of the National Park Service’s Organic Act of 1916, which calls for the preservation of the native biota of National Parks. However, early acceptance of sport fishing as a legitimate use of park resources created a management dilemma regarding the artificial enhancement of fishery resources. Gunsight Lake was originally fishless because of a downstream waterfall which blocked upstream migration. It was stocked between 1916 and 1936 with over 200,000 hatchery rainbow trout from California.

Rainbow trout represent an insidious threat to native westslope cutthroat trout because they hybridize with them. The offspring from such hybridization have lower survival rates than the native westslope cutthroat trout. Nevertheless, hybridization has spread rapidly because hybrids are more likely to disperse than native trout, and they thereby extend the hybridization to other populations. Today, less than 10% of the native range of westslope cutthroat contains non-hybridized populations.

Headwater lakes harboring rainbow trout, such as Gunsight, represent an ongoing threat because rainbow trout can swim downstream and hybridize with westslope cutthroat. There are currently very few non-hybridized populations of westslope cutthroat east of the Continental Divide.

The primary threat to bull trout is climate change. Gunsight Lake is a potentially valuable habitat for bull trout because its high altitude provides the cold-water habitat necessary for bull trout.

There are valid reasons to restore Gunsight Lake to its historical fishless condition. For example, this lake also provides important potential habitat for three native amphibians: long-toed salamanders, western toads, and Columbia spotted frogs. Surveys have found that long-toed salamanders are less likely to breed if trout are present. However, no such effects of trout have been found on either western toads or Columbia spotted frogs.

The overall threat of native trout populations to other species in Gunsight Lake is minimal. In contrast, establishment of secure populations of westslope cutthroat trout and bull trout in Gunsight Lake would be an important step in the recovery of these species on the east side of the Continental Divide.