Cooperative Fish and Wildlife Research Units Program: Montana Fishery
Education, Research and Technical Assistance for Managing Our Natural Resources

Yellowstone Lake has been the site of intensive efforts to conserve native Yellowstone cutthroat trout Oncorhynchus clarkii bouvieri and restore natural ecological function since invasive lake trout Salvelinus namaycush were first discovered there in 1994 (Kaeding et al. 1996). Gillnetting was implemented in 1995 to suppress the lake trout population, but despite annual increases in gillnetting effort, the population expanded throughout Yellowstone Lake (Syslo et al. 2020). The lake trout population of Yellowstone Lake is highly resilient to gillnetting, probably because of high early life history survival. Interstitial embryo predators, which are a common source of embryo mortality in the native range of lake trout, do not inhabit Yellowstone Lake.

Cutthroat trout abundance declined precipitously in Yellowstone Lake until lake trout suppression efforts reached sufficient levels to reduce lake trout abundances in 2012 (Koel et al. 2020). The number of cutthroat trout caught during annual long-term gillnetting assessments varied subsequently, with mean catch-per-unit-effort (CPUE) ranging from a low of 12.5 per 100-m net night in 2011 to highs of 27.3 and 26.4 in 2014 and 2018, respectively. Lake trout predation was also associated with a long-term shift in cutthroat trout lengths from dominance by small (100–280 mm) and midsized (290–390 mm) individuals to dominance by large individuals (400+ mm) in annual gillnetting assessments. The mean CPUE of small and midsized cutthroat trout declined from 18.6 per 100-m net night and 15.1, respectively, in the 1980s to just 6.9 and 3.9, respectively in the 2010s. Concurrently, the mean CPUE of large cutthroat trout nearly doubled, from 7.5 in the 1980s to 14.6 in the 2010s. Lake trout also caused increases in individual weights and condition of cutthroat trout. The average weight of midsized and large cutthroat trout increased from 408.0 g and 682.8 g, respectively, in the 1980s to 463.4 g and 1418.6 g, respectively, in the 2010s. Relative weights (condition factors) of individual cutthroat trout also increased during this period. Mean relative weights of small, midsize, and large cutthroat trout were 58.8, 56.5, and 55.8, respectively, in the 1980s and increased to 68.4, 70.4, and 67.7, respectively, in the 2010s. Lower densities of cutthroat trout with higher individual weights and conditions should have higher fecundity, which should aid further recovery.

Cutthroat trout abundance of all sizes remains below recovery benchmarks, but large individuals (> 400 mm) have become more abundant and individual weights have doubled, probably because of reduced density. A large (400+ mm) cutthroat trout in 2020 weighs twice what it did in 1980, prior to the lake trout invasion. Currently, the benchmarks for cutthroat trout recovery described in the 2010 conservation plan are all based upon abundance, and include gillnet CPUE, angler catch per hour, and spawner counts in streams (Koel et al. 2010). There is strong evidence that annual growth of cutthroat trout individuals greatly increased following the lake trout-driven decline in the cutthroat trout population. Existing recovery benchmarks do not account for shifts in individual growth. There is a need to examine patterns in cutthroat trout growth relative to periods of lake trout invasion over the past four decades on Yellowstone Lake. Results will be used to refine cutthroat trout recovery benchmarks to account for shifts in growth, greater individual weights, and overall population biomass.