Development of a standardized monitoring protocol to assess the efficacy of nonnative fish suppression in the Lamar River Watershed, Yellowstone National Park

Abstract

Hybridization between native Cutthroat Trout and introduced Rainbow Trout is pervasive throughout western North America, and this hybridization has resulted in reduced abundances and range contractions of native Cutthroat Trout subspecies. In the Lamar River watershed in Yellowstone National Park, Yellowstone Cutthroat Trout x Rainbow Trout hybrids are abundant in the lower Lamar River watershed because of past stocking efforts. To mitigate the threat of hybridization in the Lamar River watershed, the National Park Service has acted to remove Rainbow Trout and hybrids and block the upstream movement of these nonnative taxa into the upper watershed. A standardized monitoring protocol is desired to assess the response of fish populations to these management actions and to monitor existing populations of Yellowstone Rocky Mountain Cutthroat Trout. We evaluated the efficacy of electrofishing, snorkeling, and angling to estimate the absolute abundances and catch-per-unit-effort of mixed-stock aggregations of trout in main-stem locations. We also evaluated the efficacy of targeted, juvenile sampling to estimate the abundances of juvenile trout of each taxon. We also assessed the accuracy and limitations of a field-identification key developed for differentiating among Yellowstone Rocky Mountain Cutthroat Trout, Rainbow Trout, and Yellowstone Rocky Mountain Cutthroat x Rainbow Trout hybrids in the Lamar River watershed. We observed agreement between the proportion of nonnative trout sampled by electrofishing and snorkeling in Slough Creek. Catch-per-unit effort estimates were highly variable for Yellowstone Rocky Mountain Cutthroat and nonnative trout, but error was reduced when minimum section lengths of 3,600 m in Slough Creek and 2,200 m in the Lamar River were sampled. In both streams, error in mark-recapture estimates was reduced when angling was incorporated. Statistical power to detect declines in Yellowstone Cutthroat and nonnative trout abundance was low for CPUE and mark- recapture surveys unless 50% declines occurred for both taxa. Electrofishing surveys are not feasible for estimating the abundance of juvenile trout because of high error in taxa identification (30% correct-identification rate). However, we differentiated among adult fish with high rates of accuracy (98%). These results will guide future long-term monitoring of trout populations in the Lamar River watershed.