Genetics Clarify Non-Native Threat to Lahontan Cutthroat Trout Reintroduction in Fallen Leaf Lake, California

Abstract

Non-native introductions complicate native species recovery, and documenting successful native species reintroductions remains an ongoing challenge. Brown trout (Salmo trutta) pose a piscivory threat to native Lahontan cutthroat trout (LCT; Oncorhynchus clarkii henshawi) while rainbow trout (O. mykiss) and LCT readily hybridize. Brown and rainbow trout are present in Fallen Leaf Lake and the upper Glen Alpine Creek watershed which drains into the lake. Therefore, the presence of these species within this drainage may represent a significant threat to LCT reintroduction. Using polymorphic microsatellites and the Bayesian clustering program STRUCTURE we evaluated the genetic population structure of naturalized rainbow and brown trout throughout the Glen Alpine Creek watershed. Results indicated genetic connectivity between the upper drainage and Fallen Leaf Lake, and identified a self sustaining subpopulation of rainbow trout persisting in Fallen Leaf Lake. We tested for hybridization where rainbow and cutthroat trout were sympatric using species specific biparentally inherited markers to evaluate redds and YOY. We documented limited hybridization which resulted in adaptive management by USFWS and the placement of weirs. Our results suggest that establishment of a self-sustaining lacustrine population of Lahontan cutthroat trout in Fallen Leaf Lake is possible, but confounded by the presence of non-native fishes.