Rimrock Lake Bull Trout Population Group
Overview
The Rimrock Population Group consists of the three bull trout populations that occupy tributaries to Rimrock Lake. Detailed information on each of these can be found here:
These three populations all make use of shared foraging, migration and overwintering (FMO) habitat in Rimrock Lake, and historically would have used downstream habitat in the Tieton and Naches watersheds. Information on conditions, threats and actions for this shared FMO habitat are detailed in the Rimrock Lake FMO page.
Population Genetics
The three Rimrock local populations are moderately genetically differentiated from one another (M. P. Small et al., 2009, p. 13). The level of differentiation within Rimrock is high considering the close proximity of the three spawning tributaries, shared usage of Rimrock Lake as FMO habitat, historic lack of barriers between all three tributaries, and present-day lack of barriers between South Fork Tieton River and Indian Creek. These findings have been broadly interpreted to support the conclusion that there is high site fidelity, and correspondingly, low migration between populations and demographic independence. However, metrics of genetic connectivity are easily misinterpreted as indicators of demographic connectivity when population sizes are large or small (Gagnaire et al., 2015, p. 770; Lowe & Allendorf, 2010, p. 3042). Populations can maintain high differentiation despite substantial migration in the case of low effective population size, and while natal site fidelity is high within Rimrock, it is not complete. Small numbers of individuals collected in each of the Rimrock spawning tributaries originate from one of the other Rimrock populations and other individuals demonstrate admixture suggestive of past gene flow among Rimrock populations (M. Small et al., 2011, p. 17; M. P. Small & Martinez, 2011, p. 6).
Connectivity and thus the potential for genetic exchange with downstream populations in the Naches River fluvial system was reduced by the construction of Tieton Dam in 1925. Recent work by USFWS (Beebe et. al 2024) suggest entrainment continues to occur in the Yakima Basin, however it is more well-documented in Upper Yakima when compared with Tieton dam. Although recent trap and haul efforts have not yielded a high sample size of bull trout captured below Tieton dam, Small et. al 2009 documented one-way gene flow out of Tieton basin into the nearest tributary, Rattlesnake Creek. It is possible entrained bull trout may be spawning with other fluvial populations in the Naches river system but the rate of downstream gene flow has not been well characterized. The dam has eliminated upstream passage and genetic contributions to Rimrock Lake populations from Naches fluvial populations.
Population Monitoring
In a 2008 study by Mizell and Anderson, five adult bull trout were captured and radio-tagged in the N.F. Tieton River between Clear Lake and Rimrock Reservoir. An additional twelve Bull Trout were captured and tagged from the stilling basin below Tieton Dam during a fish salvage operation. All salvaged fish from below the dam were transported 22 miles downstream of Tieton Dam and released near the Wapatox Irrigation Diversion on the Naches River. The tagged N.F. Tieton Bull Trout migrated downstream and overwintered in Rimrock, and made spawning migrations toward Clear Creek Dam and into Indian Creek the following year. Radio tagged bull trout that were transported downstream to the Naches River primarily moved up the Naches toward the American River and Rattlesnake Creek. One bull trout attempted to go back up Tieton River but turned around and went back to the Naches river. The authors suggested that these fish essentially integrated within the existing fluvial populations in the Naches River. During an earlier bull trout tagging operation (Mizell et al. 2008) bull trout were tagged and immediately released below Tieton Dam. These fish did not leave the area, but instead they stayed in or near the stilling basin.
Early 90s helicopter rescue of bull trout in shrinking rimrock pool. Transfer to Clear Lake? Find Report.
Shared Threats
Find shared threats to the Rimrock Bull Trout Populations under the Rimrock Lake FMO Habitat page.