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:

Indian Creek Population

South Fork Tieton Population

North Fork Tieton Population

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.

Early 90s helicopter rescue of bull trout in shrinking rimrock pool. Transfer to Clear Lake? Find Report.

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.

Shared Threats

MOVE TO FMO SECTION?

Rimrock Lake bull trout are entrained in the outlet works of Tieton Dam. The population-scale impact of entrainment on the Indian Creek population or either of the other two adfluvial populations residing in the lake has not been quantified. Estimates have been derived on the number of fish entrained. Three years of entrainment monitoring occurred in the early 2000s. James (2002b) estimated that the number of bull trout entrained ranged from 46–87 in 2001 and Hiebert et al. (2002; 2003) estimated the entrainment loss to be 145 bull trout in 2002 and 140 in 2003. Underwood and Cramer (2007) used data from these entrainment studies, as well as James’ population data to create a life cycle model that was used to simulate effect of entrainment on long-term population dynamics. Any updated entrainment data?

During the 2005 construction of the Tieton hydroelectric project a fish salvage effort was conducted; 37 bull trout were captured in the stilling basin directly below Tieton Dam (Ackerman 2005). Genetic samples were collected from these fish, and 11 were assigned to the Indian Creek population (Small et al. 2009). Additional evidence of entrained bull trout surviving and migrating within the Yakima Basin comes from a single fish that was captured and sampled at Roza Dam in 2004. This fish genetically assigned to the Indian Creek population (see Genetics Baseline). Courter and Cramer subsequently reported on higher survival estimates for entrained bull trout post-construction of the hydroelectric project, with estimated mortality reduced from 85% to 45% (Courter and Vaughan 2011).