Hybridization between bull trout and brook trout across idahoperiod of performance: 4 30 2025 - 05 31 2027SUMMARY: bull trout is listed as a threatened species in the coterminous united states

This Cooperative Agreement award from the U.S. Geological Survey's Cooperative Research Units (CFDA 15.812) program provides $167,138.70 to the University of Idaho to investigate the hybridization between threatened bull trout and non-native brook trout in Idaho over a 2-year period from April 2025 to May 2027. The key objectives are to (i) evaluate the frequency, distribution, and habitat associations of bull trout-brook trout hybrids, and (ii) determine the direction of hybridization (i.e., whether bull trout are spawning with brook trout or vice versa) across multiple streams in Idaho. The project will utilize genetic analysis techniques to identify hybrids and assess the impacts of brook trout on bull trout populations, with the goal of informing conservation and recovery efforts for the threatened bull trout species.

Purpose

The overall purpose of this work is to provide information that can be used to assist in the delisting of bull trout. Specifically, the objectives are to (i) evaluate patterns in the frequency, distribution, and habitat associations of bull trout brook trout hybrids, and (II) evaluate patterns in the direction of bull trout brook trout hybridization across multiple ( 30 streams) bull trout streams in idaho.

Activities

Genetic analysis techniques will be used to estimate the number of hybrids in each population. The direction of hybridization (i.e., is a fishs mother a brook trout or bull trout) will also be investigated. The ratio of brook trout to bull trout within each stream and the SEX ratio of brook trout within occupied bull trout habitat will be used to determine the influence of brook trout on the number of hybrids. The location of hybrids in invaded streams relative to the location of brook trout and bull trout in each stream will be described.deliverables and expected outcomes: this project will provide information that can be used to assist in the delisting of bull trout by greatly improving understanding of the threat that brook trout pose to bull trout, how to assess threats, and how to manage brook trout on the ground. Information on the location and extent of hybridization will be used to identify brook trout populations that are a high priority for active management by state and federal resource managers with an overall goal of removing bull trout from the endangered species act. Quarterly reports will be provided to summarize project progress and outline short-term goals. An annual report will be provided at the end of each calendar year that provides guidance on areas that can be targeted for active management. Peer-reviewed scientific publications will be compiled into a final report. A copy of this final report along with all raw data and any other requested information will be provided to the USFWS.

Intended beneficiaries

This work will be conducted in direct collaboration with idfg and USFWS personnel. The work directly support efforts by USFWS and states (primarily idaho) to recover and delist bull trout. Considerable progress has been made in this effort over the last 5 years and this work will directly contribute to that progress.