Nonnative Brook Trout (Salvelinus fontinalis) have been introduced throughout western North America, where they can displace, outcompete, and hybridize with native trout and char. Traditional suppression methods such as electrofishing removal and chemical treatment are labor intensive, rarely achieve complete eradication, and can harm non-target species. Over the past decade, fisheries researchers have developed an alternative approach using MYY Brook Trout: hatchery-reared males that carry two Y chromosomes. When MYY males spawn with wild females, all resulting offspring are male, progressively skewing the population's sex ratio until females are eliminated and reproduction ceases.
As part of a multi-state effort coordinated by the Western Association of Fish and Wildlife Agencies, the Native Fish Investigations Section is conducting a large-scale field evaluation of the MYY approach. The study site is the Upper South Fork Sprague River, a headwater sub-watershed of the Klamath River Basin in southeastern Oregon, where nonnative Brook Trout occupy approximately 28 km of stream habitat. This study combines annual stocking of MYY Brook Trout with annual electrofishing removal of wild Brook Trout to accelerate the shift in sex ratio. We monitor MYY Brook Trout distribution and survival using PIT tag arrays and periodically collect tissue samples from wild age-0 Brook Trout to genotype and assess changes in sex ratio and ancestry. In 2021, the Bootleg Fire burned much of the study area; despite the intensity of the fire, riparian habitat remained relatively intact and the study continued.
From 2019 through 2025, we removed 68,080 wild Brook Trout and stocked 190,453 MYY Brook Trout into the study area. Monitoring indicates that stocked MYY fish are distributing beyond their release sites, which should facilitate spawning with wild females throughout the system. Preliminary genetic results show the proportion of males in the wild population increased from 51% before treatment to 61% after three years, and that approximately 10–18% of wild Brook Trout sampled in 2022 were offspring of MYY males. Tissue samples collected in 2025 are currently being analyzed to evaluate further changes in sex ratio and ancestry.