YY Male Brook Trout Stocking and Population Monitoring in Tyee Springs

Tyee Springs is located directly upstream of Carson National Fish Hatchery and is home to a thriving population of nonnative Brook Trout. This population has been a hatchery management concern and past attempts at suppressing or eradicating the Brook Trout population have been unsuccessful In 2020, the Columbia River Fish and Wildlife Conservation Office, Abernathy Fish Technology Center and Carson National Fish Hatchery initiated a long-term project using YY male Brook Trout as a tool to eradicate nonnative Brook Trout in Tyee Springs. This method involves producing male Brook Trout with two Y-chromosomes which are then released into the population targeted for eradication. Offspring of YY males (Myy) and resident females (XX) are all male (XY), so over time, the population becomes skewed toward a single sex, leading to extirpation of the target population. To maximize the effectiveness of this method, annual population suppression (i.e., fish removal) is recommended prior to stocking Myy Brook Trout. Population suppression and Myy stocking in Tyee Springs began in 2020. We removed a total of 1,145 Brook Trout in 2020 and 2,314 in 2021, which was estimated to be 15% and 46% of the population respectively. An electrofishing tow-barge was the most effective capture method in all three habitat units, capturing 74% of total Brook Trout in 2020 and 88% in 2021. A total of 600 age-1 Myy Brook Trout were stocked into Tyee Springs in 2020. All fish were adipose clipped and PIT tagged prior to release. In 2021, 1,923 age-0 Myy Brook Trout were stocked into Tyee Springs. Fifty-two percent of Myy individuals (999) were PIT tagged and all were marked via an adipose fin clip prior to release. Although Myy Brook Trout were observed and detected with a floating PIT antenna in the following months after stocking, no Myy released in 2020 were captured during the 2021 suppression event, suggesting poor overwinter survival. In contrast, five Myy Brook Trout stocked in 2021 were recovered during fish removal efforts in 2022, indicating a low level of overwinter survival. Based on current fish suppression rates, sex ratios and a population model developed in 2019, we estimate females may be extirpated from the Tyee Springs population in 2033. Moving forward, we intend to continue monitoring the resident and Myy Brook Trout population by analyzing tissue samples for genetic parentage analysis, tracking population sex ratios, and revising abundance estimates. These data will be used to refine the Tyee Spring population model which will in turn help assess the effectiveness of the project and the Myy technique as a potential biocontrol strategy.