Streamwide evaluation of survival and reproduction of MYY and wild Brook Trout populations

Brook Trout Salvelinus fontinalis have been introduced across the western USA, where the species competes with and often replaces native salmonids. Nonnative Brook Trout are difficult to eradicate; thus, new removal strategies are needed. One novel methodology couples the partial suppression of wild Brook Trout with the replacement of M_YY Brook Trout (males with two Y chromosomes). If M_YY fish survive to reproduce with wild female Brook Trout, their progeny will be 100% male, which eventually shifts the sex ratio and theoretically extirpates the population. However, the effectiveness of this approach depends on survival and reproduction of M_YY fish relative to the surviving wild conspecifics. From 2018 to 2020, we annually removed an estimated 45.7% of wild Brook Trout from three streams in New Mexico and stocked fingerling M_YY Brook Trout (mean TL = 94 mm; range = 61–123 mm) targeting 50.0% of wild annual abundance estimates. Annual survival for M_YY and wild Brook Trout was similar in Leandro Creek (M_YY = 0.63 and wild = 0.63) and Rito de los Piños (M_YY = 0.37 and wild = 0.46) but differed in Placer Creek (M_YY = 0.28 and wild = 0.75). During spawning, we evaluated the reproductive potential of M_YY Brook Trout by comparing the percentage of sexually mature male Brook Trout comprised of M_YY fish to the percentage of hybrid (M_YY × wild) F₁ progeny. By the second spawning season (2019), M_YY fish comprised 59.8, 50.4, and 34.5% of milt-producing Brook Trout, which resulted in 55.1, 33.3, and 0% hybrid progeny in Leandro Creek, Rito de los Piños, and Placer Creek, respectively. We demonstrated that M_YY fish exhibit similar vital rates compared with wild conspecifics in two of three streams; however, differences among streams highlights unforeseen variables that influence M_YY survival and reproduction. The study offers promising results of the M_YY approach for potentially eradicating unwanted Brook Trout populations.