Survival and Reproductive Success of Hatchery YY Male Brook Trout Stocked in Idaho Streams

Nonnative Brook Trout Salvelinus fontinalis were introduced throughout western North America in the early and mid 1900s, resulting in populations that are difficult to eradicate and that often threaten native salmonids. Male YY Brook Trout (M-YY), created in the hatchery by feminizing XY males and then crossing them with normal XY males, comprise a novel approach to eradicating undesirable Brook Trout populations. If stocked M-YY Brook Trout survive and reproduce with wild females, it could eventually drive the wild population sex ratio to 100% males, at which point the population would be unable to reproduce and would be eradicated after stocking ceased. In this study, we stocked the limited number of catchable-size (mean TL=229mm) M-YY hatchery Brook Trout available from an established M-YY broodstock into four streams. In two streams, the wild Brook Trout population was suppressed via electrofishing prior to stocking to determine whether diminished competition with wild fish would increase the survival of hatchery M-YY fish. We used genetic assignment testing to identify the successful reproduction of stocked M-YY fish. Apparent survival of M-YY Brook Trout averaged 18% in streams with wild population suppression (mean suppression, 17%) and 9% in streams without suppression, suggesting that suppression of the wild population before stocking increased M-YY survival poststocking. Hatchery M-YY Brook Trout comprised an estimated 3.1% of all adult Brook Trout during spawning. Genetic assignment tests identified successful reproduction of M-YY fish in all streams in which they were stocked, with an average of 3.7% of fry being the progeny of M-YY fish. Our results confirm that hatchery M-YY fish stocked in streams can survive and spawn successfully with wild fish and produce all-male progeny. Despite the slightly reduced fitness of M-YY Brook Trout, this technology may be a viable method for eradicating undesirable nonnative Brook Trout populations.