First successful use of YY males to eradicate invasive Brook Trout populations

Abstract Objective Eradication of invasive Brook Trout Salvelinus fontinalis from stream ecosystems is often constrained by the inefficiency of electrofishing alone and limitations of piscicide use. The YY male (Trojan Y chromosome) approach has been proposed as a genetic biocontrol tool to assist in eradication by shifting population sex ratios toward males, but population-scale evaluations remain limited. We assessed whether combining annual electrofishing suppression with annual stocking of hatchery-produced YY male Brook Trout could eradicate wild Brook Trout from two isolated headwater streams in central Idaho, and we evaluated whether density-dependent sex change (DDSC) could compromise eradication efforts as female abundance declined. Methods In two small streams (2.6–2.9 km in length; mean wetted width of 0.8–2.6 m), wild Brook Trout populations were annually suppressed prior to YY male stocking. All fish sampled for genetic analyses were evaluated for phenotypic sex under laboratory conditions and were genotyped using validated sex-specific genetic markers. Results Annual suppression of wild Brook Trout (≥100 mm total length) exceeded 50% for both streams in all years (mean = 91%), and stocking rate was equivalent to about 90% of the initial wild Brook Trout (≥100 mm) population sizes. Following initiation of YY male stocking, sex ratios became rapidly skewed toward males, reaching 100% male within 6 years in both streams. Surprisingly, growth, survival, and reproductive contribution of hatchery YY males were comparable to those of wild males, likely because competition with wild fish was diminished by the suppression effort. Genetic markers showed complete concordance between genotypic and phenotypic sex and no evidence of sex reversal in either stream over 10 years of extreme population suppression, thus providing no evidence of DDSC. Conclusions Our results confirm for the first time that integrated electrofishing suppression and YY male stocking can eradicate invasive Brook Trout within a management-relevant time frame and that DDSC is unlikely to hinder this approach for this taxon. We provide recommendations for a strategic integrated pest management approach for future Brook Trout eradication projects involving YY male stocking.