Fitness Regulates Non-Native Species Recovery in a Simulated Trojan MYY Brook Trout Application.

Trojan Y chromosome methodologies have been developed for bio-control of non-native freshwater fish. Individuals with an altered sex chromosome genotype are introduced into the environment to mate with wild conspecific and change the population sex ratios, with the goal of eradicating local populations. This strategy preserves native stocks and can be effective when other methods are impracticable. Applications employing Trojan males with two Y chromosomes (MYY) to control non-natives have shown promise for eradication at small spatial scales when used with manual suppression. Failure to eradicate may have consequences for the fitness and resiliency of the recovering population and remains untested. Population bottlenecks due to suppression and MYY introductions may cause inbreeding depression while admixture with hatchery MYY may counteract inbreeding depression and result in outbreeding depression. We used a spatial demogenetic and individual-based simulation model to examine the fitness consequences of inbreeding and outbreeding depression on the recovery of a non-native Brook Trout (Salvelinus fontinalis) population targeted for removal in northeastern Washington State, USA. Simulations predicted genetic and fitness consequences, as well as the extent and timing of population recovery following suppression and introduction of MYY. We found that inbreeding depression rapidly decreased overall population size and outbreeding depression was important for counteracting the fitness advantage of admixture from genetically divergent MYY. Additionally, combined effects of inbreeding and outbreeding depression prevented the population from recovering to pre-suppression abundance compared to each in isolation. This represents the first study demonstrating the demogenetic and fitness consequences of inbreeding and outbreeding depression from a MYY program when non-native species eradication is not achieved. Results suggest that the combination of negative fitness effects may limit the ability of the surviving non-native population to recover and reestablish once treatments cease, indicating that such treatments may be beneficial even if eradication is not ultimately achieved.