The practicality of Trojan sex chromosomes as a biological control: an agent based model of two highly invasive Gambusia species

Invasive fish species are a primary threat to aquatic ecosystems. Owing to the high fecundity of some fish, conventional control methods (e.g. specific removal) can be ineffective and the use of poisons is not desirable due to their non-specificity. Trojan sex chromosomes (TSC) are a theoretical method of invasive species control, where sex-reversed fish that are only able to produce male offspring are released into the target population. These Trojan individuals subsequently breed, causing a male skewed population sex ratio and eventually population collapse. Previous publications have explored TSC as an invasive species control, but assume that wild-type and Trojan fish have equal fitness, an assumption that may not be valid. What is more, models from closely related fields suggest that differential fitness between Trojans and wild-type fish maybe influential in the efficacy of TSC as a bio-control. Here we use agent based modeling to test how effectively TSC can be used to control two common invasive species of mosquitofish (Gambusia affinis and G. holbrooki) when Trojans have compromised fitness. We manipulated the fecundity, probability of mating and offspring survival of Trojan fish. Overall, our models found that fecundity holds the most influence over how effectively TSC theory can be used to control fish populations. However, a recent meta-analysis demonstrates that the fecundity of sex-reversed fish is compromised. It may be possible to compensate for reduced fecundity by increasing the rate of Trojan introductions. Surprisingly, our models also found that Trojans are a more effective bio-control when consistently introduced into the same place, rather than being randomly distributed at introduction.