Evaluating the Trojan Y chromosome strategy for the removal of invasive Sacramento Pikeminnow from the Eel River, California

ABSTRACT Objective The introduction and spread of Sacramento Pikeminnow Ptychocheilus grandis in northern California's Eel River basin represents a significant threat and impediment to the recovery of several threatened native fish species. This study was undertaken to evaluate the Trojan Y chromosome (TYC) strategy for the extirpation of pikeminnow from the basin. The TYC strategy is a biocontrol method that relies on the production and stocking of fish with YY sex chromosomes, which may be phenotypically male (YY male or Myy) or female (YY female or Fyy). These YY fish produce all-male offspring when mating with their wild conspecifics, potentially leading to the extirpation of wild-type females over time. The specific study objectives were to (1) estimate the amount and time scale of YY Sacramento Pikeminnow stocking needed to achieve elimination; (2) evaluate how a TYC program would be affected by the rate of manual pikeminnow suppression, the relative survival of YY pikeminnow, and the phenotype of stocked YY fish (i.e., Fyy or Myy); (3) evaluate the mathematical feasibility of a TYC program for population reduction in an open system where elimination would not be possible due to influx from outside sources (e.g., a river tributary); and (4) identify key biological uncertainties for future investigation. Methods To evaluate the strategy, we constructed a mechanistic, individual-based population model to simulate a TYC Sacramento Pikeminnow stocking program. Results Effectiveness of the TYC strategy (time to extirpation and chance of extirpation) was found to be highly sensitive to the stock–recruitment productivity parameter, an unknown quantity for this species. In a closed system, if the species’ productivity was low, stocking of YY males alone was found to be a promising option for eliminating the population. If productivity was high, stocking of YY males alone was found to be a poor option, never achieving extirpation. However, stocking of YY females was found to be effective in either productivity situation. Factors that improved the relative success of a TYC program included stocking more YY fish, higher YY survival in the wild, more concurrent manual suppression, and stocking of YY females. Long-term population suppression in an open system was found to be possible only if the species’ productivity was low and only if YY males alone were stocked. Conclusions Stocking of YY female Sacramento Pikeminnow to achieve the species’ extirpation from the Eel River is an option that warrants further exploration, and there is a possibility of stocking YY males for population reduction. However, additional research is needed to explore Sacramento Pikeminnow movement and stock–recruitment productivity and to better understand their effect on native fish populations.