Genetic risk involved in stock enhancement of fish having environmental sex determination

Environmental sex determination is reported in various fish species, including some fishes subject to stock enhancement. We studied the influence of stock enhancement on the sex-determining system of fish with both genotypic and environmental sex determination. We constructed and analyzed recurrence formula models for the dynamics of gene frequency in the male heterogametic (XX female and XY male) sex-determining system of the Japanese flounder (Paralichthys olivaceus). In this species, an XX individual can develop as a phenotypic male (sex-reversed male) depending on the conditions experienced by a juvenile. We show that the release of sex-reversed males may result in the extinction of the Y gene that determines sex. We also studied how the risk depends on the kind of hatchery broodstock used (e.g., fish collected from the wild or fish from a lineage established in a hatchery), sex-reversal rates in the hatchery and in the wild, and the relative contribution of released fish to reproduction in the wild. We propose some ways to reduce the risk.