PRODUCTION OF GREATER QUANTITIES OF FEMALE SOUTHERN FLOUNDER FOR FOODFISH CULTURE AND STOCK ENHANCEMENT

Gender determination of Southern Flounder is subject to internal factors (genetics, circulating sex steroids, stress hormones) but is also subject to modification by external factors (temperature). Warming trends in coastal waters have significantly skewed the sex ratios of Southern flounder in many areas. For example, a recent survey found sex ratios of Southern flounder to be skewed 16 males: 1 female in Galveston Bay, Texas. Flounder hatcheries and producers are often left to the mercy of Mother Nature due to difficulties associated with heating or cooling large volumes of water. Temperatures as little as 1°C greater than 22°C or less than 21°C during the period of sexual differentiation typically result in much larger percentages of males than females. This skewed sex ratio leads to limited successes in Southern flounder culture as males grow slowly and are difficult to rear to marketable size. Large-scale commercialization as a cultured foodfish, or production of females in quantities necessary to impact wild populations through stock enhancement, has not been realized for Southern flounder.

The majority of flounder produced for stock enhancements tend to be males due to several exogenous and endogenous factors that collectively determine gender in flounder, but release of predominately females would have a greater population impact on the target population through increased fecundity. To resolve this bottleneck, we evaluated the effects of UV irradiation of sperm followed by exposure to either pressure or temperature shock to suppress meiotic division and create heterozygous meiotic diploid offspring. The meiogynogenic juveniles can then be sex-reversed to become functional males by rearing at high temperatures or feeding methyltestosterone during the period of sex determination. The resulting XX functional males can then be spawned with normal females to produce 100% genetically female offspring. Creation of meiogynogens is only necessary one time, as when new XX functional male broodstock are needed, all-female progeny from the original meiogynogens can simply be sex-reversed using temperature or feeds containing hormones.

Two controls and two experimental treatments were evaluated to assess gamete quality, fertilization rate, embryo development, hatch rate, and total larvae produced. Control 1 served as the 'normal' control with untreated milt and eggs, while control 2 served as an irradiated control to ensure irradiated sperm could not result in embryo production. Treatment 1 utilized irradiated milt followed by 8,500 psi hydrostatic pressure shock, while treatment 2 utilized 4°C cold shock to induce meiogynogenesis. While both experimental treatments resulted in the production of meiogynogens, the 8,500 psi hydrostatic pressure shock yielded greater hatch rates and total larvae produced from the same spawns. Creation of meiogynogenic Southern flounder larvae currently appears to be a viable method to improve production for this species.