Manipulation of Gender in Southern Flounder to Improve Stock Enhancement Programs

Two constraints to large-scale Southern flounder production for stock enhancement are the reliance on collection of wild broodstock and how to maximize system impacts by releasing the largest number of females possible. Reliance on wild broodstock is due to size dimorphism between genders in Southern flounder; in confinement, the females often eat the smaller males after spawning to regain lost nutrients from the spawn, so more males must often be captured from the wild. The majority of flounder produced in Gulf Coast 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. This project seeks to remedy these key constraints to stock enhancement programs. We will evaluate the effects of temperature, UV irradiation, hormone exposure and explore the feasibility of creating captive brood populations of the same size that can produce all female stocks or the creation of predominately female populations . The first study will utilize UV irradiation of milt and temperature shock to create meiogynogens that could result in an entirely female population of larval flounder. In the second study, larvae will be treated with three concentrations of 17a-Methyl-1-testosterone for 72 hours at 30 hours post-hatch, and then fed one of three concentrations of 17a-Methyl-1-testosterone for 90 days. Any resultant gynogenetic males will then be grown out as a broodstock population and eventually spawned with typical females creating an all-female population. The third study will focus on temperature sex determination with an hormone treatment to create all female stocks. Three different concentrations of 17B-oestradiol will be fed to fish 30-60mm in length for 30 days at a low (21C) temperature to explore the potential of the combination of hormone application and temperature sex determination to produce an entirely female population of southern flounder. Creation of all female broodstock will eliminate production inefficiencies caused by males during culture of Southern flounder and benefit stock enhancement programs by releasing more females.