Improving Southern Flounder Paralichthys lethostigma Management in the Southeastern United States through Characterization of Habitat Effects on Juvenile Sex Ratios

Southern flounder support valuable fisheries in the southeastern US. Importantly, growth is sexually dimorphic and these fisheries are heavily dependent on the larger females.  Southern flounder also show temperature dependent sex determination, with warmer culture conditions producing predominantly males. These two factors make accurate estimation of sex ratios critical for assessing spawning stock biomass and management. However, extensive studies of wild juvenile sex ratios have not been undertaken. We assessed juvenile sex ratios using sex-specific gene expression markers in developing gonads (aromatase, FoxL2, and MIS) in conjunction with water temperature recordings in both Texas and North Carolina in 2014.  Texas had cooler than average water temperatures during the sex determination window, with later temperatures warming to masculinizing range, however wild caught Texas flounder had 43% males and 46% females overall.  North Carolina showed wide variability in temperature, but similar in all locations, however northern sites had 46% females while Neuse River estuary and southern sites only had 21% females. Collections will continue for the 2015 season.  This study represents a new approach to assessing habitat and yearly temperature variation effects on sex ratios for predicting spawning stock biomass in these valuable fisheries that are critically dependent on production of females.