Estimating Length-at-Age and Length-at-Maturity for the North-Central Gulf of Mexico Southern Flounder Stock

Southern Flounder, Paralichthys lethostigma, is the most commonly harvested flatfish in the north-central Gulf of Mexico, yet knowledge of the species’ growth and reproduction is greatly limited. Our objectives were to describe the length-at-age relationship, to estimate mean length-at-50%-maturity, and to assess how changes in these estimates affect spawning stock biomass per recruit models for female Southern Flounder. We collected measurements of total length (TL, mm), otoliths (n = 313) and gonadal tissue samples (n = 221) during September 2014 to February 2016. Otoliths were sectioned to estimate age, and multiple length-at-age models were fit to the TL and age data. Histological analysis was used for maturity classification and logistic models were fit to binomial maturity data. Model fit was evaluated using Akaike information criteria and we found that the three-parameter von Bertalanffy growth function best described the data (L_∞ = 509 mm, k = 0.70 y^-1, t₀ = -0.46 y). The estimated mean length-at-50%-maturity was 308 mm TL, which is the current minimum length limit for the Mississippi stock. Because spawning stock biomass is sensitive to changes in mean parameter estimates, we anticipate that these results will inform future stock assessments for this species.