Spatial Variability in Growth Rates of Southern Flounder

Delineation of stock structure is important for understanding the ecology and management of many fish populations, particularly those with wide ranging distributions and high levels of harvest. Southern flounder are a popular commercial and recreational species along the southeast Atlantic coast and Gulf of Mexico. Recent studies have provided genetic and otolith morphology evidence that the Gulf of Mexico and Atlantic Ocean stocks differ. Using age and growth data from three states (Texas, South Carolina, and North Carolina) we expanded upon the traditional Von Bertalanffy model in order to compare growth rates of putative geographic stocks of southern flounder. We improved the model fitting process by log transforming the three model parameters (L_∞, K, t₀), in addition to allowing each parameter to vary as a random effect. Multiple comparisons of parameters showed that growth rates varied significantly at small scales (within states) for females, but not for males. Growth rates were also consistent through time, when the data were available. Because within-basin populations are thought to be genetically well mixed, our results suggest that consistent small-scale environmental conditions (i.e., within estuaries) likely drive growth rates and should be considered when developing broader scale management plans.