Southern flounder (Paralichthys lethostigma) stock structure inferred from otolith shape analysis

Examination of otolith morphometric variation has been shown to provide improved descriptions of stock structure for several marine fish species. We examined spatial variation in otolith shape of southern flounder (Paralichthys lethostigma) to understand population structure at the following geographic levels: ocean basin (Atlantic and Gulf of Mexico); regional coastal waters (Texas, Florida) and (Georgia, South Carolina); and local coastal waters (North Carolina). To reduce variability, we considered only age-1 female fish. From digitized otolith images, we extracted descriptions for common shape indices and elliptical Fourier coefficients and found strong evidence for differences at the ocean basin scale, but only weak evidence for structure at either within-basin (i.e., among states) or within-state (local) spatial scales. Our finding of inferred stock structure differences between the ocean basins aligns with the geographic break in the distribution of this species the absence of this species from the southern portion of Florida as well as with recent genetic findings. Currently, state-level management of southern flounder in both areas does not account for any basin-wide population mixing and, therefore, by default, assumes a separate unit stock for each state, although our findings indicate that mixing could be extensive. Additional sources of information (e.g., genetics, life history traits) collected at appropriate spatial scales should be examined to confirm suspected levels of mixing and to determine suitable management strategies for the conservation of southern flounder stocks throughout their ranges.