Southern Flounder Life History Diversity and Contributions to Fisheries from Differing Estuarine Salinity Zones

Otolith chemistry is a useful natural tracer for discerning habitat-use of estuarine fishes. For Southern Flounder Paralichthys lethostigma, recent otolith chemistry studies have revealed a diversity of residency patterns across salinity gradients. However, the contribution of recruits with specific residency patterns to fisheries is poorly understood. The objectives of this study were to (1) use otolith chemistry from fishery-independent and fishery-dependent collections in Mobile Bay, Alabama, to classify lifetime residency patterns (i.e., freshwater, transient, estuarine) in Southern Flounder collected across a large estuarine salinity gradient (0–30 psu); (2) to test if Southern Flounder exhibited resident or migratory behavior by determining if lifetime residency patterns in fishery-independent samples matched expected salinity patterns in the region of collection after accounting for annual variation in river discharge; and (3) to examine which residency patterns contributed to the commercial and recreational Southern Flounder fisheries in nearby coastal waters. Age-0 residency patterns in fishery-independent samples were strongly correlated with region of collection and annual river discharge, suggesting that the majority of Southern Flounder had resided in the same region in which they spent their age-0 growth phase. Southern Flounder with a combination of freshwater and estuarine salinity signals and classified as “transient” did not appear to have conducted large-scale movements across salinity gradients, but instead resided in regions of the estuary experiencing seasonal fluctuations in salinity. The majority (57%) of commercially and recreationally harvested Southern Flounder were transients, while a minority (39%) were estuarine residents and lifetime freshwater residents (4%) were rarely harvested. Results from this study suggest that Southern Flounder settle in and remain in the certain habitats during the estuarine residency phase. Given the lack of movement across habitats, future efforts to understand how habitat-specific conditions (e.g., abiotic, biotic, fishing exploitation) affect vital rates seems warranted for a species currently experiencing population declines.