Freshwater Residency Patterns of Southern Flounder on the Texas Gulf Coast

Southern flounder (Paralichthys lethostigma) populations have declined over the last 25-30 years throughout its range. With this rapid decline, the sustainability of the southern flounder fishery and population viability of this commercially and recreationally important fish has come into question. Previous research conducted in the Northern Gulf of Mexico and North Carolina, has shown that southern flounder often reside in freshwater for significant periods of time during the juvenile life history stage. Juvenile southern flounder have been collected at salinities below 10 ppt in Aransas Bay, suggesting that Texas southern flounder might also have critical periods of freshwater residency. However, the presence of a freshwater residency period in southern flounder in Texas has not previously been tested. Patterns of freshwater residence will be determined using otolith microchemistry, using both stable isotope (⁸⁷Sr/⁸⁶Sr) and trace element (Sr/Ca, Ba/Ca) ratios to determine movements across salinity boundaries. Otoliths will also be used to establish age, hatch date, and growth rate of southern flounder. This will allow us to investigate the implications of a freshwater residency period and determine if the presence or absence of this period results in differential recruitment into the fishery. Water samples collected from the major tributaries to the Aransas, Copano, San Antonio, and Mission Bay systems indicate that δ¹⁸O and trace element (Sr/Ca and Ba/Ca) values show significant variation among locations. These results indicate that if southern flounder in Texas do exhibit a freshwater residency period, it should be possible to assign an individual fish to a particular freshwater habitat. This work will provide fisheries managers with a more informed understanding of habitat requirements of juvenile southern flounder, leading to the implementation of more comprehensive and effective conservation and management strategies.