Several flatfish species including southern flounder (Paralichthys lethostigma) recruit to estuaries during early life. Therefore, the evaluation of estuarine sites and habitats that serve as nurseries is critical to conservation and management efforts. I used biochemical condition and growth measurements in conjunction with catch-density data to evaluate settlement sites used by southern flounder in the Galveston Bay Estuary (GBE). In 2005, beam-trawl collections were made in three major sections of the GBE (East Bay, West Bay, Galveston Bay), and three sites were sampled in each bay. Within each sampling site, replicate collections were taken from 1) the marsh edge, 2) an intermediate zone, and 3) the open bay. The average size of southern flounder collected was between 12 and 19 mm standard length, and peak densities occurred in January and February. Catch data indicated that numeric densities of southern flounder were significantly greater in East Bay (2.75 per 100 m2) than in West Bay (0.45 per 100 m2) or in Galveston Bay (0.91 per 100 m2). Habitat-specific variation in density was not found. Otolith-based estimates of age indicated that the majority of southern flounder collected were 35-45 days old and derived from early December to early January hatch-dates. Growth rate differences were negligible across bays and habitats, with the average growth rate being 0.40 mm/day (range 0.21-0.76 mm/day). RNA:DNA ratios indicated that newly settled southern flounder in the GBE were in relatively high condition. Habitat-specific differences in RNA:DNA ratios were not observed; however, ratios were significantly lower in West Bay (average 8.0) than in East Bay (average 9.5) or in Galveston Bay (average 9.8), suggesting the condition of new recruits may vary across the GBE. Findings from this study indicate that southern flounder use a variety of habitats within the GBE during early life, and survival and recruitment success appear favorable regardless of settlement site. As a result, recruitment success of southern flounder may be less a function of the quality of nursery sites/habitats within the GBE than of other factors (e.g., larval supply to the estuary).

Essential fatty acids are necessary for growth, survival, and development of larval fishes, but there is limited information on the essential fatty acid requirements of larval southern flounder (Paralichthys lethostigma). The objectives of this study were to elucidate connections between dietary supply of docosahexaenoic acid (DHA) and arachidonic acid (ARA) and deposited fatty acids in the head or body, and then link diet and stored fatty acids in the head or body with larval quality traits. From 4-15 days posthatch (dph), southern flounder larvae were fed rotifers enriched with four different combinations of DHA-rich Algamac 3050 and ARA-rich Algamac ARA. Fatty acid concentrations in the head and body were measured at 15 dph, and relationships between fatty acids in head or body and in the diet were determined. Larval quality traits, including specific growth rate (SGR), survival, and eight behavioral performance variables were measured. Results showed that concentrations of DHA and ARA in the head and in the body were correlated with concentrations of DHA and ARA in the diet. Growth rate did not vary among the four diets, but survival was positively correlated with the amount of lipid in the diet. Responsiveness to a visual stimulus was positively correlated with the concentration of DHA in the diet, the ratio DHA:EPA in the head, and total energy content of the diet. Turning rate during routine swimming was correlated with body DHA. This study demonstrates the influence of DHA content, total lipid content, and energy levels in the diet of southern flounder and provides a foundation for future studies examining causal factors of recruitment variability or larviculture production success. / text

In marine flatfish of the genus Paralichthys, temperature plays a large role in sex determination. Thus, global climate change could have significant effects on southern flounder (Paralichthys lethostigma), a commercially and recreationally important flatfish whose populations have steadily declined in Texas in the last 25 years. The most susceptible areas to global climate change are shallow water environments, particularly estuaries, which serve as essential nursery habitats for juvenile southern flounder. While in the estuaries, juveniles develop, and sex is determined. Juvenile southern flounder possess genotypic sex determination; however, the sex of females is highly influenced by temperature and can result in sex reversal. The temperature-sensitive enzyme complex responsible for estrogen biosynthesis in vertebrates is aromatase cytochrome P450 (P450arom), a critical component in ovarian differentiation that can be used to measure presumptive males and females exposed to a gradient of temperatures. This research identifies that sex is influenced by temperature between 35 and 65 mm total length (TL) and establishes that increases in temperature from 18 °C during this size range produce increasingly male skewed sex ratios in southern flounder from Texas. The findings presented here are critical for optimizing production of females in culture and for developing stock enhancement programs of southern flounder in Texas. / text

Southern flounder (Parlichthys lethostigma) is a recreationally and commercially important flatfish species found in the Gulf of Mexico, and recent analyses indicate that the northern Gulf of Mexico population is in decline. For proper management, knowledge of habitats used throughout the juvenile stage is needed. The aim of the current study is to examine habitat use of young-of-year (YOY) southern flounder in the Galveston Bay complex using habitat distribution models and acoustic telemetry. A set of habitat distribution models examined how habitat use changes during the first year of life. In addition, southern flounder were tagged with acoustic telemetry transmitters and monitored with a novel receiver array that allows for measurements of fine-scale movements. These movements were compared to habitat maps to examine habitat selection. Habitat distribution models determined that habitat requirements for southern flounder change with ontogeny and season. Newly settled southern flounder were most influenced by physicochemical parameters and the presence of seagrass beds. YOY southern flounder, however, showed increased occurrence at freshwater inlets during summer and fall months, and occurrence decreased at tidal inlets during the fall. Predictions of habitat suitability across the Galveston Bay complex indicate that the factors influencing occurrence of southern flounder change with season, ontogeny, and availability of suitable habitats. With acoustic telemetry, it was apparent that habitat use by southern flounder was nonrandom and influenced by benthic and other physicochemical conditions. Habitat analyses indicated that southern flounder used sand habitats more frequently than seagrass, oyster reef, or salt marsh habitats. Telemetry results also indicated that depth and water temperature were important determinants of habitat suitability for YOY southern flounder, with individuals preferring deeper and cooler regions of the water column in Christmas Bay. Both model and telemetry analyses indicate that habitat use by YOY southern flounder is dynamic across multiple spatial and temporal scales, with distributions and movements influenced strongly by ontogenetic changes in habitat associations, temporal and spatial variability in physicochemical conditions, and tidal cycles.