Thermal Tolerance of Age-0 Gulf of Mexico Striped Bass (\u3ci\u3eMorone saxatilis\u3c/i\u3e): Ontogenetic and Genetic Effects

Striped bass, Morone saxatilis, were historically abundant in the Gulf of Mexico region but were largely extirpated from most Gulf rivers by the early 1960s. Since 1967 hatchery stocking has supported populations in Mississippi rivers and to date there is no evidence of natural reproduction. Intolerance of striped bass to high water temperature is a potential factor negatively impacting Gulf striped bass survival and reproductive fitness, and may be a limiting factor to reintroduction of the species in Mississippi. This work contributed to restoration efforts by establishing molecular tools necessary for genetic monitoring of striped bass restoration, and evaluating the upper thermal tolerance of Gulf striped bass larvae and juveniles. Multiplex assays for eleven microsatellite loci were optimized and the suitability of the obtained panel for genetic tagging and monitoring was evaluated. Simulation analysis employing a likelihood ratio approach to assign offspring to parents in the hatchery showed that the panel allowed identification of 95% of hatchery fish while maintaining the rate of false identification (incorrect classification of wild fish as hatchery fish) lower than 2.5% when genotypes of all hatchery broodfish are known. Significant departure from Hardy-Weinberg expected genotype frequencies was detected at most of the loci and appeared to reflect in part amplification artifacts occurring at up to five loci and possibly impacting inferences on parentage. Thermal tolerance challenges were applied in triplicate to offspring mixtures from different crosses. Challenge protocols were based on the Critical Thermal Maximum (CTM) method and were applied on yolk sac larvae (48h post hatching) or on phase I (2.5 inch) release-sized juveniles. The effect of temperature on larval yolk resorption time was characterized at a series of elevated temperatures. Thermal tolerance challenges revealed a positive ontogenetic shift in thermal tolerance from the larval (CTM estimate 25.4°C±1.2°C) to the phase I juvenile stage (CTM estimate 35.8°C±0.7°C). Available water temperature data indicated that upper thermal tolerance of age-0 Gulf striped bass is likely not exceeded in culture ponds or the lower Pearl and Pascagoula Rivers. Exposure to elevated temperature (22.1°C to 24.0°C) led to a 23% reduction of the larval yolk resorption time resulting in a substantially shorter time window when larvae can initiate exogenous feeding in comparison to groups maintained at a control temperature of 19°C. No significant genetic effects on the thermal tolerance of larvae were detected during analysis of the thermal tolerance of 48h old larvae from 11 crosses. Further assessment of striped bass thermal tolerance at older life stages and including genetic characterization of a larger number of crosses is warranted