Understanding and Managing Living Resources

The Lemon Shark (Negapion brevirostris) is a member of the large coastal shark complex found throughout the Atlantic Ocean and Gulf of Mexico. The species is well studied in a portion of their range, but their population is currently listed as vulnerable by the IUCN because of population declines from fishing pressure and habitat loss. This study aims to understand the extent of Lemon Shark use of Mississippi coastal waters as potential nursery habitat and identify key habitat characteristics associated with the species within this region. The species is known to use the waters surrounding the Chandeleur Islands as their northernmost nursery habitat; however, seasonal movement and the extent of their home range beyond the Chandeleur Island chain is unknown. To examine Lemon Shark use of Mississippi coastal waters, we used data from long-term gillnet monitoring (LTM) in conjunction with a focused gillnet study in shallow water habitat around Cat Island, MS. Captured elasmobranch species were identified, sexed, measured, weighed, and any Lemon sharks were surgically implanted with acoustic transmitters. In total, 28 Lemon sharks were implanted with acoustic transmitters around Cat Island between 07/2023 and 08/2024. After transmitter deployment, individuals were monitored through an acoustic array surrounding the tagging location. To identify specific temperature thresholds during capture events, HOBO temperature loggers were deployed to record daily temperature values across shallow environments. There was not a significant difference observed between temperature at time of capture and daily average, but trends in seasonal temperature changes are being monitored. Information from this study will contribute to knowledge of Lemon Shark habitat use in this previously unknown region, identify environmental conditions favorable for the species, and document potential shifts within Mississippi waters. 

Populations of Common Snook have expanded northward along both coasts of the Gulf of Mexico in recent years, driven in part by milder winters as a result of climate change. However, winter seawater temperatures in the northern Gulf still approach the thermal tolerance of Snook, prompting these sportfish to seek refuge in thermally buffered coastal river systems. During irregular cold snaps, when river temperatures drop below the Snook cold-shock limit of ~12°C, natural springs can still provide thermal refuge due to consistent year-round temperatures of ~21°C. The use of springs as thermal refuge by Snook during cold-snaps was documented in the Suwannee River, Florida, where a novel range-expanded population of the species was recently discovered. To estimate environmental influence on the availability of this unique thermal refuge habitat, we collected overwinter temperature data around two first magnitude springs in north-central Florida. Spatial random forest models were constructed for both springs using a multivariate dataset consisting of river, spring, and atmospheric data between November 2023 to January 2024. Acoustic telemetry was also implemented to track Snook use of the two springs over this same period. Our findings suggest substantial differences in thermal refuge habitat quantity between the springs; however, high river flow conditions negatively affected thermal refuge at both springs. Acoustic telemetry supported this conclusion, as Snook use of the higher-quality thermal refuge declined during high river conditions, and no Snook were present at the lower-quality thermal refuge under the same conditions. Evidence of a Snook cold shock stress event during this time suggests that the combination of high river flow and rapid decline in temperatures created the worst-case scenario for availability of this unique winter habitat. Further aquifer extraction could potentially worsen the plight of thermal refuge availability for Snook in the northern Gulf region by reducing springs flow.

Catfishes are widely cultured due to their rapid growth, high disease resistance, tolerance to extreme environmental conditions, and consumer acceptance. Catfish aquaculture rapidly expanded with the production of channel catfish (Ictalurus punctatus) in the 1960s and grew to become the largest finfish aquaculture industry in the U.S. More recent efforts have focused on the hybridization of female channel catfish and male blue catfish (Ictalurus furcatus), as their hybrids possess favorable characteristics (a phenomenon called "heterosis"). Even though catfish aquaculture primarily occurs in the southeastern U.S., cultured catfishes are still subjected to cold temperatures during the winter, as aquaculture ponds are relatively shallow (< 1.5 meters) and experience seasonal thermal fluctuations. Cold temperatures reduce metabolic processes; however, little is known regarding differences in metabolic rates, swimming performance, and blood metabolites at different temperatures among the principal types of cultured catfishes. Therefore, a swim flume was used to measure maximum metabolic rate, metabolic scope, and critical swimming speed (Ucrit) and intermittent respirometry was used to measure standard metabolic rate in juvenile channel, blue, and hybrid catfishes at 10 and 20℃. Additionally, blood metabolites were analyzed in fatigued and non-fatigued catfishes at 10 and 20℃. It was hypothesized that hybrid catfish would have greater metabolic and swimming performance than channel and blue catfishes due to heterosis. However, metabolic scope and swimming performance did not vary among channel, blue, and hybrid catfishes. Notably, swimming performance was significantly reduced among all catfishes at 10℃. Lactate and glucose concentrations were higher and blood pH was lower in fatigued catfishes, with channel catfish generally differing in blood metabolites from blue and hybrid catfishes. Results indicate that prolonged exposure of catfishes to cold temperatures limits metabolic processes and swimming capacity, ultimately requiring catfishes to allocate energetic resources to the maintenance of metabolic requirements.

Temperature can dramatically alter growth rates and maximum size of ectotherms. Understanding these impacts is critical in predicting species' responses to climate change. In the commercially harvested blue crab, Callinectes sapidus, field studies have linked increasing water temperatures with decreasing size at maturity. Laboratory studies have observed both decreasing intermolt period and growth per molt as water temperatures increase, providing a potential mechanism driving observed decreases in size at maturity. Previous studies are limited, however, in that most use only two temperature treatments, have not focused on the entire life cycle, and fail to develop a broadly applicable understanding of the molting process. We quantified molting patterns and growth rates of juvenile blue crabs across a range of temperatures. These data will be used in developing a temperature-dependent molt-process model, providing a framework for predicting growth rates, age-of-entry into the fishery, and size at maturity under current and future conditions. Further, these data will support improved assessment and management for this species through a more accurate understanding of growth and maturity.

Blue crabs support a large commercial fishery in the Gulf of Mexico. Effective management of this fishery requires a complete understanding of the blue crab population dynamics and sources of mortality. Most states require the release of undersized and ovigerous blue crabs, yet mortality of these discarded crabs is an understudied and poorly estimated component of total mortality and represents a major data gap hindering effective management and assessment. The aim of this project is to quantify discard mortality of undersized and ovigerous crabs using bycatch surveys. Bycatch surveys have been conducted monthly aboard commercial crabbing boats in three locations across the Lake Pontchartrain-Mississippi Sound coastal system in Louisiana and Mississippi. Crabs were sexed and measured and the likelihood of mortality of a subset of crabs was estimated with a reflex action mortality predictor (RAMP) assessment. Data will be analyzed using generalized linear models, focusing on effects of environmental variables (salinity, temperature, and DO) and crabber behavior (soak time) on bycatch rates and likelihood of mortality.