Dusty L. McDonald

Morphological and molecular variation among populations of tidewater (Menidia peninsulae) and inland (M. beryllina) silversides: insight into drivers of adaptation and speciation of silverside fishes

Inland (Menidia beryllina) and tidewater silversides (M. peninsulae) are the most common of the Menidia fishes along the Texas coast, USA. Though genetically distinct, these species are morphologically very similar and often segregate by salinity with M. beryllina occupying fresh to brackish salinities and M. peninsulae occupying brackish to oceanic salinities. Their ranges are known to overlap, suggesting the possibility for interspecific competition, and significant hybridization has been documented. This study utilizes an integrative approach of morphological and molecular techniques to examine if divergent environmental constraints (inland freshwater impoundments versus coastal saline bays) could potentially result in parallel changes in morphology among populations of Menidia spp. occupying similar habitat types or if Menidia spp. occupying the same areas exhibit divergent changes in morphology (i.e., character displacement) suggesting historic interspecific competition. While both coastal (sympatric) and inland (allopatric) forms of M. beryllina showed significant morphological separation from M. peninsulae, coastal populations were slightly more similar to M. peninsulae than inland populations. Intraspecific comparisons revealed significant morphological and molecular variation among M. beryllina populations and limited variation among M. peninsulae populations. These findings coupled with a review of past Menidia work suggest that salinity regimes may drive the presence of M. beryllina in coastal bays, with increased salinity restricting movement of this species among river drainages resulting in both interspecific and intraspecific isolation. Thus, morphological and molecular variation in Menidia seems to be driven primarily by environmental conditions with interspecific competitive interactions of little significance. This study is the most comprehensive comparison of these species to date and conclusions may be applied broadly to the process and sequence of speciation of Menidia fishes in the Gulf of Mexico.

The Critical Thermal Maximum of Juvenile Red Drum Reared for Out-of-Season Stocking in Texas

Abstract Hatchery-propagated red drum Sciaenops ocellatus are released by the Texas Parks and Wildlife Department (TPWD) during the spring, summer, and fall to supplement natural stocks. Reported success of red drum stocked out of season in summer has been low or nonexistent; several potential causes, including mortality due to high temperatures, have been suggested. The present study was devised to determine the critical thermal maximum (CTM) of pond-raised juvenile red drum that had previously been exposed to diurnal temperatures of the summer season. Laboratory studies designed to mimic the rate of temperature increase (0.25°C per hour) typical of TPWD rearing ponds were used to examine rates of mortality in juvenile red drum that were collected from three separate ponds exposed to the ambient fluctuating temperatures of the summer season. The CTM (temperature that was lethal to 50% of the test fish [LT50]) was significantly lower (P < 0.0001) for two of the fish trials (LT50 = 37.1°C and 37.0°C) than ...

Effect of salinity on the upper lethal temperature tolerance of early-juvenile red drum

Previous work investigating the temperature tolerance of juvenile red drum ranging 18-50mm TL found evidence for positive size dependence (smaller fish less tolerant to higher temperatures) suggesting smaller size classes (<18mm TL) potentially may succumb to extreme summer water temperatures. Here, we explored the upper lethal temperature tolerance (ULT) in smaller-sized red drum which ranged from 10 to 20mm TL across multiple salinities to further understand the thermal limitations of this propagated game fish. In order to investigate the combined effect of temperature and salinity on ULT, temperature trials were conducted under three levels of salinity which commonly occur along the coast of Texas (25, 35, and 45ppt). The rate of temperature increase (+0.25°C/h) was designed to mimic a natural temperature increase of a summer day in Texas. We determined that the lethal temperature at 50% (LT50) did not differ between the three salinities examined statistically; median lethal temperature for individuals exposed to 25ppt ranged from 36.4 to 37.7°C, 35ppt ranged from 36.4 to 37.7°C, and 45ppt ranged from 36.1 to 37.4°C. Further, LT50 data obtained here for early-juvenile red drum did not differ from data of a similar experiment examining 25mm TL sized fish.

Lower Lethal Temperature for Juvenile Cobia Rachycentron Canadum

Cobia are a migratory marine species that have recently gained popularity as foodfish in the aquaculture market and for stocking. Unfortunately, when culturing these species, aquaculturalists may unknowingly expose these fish to temperature extremes not normally experienced in situ. We set out to test for the critical thermal minimum temperatures of juvenile cobia by exposing them to a simulated freeze, at a drop rate of 0.33°C per hour. We observed and documented behavioral effects due to low temperature exposure using criteria for loss of equilibrium and death. We determined that the median temperature for loss of equilibrium was 12.1 ± 0.40°C and the median lethal temperature was 9.7 ± 0.28°C for low temperature tolerance. We recommend that precautions be taken well before water temperatures reach 13°C in a freeze scenario.

Effects of three cold weather event simulations on early life stages of Southern Flounder (Paralichthys lethostigma)

ABSTRACT To determine the minimum age/size at which Southern Flounder, Paralichthys lethostigma, can safely be moved to outdoor rearing facilities in Texas, we examined survival of simulated temperature drops in two distinct life stages: premetamorphic larvae and two size classes of postmetamorphic juveniles (small = 9.8 ± 0.3 mm in TL; large = 19.7 ± 0.6 mm). Temperature was lowered by −0.33°C/h to 4°C, 7°C, or 10°C, held for 48 h and then raised at +0.33°C/h back to normal rearing temperature. Fish were monitored daily for survival. Larger postmetamorphic flounder had high survival for all temperature treatments (89%–100% survival), whereas both premetamorphic larvae and smaller postmetamorphic juveniles had low survival (<30%) for all temperature treatments.

Critical Thermal Maximum of Juvenile Spotted Seatrout (Cynoscion nebulosus) Reared for Summer Stocking in Texas

The critical thermal maximum of juvenile spotted seatrout (SL range 18–33 mm) was determined using a temperature increase of +0.26°C per hour. The critical thermal maximum (water temperature that was lethal to 50% of the test fish [LT50]) for trial 1 was LT50 = 38.8°C, LT50 = 39.4°C for trial 2, and LT50 = 38.9°C for trial 3. Critical thermal maximums differed significantly (P < 0.05) between trials 2 and 3, whereas trial 1 did not differ among trials. This difference correlated with body size, where fish in trial 2 were significantly larger (P < 0.05) (mean = 27.6 ± 2.0 mm in SL) (mean ± SE) than the fish of trials 1 (mean = 23.1 ± 0.5 mm in SL) and 3 (mean = 21.5 ± 0.7 mm in SL), suggesting positive size dependence in the critical thermal maximum.