Joel D. Anderson

Genetic Diversity and Natal Origins of Green Turtles (Chelonia mydas) in the Western Gulf of Mexico

Abstract Chelonia mydas (Green Turtle) foraging areas, where juveniles, subadults, and adults of diverse natal origins coalesce and spend a large portion of their lives, can be located thousands of kilometers from nesting beaches. Unfortunately, the natal origin of turtles in many foraging areas remains unknown. Resolution of this issue was recently listed among the conservation priorities for the species by a global panel of Green Turtle researchers. We examined the genetic diversity and natal origins of Green Turtles from a well-known foraging area in the western Gulf of Mexico. Bayesian mixed-stock analysis of mitochondrial DNA haplotypes was used to demonstrate that an overwhelming percentage (∼95%) of individuals in the western Gulf of Mexico foraging group likely originate in other Gulf of Mexico and northern Caribbean rookeries, with smaller contributions from the western and southern Caribbean, and potentially the Mediterranean Sea. Management of Green Turtles in the western Gulf of Mexico will be improved by linking conservation efforts aimed at this foraging group to turtle aggregates occurring in other critical habitats within the recently defined northwest Atlantic Green Turtle regional management unit.

Population Structure and Evolutionary History of Southern Flounder in the Gulf of Mexico and Western Atlantic Ocean

Abstract The southern flounder Paralichthys lethostigma is a key species in recreational flatfish fisheries in the Gulf of Mexico and the northwestern Atlantic Ocean. Effective management of this species relies on knowing how many populations occur within the range of the species, as knowledge of the underlying genetic structure may be used in stock assessments or in the establishment of management units. Here, the evolutionary history of the southern flounder was elicited with genetic data, and the geographic extent and long-term female effective population size (Nef ) of extant populations were estimated. Mitochondrial DNA sequence data (415 base pairs, control region) were analyzed with a combination of traditional hypothesis testing (analysis of molecular variance) and the coalescent-based maximum likelihood approach (analysis with the program Migrate). Three main findings resulted from this work. First, two distinct genetic populations of southern flounder exist in the Gulf of Mexico and western Atla...

A Genetic Assessment of Current Management Strategies for Spotted Seatrout in Texas

Abstract The spotted seatrout Cynoscion nebulosus is one of the most intensively managed finfishes in the Gulf of Mexico, due primarily to its importance as a sport fish species throughout its range. As a result, genetic divergence and patterns of gene flow have previously been assessed among designated populations of spotted seatrout by using various types of genetic markers in an attempt to provide meaningful data for delineation of management units. However, genetic data can be influenced both by contemporary gene flow and historical demography, and these processes can often result in similar genetic signatures. In this study, a nested clade analysis (NCA) was used to disentangle the effects of historical and contemporary processes on the distribution of mitochondrial DNA sequence haplotypes of spotted seatrout in the western Gulf of Mexico. The NCA was coupled with traditional F-statistics and a Mantel matrix procedure to compare the results of multiple analytical frameworks. Overall, genetic divergence among populations was low but significant and was highest between populations that were far apart geographically. Correlation between genetic divergence and geographic distance was supported by a significant Mantel matrix correlation coefficient, as well as two nested clades, which had distributions that correlated significantly with latitude. All three statistical procedures suggest that the genetic structure of spotted seatrout in the western Gulf of Mexico can best be described by continuous change and isolation by distance rather than representing discrete populations. These results are compared to the results of several previous studies using alternative types of genetic data and analytical procedures and are also used to put current management strategies for spotted seatrout into a genetic context.

Genetic Assessment of Sheepshead Stock Structure in the Northern Gulf of Mexico: Morphological Divergence in the Face of Gene Flow

Abstract Strategies for conservation and management of marine species are increasingly relying on molecular genetic data for the delineation of independent fishery stocks. These data may be more reliable and less subjective than morphological or ecological variables, although their biological interpretations can be problematic. Here, we review existing morphological data synergistically with a new molecular data set for sheepsheads Archosargus probatocephalus. Two named subspecies of sheepshead exist in the northeastern Gulf of Mexico (A. probatocephalus probatocephalus and A. probatocephalus oviceps). Although these subspecies exhibit divergent morphology, it is unclear whether morphological divergence represents meaningful differentiation for management. We show that frequency distributions of each of five meristic counts are significantly different between the subspecies (P < 0.05). However, Bayesian structure analysis of microsatellite genotypes indicated that all Gulf of Mexico sheepshead populations...

Spatial genetic features of eastern oysters (Crassostrea virginica Gmelin) in the Gulf of Mexico: northward movement of a secondary contact zone.

The eastern oyster (Crassostrea virginica Gmelin) is an economically and ecologically valuable marine bivalve occurring in the Gulf of Mexico. This study builds upon previous research that identified two divergent populations of eastern oysters in the western Gulf of Mexico. Allelic and genotypic patterns from 11 microsatellite markers were used to assess genetic structure and migration between the previously described oyster populations in Texas. The main findings are as follows: (1) there are two distinct populations (FST = 0.392, P < 0.001) of oysters that overlap in the Corpus Christi/Aransas Bay estuarine complex in Texas, (2) the distribution of genotypes among individuals in the contact zone suggests limited hybridization between populations, (3) the variables of salinity, temperature, dissolved oxygen, turbidity and depth are not correlated with allele frequencies on reefs in the contact zone or when analyzed across Texas, and (4) there is little evidence of directional selection acting on the loci assayed here, although patterns at four markers suggested the influence of balancing selection based on outlier analyses. These results are consistent with long-term historical isolation between populations, followed by secondary contact. Recent hydrological changes in the area of secondary contact may be promoting migration in areas that were previously inhospitable to eastern oysters, and observed differences in the timing of spawning may limit hybridization between populations. Comparison of these findings with the results of an earlier study of oysters in Texas suggests that the secondary contact zone has shifted approximately 27 km north, in as little as a 23-year span.

A Multivariate Assessment of Factors Influencing Survival of Red Drum in Earthen Outdoor Rearing Ponds

Abstract.Extensive culture of larval Red Drum Sciaenops ocellatus in outdoor ponds occasionally results in low survival, thereby stifling stock enhancement goals. Mortality is probably tied to multiple environmental influences rather than to any singular factor. We assessed factors influencing Red Drum mortality and survival in the context of an extensive aquaculture operation in Texas. Larvae were stocked into 120 rearing ponds of various sizes in the spring, summer, and fall of 2012 and 2013. Water quality variables (temperature, salinity, dissolved oxygen, and pH) were assessed throughout culture. A principal components analysis of water quality, pond size, stocking density, and zooplankton density (number of organisms/L) was used to determine how each variable related to survival (measured as a ratio of the total number of Red Drum harvested to the total number of Red Drum stocked). Temperature, salinity, dissolved oxygen, and pH all loaded significantly onto the first ordination axis, suggesting corr...

Population Genetics of Southern Flounder with Implications for Management

Abstract The southern flounder Paralichthys lethostigma is a marine flatfish that ranges from North Carolina to the Yucatan Peninsula in the Gulf of Mexico. Management of southern flounder will be improved by proper delineation of management units based on genetic data. We used microsatellite DNA markers to determine how many populations exist across the species’ range and the geographic extent of those populations. Based on significant genetic divergence across loci (F CT = 0.035), two discrete populations of southern flounder occur. The populations inhabit the Gulf of Mexico and Atlantic Ocean, with the boundary between them located in southern Florida, an area that is generally devoid of southern flounder. Within the Gulf of Mexico, there was little evidence of discrete populations but weak evidence for isolation by distance. These data indicate that specieswide management of southern flounder should account for the independent demographic entities in the Gulf and Atlantic and that local management str...

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.

Limited genetic structure of Gulf Menhaden (Brevoortia patronus), as revealed by microsatellite markers developed for the genus Brevoortia (Clupeidae)

Long-term sustainable management of wild populations should be based on management actions that account for the genetic structure among populations. Knowledge of genetic structure and of the degree of demographic exchange between discreet [sic] populations allows managers to better define management units. However, adequate gene loci for population assessments are not always available. In this study, variable co-dominant DNA loci in the heavily exploited marine genus Brevoortia were developed with a microsatellite-enriched DNA library for the Gulf Menhaden (Brevoortia patronus). Microsatellite marker discovery was followed by genetic characterization of 4 endemic North American Brevoortia species, by using 14 novel loci as well as 5 previously described loci. Power analysis of these loci for use in species identification and genetic stock structure was used to assess their potential to improve the stock definition in the menhaden fishery of the Gulf of Mexico. These loci could be used to reliably identify menhaden species in the Gulf of Mexico with an estimated error rate of α=0.0001. Similarly, a power analysis completed on the basis of observed allele frequencies in Gulf Menhaden indicated that these markers can be used to detect very small levels of genetic divergence (Fst≈0.004) among simulated populations, with sample sizes as small as n=50 individuals. A cursory analysis of genetic structure among Gulf Menhaden sampled throughout the Gulf of Mexico indicated limited genetic structure among sampling locations, although the available sampling did not reach the target number (n=50) necessary to detect minimal values of significant structure.

Environmental Drivers of the Spatial and Temporal Distribution of Spawning Blue Crabs Callinectes sapidus in the Western Gulf of Mexico

AbstractThe blue crab Callinectes sapidus is an economically important shellfish found in the Gulf of Mexico and elsewhere. Recent declines in abundance of this species have driven progressive management actions throughout its range, including protection of spawning females by establishing “no-take” zones on the Atlantic coast. To date, no-take zones have not been established in the western Gulf of Mexico, although annual closures associated with mitigation of derelict traps exist in both Texas and Louisiana. In this study, we used long-term data acquired via fishery-independent sampling by the Texas Parks and Wildlife Department to assess spatial and seasonal distribution of spawning females in order to inform future evaluations of no-take areas. The presence of spawning females was modeled statistically by using water quality and spatial variables as predictors. Distance to the nearest Gulf pass was the most reliable predictor of presence, accounting for over 43% of the deviance observed in models from ...