Southern Flounder Research Articles

A Modeling and Field Approach to Identify Essential Fish Habitat for Juvenile Bay Whiff (Citharichthys spilopterus) and Southern Flounder (Paralichthys lethostigma) Within the Aransas Bay Complex, TX

The goal of this study was to use an ecosystem-based approach to consider the effect of environmental conditions on the distribution and abundance of juvenile bay whiff and southern flounder within the Aransas Bay Complex, TX, USA. Species habitat models for both species were developed using boosted regression trees. Juvenile bay whiff were associated with low temperatures ( 10, and moderate to high dissolved oxygen (6–9 mg O2/l, 10–14 mg/l). Juvenile southern flounder were associated with low temperatures ( 8 mg O2/l). Our results indicate that conservation measures should focus along the eastern side of Aransas Bay and the north corner of Copano Bay to protect essential fish habitat. These findings provide a valuable new tool for fisheries managers to aid in the sustainable management of bay whiff and southern flounder and provide crucial information needed to prioritize areas for habitat conservation.

Effects of a muscle‐infecting parasitic nematode on the locomotor performance of their fish host

The southern flounder Paralichthys lethostigma, host to the nematode Philometroides paralichthydis that is embedded in place of the inclinator muscles of the dorsal and anal fin elements, is hypothesized to impair two aspects of locomotor performance (swimming and burying capacity). Peak swimming acceleration and both measures of burying performance did not differ between infected and uninfected fish, whereas swimming velocity of infected fish was significantly lower than that of uninfected fish. Smaller infected fish swam at significantly slower speeds than smaller uninfected fish, whereas there was no difference among larger fish. Neither the location nor the number of worms affected either swimming or burying performance. The decrease in swimming velocity observed in smaller infected fish may be sufficient in rendering them more vulnerable to predation and environmental stressors.

Characterization of the Bycatch in the Commercial Blue Crab Pot Fishery in Georgia, November 2003– December 2006

AbstractBycatch studies have been conducted in many fisheries in Georgia, but none has focused on the commercial pot fishery for blue crab Callinectes sapidus. The purpose of this study was to identify abundance and seasonality of finfish and invertebrate bycatch species in the commercial blue crab fishery in Georgia. Between November 2003 and December 2006, observers accompanied volunteer commercial blue crab fishers randomly selected from a list of willing participants. A total of 91 trips were observed, with 5,707 commercial blue crab pots sampled. Soak times, or fishing effort, ranged from 24 to 168 h, averaging 55.8 h/trap for each trip. The number of traps sampled each trip ranged from 5 to 163, with a mean of 62.7 traps per trip. We collected 306 finfish and 4,972 invertebrates in this study period. The most numerous finfish were Southern Flounder Paralichthys lethostigma (n = 52), Atlantic Spadefish Chaetodipterus faber (n = 50), Oyster Toadfish Opsanus tau (n = 50), Hardhead Catfish Ariopsis felis (n = 46), and Southern Kingfish Menticirrhus americanus (n = 37). The most numerous invertebrates species were hermit crabs Pagurus spp. (n = 2,341), spider crabs Libinia spp. (n = 532), stone crabs Menippe mercenaria (n = 438), channeled whelk Busycon canaliculatum (n = 1,570), and knobbed whelk Busycon carica (n = 25). The similarity of bycatch by season was compared using a Morisita similarity index, and results were that Southern Flounder, Atlantic Spadefish, Oyster Toadfish, spider crabs, channeled whelk, and stone crabs varied seasonally. Of all species observed, we conclude that channeled whelk populations may be impacted by this fishery.Received October 22, 2012; accepted June 17, 2013

Isolation of novel microsatellite markers from Paralichthys lethostigma (Paralichthyidae) and their cross-species application in Pleuronectiformes

We investigated the genetic diversity of the southern flounder Paralichthys lethostigma. Microsatellite-enriched libraries were constructed and novel microsatellite markers were developed and applied for genetic detection of wild populations. Cross-species amplification was also conducted in five pleuronectiforme species. Of 45 randomly selected and sequenced clones, 43 contained a CA or GA repeat motif. Fourteen pairs of primers were designed to investigate the polymorphism and genetic structure of a wild population collected from North Carolina State coastal waters. Two loci were monomorphic and 12 loci were polymorphic. The number of alleles per polymorphic locus ranged from 2 to 16, with an average of 7.3, and the expected heterozygosity per locus ranged from 0.10 to 0.92, with an average of 0.58. Cross-species amplification showed that most of the markers could successfully amplify Paralichthys olivaceus DNAs, few markers amplified in Verasper variegatus and Verasper moseri, and none of them could amplify Scophthatmus maximus and Cynoglossus semilaevis DNAs. The isolated polymorphic markers would be useful for the genetic breeding and assessment of genetic variation within the genus Paralichthys.

Spatial and temporal shifts in suitable habitat of juvenile southern flounder (Paralichthys lethostigma)

Factors influencing suitable habitats of juvenile southern flounder (Paralichthys lethostigma) within the Galveston Bay Complex (GBC), Texas, were assessed using generalized additive models (GAM). Fishery independent data collected with bag seines throughout the GBC from 1999 to 2009 were used to predict the probability of southern flounder occurrence. Binomial GAMs were used to assess presence/absence of southern flounder and models included temporal variables, benthic variables such as distance to habitats generated within a geographic information system, and physicochemical conditions of the water column. Separate models were generated for newly settled southern flounder, young-of-the-year (YOY) southern flounder observed in the summer, and YOY southern flounder observed in fall based on size and collection month. Factors affecting southern flounder occurrence changed seasonally, as did the corresponding shifts in the spatial distribution of suitable habitat. Temporal effects (year and month) were retained in all models. Physicochemical conditions (temperature, turbidity, and measures of environmental variability), and the presence of seagrass beds were influential for newly settled southern flounder. Distance to marine and/or freshwater sources were found to be important for YOY southern flounder in the summer and fall seasons. The abundance of brown shrimp was found to only influence the distribution of YOY southern flounder in the fall, when intermediate abundances of the potential prey item increased the occurrence of southern flounder. After model completion, the availability and spatial distribution of suitable habitat within the GBC was predicted using available environmental and spatial data for 2005. Spatial distributions of predicted suitable habitat stress the relative importance of West Bay during the newly settled stage and in the fall season, and Upper Bay during the summer and fall of the first year of life. These models demonstrate the potential dynamics of suitable habitats for juvenile southern flounder and provide insight into ontogenetic shifts in habitat preference during the first year of life.

Sex determination in southern flounder, Paralichthys lethostigma, from the Texas Gulf Coast

In marine flatfish of the genus Paralichthys, temperature plays a large role in sex determination. The most susceptible areas to temperature fluctuation 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. Southern flounder exhibit genotypic sex determination; however, the sex of females is highly influenced by temperature, which can result in phenotypic sex reversal. The temperature-sensitive enzyme complex responsible for estrogen biosynthesis in vertebrates is aromatase cytochrome P450 (P450arom), a critical component in ovarian differentiation. In this study, mRNA expression of P450arom was used to identify presumptive males and females exposed to a gradient of temperatures (18, 21, 24, 27, and 30°C) beginning at 35–40 or at 65–70mm total length (TL). This research identifies that sex is influenced by temperature between 35 and 65mm 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 essential for understanding population dynamics of southern flounder and the potential effects climate change may have on wild populations.

Dual Role of IGF-II in Oocyte Maturation in Southern Flounder: Up-Regulation of Membrane Progesterone Receptor Alpha (mPRalpha) and Resumption of Meiosis.

Increasing evidence suggests a regulatory role for the insulin-like growth factor (IGF) system in teleost oocyte maturation (OM). The likely maturation-inducing steroid (MIS) in southern flounder and the mechanisms by which IGF-I or –II may be involved in the OM process were investigated. The most abundant final product of ovarian steroidogenesis assays eluted at the position of 17,20beta,21-trihydroxy-4-pregnen-3-one (20beta-S). 20beta-S was also a more sensitive and potent inducer than the other teleost MIS, 17,20beta-dihydroxy-4-pregen-3-one, of germinal vesicle breakdown (GVBD) of maturationally-competent oocytes. IGF-II (100nM) induced maturational competence, as greater GVBD was induced after incubation with IGF-II + 20beta-S compared to that of the 20beta-S +20beta-S or IGF-II + N/T group. Interestingly, incubation with IGF-II (100nM) for 4-8h increased ovarian membrane progestin receptor alpha (mPRalpha or Paqr7b) mRNA levels 12-15% and mPRalpha protein levels 75-101%. Further, the IGF-II-induced increase in mPRalpha protein concentrations was partially blocked by pretreatment with Wortmannin, a Pik3 inhibitor, and PD 098,059, a Mapk inhibitor. Both IGF-I and –II (100nM) induced meiotic resumption in maturationally-competent oocytes which was blocked by incubation with cycloheximide. Incubation with DL-Aminoglutethimide decreased IGF-II-induced GVBD but had no effect on IGF-I induced GVBD. IGF-I and –II were also able to induce meiotic resumption in maturationally-incompetent oocytes, and elicited 75 and 135% greater GVBD, respectively, than hCG + 20beta-S at100nM. In conclusion, we show that 20beta-S is the likely MIS in this species and that IGF-I and –II are also able to induce meiotic resumption. Further, IGF-II not only induces OMC but also up-regulates ovarian mPRalpha mRNA and protein through Pik3- and Mapk-dependent pathways. This is the first demonstration of mPRalpha regulation by an IGF in any vertebrate species.

Isolation and characterization of microsatellite markers in southern flounder, Paralichthys lethostigma

Thirty-six dinucleotide microsatellite markers were isolated from an enriched genomic library of southern flounder, Paralichthys lethostigma. Genotypes at all 36 microsatellites conformed to Hardy–Weinberg expectations, following Bonferroni correction for multiple tests executed simultaneously; analysis with Micro-checker indicated the possibility of null alleles at two of the microsatellites. The microsatellites characterized in this study will be useful for further evaluation of southern flounder stock structure and for assaying potential genetic impacts of stock enhancement programs.

Population Genetics of Southern Flounder with Implications for Management

AbstractThe 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 strategies should account for the upper limits to dispersal within the Gulf.Received January 12, 2012; accepted April 12, 2012

Dual role of IGF-II in oocyte maturation in southern flounder Paralichthys lethostigma: Up-regulation of mPRα and resumption of meiosis

Increasing evidence suggests a regulatory role for the IGF system in teleost oocyte maturation (OM). Our objectives were to determine if IGF-I and IGF-II regulate different stages of OM in southern flounder (Paralichthys lethostigma) and to identify the likely maturation-inducing steroid (MIS) in this species. The most abundant final product of ovarian steroidogenesis assays eluted at the position of 17,20β,21-trihydroxy-4-pregnen-3-one (20β-S). 20β-S was also more potent in inducing germinal vesicle breakdown (GVBD) of maturationally-competent oocytes than other teleost MISs. IGF-II (100nM) induced maturational competence (OMC), as greater GVBD was induced after incubation with IGF-II+20β-S compared to that of the 20β-S+20β-S or IGF-II+no treatment group. Incubation with IGF-II (100nM) for 4–8h significantly increased ovarian membrane progestin receptor alpha (mPRα or Paqr7b) mRNA levels 12–15% and mPRα protein levels 75–101%. Further, the IGF-II-induced increase in mPRα protein concentrations was partially blocked by pretreatment with Wortmannin, a Pik3 inhibitor, and PD 098,059, a Mapk inhibitor. Both IGF-I and -II (100nM) induced GVBD of maturationally-competent oocytes was blocked by incubation with cycloheximide. Incubation with d,l-Aminoglutethimide decreased IGF-II-induced GVBD but had no effect on IGF-I-induced GVBD. IGF-I and -II were also able to induce GVBD of maturationally-incompetent oocytes, and elicited 75% and 135% greater GVBD, respectively, than hCG+20β-S at 100nM. In conclusion, we show that 20β-S is the likely MIS in this species and that IGF-I and -II are also able to induce GVBD. Further, IGF-II not only induces OMC but also up-regulates ovarian mPRα mRNA and protein through Pik3- and Mapk-dependent pathways. This is the first demonstration of mPRα regulation by an IGF in any vertebrate species.

Response to “Comment on ‘Otolith Microchemistry Reveals Substantial Use of Freshwater by Southern Flounder in the Northern Gulf of Mexico’” by Pedro Morais

Lowe et al. (Estuar Coast 34:630–639, 2011) used Sr/Ca in the otoliths of juvenile southern flounder (Paralichthys lethostigma) to demonstrate the importance of low-salinity habitats in the northern Gulf of Mexico during early life. While otolith cores indicated that most southern flounder originated in high-salinity environments (elevated Sr/Ca in otolith core) and moved into lower salinity habitats (decreased Sr/Ca outside of the otolith core), approximately one third of the sampled individuals had otolith core chemistry that indicated spawning or hatching in or near freshwater habitats (low Sr/Ca throughout the otolith). Morais (Estuar Coast 34:630–639, 2012) proposes a different interpretation: he suggests that the high Sr/Ca found in 68 % of juvenile otolith cores (and low Sr/Ca just outside of the core region) may be due to females hydrating their eggs in offshore (high-salinity) waters, with spawning, hatching, and egg/larval development all actually occurring in low-salinity environments (his “sea-run mother” hypothesis). While there is ample literature supporting the importance of maternal contributions to otolith microchemistry during vitellogenesis, it is our contention that such contributions ultimately depend on the ecology and reproductive physiology of each species. Further, a mass inshore migration of adult southern flounder during their spawning season is counter to the well-documented life history patterns for this species throughout its range. While we appreciate Morais' perspective, we continue to stand by the conclusion of our original paper.

Comments on Lowe et al. “Otolith Microchemistry Reveals Substantial Use of Freshwater by Southern Flounder in the Northern Gulf of Mexico”

Lowe et al. (Estuaries and Coasts, 34:630–639, 2011) hypothesized that juvenile southern flounder Paralichthys lethostigma (Jordan and Gilbert 1884) would migrate from the Gulf of Mexico into the Mobile-Tensaw River Delta (AL, USA) and use low-salinity (oligohaline/freshwater) habitats during, at least, a portion of their first year of life. Thus, they analyzed the Sr/Ca ratio profiles along the sagittal otoliths of southern flounder collected in the Mobile-Tensaw River Delta and observed that one third of the flounders had low Sr/Ca levels in the otoliths’ core and throughout the otolith, suggesting that these fishes hatched in freshwater or low-salinity habitats where they spend the majority of their life. The other two thirds of southern flounder showed high levels of Sr/Ca ratio in the otoliths’ core following a marked decline of Sr/Ca ratio, which then maintained along the remainder of the otolith. This pattern was interpreted as larvae hatched in higher salinity waters before entering the Mobile-Tensaw River Delta; however, in this paper, I list several arguments to support an alternative interpretation for this pattern. I suggest that the high levels of Sr/Ca ratios in the otoliths’ core of southern flounder does not reflect the saline conditions where larvae hatched, instead it reflects the location where the female progenitor hydrated the eggs. Thus, adding my interpretation on the data of Lowe et al. (Estuaries and Coasts, 34:630–639, 2011), it seems that southern flounder might hatch in or near freshwater habitats and the migration of southern flounder into an estuarine ecosystem to spawn might exist.

Histological Analysis Reveals Larger Size at Maturity for Southern Flounder with Implications for Biological Reference Points

AbstractFor fish stocks managed using biological reference points based upon spawning biomass, it is critical to have accurate maturity schedules. We investigated size‐ and age‐dependent patterns in maturity for southern flounder Paralichthys lethostigma, a flatfish supporting valuable coastal fisheries in North Carolina and throughout its range. We evaluated both macroscopic and histological methods over two consecutive reproductive seasons. Histological analyses revealed that maturity occurred at larger sizes and older ages than previously estimated. Length at 50% maturity (L 50) was estimated at 408 mm total length (TL), which was more than 60 mm larger than currently assumed, and was relatively stable between study years. We found that only 44% of age‐1 southern flounder were mature compared with an estimated 74% in an earlier study. We suspect that most of the differences in maturity timing of southern flounder between our findings and previous studies stem from macroscopic assignment error. During this study, only 61% of fish staged macroscopically as developing were found to be mature based on histological analysis. Assuming incorrectly that all of these fish were mature would have resulted in an L 50 of 375 mm TL, which is closer to previous estimates. Analysis of spawning stock biomass per recruit demonstrated that biological reference points (e.g., F SPR) could be affected considerably by shifting maturity schedules, and the effects could be magnified at larger sizes at entry and higher harvest rates. Given the life history strategy of southern flounder and the lack of a developed offshore fishery or sampling program, which combine to prevent access to fish on the spawning grounds, it is probably most judicious to routinely analyze reproductive tissue samples histologically to ensure accurate information on the timing of maturity.Received April 6, 2012; accepted July 28, 2012

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

AbstractThe 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 Atlantic Ocean, and there is a measured genetic divergence (i.e., FST) of 0.374 between them. Within ocean basins, there was little significant genetic structure among samples. Second, coalescent modeling suggests that the two populations of southern flounder have expanded rapidly in numbers in the past 10,000–22,000 years, timing that approximately coincides with the end of the last North American glacial retreat. Third, historical population growth is coupled with high estimates of long‐term Nef, both of which contrast with present‐day declines in abundance measured by stock assessments. Present‐day southern flounder populations persist in two independent evolutionarily significant units that have divergent evolutionary backgrounds.

Effects of Replacement of Menhaden Fish Meal Protein by Solvent‐Extracted Soybean Meal Protein Supplemented with or without l‐Methionine and l‐Lysine in the Diet of Juvenile Southern Flounder

AbstractTwo feeding experiments were conducted to study the effects of different levels of solvent‐extracted soybean meal on the growth and body composition of juvenile southern flounder Paralichthys lethostigma. In experiment 1, seven diets were formulated, replacing menhaden fish meal protein (FMP) with soybean meal protein (SBP) at 0, 10, 20, 30, 40, 50, and 60% without supplementing amino acids. In experiment 2, eight diets were formulated, replacing FMP with SBP at 0, 10, 20, 30, 40, 50, 60, and 70% and supplementing with l‐methionine and l‐lysine. In the test diets, l‐methionine and l‐lysine were supplemented to approximate the methionine and lysine level found in the 45% whole‐body protein of southern flounder. All diets were formulated to have the same crude protein (45%) and lipid levels (12%), and all diets contained 9% squid meal and 5% krill meal to improve palatability. Each of the test diets was fed two times a day to triplicate groups of flounder (average weight = 0.61 g for experiment 1 and 1.63 g for experiment 2). Experiments 1 and 2 were conducted for 42 and 60 d, respectively. Fish were held in 75‐L recirculating seawater tanks at 15 fish per tank. In both experiments, no significant differences were observed in body weight gain (WG), specific growth rate, feed intake, feed conversion and protein efficiency ratios, and the whole‐body proximate composition among fish fed diets replacing 0–40% of FMP with SBP. When compared with fish fed 0% SBP, WG decreased significantly for the fish fed more than 40% SBP. Broken‐line regression showed that the optimum levels of FMP replacement with SBP in the diet of southern flounder without and with supplemental methionine and lysine were 35.1% and 38.9%, respectively, when diets contained 9% squid meal and 5% krill meal.

Use of molecular tools in identification of philometrid larvae in fishes: technical limitations parallel our poor assessment of their biodiversity

The usefulness of the polymerase chain reaction with restriction fragment length polymorphism (PCR-RFLP) and partial sequencing of the cytochrome oxidase I (COI) gene was tested regarding the utility of these techniques in unraveling philometrid life cycles and, in particular, to determine putative paratenic host species. Our focus was to study three species of philometrids commonly found in the estuaries of South Carolina: Philometroides paralichthydis and Philometra overstreeti from the southern flounder, Paralichthys lethostigma and Philometra carolinensis from the spotted seatrout, Cynoscion nebulosus. A total of 473 fish in 19 species known to be potential prey of the spotted seatrout and the southern flounder were dissected. Of all nematode larvae found in the mesenteries of 53 fish of 10 species, 21 specimens were determined to be philometrids using PCR. The use of PCR-RFLP allowed the identification of larvae of P. carolinensis and P. overstreeti in the freshwater goby, Ctenogobius shufeldti, and P. overstreeti in mummichog, Fundulus heteroclitus. However, 12 RFLP profiles could not be matched to control species, thus demonstrating the limitation of this technique in areas where diversity of philometrids is not well known and higher than anticipated. Similarly, COI procedures provided unknown sequences that did not match those of nine philometrid species used as controls. We concluded that although both techniques showed some usefulness and promise, at this point, however, they demonstrate the need of increasing our knowledge of marine and estuarine philometrid biodiversity.

Postrelease Survival of Sublegal Southern Flounder Captured in a Commercial Gill-Net Fishery

Discards from commercial fisheries may be subject to high rates of mortality that vary over time and space, contributing to inaccuracies in stock assessments. We present the results from a field experiment in the New River estuary, North Carolina, designed to estimate the postrelease survival of commercially gill-netted, sublegal southern flounder Paralichthys lethostigma. Large, replicate field enclosures were used to monitor gill-netted southern flounder for a 3-d period after capture. The survival of observed discards showed significant variation among seasons. During fall and spring, overall survival was estimated to fall between 0.74 and 0.87, while during summer survival was estimated to fall between 0.22 and 0.30 and included fish that were dead when gill nets were retrieved. Logistic regression analysis was used to evaluate the relative effects of individual traits on the postrelease survival of southern flounder in the commercial gill-net fishery. Model selection indicated that season and fish condition were the best characteristics with which to predict postrelease survival and that body size may also influence postrelease survival. Low seasonal estimates of the survival of discarded southern flounder suggest that management measures that mitigate poor discard survival will be most effective during the summer. Received November 11, 2010; accepted February 17, 2011

Gynogenetic induction in southern flounder (<I>Paralichthys lethostigma</I>) by cryopreserved sperm of <I>Lateolabrax japonicas</I>

Female southern flounder( Paralichthys lethostigma) grow larger than males.Therefore,all-female production will maximize profit potential for the culture of this species.It may be effective to develop protocols to produce all-female southern flounder through induction of meiotic gynogenesis with the cryopreserved heterologous sperm of Lateolabrax japonicas.To test methods for inducing diploid gynogenesis in southern flounder using heterogenous sperm,the UV was used to inactivate sperm and pressure shock was used to prevent extrusion of the second polar body.The results of experiments showed that gynogenetic diploid can be induced by inactivated heterogenous sperm because that hybrid would die before hatching.Diploid gynogenesis was induced by activating egg development with UV irradiated sperm( 80 mJ/cm2) 4-5 min after fertilization ( 18 ℃) ,and then 65 MPa pressure treatment for 6 min.Flow cytometry and microsatellite DNA analysis were used to demonstrate that the gynogenetic fry are diploidy.These results indicate that the use of UV irradiated sperm from Lateolabrax japonicas for activation of flounder eggs and pressure shock for polar body retention is an effective method to produce gynogenetic diploid in southern flounder.

Philometrids of the Southern Flounder Paralichthys lethostigma: A Multidimensional Approach to Determine Their Diversity

Two species of philometrid nematode, Philometra overstreeti and Philometroides paralichthydis, infect the southern flounder, Paralichthys lethostigma. Individuals of P. overstreeti are located between the teeth and inside the bony part of the branchial arches of the fish. Individuals of P. paralichthydis are associated with the bones of the buccal cavity and among muscles that control the dorsal and anal fins. Sequencing of part of the cytochrome oxidase I gene revealed 4 distinct genetic clades, each corresponding exactly to the 4 respective locations of the parasites in the host, suggesting the need for taxonomic revision. We hypothesized that each clade represented a separate species and, because the worms are morphologically indistinguishable, compared population level parameters of the clades comprising each currently recognized species. For each currently recognized species, the presence of worms from 1 clade was negatively correlated with the presence of worms from the other. Results also indicated significant differences between the clades in prevalences relative to both biotic and abiotic factors. Results clearly indicated major differences in the ecology of the philometrids constituting each clade. Taken as a whole, molecular and ecological data support the contention that the 4 genetic clades are likely 4 distinct species.

Assessing southern flounder ( Paralichthys lethostigma) long-term population trends in the northern Gulf of Mexico using time series analyses

A long-term fisheries independent data set (1975–2008) was used to assess population trends of juvenile and adult southern flounder ( Paralichthys lethostigma) along the Texas coast in the northern Gulf of Mexico, USA. The dataset contained a total of 46,784 sites that were sampled with bag seines to monitor small nekton abundance and 22,870 sites that were sampled with gill nets to assess adult fisheries trends. These data were examined for age-specific population trends using generalized least squares and extended with non-parametric bootstrapping to obtain interval estimates of regression parameters (juveniles) and linear regression (adults). These data showed long-term declines in juvenile southern flounder abundance (1.3% per year). For adult southern flounder, rate of decline was much more rapid (2.5% per year). Results suggest that survival of post-juvenile flounder has decreased during the time series. This precipitous decline has prompted increasingly stricter harvest restrictions along the Texas coast. However, these management measures have been insufficient to curb declines.