Alligator Gar Atractosteus Spatula Research Articles

Applying Floodplain Inundation Modeling to Estimate Suitable Spawning Habitat and Recruitment Success for Alligator Gar in the Guadalupe River, Texas

We developed a floodplain inundation model to extract specific flood extent and depth parameters and combined these with vegetation land cover and historic flow data to quantify spatial habitat suitability and temporal hydrologic metrics that support Alligator Gar Atractosteus spatula spawning within a 257 km segment of the lower Guadalupe River, Texas, USA. We modeled nine flows across a range of flood frequency recurrence intervals from 257 m3s−1 to ~4997 m3s−1 and estimated the availability of suitable spawning water depths (0.2 to 2 m) and lateral connectedness between the river and suitable floodplain landcover types. We estimated the ages via otoliths of 95 Alligator Gar collected in the reach to determine the year that they were recruited into the system. We analyzed a total of 30 Indicators of Hydrologic Alteration flow metrics to examine how the spatially derived suitable habitats related to the temporal aspects of flow occurrence during the spawning season for the period of flow record April–July (1935–2020) and to the years spanning the recruitment data of the Alligator Gar (1981–2010). A non-linear relationship existed between suitable spawning habitat area and the flow regime, with the most habitat availability corresponding to the 10–20-year flood recurrence interval frequency with peak flows of 2057–3108 m3s−1, respectively. The Alligator Gar recruitment data indicated that six years provided high recruitment, which correlated with peak flows of ~5-year frequency with an available spawning area of ~9000 Ha, moderate recruitment years related to peak flows with ~3-year frequency with an available spawning area of 6000 Ha, and low recruitment years where spawning was likely to occur at least every other year with at least 2500 Ha of available spawning area. The results of this model support the development of legislatively mandated environmental flow standards for the Guadalupe River Basin, inform field-based efforts for collecting empirical and observational data on the species’ reproduction, and provide spatial and temporal information for designing conservation strategies for Alligator Gar.

Predicting the Potential Distribution of the Alien Invasive Alligator Gar Atractosteus spatula in China

Alligator gar Atractosteus spatula originates from North America but has been introduced into China recently. Considered an invasive fish, it may cause losses in the diversity and number of local species and in fish catch due to its predation on numerous aquatic animals in non-native habitats. A comprehensive study of this alien invasive species’ existing spatial patterns in relation to climatic variables is critical to understanding the conditions amenable to its distribution and controlling its further spread into potential range areas. We used MaxEnt and QGIS species distribution modeling to estimate the likely biogeographical range of A. spatula in China based on 36 validated distribution records and seven selected environmental variables. The highly suitable area was found primarily in a series of provinces extending from inland to coastal regions, covering southwest to south, central and east China. The model identified the minimum temperature of the coldest month (Bio6) and mean temperature of the warmest quarter (Bio10) as the strongest predictors of A. spatula distribution. The findings could offer scientific guidance for managing and preventing the spread of this invasive fish and hint at controlling invasive aquatic fauna.

Modeling the Population Response of Alligator Gar in Texoma Reservoir to Harvest and Discard Mortality

AbstractThe popularity of angling for Alligator Gar Atractosteus spatula is an essential component of fisheries management and can affect population resilience. In most cases with Alligator Gar, development of biological assessment models to assess stocks is often unavailable. For this reason, we aimed at describing Alligator Gar demographic rates and population size in Texoma Reservoir, Oklahoma. We used these data to parameterize an age‐structured simulation model to determine population responses of maximized yield, total catch, catch of trophy fish, and fishery sustainability to a range of instantaneous finite fishing mortality rates (FM) and discard mortality rates (D) in approximating ideal harvest strategies. Our simulations suggest that the Alligator Gar population is highly sensitive to increases in exploitation. Under the current management regulations, an FM of 0.00–0.05 would have a limited impact on sustainability, which would result in negligible growth as an effect of stochastic recruitment. If FM reaches or exceeds 0.06, the current regulations would not be effective at sustaining the population (spawning potential ratio <0.30). At FM values greater than 0.10, the probability of the population declining from the current population estimate (1,947 fish) to less than 500 individuals would increase from 20% to 100%, resulting in the collapse of the population over 100 years, especially if D is greater than 0.10. Use of a 1,524‐mm minimum length limit had the potential to allow FM values of up to 0.10 while absorbing a low amount of D. The use of length limits may provide some options for sustaining the Alligator Gar population, although their efficacy is limited by D when FM exceeds 0.06. Given the sensitivity of long‐lived, periodically recruiting fishes to exploitation and given the benefits that possible length‐based regulations or a tag/lottery system can provide, we encourage managers to continue monitoring Alligator Gar populations, and when concerning trends are observed, managers should consider such regulations to increase resilience to FM and increase sustainability.

Nonlethal Fin Clip Model Validation for Stable Isotope Analysis of Spotted Gar and Alligator Gar

AbstractStable isotope analysis (SIA) can be a powerful tool for investigating trophic ecology and energy fluxes through an ecosystem. In fish, muscle tissue is generally preferred for SIA due to its intermediate turnover rate, even though sampling muscle is often lethal. Here, we evaluated the feasibility of a fin clip model using fin tissue as a nonlethal alternative to using muscle tissue for SIA (nitrogen, δ15N and carbon, δ13C) in Spotted Gar Lepisosteus oculatus and Alligator Gar Atractosteus spatula. We also investigated the effects of locality, age, and size of Spotted Gars on the robustness of the fin clip model. Muscle tissue plugs and fin tissue clips were collected from Spotted Gars (n = 104) and Alligator Gars (n = 16) at four localities in southeastern Louisiana in 2019. Both Spotted Gars and Alligator Gars showed strong positive Pearson’s correlations in δ15N and δ13C between the fin and muscle tissues, and a linear regression analysis confirmed that the correlations were significant (Spotted Gar; δ15N: R2 = 0.88; δ13C: R2 = 0.95, P < 0.001; Alligator Gar; δ15N: R2 = 0.74; δ13C: R2 = 0.94, P < 0.001). The fin clip model for Spotted Gar was affected by locality but not by size or age. Overall, our results suggest that fin tissue can be used as a nonlethal surrogate for muscle tissue for SIA for both Spotted Gars and Alligator Gars.

Effects of Training on Side-Scan Sonar Use as a Fish Survey Tool: A Case Study of Alligator Gar

Abstract Consumer-grade side-scan sonar has become a versatile fisheries management tool. First applied to assess habitat, its use has expanded to surveying fishes in recent years. However, an important consideration is the skill and experience of users, which can affect both the accuracy and comparability of surveys. To this end, we characterized the ability of a small sample of novice users (N = 8) to identify Alligator Gar Atractosteus spatula in imagery, as well as the effect of a 2-h training exercise on user performance. Prior to training, mean accuracy (expressed as the difference between observed and expected counts) among participants ranged from −2.6 to 1.3 fish and precision ranged from ±1.2 to ±2.4 fish, with the majority of participants underestimating the number of Alligator Gar present in the imagery. False positives (i.e., identifying Alligator Gar in imagery when none were present) were common among participants. Posttraining mean accuracy ranged from −3.1 to 0 among participants and precision ranged from ±1.6 to ±3.2 fish. The frequency of false positives was significantly reduced following training, and participants reported significant increases in confidence associated with image interpretation. The relatively high accuracy and precision we observed prior to training indicated that side-scan sonar can be easily incorporated into large-scale fishery monitoring efforts for Alligator Gar. However, our results also suggested that a rather minimal investment in training can further improve consistency and reduce uncertainty among novice users.

Movements and Macrohabitat Use of Alligator Gar in Relation to a Low Head Lock and Dam System

AbstractLoss of habitat, targeted removals, and overharvest has contributed to the decline of Alligator Gar Atractosteus spatula. There is agreement among commercial fishers, some recreational anglers, conservationists, and natural resource managers that this species warrants conservation. Appropriate management actions require research to further understand the life history and ecology of Alligator Gar populations. We conducted a movement study of 16 Alligator Gar in the lower Ouachita River, Arkansas–Louisiana, for 1 year using acoustic telemetry. The Alligator Gar were stationary at least 28% of the year. We documented 1,552 movement events during the study. The mean (SD) movement rate during the movement events was 1.13 (0.18) km/h, but it varied among individuals. Only 2 of the 16 Alligator Gar were detected upstream of Felsenthal Dam. The tailwater habitat just below Felsenthal Dam was used heavily by the Alligator Gar. The linear home range was 11.9 (7.1) river kilometers. Due to the hydrology of the system, access to the floodplain is not a hindrance to the lateral movement of Alligator Gar in the Ouachita River. The Felsenthal Dam did seem to hinder longitudinal movement, even though high water allowed the fish to move over and around the dam during spring and early summer. Management plans to rebuild Alligator Gar populations in regulated river systems should consider the negative consequences of barriers to lateral or longitudinal movement.

Advances in Conservation and Management of the Alligator Gar: A Synthesis of Current Knowledge and Introduction to a Special Section

AbstractGrowing appreciation of biodiversity and the role of apex predators, along with the increasing popularity of multispecies and trophy‐oriented angling, has elevated the status of gars—in particular, the Alligator Gar Atractosteus spatula—among anglers and biologists alike. As a result, considerable effort has been spent in recent years to gain a working knowledge of the biology and ecology of the species in order to advance science‐based management. In January 2019, the Alligator Gar Technical Committee of the Southern Division of the American Fisheries Society hosted a symposium entitled “Advances in the Conservation and Management of North American Gars” to showcase the results of research and management efforts conducted throughout the species’ range. Fishery researchers and managers presented a diversity of works, furthering our understanding of population dynamics, stock assessment, genetics, hatchery practices and stocking, habitat use, and angler desires associated with Alligator Gar fisheries. In our introduction to this special section, we introduce these works and provide a synthesis of the current state of scientific knowledge regarding the Alligator Gar. We hope that this will provide context to the works presented in the symposium and will serve to guide the development of future research that addresses the remaining knowledge gaps concerning the species and its growing fishery.

Hydrologic Correlates of Reproductive Success in the Alligator Gar

AbstractPrevious work relating adult age data to historic hydrology indicated that strong year‐classes of Alligator Gar Atractosteus spatula were rare, tied to major flooding events, and linked to the inundation and connectivity of floodplain habitats. Although spawning habitat suitability criteria have been correlated with successful recruitment in Texas Alligator Gar populations, direct evaluation of recruitment documenting young‐of‐the‐year (age‐0) presence or abundance has not been reported. To specify conditions associated with successful spawning and recruitment of Alligator Gar, we sampled for age‐0 fish monthly from May to July 2013–2017 in the Trinity River and from May to July 2015–2017 in Choke Canyon Reservoir. Suitability criteria predicted strong year‐classes in 2015 and 2016 for the Trinity River and in 2015 for Choke Canyon Reservoir; the predictions were confirmed by the presence of abundant age‐0 Alligator Gar in those systems and years. Hatch dates of juvenile Alligator Gar were estimated from otoliths and compared to corresponding environmental and hydrologic conditions. Hatch date analyses indicated a protracted spawning duration in 2015 associated with extended widespread flooding and a narrower range of spawning dates in 2016. Due to rapid growth and variable gear efficacy relative to fish size, multiple gears were required to assess the full size range of age‐0 Alligator Gar. A standardized sampling framework that included visual searching and floating mini‐fyke nets effectively sampled age‐0 Alligator Gar from 15 to 650 mm TL. We recommend a two‐tiered approach for predicting and monitoring Alligator Gar recruitment. First, the monitoring of hydrology and the application of suitability criteria can provide an early predictor of potential Alligator Gar reproductive success. If conditions are favorable for spawning, then sampling for age‐0 Alligator Gar can confirm recruitment and can give an indication of the spatial extent (visual searching) and relative abundance (visual searching and/or mini‐fyke nets) of successful reproduction.

Feeding Ecology of Age‐0 Gar at Texoma Reservoir Inferred from Analysis of Stable Isotopes

AbstractConservation and restoration of gar (Lepisosteidae) populations in North America are increasingly of interest to fisheries managers. Alligator Gar Atractosteus spatula are being stocked as age‐0 fish in efforts to re‐establish extirpated populations. However, gars are known to be highly cannibalistic in hatcheries, suggesting that age‐0 Alligator Gar introduced into natural habitats may face predation pressures from other gar species, limiting the likelihood of released fish recruiting to the population. Furthermore, introduced age‐0 gar may not have the proper prey resources for rapid growth that would facilitate recruitment to adulthood. Texoma Reservoir, located on the Oklahoma–Texas border, hosts four native gar species, including the Alligator Gar, whose population is supported by supplemental stocking of age‐0 fingerlings. We investigated feeding by age‐0 gar to obtain a baseline level of trophic ecology for this group of fishes, including the potential for poststocking cannibalism. Food webs were reconstructed via analysis of carbon (δ13C) and nitrogen (δ15N) isotopes using Bayesian mixing model approaches. Isotopic values for age‐0 fish of all four gar species were similar. Invertebrates represented a large fraction of the diet for all gars, followed by Common Carp Cyprinus carpio and Grass Carp Ctenopharyngodon idella. Competitive interactions among Bluegill Lepomis macrochirus, White Bass Morone chrysops, White Crappie Pomoxis annularis, and young gar may occur for invertebrates, shads Dorosoma spp., and Western Mosquitofish Gambusia affinis but are likely short‐lived due to the rapid growth of age‐0 gar after their transition to piscivory. Trophic position of age‐0 Alligator Gar, Longnose Gar Lepisosteus osseus, and Spotted Gar Lepisosteus oculatus was positively related to TL, but this relationship was not observed for Shortnose Gar Lepisosteus platostomus. In Texoma Reservoir, the four native gar species, including stocked Alligator Gar, appear to have adequate food resources for recruitment, with little indication of within‐family or interspecific predation. This suggests that stocked Alligator Gar quickly acclimated to their new environment.

Evaluation of Alligator Gar Broodfish Collections in the Lower Mobile–Tensaw Delta, Alabama

AbstractIn Alabama, Alligator Gar Atractosteus spatula populations have experienced a decline in abundance and due to their limited distribution are classified as a nongame species of moderate conservation concern. Conservation management plans have included creel limits and stocking to augment a diminished population within the lower Alabama River. Our objective was to evaluate gill‐net efficacy on catch and the environmental variables associated with improving broodfish capture rates for hatchery propagation. Broodfish collections were initiated and completed over 75 sample dates between March 2005 and April 2015. Collectors utilized multifilament gill nets consisting of three mesh sizes (101.6, 127.0, and 152.4 mm) and three net lengths (22.9, 30.5, and 61.0 m). Gill nets (n = 569) expended 2,173 h of soak time, resulting in the capture of 101 individuals (TL range = 1,020–2,210 mm). Catch was highest for the smallest mesh size, but net panel length did not influence catch rates. Median fish lengths increased with larger mesh sizes. Catch declined more than sevenfold for net soaks of 1 h or more, and the proportion of successful gill nets declined threefold for longer soak times. Examination of environmental conditions during and prior to the setting of gill nets demonstrated that fish capture success was in part influenced by various aspects of tidal current and water temperature. Catch success was 68% when nets were set at water temperatures of at least 20°C and when at least 50% of net soak time occurred during rising tides. However, success was lower when nets were set at temperatures less than 20°C, capturing Alligator Gar in 44% of all soaks and 37% for those on falling or slack tides. Agencies involved in hatchery propagation programs should continually evaluate field sampling methods, use of collection gear, and environmental conditions to increase broodfish catch rates and improve staff efficacy.

Using Remote Sensing to Assess Alligator Gar Spawning Habitat Suitability in the Lower Mississippi River

AbstractFloodplains are an important part of large‐river ecosystems; the frequency, timing, duration, and spatial extent of inundation drive habitat quality and determine the suitability of these habitats for both aquatic and terrestrial organisms. Managers have traditionally had very limited information with which to evaluate and quantify the dynamics of large‐river floodplains. Alligator Gar Atractosteus spatula use floodplains in the lower Mississippi River for spawning and have experienced declines in historic range that have been partly attributed to declines in spawning habitat availability. The Alligator Gar has therefore been identified by the American Fisheries Society, the U.S. Fish and Wildlife Service, and many state agencies as a species of concern in the lower Mississippi Alluvial Valley. The goal of this study was to develop landscape‐level spatial data to determine the extent and quality of floodplain habitat that may be available for Alligator Gar spawning. Multi‐temporal analysis of remote sensing imagery was used to develop spatial data products that defined floodplain inundation extent, inundation frequency, and temperature. These products were combined with existing layers of physical habitat structure to define and quantify spawning habitat suitability throughout the entire area subject to direct inundation by the lower Mississippi River. Habitat suitability categories were defined based on meeting unique combinations of inundation, temperature, and physical structure so that the most suitable conservation measures can be applied to improve local conditions.

Sibship Analysis to Characterize Alligator Gar Reproductive Contributions in Two Texas Systems

AbstractWe used sibship analysis to further our understanding of Alligator Gar Atractosteus spatula reproductive biology. Quantifying the relative contributions of spawners and spatial patterns of reproduction is important in making management decisions, especially with regard to long‐lived, large‐bodied fish like Alligator Gar, for which exploitation of large females may negatively impact recruitment. Using genetic data from age‐0 fish, we estimated the effective number of breeders (Nb) and the number of spawning adults (NS) at multiple spatial and temporal scales in two Texas systems over a 2‐year period. Analysis of 265 fish collected in 2015 from six sites within an approximately 3,000‐ha floodplain wetland system on the Trinity River identified no full siblings or half siblings with a probability greater than 0.90. Estimates of Nb (24–30) and NS (21–33) were comparable among sites, with low levels of relatedness among individuals within sites. Analysis of an additional 136 age‐0 Alligator Gar collected during 2015 from six other sites distributed over 275 river kilometers downstream of the wetland system revealed similar patterns. Age‐0 fish collected in Choke Canyon Reservoir (N = 131) in 2015 also demonstrated close correspondence between estimates of Nb and NS (54 and 56, respectively). Age‐0 Alligator Gar (N = 88) were again collected at three sites in the Trinity River system during 2016; results were comparable to those of the previous year, with no evidence of repeat spawning at each site. The similarities we observed between the estimated Nb and NS indicated little variance in the number of offspring produced among individual spawners. Because individual spawners appear to contribute somewhat equally to the year‐classes and spawning appears to occur over a broad geographic extent, there is likely little risk of year‐class failure from harvesting a sustainable number of spawning adults.

Occurrence and Co‐Occurrence Patterns of Gar in River–Floodplain Habitats: Methods to Leverage Species Coexistence to Benefit Distributional Models

AbstractHabitat segregation and hypothesized biotic interactions among coexisting gar species may confound attempts to describe the distributions of these species by using only macrohabitat availability in the presence of conspecifics. However, the strength of interactions among gar species and the spatial scale at which they occur are largely unknown. We used an existing data set to evaluate the co‐occurrence patterns (i.e., random assemblages versus species co‐occurring more or less than expected at random) of three gar species in 62 dynamic river–floodplain habitats associated with the lower Mississippi River and its major tributaries. A novel parameterization of a multispecies occupancy model was utilized to examine the spatial relationships among the full array of possible gar assemblages across a gradient of floodplain habitats. Spotted Gar Lepisosteus oculatus were the most abundant and frequently encountered species (~78% of samples). Shortnose Gar L. platostomus (~27% of samples) and Longnose Gar L. osseus (~12% of samples) were relatively uncommon, and no Alligator Gar Atractosteus spatula were captured in over 600 electrofishing transects. Estimated detection probabilities of the encountered species varied (range = 0.27 [Longnose Gar] to 0.80 [Spotted Gar]), and five species‐specific environmental and sampling covariates predicted detection. Relatively strong co‐occurrence patterns between Shortnose Gar and Longnose Gar were significantly influenced by the proximity of a floodplain lake to a river (i.e., river : lake elevation change and linear distance) and by lake latitude. In contrast, the occurrence patterns of Spotted Gar were largely independent of the other species and had minor influences on the relationship between Longnose Gar and Shortnose Gar. We demonstrate how species co‐occurrence patterns can be leveraged to reduce uncertainty associated with species‐specific occupancy estimates and how species distribution models can be improved by utilizing abiotic and biotic features of the target ecosystems.

Effects of Temperature on Hatching Rate and Early Development of Alligator Gar and Spotted Gar in a Laboratory Setting

AbstractWater temperature influences both morphological and physiological development in fishes. However, the effects of water temperature on the early development of Alligator Gar Atractosteus spatula and Spotted Gar Lepisosteus oculatus are not well understood. Both gar species were collected from natural environments and spawned in a hatchery setting. After spawning, fertilized embryos were collected and transferred to the Oklahoma Fishery Research Laboratory, where the embryos (50–72 embryos/treatment) were placed into one of five water temperature treatments (15.5, 20.0, 23.8, 27.5, and 32.2°C) and observed over time to estimate the time to hatch and the time to reach the free‐swimming stage. Both species showed an inverse relationship between temperature and the timing of hatch and advancement to free‐swimming fingerlings for all treatments. In addition, Alligator Gar embryos did not develop at the coldest water temperature tested, and Alligator Gar juveniles held at the warmest temperature tested were observed with developmental abnormalities, potentially affecting their survival. The same temperature extremes had no comparable negative effect on Spotted Gar. The results of this study are useful for understanding early life history dynamics of these two species in their natural environments and can also be used by hatchery managers who are seeking to optimize their production protocols.

Population Contributions of Large Females May be Eroded by Contaminant Body Burden and Maternal Transfer: A Case Study of Alligator Gar

AbstractWithin a population, larger individuals tend to have higher fecundity and produce higher quality eggs. For this reason, larger, older females often disproportionately contribute to recruitment. However, increases in body size and age have also been shown to correlate with increased bioaccumulation of environmental contaminants, which negatively impact reproduction and larval survival. In this study, we tested for maternal age and size effects on fecundity, egg size and quality, contaminant body burden, and maternal transfer in Alligator Gar Atractosteus spatula to assess the potential for contaminant bioaccumulation to offset reproductive benefits of large females. We measured fecundity and egg quality (via lipid content) as well as tissue concentrations of mercury, polychlorinated biphenyls (PCBs), and organochlorine pesticides (OCPs) in female Alligator Gar from the Trinity River, Texas. Fecundity was positively correlated with body size, but egg lipid was not significantly correlated with body size or with fecundity. Total mercury (THg) in both liver and muscle was positively correlated with maternal body size, and egg THg was positively correlated with maternal body size and liver THg. All liver and egg samples contained detectable levels of PCBs and OCPs, which were positively correlated with body size. Concentrations of PCBs and OCPs were positively correlated between tissues for each female, with egg concentrations being generally higher than liver concentrations. Based on concentrations of Hg, PCBs, and OCPs known to cause biological impairment in adult and larval fishes, the concentrations observed in this study suggest that adult reproductive function and behavior—as well as egg hatching success, larval development, and survival—could be compromised. Our results provide data that are relevant for managing the lower Trinity River Alligator Gar population and also serve as a case study demonstrating that the reproductive benefits of larger, older females may be eroded by contaminant accumulation and maternal transfer.

Otolith‐Based Age Estimates of Alligator Gar Assessed Using Bomb Radiocarbon Dating to Greater than 60 Years

AbstractConsiderable efforts have been undertaken to characterize population dynamics of the Alligator Gar Atractosteus spatula. Accomplishing this task required identification of suitable structures to estimate age. Buckmeier et al. (2012) validated sagittal otoliths to 31 years—the oldest fish in their study. However, they noted that interpreting annuli became increasingly difficult with age and cautioned that difficulties may result in underestimates of age for older fish. In recent years, sampling efforts have resulted in the collection of otoliths from a number of large (>1,800 mm TL) individuals. Age estimates of these fish ranged to 68 years; however, the accuracy of these ages was unknown. Therefore, we used bomb radiocarbon dating to assess accuracy. The technique measures the time‐specific increase in the radioactive carbon isotope (14C) in calcified aging structures resulting from the testing of thermonuclear devices in the 1950s and 1960s. An abrupt, worldwide increase in environmental radiocarbon occurred in approximately 1958 and acts as a time stamp, providing a reliable means of corroborating the age estimates of long‐lived fishes. Core material (i.e., hatch year), along with a subsample from early growth years, was extracted from the otoliths of 24 Alligator Gar (1,805 to 2,540 mm TL) and analyzed for Δ14C. Estimated age from growth zone counting ranged from 26 to 68 years, corresponding to hatch years spanning from 1946 to 1989. Otolith Δ14C was then plotted against the corresponding hatch years and compared to reference curves for environmental 14C levels across time. Despite evidence of regionally specific trends attributed to differences in groundwater and surface inflows, Δ14C levels in the otoliths were highly correlated with reference data, indicating that age estimates were accurate to within a few years. Thus, our results extend the utility of methods developed by Buckmeier et al. (2012) for Alligator Gar in excess of 60 years and provide greater insight on longevity.

Hybridization and Population Genetics of Alligator Gar in Lake Texoma

AbstractThe Alligator Gar Atractosteus spatula (AG) is a long‐lived fish of growing management and conservation interest. Situated on the border of Texas and Oklahoma, Lake Texoma supports one of the last robust AG populations in Oklahoma; however, a genetic evaluation of this population is lacking. We genotyped AG individuals with 17 microsatellite loci, 7 of which also cross‐amplified in three sympatric Lepisosteus species: the Longnose Gar L. osseus (LN), Shortnose Gar L. platostomus (SN), and Spotted Gar L. oculatus (SP). Bayesian assignment analyses conducted in STRUCTURE and NewHybrids confirmed that a field‐identified hybrid was an F1 AG × LN and identified five other individuals that were suspected backcrosses (three LN × SN; two SN × SP). Alligator Gar had the lowest observed heterozygosity (0.179) and the lowest allelic richness (1.682) among the nonhybrid individuals of the four gar species examined. We also examined the potential for population structure and differences in pairwise relatedness (r) between two areas where AG are commonly encountered within Lake Texoma: the Red River and Washita River arms. No population structure was detected using noninformative or location priors in STRUCTURE, and estimates of r produced by the TrioML estimator in COANCESTRY were not significantly different between arms (overall mean r = 0.199). Point estimates of effective population size ranging from 16.3 to 29.2 suggested that the AG population may be vulnerable to the effects of inbreeding depression and random genetic drift. Results provide a genetic status assessment of AG in Lake Texoma and a baseline for future management and conservation decisions within Lake Texoma and surrounding regions.

Validation of Annuli and Identification of Discontinuities in Sagittal Otoliths of Juvenile Alligator Gar

AbstractAnnual increment periodicity has been validated in sagittal otoliths of Alligator Gar Atractosteus spatula; however, discontinuities (often referred to as false annuli or checks) are common in early years, potentially affecting both accuracy and precision. We used up to four injections of oxytetracycline (OTC) to induce time stamps associated with annuli (opaque zones) in the otoliths of juvenile Alligator Gar (ages 2–6; n = 68). These OTC marks allowed us to test a reader's ability to distinguish discontinuities (hereafter, “checks”) from annuli by using established criteria. In addition, the OTC marks were used to infer the timing of translucent zone and opaque zone formation. Although the reader's error rate was low (6%), some checks were essentially indistinguishable from annuli. Checks were common, occurring in over half of the otoliths examined and within all annual increments in these juvenile fish. The association of all OTC marks with the opaque zones of the annual increment further validates annulus formation in juvenile Alligator Gar and indicates that these narrow zones formed approximately between November and June. Subsequently, it can be inferred that the much wider translucent zones formed from about June to November. Although opaque zone formation was complete by June, new annuli were not distinguishable from the ventral edge of the otolith until translucent material was deposited beyond the annuli. In our samples, only 22.2% of the otoliths from fish sacrificed on June 30 (n = 9) had fully visible annuli along the edge of the otolith. Based on multiple studies, it is apparent that annual increments form in the sagittal otoliths of Alligator Gar throughout their lives; however, the interpretation of these increments can be very difficult. In our experience, the OTC marking of fish can provide a valuable tool for training and verification programs needed for this species and for other long‐lived fishes that pose difficulties in age estimation.

Histopathological Changes Induced by Chronic, Sub- Lethal Diazinon Exposure in Alligator Gar (Atractosteus Spatula) Tissues

Diazinon is a widely used household and agricultural pesticide that accumulates in the aquatic environment and adversely affects non-target organisms such as mammals, birds, and fish. Sub-lethal pesticide levels occur in natural waters, and can impact the health, physiology and fitness of fish populations. This study was conducted to assess the effects of chronic, sub-lethal diazinon exposure to skin, liver, kidney, spleen, heart, gut, intestine, and gas bladder tissues of alligator gar, Atractosteus spatula. In two studies, gar were exposed to sub-lethal concentrations of 0.01, and 0.1 mg/L diazinon for 15 and 30 day periods. Skin, gills, liver, and kidney of exposed fish demonstrated remarkable microscopic lesions. These changes included skin lesions in the head and body, which started as white spots and progressed into deep ulcerations, hepatic vacuolation, swollen hepatocytes, steatosis, aggregation of macrophages, necrosis, and hepatic fibrosis. Gill tissues demonstrated epithelial hyperplasia in the secondary lamellae. Vacuolar degeneration was also present in the hematopoietic tissues of the kidney. Lesion occurrence and severity were correlated to increased diazinon dose and exposure time. Our findings revealed the harmful effects of chronic, sub-lethal diazinon exposure on alligator gar, and suggest other aquatic organisms may also be affected by sub-lethal concentrations of pollutants in their environment.

Sex‐Specific Dynamic Rates in the Alligator Gar: Implications for Stock Assessment and Management

AbstractGrowth and mortality are commonly estimated for fish populations; however, potential differences between males and females receive less attention. Quantifying sex‐specific dynamic rates can be important as they can lead to differential exploitation, altered sex ratios, and reduced reproductive potential. We examined sex‐specific growth, size and age structure, maximum observed age, longevity, and total mortality using total length (TL), age, and sex collected from 549 Alligator Gar Atractosteus spatula from multiple Texas systems. Males (n = 324) ranged from 353 to 2,051 mm TL and 1–61 years of age, whereas females (n = 225) were 385 to 2,420 mm TL and 1–60 years old. We found significant differences in population‐specific size and age distributions between sexes. Greater than 91% of males were less than 1,500 mm TL, whereas over 63% of females sampled were greater than this length. Sixty‐four percent of males were ≤ 10 years old as compared with only 20% of females. Sex‐specific estimates of longevity and total mortality indicated that females lived 10–15 years longer than males and exhibited 25% to 40% lower total mortality. Despite similar growth rates for males and females prior to age 3, female Alligator Gar growth was greater than males at older age‐classes, resulting in a 617‐mm difference in theoretical maximum length based on the double von Bertalanffy growth model. The considerable differences we observed between sexes have important implications for population sampling and stock assessment and highlight the importance of collecting sex‐specific data.