Bycatch of the endangered pallid sturgeon (Scaphirhynchus albus) in a commercial fishery for shovelnose sturgeon (Scaphirhynchus platorynchus)

We quantified the bycatch of pallid sturgeon Scaphirhynchus albus in Tennessee's shovelnose sturgeon ( Scaphirhynchus platorynchus) fishery by accompanying commercial fishers and monitoring their catch on five dates in spring 2007. Fishers were free to keep or discard any sturgeon they collected in their gillnets and trotlines and we were afforded the opportunity to collect meristic and morphometric data and tissue samples from discarded and harvested specimens. Fishers removed 327 live sturgeon from their gear in our presence, of which 93 were harvested; we also obtained the carcasses of 20 sturgeon that a fisher harvested out of our sight while we were on the water with another fisher. Two of the 113 harvested sturgeon were confirmed pallid sturgeon based on microsatellite DNA analyses. Additionally, fishers gave us five, live pallid sturgeon that they had removed from their gear. If the incidental harvest rate of pallid sturgeon (1.8% of all sturgeon harvested) was similar in the previous two commercial seasons, at least 169 adult pallid sturgeon were harvested by commercial fishers in the Tennessee waters of the Mississippi River in 2005–2007. If fishers altered their behavior because of our presence (i.e. if they were more conservative in what they harvested), the pallid sturgeon take was probably higher when they fished unaccompanied by observers. While retrieving a gill net set the previous day, a fisher we were accompanying retrieved a gillnet lost 2 days earlier; this ghost net caught 53 sturgeon whereby one fish was harvested but most fish were dead, including one confirmed pallid sturgeon.     

Genetic evidence for hybridization of pallid and shovelnose sturgeon

To determine the genetic origin of individual sturgeon that are morphologically intermediate to pallid (Scaphirhynchus albus) and shovelnose (Scaphirhynchus platorhynchus) sturgeon, we combined previously published mitochondrial DNA (mtDNA) and microsatellite data with additional microsatellite data. Two sympatric populations of pallid and shovelnose sturgeon from the upper Missouri River and a sympatric population containing pallid, shovelnose, and putative pallid-shovelnose hybrids from the Atchafalaya River were analyzed using an index of hybridization and a principle components analysis of individual relatedness scores. The addition of new microsatellite data improved our ability to genetically differentiate individual pallid and shovelnose sturgeon collected in both areas. Our methods distinguished morphologically intermediate Atchafalaya River sturgeon, which appear to be genetically intermediate between pallid and shovelnose sturgeon. The results support a hybrid origin for morphologically intermediate individuals, although it is unclear whether they are all first-generation hybrids or if some are the result of subsequent backcrossing with the more common shovelnose sturgeon.     

Functional morphology of prey capture in the sturgeon,Scaphirhynchus albus

Acipenseriformes (sturgeon and paddlefish) are basal actinopterygians with a highly derived cranial morphology that is characterized by an anatomical independence of the jaws from the neurocranium. We examined the morphological and kinematic basis of prey capture in the Acipenseriform fish Scaphirhynchus albus, the pallid sturgeon. Feeding pallid sturgeon were filmed in lateral and ventral views and movement of cranial elements was measured from video sequences. Sturgeon feed by creating an anterior to posterior wave of cranial expansion resulting in prey movement through the mouth. The kinematics of S. albus resemble those of other aquatic vertebrates: maximum hyoid depression follows maximum gape by an average of 15 ms and maximum opercular abduction follows maximum hyoid depression by an average of 57 ms. Neurocranial rotation was not a part of prey capture kinematics in S. albus, but was observed in another sturgeon species, Acipenser medirostris. Acipenseriformes have a novel jaw protrusion mechanism, which converts rostral rotation of the hyomandibula into ventral protrusion of the jaw joint. The relationship between jaw protrusion and jaw opening in sturgeon typically resembles that of elasmobranchs, with peak upper jaw protrusion occurring after peak gape.     

Gender identification of shovelnose sturgeon using ultrasonic and endoscopic imagery and the application of the method to the pallid sturgeon

Monthly sampling of shovelnose sturgeon Scaphirhynchus platorynchus , a biological surrogate for the endangered pallid sturgeon Scaphirhynchus albus , was conducted to develop a multi‐seasonal profile of reproductive stages. Data collected included histological characteristics of gonads from wild caught fish and laboratory and field ultrasonic and endoscopic images. These data were used to compare effectiveness of ultrasonic and endoscopic techniques at identifying gender of adult shovelnose sturgeon at different reproductive stages. The least invasive method ( i.e . ultrasound) was least effective while the most invasive ( i.e . endoscope through an abdominal incision) was the most effective at identifying shovelnose sturgeon gender. In most cases, success rate for identifying males was greater than females, with success at identifying both genders greater in more advanced reproductive stages. Concomitantly, for most months average reproductive stage was more advanced for males than females. April and May were the months with the most advanced reproductive stage, and were the months when ultrasound was most effective. Methods were also applied in the Upper Missouri River to validate their use on pallid sturgeon Scaphirhynchus albus . Ultrasound was successful at identifying pallid sturgeon gender, however, endoscopic examination through the urogenital duct was only successful at identifying pallid sturgeon gender when the urogenital duct was not opaque.     

Use of genetic tags to identify captive-bred pallid sturgeon (<Emphasis Type="Italic">Scaphirhynchus albus</Emphasis>) in the wild: improving abundance estimates for an endangered species

The ability to distinguish captive-bred and natural-origin individuals in the wild is critical for evaluating the impact of captive breeding programs on natural populations. Continued persistence of endangered pallid sturgeon (Scaphirhynchus albus) in the Missouri River is largely dependent on captive breeding efforts that spawn natural-origin adults in fish hatcheries and release their progeny into the wild. Prior to release, hatchery-origin individuals are physically marked so they can be distinguished from natural-origin individuals when recaptured. During the years 2004–2006, 24 unmarked juvenile pallid sturgeon tissue samples were collected in the Missouri River downstream of Gavins Point Dam, South Dakota, USA that were presumed natural-origin. However, these individuals were similar in size to hatchery-origin fish released in this area raising concerns that these individuals were actually hatchery-origin fish with lost or malfunctioning tags. We used microsatellite based parentage analysis to determine if the unmarked fish were members of hatchery families that had been released in this area. This retrospective genetic tagging approach revealed that 23 of 24 unmarked fish were indeed hatchery-origin. The origin of the remaining individual remains unknown because genetic samples were not available from all of the families released below the dam and the unassigned fish may have originated from one of these un-sampled families. These results provide important insight into the conservation status of endangered pallid sturgeon as well as provide data important for guiding management decisions. Our results also demonstrate the efficacy of using genetic tags as an alternative or complimentary approach to physically marking individuals.     

Genetic discrimination of middle Mississippi River <Emphasis Type="Italic">Scaphirhynchus</Emphasis> sturgeon into pallid,shovelnose, and putative hybrids with multiple microsatellite loci

The pallid sturgeon (Scaphirhynchus albus), which is protected under the US endangered species act, and shovelnose sturgeon (S. platorhynchus), which is legally harvested in some locations, are sympatric throughout the range of pallid sturgeon. There is considerable morphological overlap between the species making discrimination problematic. The inability to reliably differentiate between species across all life stages has hampered pallid sturgeon recovery efforts. Furthermore, the two species are believed to hybridize. This study used allele frequency data at multiple microsatellite loci to perform Bayesian and likelihood-based assignment testing and morphological measures and meristics to discriminate pallid, shovelnose, and putative hybrid sturgeons from the middle Mississippi River. Bayesian model-based clustering of the genetic data indicated that two natural genetic units occur in the region. These units correspond to morphologically identified pallid and shovelnose sturgeon. Some individuals were morphologically intermediate and many of these failed to strongly assign genetically as either pallid or shovelnose sturgeon, suggesting they may be hybrids. These data indicate that pallid sturgeon and shovelnose sturgeon are genetically distinct in the middle Mississippi River (F _ST = 0.036, P < 0.0001) and suggest that hybridization between pallid sturgeon and shovelnose sturgeon has occurred in this region with genetic distance estimates indicating the greatest distance is between pallid and shovelnose sturgeon, while hybrid sturgeon are intermediate but closer to shovelnose. This study demonstrates that assignment testing with multiple microsatellite markers can be successful at discriminating pallid sturgeon and shovelnose sturgeon, providing a valuable resource for pallid sturgeon recovery and conservation.

Life history and status of the shovelnose sturgeon, Scaphirhynchus platorynchus

The shovelnose sturgeon, Scaphirhynchus platorynchus, is a freshwater sturgeon of the Mississippi and Missouri rivers and their tributaries. It is one of the smaller North American sturgeons, seldom weighing more than 2.5 kg over most of its range except in the upper Missouri River, where individuals of over 7 kg have been found. Spawning occurs in spring at temperatures between 17 and 21 °C over rock or gravel substrate downstream from dams, near rock structures, or in tributaries. most males reach sexual maturity at 5 years, most females at 7 years. Adults do not spawn every year. Shovelnose sturgeon are found in large, turbid rivers and frequently concentrate in areas downstream from dams or at the mouths of tributaries. Population densities range up to 2500 fish per km. They are commonly found in areas of current over sandy bottoms or near rocky points or bars, where they feed primarily on aquatic invertebrates. The shovelnose sturgeon is classified as a sport species in 12 of 24 states where it occurs. Commercial harvest is allowed in seven states, where fresh shovelnose sturgeon sell for 55 to 88 cents per kg, smoked shovelnose for about $5.75 per kg, and roe from 33 to 110 dollars per kg. About 25 tons of shovelnose sturgeon are harvested commercially each year. Shovelnose sturgeon are considered extirpated in three states, fully protected in four states, and rare, threatened, or of special concern in eight states. Populations are considered stable throughout most of the upper Mississippi, lower Missouri, Red, and Atchafalaya rivers. Three states, Wyoming, West Virginia, and New Mexico, have developed plans to reintroduce the species into rivers where it has been extirpated.     

Vulnerability of age-0 pallid sturgeon Scaphirhynchus albus to fish predation

Stocking is a commonly employed conservation strategy for endangered species such as the pallid sturgeon, Scaphirhynchus albus . However, decisions about when, where and at what size pallid sturgeon should be stocked are hindered because vulnerability of pallid sturgeon to fish predation is not known. The objective of this study was to evaluate the vulnerability of age-0 pallid sturgeon to predation by two Missouri River predators under different flow regimes, and in combination with alternative prey. To document vulnerability, age-0 pallid sturgeon (<100 mm) were offered to channel catfish Ictalurus punctatus and smallmouth bass Micropterus dolomieu in laboratory experiments. Selection of pallid sturgeon by both predators was measured by offering pallid sturgeon and an alternative prey, fathead minnows Pimephales promelas, in varying prey densities. Smallmouth bass consumed more age-0 pallid sturgeon (0.95 h⁻¹) than did channel catfish (0.13 h⁻¹), and predation rates did not differ between water velocities supporting sustained (0 m s⁻¹) or prolonged swimming speeds (0.15 m s⁻¹). Neither predator positively selected pallid sturgeon when alternative prey was available. Both predator species consumed more fathead minnows than pallid sturgeon across all prey density combinations. Results indicate that the vulnerability of age-0 pallid sturgeon to predation by channel catfish and smallmouth bass is low, especially in the presence of an alternative fish prey.     

Spatial analysis of pallid sturgeon Scaphirhynchus albus distribution in the Missouri River, South Dakota

Movement and distribution of the endangered pallid sturgeon Scaphirhynchus albus has generally been documented using radio telemetry. However, because of the time and cost involved in tracking individual fish (i.e. small sample size), it is often difficult to evaluate spatial distribution of groups of fish over long time periods (> 3 years). Standardized sampling for pallid sturgeon, which relies on a variety of gear types, has been conducted on the Missouri River downstream of Fort Randall Dam annually since 2003. Using catch data from 2003 to 2006, the spatial distribution of juvenile pallid sturgeon was evaluated using spatial scan statistics. Presence/absence of pallid sturgeon was summarized from a variety of gear and distribution patterns were analyzed based on: (i) each gear per season, (ii) all gear pooled per season, (iii) each gear pooled across seasons, and (iv) pooled data from all gear and years combined. Three significant clusters of pallid sturgeon captures were identified when all gear and years were pooled. Distribution patterns identified using data from summer trammel nets agreed well with the overall pooled dataset and could be used to identify areas with a high probability of pallid sturgeon presence. This methodology can be used to identify areas where pallid sturgeon are likely to occur, thus improving sampling efficiency for monitoring vital statistics for this endangered species. Moreover, this approach could be applied to other reaches of the Missouri River using existing data from the Pallid Sturgeon Monitoring and Assessment Program. Once identified, these areas could then be evaluated to better understand the habitat requirements of pallid sturgeon.     

Improved genetic identification of acipenseriform embryos with application to the endangered pallid sturgeon Scaphirhynchus albus

We produced pallid sturgeon Scaphirhynchus albus embryos at five pre-hatch developmental stages and isolated and quantified genomic DNA from four of the stages using four commercial DNA isolation kits. Genomic DNA prepared using the kit that produced the largest yields and concentrations were used for microsatellite DNA analyses of 10–20 embryos at each of the five developmental stages. We attempted to genotype the hatchery-produced embryos at 19 microsatellite loci and confirmed reliable genotyping by comparing the microsatellite genotypes to those of known parents. Embryos at stages 5 and 8 did not produce reliable genotyping while those at stages 14, 24 and 33 did. We used the same DNA isolation method on 262 wild-caught acipenseriform embryos collected from the lower Yellowstone River. A total of 200 of the wild embryos were successfully identified to stages 8 to 34 and the rest could not be staged. Using a combination of single nucleotide polymorphism and microsatellite markers, 249 of the wild-caught embryos were genetically identified as paddlefish Polyodon spathula, five were identified as shovelnose sturgeon Scaphirhynchus platorynchus and eight failed to amplify. None were identified as pallid sturgeon. This study demonstrates that early-stage wild-spawned acipenseriform embryos can be genetically identified less than 24 h post-spawn. This methodology will be useful for recovery efforts for endangered pallid sturgeon and can be applied to other acipenseriform species.     

Microsatellite variation among River Sturgeons of the genus Scaphirhynchus (Actinopterygii: Acipenseridae): a preliminary assessment of hybridization*

Microsatellite variation from 13 disomic loci is reported for a total of 208 individuals of the genus Scaphirhynchus. This includes 105 individuals of the pallid sturgeon (Scaphirhynchus albus) from the lower Mississippi River, 11 pallid sturgeon from the Upper Mississippi and Missouri rivers, 65 shovelnose sturgeon (S. platorynchus) from the lower Mississippi River, six Alabama sturgeon (S. suttkusi), and 21 individuals of sturgeon identified as intermediate between S. albus and S. platorynchus. Results indicate that all five of the above population/species units are significantly differentiated from one another based on pairwise F_ST estimates. Locus Spl‐7 was diagnostic for the Alabama sturgeon and serves to further differentiate this allopatric species from other Scaphirhynchus. Classification of genotypes with and without a priori designations failed to clearly delineate the species and intermediates in the latter case but was successful for the species but not in the intermediates in the former case. The presence of six unique alleles in five of the 21 morphologically ‘intermediate’ sturgeon examined requires additional evaluation but suggests that these individuals are possibly not the result of hybridization. We hope that raising these important issues will bring all stakeholders to the table to establish a concerted effort needed for both morphological and molecular analyses to adequately address the question of hybridization and the origin of the morphological variation in these fishes.     

Genetic variation and relationships of seven sturgeon species and ten interspecific hybrids

Background

Sturgeon cultivation is important for both industry and aquaculture in China. To date, more than 17 species or strains have been farmed for fillets and caviar production. Crossbreeding among different sturgeon species is frequent and the F2 hybrids are fertile. However, large-scale farming can have negative impacts on wild populations i.e. escape of exotic sturgeons and must be taken into consideration. Escape of exotic sturgeons can cause severe ecological problems, including threatening native sturgeon species once the exotic varieties become established or hybridize with native individuals. However, little is known about their genetic resources and variation.

Methods

Genetic diversity and introgression of seven sturgeon species were analyzed using mitochondrial DNA cytochrome oxidase subunit I (COI) and nine microsatellite markers. This study included 189 individuals from seven sturgeon species and 277 individuals from ten lineages of F2 hybrid strains.

Results

MtDNA COI sequences (632 bp long) were generated from 91 individuals across the 17 sturgeon strains and produced 23 different haplotypes. Haplotype diversity was high (h = 0.915 ± 0.015) and nucleotide diversity was low (π = 0.03680 ± 0.00153) in the seven sturgeon species and ten interspecific hybrids. Phylogenetic analyses resulted in almost identical tree topologies, and different haplotype structures were mainly related with sturgeons of different female parents. Analysis of molecular variance revealed that 81.73% of the genetic variance was due to matrilineal differences, while 9.40% resulted from strain variation. Pairwise Fst values obtained with POLYSAT software, were high among strains and ranged from 0.031 to 0.164. Admixture analysis assigned seven distinct groups and ten genotypes of admixed clusters composed of hybrid strains using STRUCTURE when assuming K = 7.

Conclusions

The interspecific mtDNA gene tree corresponded to the expected taxonomic divisions. These relationships were also supported by the results from the microsatellite analysis and contributed to unambiguously identify seven sturgeon species and ten F2 hybrid strains from sturgeon farms in China. Moreover, we found that introgressive hybridization is pervasive, exists in both purebred and hybrid sturgeons, and may reflect widespread mismanagement in sturgeon breeding in China.     

Stock structure of pallid sturgeon analyzed with microsatellite loci

Recovery efforts for the endangered pallid sturgeon (Scaphirhynchus albus) include supplementation of wild stocks with hatchery reared progeny. Identifying the extent of genetic stock structure, which has previously been detected in samples from the range extremes, will help to determine whether stock transfers might be harmful. DNA microsatellite genotypes were screened in pallid sturgeon from the upper Missouri River, lower Missouri River, middle Mississippi River and Atchafalaya River and analyzed using a combination of Bayesian model‐based and more traditional F‐statistic based methods to characterize genetic differentiation. Scaphirhynchus specimens were collected by researchers active in the recovery effort and genotypes were screened at 16 microsatellite loci. Because there is considerable genetic and morphological overlap between pallid sturgeon, shovelnose sturgeon, and their hybrids, a combination of morphological and genetic techniques were used to eliminate shovelnose and possible hybrids from the sample. Genetic differentiation was detected among samples (overall θ = 0.050, P = 0.001). Pairwise θ, genetic distances, and Bayesian assignment testing reveal that pallid sturgeon from the upper Missouri River are the most distinct group with pairwise comparisons of pallid sturgeon among all the remaining samples exhibiting lower θ values, higher genetic distances, and self assignment scores. Our results indicate that using local broodstock, when available, should be used for pallid sturgeon propagation. If local broodstock are not available, geographically proximate individuals would limit genetic differences between native and stocked individuals.     

Use of behavioral and physiological indicators to evaluate Scaphirhynchus sturgeon spawning success

Thirty gravid, female shovelnose sturgeon (Scaphirhynchus platorynchus) were captured in the Lower Missouri River in March 2004 to evaluate the effectiveness of physiology, telemetry and remote sensor technology coupled with change point analysis in identifying when and where Scaphirhynchus sturgeon spawn. Captured sturgeons were instrumented with ultrasonic transmitters and with archival data storage tags (DST) that recorded temperature and pressure. Female sturgeon were tracked through the suspected spawning period. Thereafter, attempts were made to recapture fish to evaluate spawning success. At the time of transmitter implantation, blood and an ovarian biopsy were taken. Reproductive hormones and cortisol were measured in blood. Polarization indices and germinal vesicle breakdown were assessed on the biopsied oocytes to determine readiness to spawn. Behavioral data collected using telemetry and DST sensors were used to determine the direction and magnitude of possible spawning‐related movements and to identify the timing of potential spawning events. Upon recapture observations of the ovaries and blood chemistry provided measures of spawning success and comparative indicators to explain differences in observed behavior. Behavioral and physiological indicators of spawning interpreted along with environmental measures may assist in the determination of variables that may cue sturgeon reproduction and the conditions under which sturgeon successfully spawn.     

Morphometric variation among river sturgeons (Scaphirhynchus spp.) of the Middle and Lower Mississippi River

Pallid sturgeon (Scaphirhynchus albus) captured in the Middle and Lower Mississippi River (i.e. below St. Louis, MO, USA) are morphologically very similar to shovelnose sturgeon (Scaphirhynchus platorynchus). Available empirical data are limited to a few studies based on low sample sizes from disjointed populations. Geneticists are currently searching for markers that will differentiate the two species, but the need for unequivocal species‐specific field characters remains. Continuation of commercial fishing for shovelnose sturgeon in some states necessitates an immediate means for accurate field identifications. Previous studies of lower basin river sturgeon classified individuals with simple morphometric character indices and interpreted intermediacy as interspecific hybridization. In this study, morphometric variation among Scaphirhynchus specimens from the Middle and Lower Mississippi River is examined for evidence of hybridization. Data are compared for large (>250‐mm standard length) hatchery‐reared and wild pallid specimens and wild shovelnose specimens. Specimens are compared using two morphometric character indices, two morphometric/meristic character indices and principal components analysis. Results indicate substantial morphological variation among pallid sturgeon below the mouth of the Missouri River. The amount of variation appears to decrease downstream in the Mississippi River. Sheared principal components analysis of morphometric data shows complete separation of shovelnose and pallid sturgeon specimens, whereas character indices indicate overlap. Both character indices and sheared principal components analysis demonstrate that pallid sturgeon in the Lower Mississippi River are morphologically more similar to shovelnose sturgeon than are pallids from the Upper Missouri River. This similarity, explained in previous studies as hybridization, may be the result of latitudinal morphometric variation and length‐at‐age differences between populations of the upper and lower extremes of the range.     

Development of PCR assays to detect iridovirus infections among captive and wild populations of Missouri River sturgeon

The Missouri River sturgeon iridovirus (MRSIV) is an important factor contributing to losses during the hatchery rearing of juvenile pallid Scaphirhynchus albus and shovelnose S. platorynchus sturgeon. As the virus has not been isolated in cell culture, current detection procedures rely upon a combination of light and electron microscopy. Detection of characteristic virus-infected cells in the integument, usually of the fins, in hematoxylin and eosin (H&E)-stained tissue sections provides a presumptive finding. Confirmation requires observation by electron microscopy of characteristic doubly enveloped hexagonal virions of the appropriate size in the host cell cytoplasm. To improve these diagnostic procedures, a conventional polymerase chain reduction (PCR) assay was developed as a sensitive and specific method for detection of MRSIV DNA as found in numerous tissues of both naturally and experimentally infected pallid and shovelnose sturgeon. Sequences of amplicons obtained from testing of wild-caught shovelnose sturgeon and juvenile pallid sturgeon during hatchery outbreaks were identical, suggesting that the viruses found in both sturgeon are similar or closely related. In addition, a TaqMan PCR was developed that allowed estimates of the concentrations of MRSIV DNA present in the tissues of pallid and shovelnose sturgeon during acute and persistent infection. These new PCR assays are improved methods to detect MRSIV, but equally importantly, they provide insights into to the biology of the agent for more effective management of viral diseases in captive and wild Missouri River sturgeon populations.     

Genome duplication events and functional reduction of ploidy levels in sturgeon (Acipenser, Huso and Scaphirhynchus).

Sturgeon (order Acipenserformes) provide an ideal taxonomic context for examination of genome duplication events. Multiple levels of ploidy exist among these fish. In a novel microsatellite approach, data from 962 fish from 20 sturgeon species were used for analysis of ploidy in sturgeon. Allele numbers in a sample of individuals were assessed at six microsatellite loci. Species with approximately 120 chromosomes are classified as functional diploid species, species with approximately 250 chromosomes as functional tetraploid species, and with approximately 500 chromosomes as functional octaploids. A molecular phylogeny of the sturgeon was determined on the basis of sequences of the entire mitochondrial cytochrome b gene. By mapping the estimated levels of ploidy on this proposed phylogeny we demonstrate that (I) polyploidization events independently occurred in the acipenseriform radiation; (II) the process of functional genome reduction is nearly finished in species with approximately 120 chromosomes and more active in species with approximately 250 chromosomes and approximately 500 chromosomes; and (III) species with approximately 250 and approximately 500 chromosomes arose more recently than those with approximately 120 chromosomes. These results suggest that gene silencing, chromosomal rearrangements, and transposition events played an important role in the acipenseriform genome formation. Furthermore, this phylogeny is broadly consistent with previous hypotheses but reveals a highly supported oceanic (Atlantic-Pacific) subdivision within the Acipenser/Huso complex.     

Genetic structure of hybrid zones between Silene latifolia and Silene dioica (Caryophyllaceae): evidence for introgressive hybridization

Natural hybrid zones provide a valuable tool to study introgressive hybridization, because they can contain a wide variety of genotypes that result from many generations of recombination. Here we used molecular markers and morphological variation to describe the structure of two natural hybrid zones between Silene latifolia and Silene dioica in the Swiss Alps. Populations in both hybrid zones consisted of few intermediate hybrids and were dominated by backcross hybrids. The latter were also found in the parental populations at the margins of the hybrid zones. Out of 209 amplified fragment length polymorphism (AFLP) markers scored in 390 individuals, only 7 (3.3%) were species specific. These results indicate that introgression between S. dioica and S. latifolia is extensive, and that hybrid zones act as bridges to gene flow between these two species. Analysis of linkage disequilibrium identified few populations in which hybridization is ongoing, whereas in most populations linkage disequilibrium has eroded. Where hybridization is ongoing, strong changes in species-specific marker frequencies and morphological traits were observed. Plastid introgression into the hybrid zone was found to be bidirectional, but only the S. latifolia plastid haplotype was found in a nuclear S. dioica background. This unidirectional plastid introgression from S. latifolia into S. dioica is most likely due to pollen-flow from S. dioica onto S. latifolia, and results in plastid capture. Comparisons between the molecular and the morphological hybrid indices revealed that morphology in this study system is useful for identifying hybrids, but not for detailed analysis of hybrid zone structure.     

Introgressive hybridization between two Iberian endemic cyprinid fish: a comparison between two independent hybrid zones

Pseudochondrostoma duriense and Achondrostoma oligolepis are two Iberian endemic cyprinid fish species that occur in sympatry over most of their distribution range and that are suspected to hybridize in nature. Here, we employed a combination of mitochondrial and microsatellite markers to explore the extent of introgressive hybridization between these fishes. Two natural hybrid zones were identified in different river basins. Introgression was bi‐directional and both hybrid zones consisted mostly of parental genotypes/phenotypes (i.e. bimodal hybrid zones). Yet, they appeared to differ in the extent and direction of introgression, which supports the view that they constitute independent outcomes of different hybridization processes probably influenced by environmental features. Several discordances were found between mtDNA and microsatellite results, suggesting that this hybridization process has complex consequences and illustrating the importance of using independent markers to define accurately the hybrid status of individuals in the presence of high levels of backcrossing.     

Post‐stocking pallid sturgeon Scaphirhynchus albus growth,dispersal, and survival in the lower Missouri River

Hatchery augmentation has been used to mitigate declines in fish populations worldwide, especially for sturgeon species. Information regarding stocking success including survival, dispersal, and growth of sturgeon post‐stocking may refine sturgeon augmentation programs and facilitate recovery. Pallid sturgeon Scaphirhynchus albus populations have been supplemented by hatchery‐reared stocks for 25 years in the Missouri River, USA. We assessed survival, dispersal patterns, and growth characteristics of post‐stocked pallid sturgeon in the lower Missouri River. Hatchery‐reared pallid sturgeon stocked at age‐1 (4.1%) and > age‐1 (2.9%) were recaptured at a higher frequency than fish stocked at age‐0 (0.3%). Post‐stocking dispersal patterns suggested dispersal range increase as age increased, but individuals tended to remain in the same river segment as their stocking location. Growth rates varied by year class with younger year classes having truncated growth trajectories compared to older year classes. Post‐stock survival of pallid sturgeon varied by age‐at‐stocking and suggest age‐1 survival has declined through time. Augmentation of pallid sturgeon may benefit from considering dispersal from stocking location and by stocking older individuals which appear to have increased survival post‐stocking. A better understanding regarding environmental drivers of growth and specific habitat features used is needed to better predict optimal timing and location of future stockings.