Age and growth of pallid sturgeon in the free-flowing Mississippi River

Trotlines were used to capture pallid sturgeon in the free‐flowing Mississippi River, which extends from the Gulf of Mexico to the mouth of the Missouri River. Trotlines were baited with worms, and set overnight usually along the channel border. The pectoral fin rays of 165 pallid sturgeon caught in the Mississippi River were aged; 118 were from the lower Mississippi River (LMR) between the Gulf and mouth of the Ohio River, and 47 were from the middle Mississippi River (MMR) between the mouths of the Ohio and Missouri rivers. Initial agreement within ±1 year between two readers ranged from 53% for the LMR specimens, which were read first, to 84% for the MMR. Final age was agreed upon by both readers. For LMR pallid sturgeon, final age estimates ranged from 3 to 21 years with a mean (±SD) of 11.0 ± 4.7. For MMR pallid sturgeon, final age estimates ranged from 5 to 14 years with a mean of 9.5 ± 2.1. Seven pallid sturgeon marked with coded wire tags (CWT), indicating hatchery origin, were collected in the MMR. Age estimates for CWT fish were 7–8 years representing 1997 stocked fish, and 11–12 years representing 1992 progeny stocked in 1994. Von Bertalanffy growth equations for length indicated that pallid sturgeon in the MMR had higher growth rates for a given age than pallid sturgeon in the LMR. However, there were no significant differences (anova , P > 0.5) in the length–weight relationships between reaches. In the LMR, pallid sturgeon fully recruited to trotlines at age 11 and instantaneous total mortality (Z; slope of catch curve) was estimated at −0.12 (n = 10 year classes, r² = 0.55, P = 0.01). Of the 118 sectioned rays from the LMR, 28 could not be reliably aged (only one section from the MMR could not be aged). Therefore, age was predicted from length using the von Bertalanffy equation. The catch curve was re‐calculated using the predicted ages of the 28 pallid sturgeon in the LMR resulting in Z = −0.07. In the MMR, pallid sturgeon fully recruited to trotlines at age 9 and Z was estimated at −0.36 (n = 6 year classes, r² = 0.67, P = 0.04), which was significantly higher (anova , P = 0.04) than the LMR estimate. Higher mortality in the MMR may be due to habitat limitations compared to a larger, more diverse channel in the LMR, and incidental take of larger, older individuals during commercial harvesting of shovelnose sturgeon. Commercial take of shovelnose does not occur in the LMR except in the northern portion of the reach. Considering the presence of pallid sturgeon with CWT, recruitment of older individuals in the MMR may have been influenced by stocking a decade earlier. Management strategies for this endangered species should consider the differences in mortality rates among reaches, the impacts of commercial fishing on recovery of pallid sturgeon in the MMR, and the long‐term effects of hatchery fish now recruiting into the free‐flowing Mississippi River.     

Distribution, relative abundance and movements of pallid sturgeon in the free-flowing Mississippi River

A multiyear study of pallid sturgeon distribution and relative abundance was conducted in the lower and middle Mississippi river (LMR and MMR, respectively). The LMR and MMR comprise the free‐flowing Mississippi River extending 1857 river kilometers (rkm) from its mouth at the Gulf of Mexico upstream to the mouth of the Missouri River. A total of 219 pallid sturgeon and 6018 shovelnose sturgeon was collected during the periods 1996–1997 and 2000–2006. Trotlines baited with worms were the primary collecting gear. The smallest pallid sturgeon captured on trotlines was 405 mm FL and the largest was 995 mm FL. Mean size of pallid sturgeon was statistically smaller in the Mississippi River below the Atchafalaya River near Baton Rouge, LA (621 mm FL). Mean abundance (catch per trotline night) of pallid sturgeon was highest at water temperatures around 10°C. There was a latitudinal trend in mean abundance of pallid and shovelnose sturgeon, but the pattern differed between species. Pallid sturgeon abundance was statistically (P < 0.05) higher (0.3 fish per trotline night) in the lower reach between the Atchafalaya River and New Orleans (rkm 154–507), and at the Chain of Rocks (COR), a low water dam near the mouth of the Missouri River. Pallid sturgeon abundance between these two locations was statistically the same (0.12–0.23). Shovelnose sturgeon abundance increased going upstream, but was disproportionally higher at the COR (22 fish per line compared with <6 fish per line in other reaches). Overall, the ratio between pallid and shovelnose sturgeon varied from a high of 1 : 6 at the lower reach, and gradually decreased upstream to a low of 1 : 77 at the COR. Based on differences in sturgeon abundance, size and habitat characteristics, the free‐flowing Mississippi River can be divided into two reaches in the MMR (i.e. COR is a separate location), and four reaches (i.e., including the Atchafalaya River) in the LMR where management goals may differ.     

Hybridization between pallid sturgeon Scaphirhynchus albus and shovelnose sturgeon Scaphirhynchus platorynchus

This study found that introgressive hybridization of the pallid sturgeon Scaphirhynchus albus with the common shovelnose sturgeon Scaphirhynchus platorynchus has probably occurred across the range of S. albus. Bayesian clustering found evidence of hybridization in all management units of S. albus. Some individuals were intermediate at both genetic and morphological characters, and some had discordant results. The results support introgressive hybridization throughout much of the range of S. albus, yet individuals consistent with being pure members of each species were detected in all management units. Simulations demonstrated that it would be very difficult to distinguish introgressed individuals from pure specimens after multiple generations of backcrossing with these microsatellite markers. Using hybrid or backcross fish as broodstock could artificially accelerate the loss of unique genetic variation in S. albus. Additional microsatellite loci or additional genetic markers, along with morphological data may be required to ensure that hybrid or backcross fish are not used. Introgressive hybridization requires at least two generations and generation lengths of S. albus are long, perhaps as long as 30 years. The proportion of individuals consistent with introgressive hybrid origins indicates that hybridization between S. albus and S. platorynchus probably has occurred for several generations and is not a recent phenomenon.     

Population viability analysis of Lower Missouri River shovelnose sturgeon with initial application to the pallid sturgeon

Demographic models for the shovelnose (Scaphirhynchus platorynchus) and pallid (S. albus) sturgeons in the Lower Missouri River were developed to conduct sensitivity analyses for both populations. Potential effects of increased fishing mortality on the shovelnose sturgeon were also evaluated. Populations of shovelnose and pallid sturgeon were most sensitive to age‐0 mortality rates as well as mortality rates of juveniles and young adults. Overall, fecundity was a less sensitive parameter. However, increased fecundity effectively balanced higher mortality among sensitive age classes in both populations. Management that increases population‐level fecundity and improves survival of age‐0, juveniles, and young adults should most effectively benefit both populations. Evaluation of reproductive values indicated that populations of pallid sturgeon dominated by ages ≥35 could rapidly lose their potential for growth, particularly if recruitment remains low. Under the initial parameter values portraying current conditions the population of shovelnose sturgeon was predicted to decline by 1.65% annually, causing the commercial yield to also decline. Modeling indicated that the commercial yield could increase substantially if exploitation of females in ages ≤12 was highly restricted.     

Entrainment of shovelnose sturgeon by towboat navigation in the Upper Mississippi River

Estimated number of shovelnose sturgeon Scaphirhynchus platorynchus impacted annually by towboat entrainment in navigation pools of the Upper Mississippi River were compared against estimates of fishery harvest and ambient population densities to evaluate the relevance of entrainment at the population level. Mean number of sturgeon entrained per kilometer of navigation was estimated at 0.02, and mean number entrained annually considering towboat traffic was estimated at 0.38 sturgeon/ha. Losses associated with entrainment were mostly lower than fishery harvest, although differences were not large. The two sources of mortality combined could potentially reduce the mature adult population to a level where it no longer has the reproductive capacity to replenish itself. Thus, through a combination of entrainment and fishing mortality shovelnose sturgeon may be looming near unsustainable population levels. These estimates are preliminary considering the many uncertainties associated with quantifying entrainment and its effects. Additional research is needed not only to derive better estimates, but also to develop options for managing entrainment.     

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.     

Gonadal development and sex-specific demographics of the shovelnose sturgeon in the Middle Mississippi River

Harvest of the shovelnose sturgeon, Scaphirhynchus platorynchus for caviar has increased. To determine whether populations can withstand increased harvest, detailed information regarding sexual demographics is needed. We describe gender and reproductive development of 306 shovelnose sturgeon from the Middle Mississippi River (River km 0–322) during September 2001 through December 2003. Using dissection and histology, we identified three of the four gonadal stages described previously for male lake sturgeon and all seven stages for females. Males reached maturity at a smaller size than did females. Gonads can be rapidly inspected for sex and stage of development for the shovelnose sturgeon. The sex ratio was not different from 1 : 1. Seven intersexual fish occurred. Female fecundity was positively related to body weight (number of eggs = 30.24 × body weight − 8392; P = 0.013; r² = 0.45) and weakly related to fork length (number of eggs = 146.37 × fork length − 66 176, P = 0.053, r² = 0.23).     

Diet composition of larval and young-of-year shovelnose sturgeon in the Upper Missouri River

Obtaining food following the transition from endogenous to exogenous feeding and during the first year of life is a critical event that strongly influences growth and survival of young‐of‐year fishes. For shovelnose sturgeon Scaphirhynchus platorynchus, limited information is available on food habits during the first year of life. The objective of this study was to quantify diet components of shovelnose sturgeon during the transition from endogenous to exogenous feeding and during the young‐of‐year life stage in the North Dakota and Montana portions of the Missouri River. Young‐of‐year shovelnose sturgeon were sampled between early August and early September 2003. Shovelnose sturgeon initiated exogenous feeding by 16 mm, and individuals 16–140 mm fed exclusively on two macroinvertebrate orders (Diptera and Ephemeroptera). Young‐of‐year shovelnose sturgeon exhibited an apparently high feeding success as 99 of 100 individuals contained food in the gut. The number of organisms in the gut increased exponentially with fish length for larval Diptera (r² = 0.73, P < 0.0001) and linearly (r² = 0.12, P = 0.0006) for larval Ephemeroptera, but the number of Diptera pupae in the gut was not significantly related (P = 0.55) to length of young‐of‐year shovelnose sturgeon. The length of ingested prey was linearly related to fish length for Diptera larvae (r² = 0.20, P = 0.002), whereas the relationship between lengths of ingested Ephemeroptera larvae and lengths of young‐of‐year shovelnose sturgeon was best described by a power function (r² = 0.50, P < 0.0001). These results provide the first quantification of feeding dynamics for young‐of‐year shovelnose sturgeon in a natural river environment.     

Seasonal changes in habitat suitability for adult shovelnose sturgeon in the lower Mississippi River

Shovelnose sturgeon Scaphirhynchus platorynchus are a large‐river fish distributed throughout the Mississippi River basin, including the lower 1,533 km of the Mississippi River where riverine habitat has been and continues to be modified for navigation and is a potential site for development of instream hydrokinetic electric power generation. Information about habitat use and preference is essential to future conservation efforts. Shovelnose sturgeon have previously been found to select particular habitat types, and these selected habitats vary seasonally; although these past analyses do not consider the selected habitats in a landscape context. We used ecological niche factor analysis (ENFA) that uses distributions of telemetry locations and environmental variables to model habitat suitability in a landscape context. We recorded 333 locations of shovelnose sturgeon during July–December 2013 that included periods of relatively high and low river stages. The ENFA analysis indicated high‐suitability locations were in or near deep water during both high and low river stages. During high river stages, high‐suitability locations were near island tip habitat, deep water, and steep bottom slope and far from main channel habitat. During low stages, high‐suitability locations were in or near deep water and main channel habitat and far from secondary channel and wing dike habitats. This landscape‐scale analysis supports seasonal shifts in habitat use and provides insights that can be used to inform habitat conservation and management to benefit shovelnose sturgeon in the lower Mississippi River and possibly other large rivers.     

Pallid sturgeon seasonal habitat selection in a large free-flowing river,the lower Mississippi River

Pallid sturgeon Scaphirhynchus albus (Forbes & Richardson, 1905, Bulletin of the Illinois State Laboratory of Natural History, 1905, 7, 37) are an endangered riverine sturgeon native to the Mississippi and Missouri rivers, and declining numbers have been attributed to multiple stressors, including habitat loss and alteration. The lower Mississippi River provides a useful context to assess pallid sturgeon habitat selection because, although altered for flood control and navigation, it provides a free-flowing system with a diversity of habitats and a minimally altered hydrograph. A discrete choice model of data collected year-round from two reaches for 3–5 years revealed changes in habitat selection across water temperatures and river stages representative of seasonal variation in habitat for 116 telemetry-tagged pallid sturgeon. Natural bank, island tip, and secondary channel were positively selected and main channel, although frequently used, was avoided. The degree of selection varied among river stages, water temperatures, and reaches. Habitat selection appears to be strongly influenced by preference for locations with moderate depth (median 11.7 m; lower and upper quartiles 8.1 m and 16.3 m) and moderate current velocity (median 0.9 m/s; lower and upper quartiles 0.7 m/s and 1.2 m/s).     

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.     

Larvae provide first evidence of successful reproduction by pallid sturgeon, Scaphirhynchus albus, in the Mississippi River

The pallid sturgeon (Scaphirhynchus albus) was not described until 1905, when it was commonly caught by commercial fishers. This species began to decline in the early 1900s presumably because of overharvest and habitat degradation. The U.S. Fish and Wildlife Service listed S. albus as an endangered species in 1990. Because S. albus live in deep, turbid rivers that are difficult to sample, very little is known about its reproductive timing and spawning habitat. The act of spawning has never been observed in nature. Captures of wild young S. albus verifying natural reproduction are rare, the last being a 4‐year‐old fish taken in 1978. In this paper, we describe the first collection of a larval S. albus from the wild and subsequent larval collections in the Mississippi River from 1998 to 2000 using a modified slingshot balloon trawl (the Missouri Trawl) designed to capture small fishes in deep, turbulent rivers. We captured larval Scaphirhynchus spp., including verified S. albus, in association with island habitats often in heavy detritus, especially at downstream tips. We postulate that Scaphirhynchus spp. spawned at the heads of islands upstream from where we collected larvae, but we cannot be certain. The capture of larval S. albus verifies reproduction possibly from the lower Missouri River to the upper and lower Mississippi River. However, we found no evidence of recruitment of S. albus from 1998 to 2000 as we were unable to capture juveniles after 374 trawl hauls that captured over 21 735 fish.     

Habitat use and movements of shovelnose sturgeon in Pool 13 of the upper Mississippi River during extreme low flow conditions

We monitored habitat use and movement of 27 adult shovelnose sturgeon in Pool 13 of the upper Mississippi River, Iowa-Illinois, by radio-telemetry in April through August 1988. Our objective was to determine the response of this species to unusually low water conditions in the upper Mississippi River in 1988. Most (94%) telemetry contacts were made in 3 habitat types: main channel (50%), main channel border where wing dams were present (29%), and tailwaters of Lock and Dam 12 (15%). Habitat use in spring was affected by the extreme low flows. We often found tagged shovelnose sturgeon in the main channel and tailwaters during the spring period (11 March–20 May) where water velocities were highest. This was in contrast to other studies where shovelnose sturgeon did not occupy those areas during years with normal spring flows. Shovelnose sturgeon were typically found in areas with a sand bottom, mean water depth of 5.8 m, and mean bottom current velocity of 0.23 m sec^-1. They occupied areas of swifter current but were not always found in the fastest current in their immediate vicinity. Tagged shovelnose sturgeon tended to remain in the upper, more riverine portion of the pool, and we observed no emigration from the study pool. Linear total range of movement from the tagging site ranged from 1.9 to 54.6 km during the study period.     

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.     

Growth rates of young-of-year shovelnose sturgeon in the Upper Missouri River

Information on growth during the larval and young‐of‐year life stages in natural river environments is generally lacking for most sturgeon species. In this study, methods for estimating ages and quantifying growth were developed for field‐sampled larval and young‐of‐year shovelnose sturgeon Scaphirhynchus platorynchus in the upper Missouri River. First, growth was assessed by partitioning samples of young‐of‐year shovelnose sturgeon into cohorts, and regressing weekly increases in cohort mean length on sampling date. This method quantified relative growth because ages of the cohorts were unknown. Cohort increases in mean length among sampling dates were positively related (P < 0.05, r² > 0.59 for all cohorts) to sampling date, and yielded growth rate estimates of 0.80–2.95 mm day⁻¹ (2003) and 0.44–2.28 mm day⁻¹ (2004). Highest growth rates occurred in the largest (and earliest spawned) cohorts. Second, a method was developed to estimate cohort hatch dates, thus age on date of sampling could be determined. This method included quantification of post‐hatch length increases as a function of water temperature (growth capacity; mm per thermal unit, mm TU⁻¹), and summation of mean daily water temperatures to achieve the required number of thermal units that corresponded to post‐hatch lengths of shovelnose sturgeon on sampling dates. For six of seven cohorts of shovelnose sturgeon analyzed, linear growth models (r² ≥ 0.65, P < 0.0001) or Gompertz growth models (r² ≥ 0.83, P < 0.0001) quantified length‐at‐age from hatch through 55 days post‐hatch (98–100 mm). Comparisons of length‐at‐age derived from the growth models indicated that length‐at‐age was greater for the earlier‐hatched cohorts than later‐hatched cohorts. Estimated hatch dates for different cohorts were corroborated based on the dates that newly‐hatched larval shovelnose sturgeon were sampled in the drift. These results provide the first quantification of growth dynamics for field‐sampled age‐0 shovelnose sturgeon in a natural river environment, and provide an accurate method for estimating age of wild‐caught individuals. Methods of age determination used in this study have applications to sturgeons in other regions, but require additional testing and validation.     

Gonadosomatic index and fecundity of Lower Missouri and Middle Mississippi River endangered pallid sturgeon estimated using minimally invasive techniques

Minimally invasive, non‐lethal methods of ultrasonography were used to assess sex, egg diameter, fecundity, gonad volume, and gonadosomatic index, as well as endoscopy to visually assess the reproductive stage of Scaphirhynchus albus. Estimated mean egg diameters of 2.202 ± 0.187 mm and mean fecundity of 44 531 ± 23 940 eggs were similar to previous studies using invasive techniques. Mean S. albus gonadosomatic indices (GSI) for reproductive and non‐reproductive females were 16.16 and 1.26%, respectively, while reproductive and non‐reproductive male GSI were 2.00 and 0.43%, respectively. There was no relationship between hybrid status or capture location and GSI. Mean fecundity was 48.5% higher than hatchery spawn estimates. Fecundity increased as fork length increased but did so more dramatically in the upper river kilometers of the Missouri River. By examining multiple fish over multiple years, the reproductive cycle periodicity for hatchery female S. albus was found to be 2–4 years and river dwelling males 1–4 years. The use of ultrasonic and endoscopic methods in combination was shown to be helpful in tracking individual gonad characteristics over multi‐year reproductive cycles.     

Genetic distinction of pallid, shovelnose, and Alabama sturgeon: emerging species and the US Endangered Species Act

The sturgeon genus Scaphirhynchus consists of threerecognized species. Pallid and shovelnose sturgeon (S. albusand S. platorynchus, respectively) are sympatric in theMissouri and lower Mississippi Rivers of the central United States. TheAlabama sturgeon (S. suttkusi) is endemic to the nearby MobileRiver drainage and is isolated geographically from the other twospecies. Pallid sturgeon and the extremely rare Alabama sturgeon arelisted as endangered under the US Endangered Species Act (ESA).In contrast, shovelnose sturgeon are relatively common and are notlisted. Despite these taxonomies and morphological evidence, somebiologists have questioned the genetic and taxonomic distinctions of thethree species, thus raising doubts concerning the validity of protectingpallid and Alabama sturgeon under the ESA. To investigate thesequestions, we compared a 436 base-pair sequence of the mitochondrial DNA(mtDNA) control region among the three species. We observed 16 mtDNAhaplotypes defined by 27 single base-pair substitutions (transitions)and one single base-pair insertion/deletion (indel) among 78individuals examined. The maximum sequence divergence among thosehaplotypes (2.06%) was less than values usually observed betweenfish species. However, Alabama sturgeon (n = 3) weredistinguished from the other two taxa (n = 75) by aunique base-pair substitution and haplotype, and pallid and shovelnosesturgeon at their northern range of natural sympatry (upper MissouriRiver) did not share any haplotypes. On the other hand, only frequencydifferences among shared haplotypes distinguished (P < 0.01)pallid and shovelnose sturgeon at their southern range of naturalsympatry (Atchafalaya River), and genetic distances between northern andsouthern localities for each species were nearly as large as thedistances between species. These latter results are consistent withseveral hypotheses, including reports (based on morphology) of putativenatural hybrids in the Atchafalaya River but not in the upper MissouriRiver. Overall, these mtDNA results indicate significant reproductiveisolation between pallid and shovelnose sturgeon in areas of naturalsympatry, and recent evolutionary divergence of Alabama sturgeon. ThesemtDNA results provide the first molecular genetic evidence fordistinguishing the three Scaphirhynchus species and, coupledwith morphological and biogeographic data, indicate that pallid andAlabama sturgeon should be evaluated as distinct species under theESA.     

Estimation of gonad volume, fecundity, and reproductive stage of shovelnose sturgeon using sonography and endoscopy with application to the endangered pallid sturgeon

Most species of sturgeon are declining in the Mississippi River Basin of North America including pallid (Scaphirhynchus albus F. and R.) and shovelnose sturgeons (S. platorynchus R.). Understanding the reproductive cycle of sturgeon in the Mississippi River Basin is important in evaluating the status and viability of sturgeon populations. We used non‐invasive, non‐lethal methods for examining internal reproductive organs of shovelnose and pallid sturgeon. We used an ultrasound to measure egg diameter, fecundity, and gonad volume; endoscope was used to visually examine the gonad. We found the ultrasound to accurately measure the gonad volume, but it underestimated egg diameter by 52%. After correcting for the measurement error, the ultrasound accurately measured the gonad volume but it was higher than the true gonad volume for stages I and II. The ultrasound underestimated the fecundity of shovelnose sturgeon by 5%. The ultrasound fecundity was lower than the true fecundity for stage III and during August. Using the endoscope, we viewed seven different egg color categories. Using a model selection procedure, the presence of four egg categories correctly predicted the reproductive stage ± one reproductive stage of shovelnose sturgeon 95% of the time. For pallid sturgeon, the ultrasound overestimated the density of eggs by 49% and the endoscope was able to view eggs in 50% of the pallid sturgeon. Individually, the ultrasound and endoscope can be used to assess certain reproductive characteristics in sturgeon. The use of both methods at the same time can be complementary depending on the parameter measured. These methods can be used to track gonad characteristics, including measuring Gonadosomatic Index in individuals and/or populations through time, which can be very useful when associating gonad characteristics with environmental spawning triggers or with repeated examinations of individual fish throughout the reproductive cycle.     

Effects of temperature and hydrology on growth of shovelnose sturgeon Scaphirhynchus platorynchus (Rafinesque, 1820) in the lower Mississippi River

We evaluated the effects of thermal and hydrologic conditions on the growth of shovelnose sturgeon (Scaphirhynchus platorynchus) in the lower Mississippi River, USA. Duration of water temperatures 12–24°C had a positive influence and temperatures below 11°C had a negative influence on annual growth increment, but these two variables accounted for less than 15% of the variation in growth. Duration of water temperatures above 28°C, duration of floodplain inundation, duration of low water, and minimum and maximum river stage did not influence annual growth increment. Growth of shovelnose sturgeon in the lower Mississippi River appears to be positively influenced by duration of moderate water temperatures but minimally influenced by hydrologic conditions. The low variation accounted for by thermal and hydrologic variables suggests annual growth increment may be largely influenced by additional abiotic or biotic factors.     

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.