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.     

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).     

Natural growth and diet of known‐age pallid sturgeon (Scaphirhynchus albus) early life stages in the upper Missouri River basin,Montana and North Dakota

Prior to anthropogenic modifications, the historic Missouri River provided ecological conditions suitable for reproduction, growth, and survival of pallid sturgeon Scaphirhynchus albus. However, little information is available to discern whether altered conditions in the contemporary Missouri River are suitable for feeding, growth and survival of endangered pallid sturgeon during the early life stages. In 2004 and 2007, nearly 600 000 pallid sturgeon free embryos and larvae were released in the upper Missouri River and survivors from these releases were collected during 2004–2010 to quantify natural growth rates and diet composition. Based on genetic analysis and known‐age at release (1–17 days post‐hatch, dph), age at capture (dph, years) could be determined for each survivor. Totals of 23 and 28 survivors from the 2004 and 2007 releases, respectively, were sampled. Growth of pallid sturgeon was rapid (1.91 mm day^?1) during the initial 13–48 dph, then slowed as fish approached maximum length (120–140 mm) towards the end of the first growing season. The diet of young‐of‐year pallid sturgeon was comprised of Diptera larvae, Diptera pupae, and Ephemeroptera nymphs. Growth of pallid sturgeon from ages 1–6 years was about 48.0 mm year^?1. This study provides the first assessment of natural growth and diet of young pallid sturgeon in the wild. Results depict pallid sturgeon growth trajectories that may be expected for naturally produced wild stocks under contemporary habitat conditions in the Missouri River and Yellowstone River.     

Reproductive biology of female shovelnose sturgeon in the upper Wabash River, Indiana

Shovelnose sturgeon Scaphirhynchus platorynchus are commercially important, supporting a viable roe fishery throughout much of the Mississippi River drainage. We examined the reproductive attributes of stage‐5 female shovelnose sturgeon captured from the upper Wabash River, Indiana, from March to June 2004. Shovelnose sturgeon were collected using boat electrofishing and experimental gill nets, measured for fork length (FL) and wet weight, and sexed externally if possible. Size‐ and age‐at‐maturity, absolute and relative fecundity, relative egg size, and gonadosomatic index (GSI) were determined for 49 female shovelnose sturgeon (range, 601‐ to 858‐mm FL). Female shovelnose sturgeon reached sexual maturity at approximately 600 mm and age‐at‐maturity ranged from ages 6 to 12 (median age = 9). Relative fecundity ranged from 11 220 to 23 956 eggs kg^?1 (mean = 18 156 eggs kg^?1). Absolute fecundity ranged from 14 294 to 65 490 eggs female^?1 (mean = 30 397 eggs female^?1) and was positively related to FL (r² = 0.76) and wet weight (r² = 0.82). The number of eggs g^?1 of ovary weight ranged from 72 to 170 (mean = 98 eggs g^?1) and was negatively correlated with GSI. GSI values ranged from 9.4 to 27.2 (mean = 19.3) and were positively correlated to FL (r² = 0.18). Our results increase our understanding of shovelnose sturgeon reproductive biology and recruitment dynamics and provide input for models to evaluate the effects of harvest on this species.     

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.     

Stock characteristics of shovelnose sturgeon in the lower Platte River, Nebraska

The objectives of this research were to evaluate the condition, size structure, and growth of shovelnose sturgeon (Scaphirhynchus platorynchus) in the lower Platte River, Nebraska. A total of 1338 shovelnose sturgeon was collected using drifted gill and trammel nets (n = 954), trot lines (n = 340), and benthic trawls (n = 44) in the spring, summer, and autumn from four reaches: (i) Two Rivers State Park, (ii) confluence of Platte and Elkhorn rivers (iii) Louisville, Nebraska, and (iv) confluence of Platte and Missouri rivers during the spring, summer, and autumn of 2000 through 2004. Structural and condition indices were compared among reaches and years. Incremental relative stock densities (RSD) for shovelnose sturgeon sampled throughout the entire lower Platte River were: stock‐quality (1), quality‐preferred (12), preferred‐memorable (82), and memorable‐trophy (5). Proportional stock values were >99 for all years. A significance was detected in RSD categories among reaches and years with larger length‐categories observed in the upstream reaches. Mean relative weight (W_r) for all shovelnose sturgeon was 86.5, indicating a fit population. Mean W_r showed no significant differences among years, but significance was detected among reaches and RSD categories. Shovelnose sturgeon in the lower Platte River appear to be in good condition and exhibit different length‐frequency distributions longitudinally.     

Food habits of the endangered Alabama Sturgeon, Scaphirhynchus suttkusi Williams and Clemmer, 1991 (Acipenseridae)

The gut contents of 12 museum specimens of the federally (USA) endangered Alabama sturgeon were analyzed. This collective series represents 32% of the known museum specimens. Gut contents were dominated by aquatic insects and fishes. At the taxonomic level of insect orders, Dipterans (174.3/fish) were the most numerically abundant food item consumed, followed by Ephemeroptera (19.0/fish), Trichoptera (4.0/fish), Coleoptera (2.4/fish), and Odonata (2.0/fish). Weight, which better reflects the caloric value of food items consumed, showed a similar consumption pattern. Dipterans (12.5%) made up the largest percentage of the total weight of insect prey, followed by Ephemeroptera (8.9%), Trichoptera (6.0%), Odonata, (4.5%) and Coleoptera (0.9%). Volumetric values were substantially higher than weights for Odonata (13.4%) and Ephemeroptera (14.3%), but were comparable for Diptera (10.6%), Trichoptera (4.7%), and Coleoptera (2.2%). The diet of the Alabama sturgeon is similar to the pallid and shovelnose sturgeon, eating both aquatic invertebrates and fish. In terms of piscivory, the volume of fish in the diet of the Alabama sturgeon (34.5%) is intermediate between that of the shovelnose (0% fish) and the pallid sturgeon (60% fish). Based upon the habitat preference of the prey family assemblages present from the 12 guts examined, probable feeding habitats of this sturgeon include sandy to rocky bottoms, slow to swift water velocities, and the water column. Conservation of fish prey may be important to the survival and recovery of the Alabama sturgeon.     

Harvest of Mississippi River sturgeon drives abundance and reproductive success: a harbinger of collapse?

Within harvested populations, relationships between harvest intensity and reproductive responses are typically unclear, rendering regulatory decisions difficult. Harvest of the commercially important shovelnose sturgeon (Scaphirhynchus platorynchus) is increasing in the upper Mississippi River; standardized seasonal sampling revealed that adult abundance is declining. Relative density of annual cohorts varied negatively with historical harvest intensity (r² = 0.84), suggesting that removal of mature adults is reducing the contribution of cohorts to population density. The results of simulation modeling suggest that this currently unregulated fishery is experiencing both growth and recruitment overfishing. Further, the current proposed multi‐state minimum length regulation was insufficient to maintain a sustainable stock. Only a more conservative minimum length limit (685 mm) produced yields that were sustainable at the current level of mortality and provided room for the fishery to grow. The annual mortality rate of the sympatric, federally endangered pallid sturgeon (S. albus) was similar to that of the shovelnose sturgeon population, raising concerns that harvest‐induced mortality is affecting this congener's vital rates.     

Diet of shovelnose sturgeon and pallid sturgeon in the free-flowing Mississippi River

Gut contents of shovelnose and pallid sturgeon from the lower and middle Mississippi River were obtained by colonic flushing, a safe and easily implemented alternative to gastric lavage. Diets of both species were dominated numerically by immature Trichoptera, Ephemeroptera, and Diptera. Primary prey, based on volume, for shovelnose sturgeon were Trichoptera, and for pallid sturgeon were various fishes. Geographic and seasonal nuances in diet were observed for both species, but the general dichotomy of shovelnose sturgeon as browser on invertebrates and pallid sturgeon as predator on fishes did not change. Data indicate that both species require hard substrates for feeding. Data demonstrate that colonic flushing is an effective technique for describing diet and inferring ecological and behavioral information about sturgeon.     

Reproductive traits of shovelnose sturgeon Scaphirhynchus platorynchus (Rafinesque, 1820) in the lower Platte River,Nebraska

We assessed reproductive status, fecundity, egg size, and spawning dynamics of shovelnose sturgeon Scaphirhynchus platorynchus in the lower Platte River. Shovelnose sturgeon were captured throughout each year during 2011 and 2012 using a multi‐gear approach designed to collect a variety of fish of varying sizes and ages. Fish were collected monthly for a laboratory assessment of reproductive condition. Female shovelnose sturgeon reached fork length at 50% maturity (FL₅₀) at 547 mm and at a minimum length of 449 mm. The average female spawning cycle was 3–5 years. Mean egg count for adult females was 16 098 ± 1103 (SE), and mean egg size was 2.401 ± 0.051 (SE) mm. Total fecundity was positively correlated with length (r² = 0.728; P < 0.001), mass (r² = 0.896; P < 0.001), and age (r² = 0.396; P = 0.029). However, fish size and age did not correlate to egg size (P > 0.05). Male shovelnose sturgeon reached FL₅₀ at 579 mm and at a minimum length of 453 mm. The average male spawning cycle was 1–2 years. Reproductively viable male and female sturgeon occurred during the spring (March–May) and autumn (September–October) in both years, indicating spring and potential autumn spawning events. Shovelnose sturgeon in the lower Platte River are maturing at a shorter length and younger age compared to populations elsewhere. Although it is unknown if the change is plastic or evolutionary, unfavorable environmental conditions or over‐harvest may lead to hastened declines compared to other systems.     

A laboratory examination of substrate, water depth, and light use at two water velocity levels by individual juvenile pallid (Scaphirhynchus albus) and shovelnose (Scaphirhynchus platorynchus) sturgeon

We investigated the influence of substrate type, water depth, light, and relative water velocity on microhabitat selection in juvenile pallid (Scaphirhynchus albus) and shovelnose (Scaphirhynchus platorynchus) sturgeon. Individual sturgeon were placed in an 18 927 L elliptical flume, and their location was recorded after a 2‐h period. Data were analyzed using exact chi‐square goodness of fit tests and exact tests of independence. Both sturgeon species used substrate, depth, and light in similar proportions. (all comparisons; P > 0.05). Specifically, pallid and shovelnose sturgeon did not use substrate in proportion to its availability (pallid: P = 0.0026; shovelnose: P = 0.0199). Each species used sand substrate more and gravel substrate less than expected based on availability. Additionally, neither species used woody structure. Both species used deep areas in greater proportion than availability while shallow areas were used less than expected based on availability (pallid; P < 0.0001; shovelnose; P = 0.0335). Pallid and shovelnose sturgeon used very dark areas in greater proportion than expected based on availability; however, very light areas were used in lower proportion than expected (P < 0.0001). Overall, neither species changed their use of habitat in relation to a change in water velocity (pallid, all comparisons P > 0.05; shovelnose, all comparisons P > 0.05). This study is the first investigation of juvenile pallid and shovelnose sturgeon habitat selection in a large‐scale artificial stream system. Field studies of microhabitat selection by juvenile pallid and shovelnose sturgeon should be carried out to substantiate the results of this study, and to identify critical habitat for recovery and management of sturgeon species. Due to the extensive range, longevity, and migratory behavior of these fishes, proper management likely requires river improvements that provide sturgeon with access to a broad range of habitat conditions over time, including system‐wide habitat diversity; natural variation in flow, velocity, temperature, and turbidity; high water quality; a broad prey base; free‐flowing river sections which provide suitable spawning and rearing sites, as well as protection from recreational and commercial harvesting.     

Habitat use and population characteristics of potentially spawning shovelnose sturgeon Scaphirhynchus platorynchus (Rafinesque, 1820), blue sucker (Cycleptus elongatus (Lesueur, 1817), and associated species in the lower Wisconsin River, USA

The goal of this study was to compare the possible locations, timing, and characteristics of potentially spawning shovelnose sturgeon (Scaphirhynchus platorynchus), blue sucker (Cycleptus elongatus), and associated species during the spring of 2007–2015 in the 149‐km‐long lower Wisconsin River, Wisconsin, USA, a large, shallow, sand‐dominated Mississippi River tributary. A 5‐km index station of two pairs of rocky shoals surrounded by sandy areas was electrofished for shovelnose sturgeon and blue sucker in a standardized fashion a total of 40 times from late March through mid‐June, the presumed spawning period. On one date in 2008 and two dates in 2012, all rocky shoals and adjacent sandy areas in the lowermost 149 km of the river were also electrofished for both species. Shovelnose sturgeon and blue sucker appeared to spawn in the limited rocky areas of the river along with at least four other species: mooneye (Hiodon tergisus), quillback (Carpiodes cyprinus), smallmouth buffalo (Ictiobus bubalus), and shorthead redhorse (Moxostoma macrolepidotum), usually at depths of 0.8–2.0 m and surface velocities of 0.4–1.0 m/s. However, apparently spawning shovelnose sturgeon were found only on mid‐channel cobble and coarse gravel shoals within a single 7‐km segment that included the 5‐km index station, whereas apparently spawning blue suckers were encountered on these same shoals but also more widely throughout the river on eroding bluff shorelines of bedrock and boulder and on artificial boulder wing dams and shoreline rip‐rap. Both species showed evidence of homing to the same mid‐channel shoal complexes across years. Blue sucker tended to concentrate on the shoals earlier in the spring than shovelnose sturgeon, usually from late April through mid‐May at water temperatures of 8.0–15.5°C along with quillback and shorthead redhorse. In comparison, shovelnose sturgeon usually concentrated on the shoals from mid‐May through early June at 13.5–21.8°C along with mooneye and smallmouth buffalo. Based on recaptures of tagged fish, at least some shovelnose sturgeon and blue sucker returned to the shoals at one‐year intervals, although there was evidence that female blue sucker may have been more likely to return at two‐year intervals. Most shovelnose sturgeon could not be reliably sexed based on external characteristics. Spawning shovelnose sturgeon ranged from 487 to 788 mm fork length, 500–2400 g weight, and 5–20 years of age, whereas spawning blue sucker ranged from 495 to 822 mm total length, 900–5100 g weight, and 5–34 years of age, although age estimates were uncertain. Females were significantly larger than males for both species although there was overlap. Growth in length was negligible for tagged and recaptured presumably spawning shovelnose sturgeon and low (3.5 mm/y) for blue sucker, suggesting that nearly all growth may have occurred prior to maturity and that fish may have matured at a wide range of sizes.     

Effects of harvest and length limits on shovelnose sturgeon in the upper Wabash River, Indiana

Shovelnose sturgeon Scaphirhynchus platorynchus are one of the few sturgeon species that currently support sustainable commercial harvest. However, harvest closures for many Eurasian sturgeons have resulted in increased exploitation of this fishery, thereby raising concerns about the sustainability of shovelnose sturgeon resources. As a result, the maintenance of self‐sustaining shovelnose sturgeon populations will require the estimation of appropriate harvest levels. This study used an age‐structured population model to examine the effects of harvest (u = 0.15–0.75) and length restrictions on population abundance, mean length‐at‐harvest, biomass, yield, and reproductive potential of female shovelnose sturgeon in the upper Wabash River, Indiana. Model simulations for four hypothetical length‐restriction scenarios (610‐ to 813‐mm reverse slot limit, and a 610‐, 635‐, and 660‐mm minimum length limit) were compared to outputs with no restriction. All population parameters within each length‐restriction scenario declined with increases in harvest level. For each harvest level, all population parameters increased as length limits became more restrictive. The reverse slot limit and 610‐mm minimum length limit provided adequate protection to allow population parameters to increase through an annual harvest level of 0.55. However, these length restrictions were not sufficiently conservative to warrant implementation due to their similarity to length‐at‐maturity of female shovelnose sturgeon. The implementation of a 635‐mm minimum length limit would protect female shovelnose sturgeon from harvest rates >0.75, allow 92% of the females to remain available for harvest, and minimize short‐term (<30 years) declines in yield. Further, sensitivity and robustness analyses suggested that the 635‐mm minimum length limit would allow population parameters to increase even at the worst‐case scenario. As a result, the 635‐mm minimum length limit was recommended as the most appropriate regulation to promote conservation and sustainable harvest of shovelnose sturgeon in the upper Wabash River.     

Effects of river stage height and water temperature on diet composition of year‐0 sturgeon (Scaphirhynchus spp.): a multi‐year study

The diet composition was quantified of year‐0 Scaphirhynchus sturgeons during sampling years (2004–2008) from the Middle Mississippi River to evaluate trends in diet. River stage height varied among years and seasons. Water temperature differed among seasons but did not differ among years. The presence of empty stomachs was evident across years, seasons, and size classes of sturgeon. The overall Multivariate Analysis of Variance for frequency of occurrence was significant, suggesting that macroinvertebrate taxa (e.g. Ephemeroptera, Diptera pupae, Chironomidae, ‘Other’) differed among size classes of sturgeon, but not seasons or years. Furthermore, Tukey's pairwise comparisons indicated Ephemeroptera frequency of occurrence differed between medium and large size classes; Diptera pupae differed between small and medium size classes and small and large size classes; Chironomidae differed between small and large size classes and medium and large size classes; the macroinvertebrate taxa category ‘Other’ differed between small and large size classes. Ultimately, because of the link between diet and growth of year‐0 Scaphirhynchus sturgeon, the early life history may affect eventual recruitment to adult stages.     

Summer diet of juvenile lake sturgeon reintroduced into the Genesee and St. Regis Rivers,New York USA

The restoration of threatened species involves understanding multiple aspects of the life history and ecology of the target species. One important consideration in the restoration of threatened species is feeding ecology. We examined the summer diet of reintroduced juvenile lake sturgeon (Acipenser fulvescens) in the Genesee (n = 119, ages 1 and 2) and the St. Regis rivers (n = 40, ages 1, 2, 5, 6, and 7) of New York State, USA, in the summer of 2005. The most common taxa consumed were Diptera (88% of prey biomass) in the Genesee River and Ephemeroptera (35% of prey biomass) and Diptera (29%) in the St. Regis River. Diptera: (Chironomidae) was the major prey taxon in both benthic communities. Selectivity analysis showed a positive selection of Diptera by Genesee River lake sturgeon and an avoidance of Diptera by St. Regis River lake sturgeon. The St. Regis River lake sturgeon showed avoidance of Coleoptera, but positive selection of Ephemeroptera, Trichoptera, and Oligochaeta. Results indicate that the reintroduced juvenile lake sturgeon are growing and successfully finding prey in these two rivers and appear to actively select specific prey types.     

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.     

Stock structure of shovelnose sturgeon analyzed with microsatellite DNA and morphological characters

Shovelnose sturgeon (Scaphirhynchus platorhynchus) caviar fisheries exist in several states throughout the Mississippi River drainage. Management of these fisheries may benefit from information about genetic stock structure. Sixteen microsatellite loci and morphological analysis were used to examine geographic stock structure of shovelnose sturgeon among seven geographic locations: five within continuous shovelnose sturgeon habitat, and two isolated by artificial barriers. Tissue samples were collected from 1999 to 2006 from the upper Missouri, Platte, lower Missouri, middle Mississippi, Ohio, Wabash, and Atchafalaya rivers. Geographic samples of shovelnose sturgeon samples could be separated into three groups with discriminate function analysis of four morphological characters. The microsatellite loci were highly variable (allelic richness range 5.65–13, observed heterozygosity range 0.64–0.89). Bayesian clustering did not identify multiple groups in the genetic data. However, significant genetic differentiation (θ_ST = 0.017, P < 0.0001) was observed among a priori defined geographic samples and all pairwise estimates of θ_ST were significant. Assignment testing among a priori defined groups indicated that the sturgeon from the upper Missouri, Platte, and Atchafalaya rivers had the highest assignment scores and thus were most distinct, while the lower Missouri and the middle Mississippi were less distinct and a larger fraction of the sturgeon from these rivers was genetically assigned to other rivers. The Ohio and Wabash rivers were genetically most similar. A Mantel test revealed a positive relationship between genetic and geographic distance (r = 0.464, P = 0.055) that was not statistically significant. The level of genetic differentiation observed at both molecular and morphological characters suggests that multiple shovelnose sturgeon populations may exist within the studied area, yet demographic factors and possible gene flow may have minimized the amount of genetic differentiation among locations.     

A comparison of four types of sampling gear used to collect shovelnose sturgeon in the Lower Missouri River

We compared variability in catch per unit effort (CPUE) and size structure of shovelnose sturgeon Scaphirhynchus platorynchus collected in 2003 and 2004 with stationary winter gill nets, drifted trammel nets, hoop nets, and otter trawls in the Lower Missouri River, USA to determine the most precise types of gear to collect all sizes of sturgeon so that refinements of long‐term monitoring protocols can be made. A total of 1947 net sets or trawls collected 8743 shovelnose sturgeon, with 67% of the fish collected during winter gill netting (16% of total samples). Mean coefficient of variation (CV) among all months for juvenile (age 3 and younger; <250 mm fork length) sturgeon was highest for gill nets and lowest for otter trawls (P = 0.0008). Mean CV of subadult and adult shovelnose sturgeon (≥250 mm) was highest in hoop nets compared to other gear types (P = 0.0002). All gear and mesh sizes collected the most common sizes of shovelnose sturgeon (500–600 mm), but only otter trawls and trammel nets collected fish <150 mm. The higher precision of winter gill nets and summer otter trawls led to fewer samples needed to detect changes in CPUE as compared to hoop and trammel nets. Sampling only with types of gear that do not collect younger shovelnose sturgeon may hinder management decisions that rely on recruitment trends to determine the effects of management actions (e.g. channel modifications).     

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.     

Evaluation of pulsed gastric lavage on the survival of captive shovelnose sturgeon

Little is known about the diet of the endangered pallid sturgeon, Scaphirhynchus albus. Because of this endangered status non‐lethal methods to ascertain their food habits must be evaluated. Tests of pulsed gastric lavage (PGL) were conducted on shovelnose sturgeon S. platorynchus, as a surrogate for the pallid sturgeon, to determine if PGL negatively affected their survival. Nine sequential trials containing eight shovelnose sturgeon (N = 72) were conducted with wild fish collected from the Platte River near Louisville, Nebraska. Each trial group was placed into one of two tanks, with each tank containing three pulsed gastric lavaged fish and one control fish. Fish were weighed, measured, and tagged with a passive integrated transponder (PIT tag) before being transported to holding tanks for acclimation and PGL treatment. Sturgeon were acclimated 4–6 days prior to PGL, then held for 15 days after treatment. Temperature, dissolved oxygen, ammonia, and nitrite were monitored daily. No significant difference between the survival of PGL fish and control fish was detected. Because the test required these fish to endure the additional stress of transport and handling, PGL seems a viable technique that can be safely used to evaluate food habits of shovelnose sturgeon, and could be considered safe for pallid sturgeon food habit studies.