Monitoring demographics of a commercially exploited population of shovelnose sturgeon in the Wabash River,Illinois/Indiana,USA

Shovelnose sturgeon (Scaphirhynchus platorynchus, Rafinesque, 1820) in the Wabash River, Illinois/Indiana, USA, provide an important recreational sport and commercial caviar fishery. In fact, it is one of the last commercially viable populations for sturgeon roe harvest. Due to increased demand in the caviar trade and endangered species legislation that protect shovelnose sturgeon in only a portion of their range, efforts of the roe harvest market may continue to divert toward unprotected populations like the shovelnose sturgeon in the Wabash River. Previous studies have shown that increased harvest pressure in this species can affect the age‐at‐maturation and result in recruitment overfishing. Therefore, it is important to closely and continuously monitor commercially exploited populations. Over the past decade (2007–2016), 13,170 shovelnose sturgeon were sampled with boat electroshocking, hoop nets, drift nets, trotlines, and benthic electrified trawls. Captured fish ranged from 61 to 910 mm fork length (FL; mean = 668 mm), with very few fish less than 550 mm FL. Although fish were found to be in a healthy condition (mean relative weight = 87), there was a decrease in the mean condition over time. In addition, we saw declines in mean FL, weight of roe‐per‐fish, and size‐at‐maturity for female fish directly impacted by harvest. The decline of these population parameters, coupled with an increase in total annual mortality and a truncated age frequency distribution, suggest that harvest is negatively impacting the demographics and recruitment of shovelnose sturgeon in the Wabash River. Considering the downward trajectory of population dynamics and high estimates of mortality, their resiliency to continued harvest and environmental changes will be limited.     

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.     

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.     

Current status and trends in shovelnose sturgeon (Scaphirhynchus platorynchus) management and conservation

The objective of this study was to update information regarding the status of shovelnose sturgeon fisheries. Although a substantial amount of shovelnose sturgeon research has been conducted in the past decade, the study purpose was not to provide a comprehensive review of the literature; the primary interest was in the status, trends, and management of Scaphirhynchus platorynchus fisheries in North America. Biologists were surveyed in all 24 states within the native distribution of the species; results indicate that commercial harvest is currently permitted in eight states, recreational harvest is allowed in 13 states, and that regulations vary within rivers and jurisdictional boundaries. Although recreational exploitation of shovelnose sturgeon is thought to be low and not a significant threat to populations, commercial harvest is a major concern in states with a commercial fishery. In the last decade harvest has increased in all states with commercial shovelnose sturgeon fisheries, but recent implementation of regulations has decreased harvest in some states. Approximately half of the states with extant shovelnose sturgeon populations conduct routine monitoring of the species, and the understanding of shovelnose sturgeon populations is increasing.     

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.     

Potential responses of the Lower Missouri River Shovelnose Sturgeon (Scaphirhynchus platorynchus) population to a commercial fishing ban

We developed an age‐structured population matrix model to perform population viability analysis for Lower Missouri River (LMR) shovelnose sturgeon (Scaphirhynchus platorynchus). We investigated potential effects of the commercial fishing moratorium put in place to help protect the similar‐appearing pallid sturgeon (S. albus). The model applies different components of total variance in life history parameters at different levels: sampling variance (parameter uncertainty) between model iterations; temporal variance (temporal environmental fluctuations) between time steps within iterations; and individual variance (individual differences) within each time‐step. The model predicted annual rates of population increase of 1.96% under historic fishing and 2.67% with removal of historic fishing. We identified combinations of fishing and harvest size restrictions that would permit a sustainable harvest of shovelnose sturgeon. Overall, the ban on commercial fishing of shovelnose sturgeon in the LMR due to similarity of appearance to pallid sturgeon should help the LMR shovelnose sturgeon population begin to rebound while decreasing any negative effects it may have had on pallid sturgeon populations.     

Unscreened Water-Diversion Pipes Pose an Entrainment Risk to the Threatened Green Sturgeon,Acipenser medirostris

Over 3,300 unscreened agricultural water diversion pipes line the levees and riverbanks of the Sacramento River (California) watershed, where the threatened Southern Distinct Population Segment of green sturgeon, Acipenser medirostris, spawn. The number of sturgeon drawn into (entrained) and killed by these pipes is greatly unknown. We examined avoidance behaviors and entrainment susceptibility of juvenile green sturgeon (35±0.6 cm mean fork length) to entrainment in a large (>500-kl) outdoor flume with a 0.46-m-diameter water-diversion pipe. Fish entrainment was generally high (range: 26–61%), likely due to a lack of avoidance behavior prior to entering inescapable inflow conditions. We estimated that up to 52% of green sturgeon could be entrained after passing within 1.5 m of an active water-diversion pipe three times. These data suggest that green sturgeon are vulnerable to unscreened water-diversion pipes, and that additional research is needed to determine the potential impacts of entrainment mortality on declining sturgeon populations. Data under various hydraulic conditions also suggest that entrainment-related mortality could be decreased by extracting water at lower diversion rates over longer periods of time, balancing agricultural needs with green sturgeon conservation.     

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.

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.     

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.     

A synopsis of lake sturgeon management in Alberta, Canada

The lake sturgeon ( Acipenser fulvescens ) is resident in the North and South Saskatchewan rivers of Alberta. Because of their confined distribution, low abundance, and vulnerability to anthropogenic impacts, lake sturgeon in Alberta have been the focus of specific management actions for nearly 60 years.
Lake sturgeon harvest was prohibited in Alberta from 1940 to 1968, after which a limited harvest was again permitted, but only as a hook-and-line sport fishery. After being reopened the fishery was primarily managed as a "trophy" fishery. Sport fishery harvest statistics have been compiled annually since 1968, through mail-out questionnaires sent to all holders of sturgeon angling licenses.
Few research studies have been undertaken on the life history or habitat requirements of lake sturgeon in the province, and biologists have relied primarily on anglers to provide information for management decisions. Preliminary population information for the South Saskatchewan River, from mark-recapture data, suggests a population size of about 3700 fish. Recent studies in the North Saskatchewan River indicate a population of about 1300 fish. Studies indicate that a portion of the population in both rivers is trans-boundary, moving between the waters of the adjoining provinces of Alberta and Saskatchewan, and creating concerns because of differences in angling regulations. During recent decades, major consumptive uses of water have also contributed to depletion of lake sturgeon habitat in the province.
After reviewing past management strategies and actions, Alberta Environmental Protection implemented a number of regulatory changes in 1987 to further protect lake sturgeon populations in the province while continuing to provide angling opportunities and maintaining a controlled harvest.     

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).     

Shovelnose sturgeon exhibit predator avoidance behaviour in the presence of a hungry predator

An experiment was designed to test whether age‐0 shovelnose sturgeon (Scaphirhynchus platorynchus) exhibited predator avoidance behaviour in response to a channel catfish (Ictalurus punctatus) predator. It was hypothesized that shovelnose sturgeon would not exhibit any innate predator avoidance behaviour because previous reports have shown a congener of the shovelnose sturgeon, the pallid sturgeon (S. albus), to be an unfavourable prey item for channel catfish. The results, however, indicated that shovelnose sturgeon generally avoided space occupied by the catfish predator and spent a greater proportion of time in the predator avoidance zone within the experimental tank. Bitten fish, in particular, spent a greater period of time in the predator avoidance zone. Of all sturgeon used in this experiment (N = 30), 73% swam within the fork length (350 mm) of the catfish predator. The results seem to indicate that shovelnose sturgeon were initially oblivious to the risk of predation by the catfish predator, but after interaction (e.g. being chased or bitten) appeared to display predator avoidance behaviour. Predator avoidance behaviour in shovelnose sturgeon may thus be suggested as a learned rather than an innate behaviour.     

The effect of temperature on embryo survival and development in pallid sturgeon Scaphirhynchus albus (Forbes & Richardson 1905) and shovelnose sturgeon S. platorynchus (Rafinesque, 1820)

The thermal response of pallid sturgeon Scaphirhynchus albus and shovelnose sturgeon S. platorynchus embryos was determined at incubation temperatures from 8 to 26°C and 8 to 28°C, respectively. The upper and lower temperatures with 100% (LT₁₀₀) embryo mortality were 8 and 26°C for pallid sturgeon and 8 and 28°C for shovelnose sturgeon. It was concluded that 12–24°C is the approximate thermal niche for embryos of both species. Generalized additive and additive‐mixed models were used to analyze survival, developmental rate and dry weight data, and predict an optimal temperature for embryo incubation. Pallid sturgeon and shovelnose sturgeon embryo survival rates were different in intermediate and extreme temperatures. The estimated optimal temperature for embryo survival was 17–18°C for both species. A significant interaction between rate of development and temperature was found in each species. No evidence was found for a difference in timing of blastopore, neural tube closure, or formation of an S‐shaped heart between species at similar temperatures. The estimated effects of temperature on developmental rate ranged from linear to exponential shapes. The relationship for rate of development to temperature was relatively linear from 12°C to 20°C and increasingly curvilinear at temperatures exceeding 20°C, suggesting an optimal temperature near 20°C. Though significant differences in mean dry weights between species were observed, both predicted maximum weights occurred at approximately 18°C, suggesting a temperature optimum near 18°C for metabolic processes. Using thermal optimums and tolerances of embryos as a proxy to estimate spawning distributions of adults in a river with a naturally vernalized thermal regime, it is predicted that pallid sturgeon and shovelnose sturgeon spawn in the wild from 12°C to 24°C, with mass spawning likely occurring from 16°C to 20°C and with fewer individuals spawning from 12 to 15°C and 21 to 24°C. Hypolimnetic releases from Missouri River dams were examined; it was concluded that the cooler water has the potential to inhibit and delay sturgeon spawning and impede embryo incubation in areas downstream of the dams. Further investigations into this area, including potential mitigative solutions, are warranted.     

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.     

Comparing aging precision of calcified structures in shovelnose sturgeon

Age estimates for population analysis must be precise. We assessed the usefulness of pectoral fin rays, sphenoids, opercula, and dorsal scutes of shovelnose sturgeonScaphirhynchus platorynchus (n = 30) as aging structures based on ease of collection, distinctness of annuli, and measures of precision both between and within readers. We also determined how age estimates from paired fin rays of individuals were related (n = 106). Pectoral fin rays generated the highest within‐reader precision (100% within 2 years) followed by sphenoids (58%), opercula (56%), and dorsal scutes (49%). Ages estimated by the pectoral fin ray also had higher between‐reader agreement (80% within 1 year) than did those from the operculum (60%), sphenoid (59%), or dorsal scute (56%). Likewise, age estimates from pectoral fin rays had the lowest mean coefficient of variation (8.2%) followed by sphenoids (9.9%), opercula (11.3%), and dorsal scutes (11.5%). Only the operculum produced biased estimates between readers. Ages from paired fin rays agreed poorly (36% exact, 30% within 1 year) although no aging bias occurred. The pectoral fin ray is typically used to age shovelnose sturgeon. Because uncertainty about accuracy and precision of age estimates from this structure remains, shovelnose sturgeon management objectives that result from age data should remain conservative.     

A life‐stage population model of the European sturgeon (Acipenser sturio L., 1758) in the Elbe River. Part II: assessment of the historic population decline

A post‐hoc analysis on the decline of the European sturgeon (Acipenser sturio) population in the Elbe River was performed for the end of the 19th century. A modeling approach was applied to determine the role of various factors potentially impacting the population. Using an age‐structured population model developed for European sturgeon, the estimates of fishing efficiency in the Elbe were verified. Catches reported from 1850 to 1920 were used as a basis to identify the population size, dynamics of the decline and the impacts contributing to the population decline. According to results of the model, the historic population size of the European sturgeon in the Elbe River exceeded 450 000 individuals. The population revealed a significant response in abundance of adults towards the fishery directed at spawning migrants, even at low fishery effort. Unsustainable fishery was the initial cause of the population decline, while increasing pollution and habitat degradation revealed significant impacts after 1860. The results represent a good example for the fisheries ‘hyperstability’ phenomenon. In the case of the Elbe fishery, an increasing fishery effort was masking the actual population decline while temporarily maintaining the overall yield. The results indicate that, for sturgeons and other species with similar life history patterns, the use of fishery data for the assessment of population dynamics and fishery sustainability has to be applied with due caution, as it might produce false images of the population size and dynamics.     

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.     

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.     

An approach for assessing paddlefish Polyodon spathula (Walbaum, 1792) populations using mark‐recapture information

Historically, management of fish populations has been achieved through the use of age‐derived estimates of growth and mortality. For long‐lived species such as the paddlefish, Polyodon spathula, the validation of calcified structures is necessary to correct for the presence of false annuli or the absence of growth rings. Regardless, numerous studies on paddlefish populations throughout their range have continued the use of un‐validated age estimates to evaluate dynamic rate functions. The use of mark‐recapture studies has been applied widely to evaluate growth of short‐lived fishes, but only recently to a few long‐lived freshwater fishes (i.e. white sturgeon, shovelnose sturgeon, and pallid sturgeon). This study provides the first simultaneous evaluation of both mark‐recapture and age‐estimate information in determining population characteristics for paddlefish. In doing so, this study has determined that the P. spathula population in the Black River below Clearwater Dam, Missouri is sustainable. Additionally, mark‐recapture information is sufficient to produce accurate and reliable assessments of paddlefish populations in lieu of validated aging structures; future management should be centered on accurate scientific methods, which is not the case when using un‐validated aging structures (e.g. scales, otoliths, fin rays, dentary bones) to determine population parameters. Mark‐recapture information can provide an accurate, alternative source of growth and mortality information for use in evaluating and managing paddlefish populations throughout their range.