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.     

Age,growth and hatch dates of ingressing larvae and surviving juveniles of Atlantic menhaden Brevoortia tyrannus

Ages, growth and hatch dates of ingressing Brevoortia tyrannus larvae were determined in a 3 year sampling survey at the mouth of the Chesapeake Bay, U.S.A. To determine if otolith‐aged cohorts had variable relative survival, hatch dates of summer‐caught young‐of‐the‐year (YOY) juveniles collected throughout the Chesapeake Bay were compared with hatch dates of ingressing larvae. Modal total length of ingressing larvae was similar among years: 28 mm in 2005–2006 and 2007–2008, and 30 mm in 2006–2007. Ages of ingressing larvae ranged from 9 to 96 days post hatch (dph); mean ages were similar among years, but significantly older in 2006–2007 (50 dph) than in 2005–2006 (44 dph) and 2007–2008 (46 dph). Larval growth rates differed among years. Earliest growth, when larvae were offshore (0–20 dph), was faster in 2006–2007 (0·62 mm day^?1), than in 2005–2006 and 2007–2008 (0·55 mm day^?1 in these years). Subsequently, from 30 to 80 dph, growth was slowest in 2006–2007. Hatch dates of ingressing larvae occurred from September to March and 90% (2007–2008) to 98% (2006–2007) had hatched prior to 31 December. In contrast, most surviving YOY juvenile B. tyrannus had hatched in January to February, suggesting selective mortality of early‐hatched individuals, apparently during the overwinter, larval to juvenile transition period.     

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.     

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.     

Recruitment in Japanese sea bass,Lateolabrax japonicas (Cuvier, 1828): effects of timing on spawning and larval quality and quantity

This study aimed to elucidate the causes of variability in larval survival and juvenile abundance (recruitment) within and among cohorts of Japanese sea bass (JSB; Lateolabrax japonicus), a winter‐spawning temperate coastal marine fish. Larvae and settled individuals (settlers) belonging to four cohorts were collected from Tango Bay (the Sea of Japan coast) during eight sampling cruises in 2007 and 2008. Larvae were sampled in January and February each year using an ichthyoplankton net, and settlers were collected in February and March each year using a beam trawl. Age of individual larva and settlers was determined and growth history was back‐calculated from otolith microstructure, and the hatch date distribution was computed. Temperature, daily growth rate, size‐at‐age, hatch date, and density data of larvae and settlers allowed elucidating the effects of the timing of spawning and larval quantity and quality (growth rate and body size) on larval survival and recruitment within and among cohorts of JSB. Results showed that cohorts that hatched earlier in the season had higher quantity of larvae, experienced higher mean temperatures and survived better than cohorts hatched later. Recruitment variability among cohorts is determined largely by the initial quantity of larvae, as this explained >97% of the variability in recruitment among cohorts. Within cohorts, larger hatched larvae grew faster than their smaller conspecifics, and the bigger and faster growing larvae survived and settled. Results from this study suggest the following scenarios for recruitment of JSB: (i) earlier spawning in the season promotes larval survival since earlier cohorts are likely to encounter a better temperature and perhaps food conditions, and therefore recruit better than later cohorts; (ii) the initial quantity of larvae appears to be an important determinant of recruitment variability among cohorts; and (iii) the size‐ and growth‐related mechanisms operating during the larval phase appear to start at the time of the hatch.     

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.     

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 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.     

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.     

Age and growth of early-life-stage Atlantic tarpon (Megalops atlanticus) from the northcentral Gulf of Mexico

Age and growth of early-life-stage Atlantic tarpon Megalops atlanticus collected from Mississippi coastal waters in the northcentral Gulf of Mexico (GOM) are described using otolith microstructure analysis. Tarpon leptocephali (n = 95, 16.0—27.8 mm standard length, L_S) collected from June throughOctober 2013—2018, ranged in age from 22 to 43 days (mean = 30.9 ± 0.5 days). Leptocephalus somatic growth rates ranged 0.46—1.24 mm day⁻¹ (mean = 0.76 ± 0.02 mm day⁻¹), and leptocephalus otolith growth rates ranged 1.78—3.97 μm day⁻¹ (mean = 2.58 ± 0.04 μm day⁻¹). Growth rates were inversely correlated to leptocephalus age, indicating the shrinkage phase associated with leptocephalus metamorphosis. Juvenile tarpon (n = 358, 50—359 mm fork length, L_F) were collected from August through December 2007—2018. Juveniles exhibited a positive allometric relationship (adjusted R² = 0.99, P < 0.001) between length and mass. The age of 100 juveniles (71—277 mm L_F) ranged from 76 to 174 days. Juvenile growth rate was estimated as 1.56 ± 0.11 mm day⁻¹. Significant (P < 0.001) linear relationships were found between juvenile age and otolith metrics, including otolith mass (R² = 0.81) and radius (R² = 0.68). Evaluation of the backcalculated hatch dates suggests that specimens in the collection hatched from late May through mid-September with slight peaks during July and August. A Rao's Spacing Test of Uniformity indicates the presence of significant lunar periodicity in leptocephalus hatch dates (n = 95, U = 250.1, P < 0.05), with 50% of the leptocephali hatched within 5 days (before or after) of the full moon. This study fills critical gaps in the scientific knowledge of tarpon and provides estimates of early-life-history metrics for an iconic game fish at the northernmost extent of its GOM range.     

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.     

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).     

A guide to the embryonic development of the shovelnose sturgeon (Scaphirhynchus platorynchus), reared at a constant temperature

Described is the sequence and timing of embryologic development of shovelnose sturgeon, (Scaphirhynchus platorynchus) reared at a constant temperature 20 ± 0.5°C. Artificially spawned, fertilized eggs were held in a recirculating system. Embryos were sampled hourly during the first 48 h of development and every 3 h thereafter. Embryos were viewed and imaged at 35× magnification. The first cleavage furrow appeared 2 h post‐fertilization; early (synchronous) cleavage was completed after 7 h. Blastulation concluded at 16 h when the dorsal blastopore lip formed. The slitlike blastopore appeared at 29 h, signifying the completion of gastrulation. At 33 h, the rudiments of the excretory system emerged, followed by closing of the neural tube at 36 h and formation of the s‐shaped heart at 60 h. The body continued to elongate with mass hatch occurring at 102 h. After hatch, larvae swam into the water column and drifted in the flow for approximately 2 days after which the larvae became positively rheotaxic. After expulsion of the pigment plug, the larvae began exogenous feeding and other structures continued to develop. Metamorphosis was completed after 26 days of development. Because the shovelnose sturgeon possesses developmental patterns similar to those of other sturgeon, we can use this species as a model for the closely related pallid and Alabama sturgeon.     

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.     

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.     

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.     

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.     

Similarities and differences in 13C and 15N stable isotope ratios in two non‐lethal tissue types from shovelnose sturgeon Scaphirhynchus platorynchus (Rafinesque, 1820)

We tested the hypothesis that δ¹³C and δ¹⁵N signatures of pectoral spines would provide measures of δ¹³C and δ¹⁵N similar to those obtained from fin clips for adult shovelnose sturgeon Scaphirhynchus platorynchus. Thirty‐two shovelnose sturgeon (fork length [FL] = 500–724 mm) were sampled from the lower Mississippi River, USA on 23 February 2013. Isotopic relationships between the two tissue types were analyzed using mixed model analysis of covariance. Tissue types differed significantly for both δ¹³C (P < 0.01; spine: mean = ?23.83, SD = 0.62; fin clip: mean = ?25.74, SD = 0.97) and δ¹⁵N (P = 0.01; spine: mean = 17.01, SD = 0.51; fin clip: mean = 17.19, SD = 0.62). Neither FL nor the FL × tissue type interaction had significant (P > 0.05) effects on δ¹³C. Fin clip δ¹³C values were highly variable and weakly correlated (r = 0.16, P = 0.40) with those from pectoral spines. We found a significant FL‐tissue type interaction for δ¹⁵N, reflecting increasing δ¹⁵N with FL for spines and decreasing δ¹⁵N with FL for fin clips. These results indicate that spines are not a substitute for fin clip tissue for measuring δ¹³C and δ¹⁵N for shovelnose sturgeon in the lower Mississippi River, but the two tissues have different turnover rates they may provide complementary information for assessing trophic position at different time scales.     

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).