Seasonal variation in the diet of juvenile lake sturgeon,Acipenser fulvescens,Rafinesque, 1817, in the Winnipeg River,Manitoba, Canada

The lake sturgeon, Acipenser fulvescens, is a threatened species across most of its range. To understand any potential shifts in diet across season or habitat, stomach contents were examined from juvenile lake sturgeon caught in the Winnipeg River, Manitoba, Canada. This information will aid in assessment of environmental impacts and conservation initiatives for this threatened species. From 2006 to 2008 gut contents were collected from juvenile lake sturgeon using a non‐lethal gastric lavage methodology. Juvenile lake sturgeon were sampled from four discrete deep water habitat types (13.7–27.4 m depths), during the months of May, June, July and October. In total, 13 066 prey items belonging to 14 prey groups were collected from 345 juvenile lake sturgeon (251–835 mm TL). Overall, juvenile lake sturgeon consumed a low diversity of organisms; insect larvae from three invertebrate orders, Trichoptera, Diptera and Ephemeroptera, accounted for 97.4% of the total numeric abundance of prey items recovered. A relationship between size of juvenile lake sturgeon and prey quantity or type was not observed. Diet of juvenile lake sturgeon varied in relation to season, with Diptera (77.6%) being the most abundant prey in May, Trichoptera the most abundant in June (n = 3,056, 60.4%) and July (n = 2,055, 52.6%). During October, 96% of stomachs examined were empty. In terms of habitat type, the standardized abundance of invertebrate prey items was highest in deep water habitats characterized by medium water velocity (depths > 13.7 m) and coarse substrate (particle sizes > 0.063 mm) in May and June when compared to deep water habitats characterized by low water velocity (<0.2 m.s^?1) and fine substrate (particle sizes < 0.063 mm). However, in July, the opposite was observed and prey abundance in juvenile lake sturgeon stomachs decreased with increasing particle size. Results suggest a high degree of dietary overlap among juvenile lake sturgeon from multiple size/age classes occupying deep‐water habitats of the Winnipeg River.     

Seasonal variation in diet of juvenile Amur sturgeon Acipenser schrenckii in the lower reaches of the Songhua River,Northeast China

We investigated seasonal variation in the diet of hatchery-reared juvenile Amur sturgeon Acipenser schrenckii in the Tongjiang reach of the Songhua River, Northeast China. The results indicated that Amur sturgeon fed mainly on Chironomidae and Ephemeroptera and secondarily on unspecified bony fish, Odonata, and Trichoptera. Diet changed throughout the year, with autumn 2017 and spring 2018 dominated by Ephemeroptera larvae, while summer 2018 prey comprised primarily Chironomidae larvae. Unspecified bony fish were observed at greater frequency in autumn 2017. Amur sturgeon captured in summer contained a greater quantity of Trichoptera larvae compared to those from autumn and spring sampling. Results were consistent with previous reports of Amur sturgeon diet in other areas of its native distribution and suggested that they may be opportunistic feeders throughout the year, although larval Chironomidae and Ephemeroptera appear to be important prey. These results represent the first reported diet composition data for juvenile Amur sturgeon in the lower Songhua River. Results of the study form part of a comprehensive investigation of feeding habits of juvenile Amur sturgeon that provides basic data for study of the biological characteristics of the species.     

Assessment of lake sturgeon (Acipenser fulvescens) recruitment in a regulated spawning tributary of Rainy Lake,Ontario

Continued study of the relationship between lake sturgeon (Acipenser fulvescens) recruitment and hydroelectric dams and operations, in a variety of river systems and habitat types is needed to improve the ability to predict and monitor impacts of the hydroelectric industry on this species. Herein, we present results of a juvenile lake sturgeon study aimed at addressing concerns over an inferred lack of recruitment resulting from spawning downstream of a hydroelectric generating station (HGS). Two years of sampling (2015 and 2016) were conducted in five sections of a 41 km long reach of the Seine River, Ontario, a lake sturgeon spawning tributary of Rainy Lake. Using an established gillnetting method, deepwater habitat was targeted to capture juvenile lake sturgeon to assess relative abundance, recruitment (cohort strength), and growth. Deepwater habitat, defined as water depths >6 m in this system, comprised only 2.1% of the wetted area in this study area. Within these habitats, a total of 331 lake sturgeon capture events were observed over the 2-years study period. The majority of the lake sturgeon catch (85%) was comprised of age-0 to age-5 individuals (both sampling years combined). Although inter-annual variation in cohort strength was apparent, each cohort between 2006 and 2016 was represented. The spatial distribution of cohorts varied among river reaches with younger individuals (age-0 and age-1) occupying reaches proximal to the Sturgeon Falls HGS, and larger, older individuals (age-2 to age-5) occupying reaches further downstream. The rarity of age-6+ individuals can likely be explained by ongoing downstream redistribution of juveniles over time, out of the Seine River and into Rainy Lake. Growth of juvenile lake sturgeon captured in the Seine River was above average relative to conspecifics from other rivers in the Hudson Bay drainage. Unfortunately, baseline data sets required to facilitate comparisons of contemporary (post-construction Sturgeon Falls HGS) versus historical (i.e. pre- Sturgeon Falls HGS) lake sturgeon recruitment, or to evaluate the influence of the Seine River Water Management Plan (2004) on lake sturgeon recruitment, are lacking. However, juvenile Lake Sturgeon are more abundant in this system than what had been surmised based on recent studies which implemented random sampling. Results indicate that juvenile lake sturgeon may reside in spawning tributaries for several years (age-0 to age-5) prior to seeking alternate habitats and highlights the value of targeted sampling (i.e. by depth) along the flow axis of rivers downstream of spawning areas when assessing lake sturgeon recruitment patterns.     

A reintroduced lake sturgeon population comes of age: A genetic evaluation of stocking success in the St. Louis River

Lake sturgeon (Acipenser fulvescens) have experienced significant declines throughout the Great Lakes, leading to stocking efforts at some locations to reestablish extirpated populations. Given the late sexual maturity of the species, assessment of stocking program success is often delayed. Lake sturgeon were extirpated from the St. Louis River in western Lake Superior. Stocking began in 1983 and continued until 1994, using the Wolf River (Lake Winnebago) in the Lake Michigan basin as the source. Stocking resumed from 1998 to 2000, using the Sturgeon River in Lake Superior as the source. Our objectives were to (a) determine the movement patterns of stocked Wolf River sturgeon, (b) determine whether stocked individuals are migrating into natural spawning populations during the spawning season, (c) identify the origins of individuals captured in the St. Louis River, and (d) assess the genetic diversity of the reintroduced St. Louis River population. All collected samples were analyzed using 12 microsatellite loci, followed by genetic assignment testing to achieve the first three objectives. Genetic diversity was compared to natural spawning populations in Lake Superior. The highest proportion of stocked Wolf River lake sturgeon was detected in western Lake Superior, close to the stocking site. However, individuals were detected throughout Lake Superior. Wolf River individuals were detected in most of the spawning populations in Lake Superior, with the greatest number in the Sturgeon River and the Goulais River. The majority of individuals captured in the St. Louis River were of stocked origin (88.1%), with 73.5% from the Wolf River and 14.6% from the Sturgeon River. These observed proportions differed from the expected proportions based on the number of sturgeon released from each source (χ² = 55.37, p < 0.00001), with a higher representation of Sturgeon River individuals. Evidence of natural recolonization in the St. Louis River was detected from Lake Huron and Goulais River individuals. Genetic diversity of the St. Louis River population was comparable to levels observed in the remnant natural populations in Lake Superior. However, the effective population size of the St. Louis River was small (N_e = 38.1) and average relatedness among individuals was relatively high (r = 0.151). Monitoring of movement of stocked Wolf River sturgeon throughout Lake Superior should continue, with careful attention to the potential for outbreeding depression in remnant populations. Genetic diversity of the St. Louis River population should also continue to be monitored to see if it improves with increased natural reproduction.     

Spatiotemporal changes in juvenile lake sturgeon abundance in a large river

Monitoring trends in a long-lived species such as lake sturgeon (Acipenser fulvescens) has it challenges. Targeting the correct life stage to monitor can provide different and contrasting results. Early life stages experience high variable mortality and are often not a good predictor of population trend. Juvenile lake sturgeon (i.e., 5–10 years) experience high survival and could be a good life stage to monitor for assessing population trends. A structured, randomized gillnetting program, Broad-scale Monitoring, which was selective to juvenile lake sturgeon, was conducted over three cycles (i.e., 5 + years apart) in three contiguous Ottawa River reaches supporting lake sturgeon. In this study, we assessed spatiotemporal changes in juvenile lake sturgeon abundance among cycles, among and within the three river reaches. Density rasters based on catch records of lake sturgeon spatially were created using multilevel B-splines. Spatial differences in abundance among cycles was determined by obtaining the difference between the rasters. Three cycles of Broad-scale Monitoring sampled 334 lake sturgeon at a mean CPUE of 0.90 (0.12 SE) sturgeon • gang⁻¹ and a mean total length 673 mm (115 SD) from all three reaches. The relative abundance of juvenile lake sturgeon did not significantly vary among sample cycles, however, CPUE varied within reaches among cycles. There was a spatiotemporal change in density with a decrease in the lowest reach whereas juvenile lake sturgeon density increased the most in the upper reach. The approach used in this study has the potential to monitor spatiotemporal changes in juvenile lake sturgeon abundance populations and may prove effective when assessing rehabilitation or fisheries management efforts.     

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.     

Adult lake sturgeon (Acipenser fulvescens Rafinesque, 1817) occurrence in the Muskegon River system,a Lake Michigan drowned river mouth,USA

Recent advancements in telemetry have allowed managers and researchers to conduct comprehensive studies on the movement ecology of lake sturgeon (Acipenser fulvescens), a species of conservation concern in most of the Laurentian Great Lakes basin. In Michigan waters of Lake Michigan, drowned river mouth systems (a protected lake-like habitat that connects a river to lake) support 4 of 11 remaining lake sturgeon populations. One of those remnant populations is supported by the Muskegon River, a drowned river mouth system consisting of both Muskegon Lake and the Muskegon River. The objectives of this 6-year telemetry study were to determine whether adult lake sturgeon occupied the Muskegon River system outside of the spawning season (defined as March to July), to quantify their use of the system annually, and to identify and characterize patterns in occurrence. A total of 21 adult lake sturgeon implanted with acoustic transmitters were passively monitored throughout the year during 2012–2017. Eighty-two percent of tagged fish at large were detected ≥1 day in the Muskegon River system annually, and tagged lake sturgeon were frequently detected during both spawning and non-spawning time periods. Residency index (i.e., no. detection days/365 days) values indicated that adult lake sturgeon were not only detected throughout the year but that they occupied the Muskegon River system for an average of 130 days each year (residency index = 0.36 ± 0.05 SE) during our most spatially intensive acoustic monitoring in 2016–2017. Additionally, 24% of tagged lake sturgeon were primary residents (i.e., residency index >0.5) of the Muskegon River system in both years. Adult lake sturgeon followed 1 of 3 patterns of occurrence based on individual detection histories, and those patterns varied temporally and by the relative amount of use (i.e., high, medium, and low). Our findings build on previous research that found drowned river mouth systems in Lake Michigan can be important nursery habitats for juvenile lake sturgeon by showing that these habitats also can be used extensively by adult lake sturgeon throughout the year.     

Movement of sturgeons (Acipenser oxyrinchus oxyrinchus and A. fulvescens) in the St. Lawrence Estuary: A 35-year tagging study

Movement of Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) and lake sturgeon (A. fulvescens) in the St. Lawrence Estuary (Québec, Canada) are not fully understood. To assess the movement extent of both species, a mark–recapture study was conducted in collaboration with commercial fishermen operating in the St. Lawrence Estuary. Between 1981 and 2015, 3,367 Atlantic sturgeon (fork length 21.8–199.5 cm) and 3,180 lake sturgeon (fork length 17.8–190.8 cm) were tagged and released. Of these, 673 Atlantic sturgeon and 42 lake sturgeon were recaptured. The maximum distances traveled between capture and recapture locations were 1,307 km for Atlantic sturgeon (8 years after initial capture) and 252 km for lake sturgeon (less than 1 year after initial capture). Statistical analyses identified differences in the dispersal distance of both species as revealed by a first component characterized by individuals with short dispersal distances (98% and <35 km for Atlantic sturgeon; 58% and <1 km for lake sturgeon) and a second component characterized by individuals with longer dispersal distances (2% and >600 km for Atlantic sturgeon; 42% and >190 km for lake sturgeon). We suggest that the short dispersal distances detected in the vast majority of Atlantic sturgeon recaptures likely reflect strong site fidelity, highlighting the importance of the St. Lawrence Estuary as a preferred habitat for juveniles and subadults. Although recaptures were low for lake sturgeon because this species is only marginally targeted by commercial fishermen in the St. Lawrence Estuary, our results also showed that this species uses estuarine habitats and that half of the population seems to exhibit strong site fidelity (67% of individuals were recaptured within 2 km).     

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.     

Age,growth, mortality,and abundance of lake sturgeon in the Grasse River,New York,USA

An increased understanding of lake sturgeon (Acipenser fulvescens) population dynamics is a key requirement for successful management efforts. Little is known regarding the Grasse River population of lake sturgeon except that it is one of a few populations in New York State where spawning has been documented. Thus our purpose was to assess the current status of lake sturgeon in the Grasse River system, including age, growth, mortality, and abundance. Age was determined for 196 of 211 lake sturgeon by examination of sectioned pectoral fin rays. Ages ranged from 0 to 32 years and the annual mortality rate for fish between ages 7 and 14 was 16.8%. The weight (W, g) to total length (TL, mm) relationship was W = 1.281 × 10^?6TL^3.202. The von Bertalanffy growth equation was TL = 1913(1?e^{?0.0294(t+9.5691)}). While the range of observed ages was similar to that of nearby St. Lawrence River populations, mean weight at age for an individual at 1000 mm TL was lower than that observed for lake sturgeon within Lake St. Francis of the St. Lawrence River. Predicted growth based on von Bertalanffy parameters was similar to that observed for the nearby Lake St. Francis. An open population estimator using the POPAN sub‐module in the Program MARK produced an abundance estimate of 793 lake sturgeon (95% CI = 337–1249).     

Documentation of lake sturgeon (Acipenser fulvescens Rafinesque, 1817) recovery and spawning success from a restored population in the Mississippi River,Missouri, USA

Lake sturgeon Acipenser fulvescens are considered rare and were nearly extirpated in the Mississippi River in Missouri by 1931 as a result of overfishing and habitat fragmentation. Propagation efforts have been implemented by the Missouri Department of Conservation since 1984 as means to restore the lake sturgeon population. Although recent population increases have been observed, a formalized evaluation to determine if lake sturgeon are self‐sustaining in the Missouri portion of the Mississippi River has not been completed. Therefore, the objectives of this study were to: (i) determine the proportion of reproductive individuals, (ii) evaluate seasonal movement patterns of adults, and (iii) validate purported spawning locations within the Mississippi River in Missouri. Lake sturgeon catch data indicated that approximately 11 percent of the population are reproductively mature. Additionally, telemetry data confirms that the greatest movement by adult lake sturgeon occurs during spring, which suggests spawning behavior. Finally, it was possible to document lake sturgeon embryos and emergent fry larvae below Melvin Price Locks and Dam 26 in the Upper Mississippi River near St. Louis, Missouri. Water velocity, depth, and substrate size were measured at this location and embryos were collected and hatched in the laboratory. River gage data suggest that spawning behavior may have been elicited by a large influx of water during a drawdown period of water above the dam. This study represents the first documented spawning of A. fulvescens in the Mississippi River and highlights the success of recovery efforts in Missouri.     

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.     

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.     

Age structure of juvenile Amur sturgeon Acipenser schrenckii and kaluga Huso dauricus in the Fuyuan reach of the Amur River,Northeast China

Amur sturgeon Acipenser schrenckiii Brandt 1869 and kaluga Huso dauricus (Georgi 1775) are critically endangered species with populations showing significant decline from historical levels due to overexploitation, yet little is known about their population structure. Adults are not often captured in the Fuyuan reach of the Amur River, Heilongjiang Province, Northeast China, and the government prioritizes juvenile sturgeon management. This study was conducted to determine the age and length/weight relationships of juvenile Amur sturgeon and kaluga in the Fuyuan reach of the Amur River. We estimated age using pectoral fin spine sections obtained from 65 juvenile Amur sturgeon and 50 juvenile kaluga. We compared the age estimates from two readers, and found 100% between‐reader agreement in 67.7% of the Amur sturgeon and 64.0% of the kaluga. The majority of differences in estimated age were within 1 year. Length/weight relationships (LWR) were calculated, and the LWR of the Amur sturgeon and kaluga were W = 0.0025L3.106 (r² = 0.966）and W = 0.0022L3.175 (r² = 0.989), respectively. Our study provides the age structure and LWR in juveniles of two sturgeon species.     

Laboratory study of the susceptibility of hatchery reared Atlantic sturgeon (Acipenser oxyrinchus Mitchill, 1815) larvae to predation

The aim of the study was to determine the susceptibility to predation of Atlantic sturgeon larvae (Acipenser oxyrinchus) reared under traditional hatchery conditions. Experiments were conducted to determine whether predators would prey on Atlantic sturgeon if alternative prey was available and if the presence of substrate on the tank bottom impacted the number of Atlantic sturgeon consumed. European perch (Perca fluviatilis) was used as the predator, and the alternative prey were three‐spined stickleback (Gasterosteus aculeatus) or gudgeon (Gobio gobio). The predators and alternative prey were obtained from the wild. The mortality of sturgeon (n = 10) and alternative prey (n = 10) caused by four predators was recorded during 15 min trials. Trials with three‐spined stickleback and gudgeon as alternative prey were performed separately. Each experimental trial was repeated five times. The predators consumed significantly more Atlantic sturgeon than alternative prey in both the experimental setups when the bottom of the tank was covered with gravel and stone substrate and when there was no substrate. In trials with three‐spined stickleback the mortality of Atlantic sturgeon in both experimental setups was 94 ± 8.94%, while that of three‐spined stickleback in the setup with substrate was 20 ± 19.23%, and without substrate it was 22 ± 10.00%. European perch also consumed more Atlantic sturgeon than they did gudgeon, and the mean Atlantic sturgeon mortality in the experimental setup with substrate was 94 ± 5.48%, while for gudgeon it was 48 ± 8.37%. In the experimental setup without substrate the predators also consumed substantially more Atlantic sturgeon than gudgeon, with a mean Atlantic sturgeon mortality of 94 ± 8.94%, while for gudgeon it was 76 ± 5.48%. The study indicated that hatchery reared Atlantic sturgeon larvae are susceptible to predation by European perch. Predation could impact the survival of juvenile Atlantic sturgeon in the natural environment, and it could be one of the factors that is impeding the restoration of this species in the Baltic Sea basin.     

Delineation of discrete population segments of shortnose sturgeon <Emphasis Type="Italic">Acipenser brevirostrum</Emphasis> based on mitochondrial DNA control region sequence analysis

Shortnose sturgeon Acipenser brevirostrum is federally listed as ‘‘an endangered species threatened with extinction’’ in the U.S. but its listing status is currently under review. As part of this process, the U.S. National Marine Fisheries Service will determine if shortnose sturgeon are divided into Distinct Population Segments (DPS) across its distribution. In this regard, we sought to determine if shortnose sturgeon occur in genetically “discrete population segments,” and if so, the boundaries of each. We used mitochondrial DNA (mtDNA) control region sequence analysis to assess the genetic discreteness of 14 of 19 river populations that were recommended as DPS in the 1998 Final Recovery Plan for Shortnose Sturgeon. Nine of the 14 proposed DPS proved significantly discrete (P < 0.05 after Bonferoni correction) from both of their bracketing populations, the exceptions being those in the Penobscot River, Chesapeake Bay, Cooper River, and Ogeechee River (our sample from the Cape Fear River was insufficient to statistically analyze). Haplotype frequencies in the newly “rediscovered” Penobscot River collection were almost identical to those in the proximal Kennebec River system. Genetic data in combination with tagging results suggest that shortnose sturgeon in the Penobscot River are probably migrants from the Kennebec. Likewise, shortnose sturgeon found today within the Chesapeake Bay appear to be migrants from the Delaware River. While haplotype frequencies in the remnant Santee River population in Lake Marion differed significantly from those in nearby Winyah Bay, they did not differ significantly from those in the Cooper River. This suggests that the Cooper River harbors descendants of the Santee River population that are unable to access their historical spawning locales. The Ogeechee River collection was not genetically distinct from that in the nearby Savannah River, suggesting that it may host descendants of hatchery-reared individuals of Savannah River ancestry. Our genetic results indicate that most, but not all, rivers with shortnose sturgeon host genetically discrete populations, constituting important information in the consideration of DPS designations. However, shortnose sturgeon migrations through coastal waters to proximal rivers and release of hatchery-reared fish may confound results from genetic studies such as ours and lead to the possible misidentification of discrete population segments.     

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.     

Rainbow Darter (Etheostoma caeruleum,Storer, 1845) predation on early ontogenetic stages of Lake Sturgeon (Acipenser fulvescens,Rafinesque, 1817)

Previous molecular diet analysis identified lake sturgeon (Acipenser fulvescens, Rafinesque, 1817) DNA in the gastrointestinal tracts of stream-resident rainbow darters (Etheostoma caeruleum, Storer, 1845) during the egg incubation, free embryo, and larval drift stages. The objectives of this experimental study were to: (a) quantify levels of predation by rainbow darters on lake sturgeon at the egg and free-embryo stages; and (b) evaluate whether predation varied as a function of substrate size and rainbow darter body size. We conducted experimental trials in 23-L polycarbonate tanks 0.41 m (L) × 0.33 m (W) × 0.30 m (D) with a standardized benthic area of 0.14 m². The tanks were randomly assigned one of two different substrate size classes: large rock (51.35 mm ± 0.91 mm) or small rock (27.68 mm ± 0.57 mm). We stocked individual rainbow darter, which were deprived of feed for 48 hr, with lake sturgeon (133 individuals/m²) in each of 12 replicates per ontogenetic stage and substrate type. The number of surviving lake sturgeon was quantified following a 24-hr predation exposure period. We used a generalized linear model with a binomial distribution to assess the influence of ontogenetic stage, substrate size, and rainbow darter body size on proportional lake sturgeon survival. Predation on lake sturgeon occurred at both egg (6.25 ± 1.16 individuals, mean ± 2SE) and free embryo (3.08 ± 1.08 individuals, mean ± 2SE) stages. Egg proportional survival was generally lower than at the free embryo stage in both substrate sizes; however, free embryo proportional survival was greater in small substrate trials. Rainbow darter total length did not affect the probability of lake sturgeon survival at either developmental stage. Results demonstrate that rainbow darters prey on early ontogenetic stages of lake sturgeon, corroborating previous results based on genetic diet analysis. Results fill a major knowledge gap concerning the vulnerability of pre-drift sturgeon to predation by an abundant river resident species that was previously discounted as a predator for early ontogenetic stages of lake sturgeon due to its small body size.     

Ontogenetic patterns in prey use by pallid sturgeon in the Missouri River, South Dakota and Nebraska

The pallid sturgeon ( Scaphirhynchus albus ) is an endangered species native to the Missouri and Mississippi rivers. To date, recovery efforts have focused on stocking juvenile fish, but little is known about ontogenetic changes in diet composition. Although diet composition for pallid sturgeon is believed to change from macroinvertebrates to fish, it is unclear at what size and/or age these ontogenetic diet shifts occur. To evaluate diet composition, 29 hatchery-stocked pallid sturgeon (range 356–720 mm fork length [FL]; mean = 549; SE = 23) were collected from the Missouri River downstream of Fort Randall Dam, South Dakota and Nebraska during summer 2006. The majority of pallid sturgeon (72%) were captured within a large delta region formed by the Niobrara River in the headwaters of Lewis and Clark Lake. Predominant prey of pallid sturgeon based on percent occurrence was Ceratopogonidae (81%), Isonychiidae (67%), Chironomidae (52%), and fishes (24%). Percent composition by wet weight showed that diets were composed of fishes (68%), Ephemeroptera (23%), Decapoda (6%), and Diptera (3%). Graphical analysis of combined data showed that mayflies, particularly Isonychiidae, were an important component of pallid sturgeon diets. Nonetheless, the percent composition of fishes in the diet increased with pallid sturgeon body size; for fish > 600 mm FL (5–7 years of age) diets were composed primarily of fish prey (66%, mostly johnny darters Etheostoma nigrum ). These findings highlight the importance of ontogenetic changes in diet composition for pallid sturgeon. Moreover, the unique habitat formed in the delta region is characterized by higher fish and invertebrate densities that may enhance foraging opportunities and thus improve recovery efforts for stocked pallid sturgeon.     

Juvenile white sturgeon (Acipenser transmontanus) habitat and distribution in the Lower Fraser River, downstream of Hope, BC, Canada

The primary objective of this study was to identify and characterize juvenile white sturgeon (Acipenser transmontanus) habitat in the Lower Fraser River downstream of Hope, BC, Canada. A secondary objective was to estimate the juvenile white sturgeon population in the Lower Fraser River. A total of 1867 white sturgeon was captured with gill nets at 26 sites in the Lower Fraser River. The greatest numbers of sturgeon were caught in three sloughs; all but three sturgeon were captured in the June to August period. These three sloughs all had water deeper than 5 m and current that was multidirectional. Turbidity ranged greatly from 1.5 NTU (Nephelometric Units) to 67.0 NTU and the substrate of most sites was fine sediments, fine sand, silt and clay. Stomach contents were mysid shrimp (Mysidacea), midge larvae (Chironomidae) and peamouth chub (Mylocheilus caurinus). We identified three of 26 sites with appreciable numbers of juvenile sturgeon, identified water quality parameters of these sites, identified any incidental species that might be prey and also determined that between 1985 and 1993 the juvenile white sturgeon population had declined.