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.     

Spawning lake sturgeon (Acipenser fulvescens Rafinesque, 1817) and their habitat characteristics in Rainy River,Ontario and Minnesota

Lake Sturgeon (Acipenser fulvescens) spawning habitat in the Rainy River was studied in light of the ongoing review of the rules governing water levels upstream that affect discharge rates through the International Falls dam. The objectives of this study were to assess the current status of spawning Lake Sturgeon below the dam by: (i) evaluating weight–length relationships, condition, age and growth; (ii) identifying spawning locations and characterizing its physical attributes; and, (iii) evaluating the effects of water surface elevation on the availability of spawning habitat. Spawning was confirmed with use of egg mats, and targeted sampling of spawning individuals was completed using gillnets and electrofishing. Physical attributes of spawning locations, including temperature, depth and water velocity were collected at a range of water levels using an Acoustic Doppler Current Profiler and representative sediment sampling. Biological characteristics and timing of spawning events were consistent with those previously reported for Lake Sturgeon. However, a wider range of depths (0.3–10.5 m) and velocities (0.01–1.9 m³/s) than reported in most single site studies was found here, with the ranges encompassing what is reported in the literature as a whole. Lake Sturgeon were found to prefer coarse spawning substrates dominated by bedrock, boulder or cobble. The availability of preferred spawning habitat varied significantly with the changing water levels observed during the study and was eventually limited at higher water levels by river channel form. Because of the implications for dewatering of nest sites, keeping flows constant during spawning is critical to Rainy River Lake Sturgeon spawning success.     

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.     

Suitability Modeling of Lake Sturgeon Habitat in Five Northern Lake Michigan Tributaries: Implications for Population Rehabilitation

The availability of lotic spawning, staging, and nursery habitats is considered a major factor limiting the recovery of Lake sturgeon ( Acipenser fulvescens ) in Lake Michigan. Despite efforts to better understand the population biology and habitat use of remnant Lake sturgeon stocks, little information exists on the quantity, quality, and spatial distribution of habitats for riverine life stages. We applied georeferenced habitat information on substrate, water depth, and stream gradient to a Lake sturgeon habitat suitability index in a geographic information system to produce spatially explicit models of life stage–specific habitat characteristics in the Menominee River, Michigan–Wisconsin; the Peshtigo, Oconto, and lower Fox rivers, Wisconsin; and the Manistique River, Michigan. High-quality Lake sturgeon spawning habitat associated with coarse substrates (≥2.1 mm) and moderate- to high-stream gradients (≥0.6 m/km) comprised 1–6% of the available habitat in each system. Staging habitat characterized by water depths greater that 2 m located near potential spawning habitat comprised an additional 17–41%. However, access to a majority of these habitat types (range 30–100%) by Lake sturgeon from Lake Michigan is currently impeded by dams. High-quality juvenile Lake sturgeon habitat associated with finer substrates, lower stream gradients, and a broad range of water depths (i.e., 0.5–8 m) was relatively ubiquitous throughout each system and comprised 69–100% of the available habitat. Our study suggests that efforts to rehabilitate Lake sturgeon populations should consider providing fish passage and creating supplemental spawning habitat to increase reproductive and recruitment potential.     

Factors influencing the spatial ecology of Lake Sturgeon and Walleye within an impounded reach of the Winnipeg River

Impoundments of free-flowing rivers for hydropower generation often confine fish to relatively small reaches that can restrict movement, limit habitat availability, and alter life history strategies. Here, acoustic telemetry was used to describe the seasonal habitat use, locomotory activity, and depth use for Lake Sturgeon (Acipenser fulvescens) and Walleye (Sander vitreus) within an impounded reach on the Winnipeg River, Manitoba, Canada. Lake Sturgeon foraged and overwintered in the riverine-lacustrine transitionary habitat as well as immediately below the tailrace of the upstream run-of-river facility. Walleye demonstrated high site fidelity to the upstream habitat situated near the tailrace of a hydropower facility. Contrary to Lake Sturgeon, that used multiple habitat types, Walleye used the tailrace for spawning, foraging, and overwintering, given their high residency rates throughout all months at this location. Activity for both species increased with water temperature and when residing in habitat types located farther upstream, but were minimally active during the winter season throughout the impounded reach. On average, Lake Sturgeon utilized 73% of the available depth while Walleye utilized 62% of the available depth across habitat types and months. Overall, the habitat located within the tailrace and below run-of-river facilities should be a conservation priority for both Lake Sturgeon and Walleye populations. There was persistent presence of Lake Sturgeon and Walleye throughout the spawning, foraging, and overwintering periods in the SSGS tailrace and within the first rkm downstream of the tailrace. The habitat proximal to run-of-river facilities generally encompasses small areas of the total potential habitat within impoundments, yet is important to both species studied here. The results provide information on the seasonal habitat use and biological responses to environmental cues for Lake Sturgeon and Walleye that will enhance management and ecological understanding for populations that are confined to impounded reaches.     

Experimental passage of adult male Gulf sturgeon around Jim Woodruff Lock and Dam on the Apalachicola River,Florida

Dams can impede access to habitats that are required for the completion of life history phases of many migratory fish species, including anadromous sturgeons. Various forms of fish passage have been developed to permit migratory fishes to move above dams, but many dams still lack such structures. Translocation of ripe, mature fish above dams has been used as a first step to determine the efficacy of potential fish passage systems. The anadromous Gulf sturgeon, Acipenser oxyrinchus desotoi, inhabits the Gulf of Mexico and coastal rivers from Florida to Louisiana, and requires upriver spawning habitats to complete its life cycle. Historic overfishing and other anthropogenic threats, including dam construction, led to species declines and subsequent listing as threatened under the Endangered Species Act. In the Apalachicola River, FL, the 1957 completion of Jim Woodruff Lock and Dam (JWLD) created Lake Seminole and blocked Gulf Sturgeon from accessing 78% of historic riverine habitat—including potential spawning habitat—in the Apalachicola-Chattahoochee-Flint River Basin. The objective of this pilot study was to determine the efficacy of passage around JWLD through the trap-and-transport of 10 male Gulf sturgeon from the Apalachicola River to the reservoir above the dam. Through the use of acoustic telemetry, we were able to assess the ability of these fish to navigate Lake Seminole, access potentially suitable spawning habitat in the Flint and River, and complete their seasonal outmigration to the Gulf of Mexico. In this study, 2 translocated sturgeon moved 69 km upstream into potential spawning habitat in the Flint River, but 6 fish fell back through the lock/spill gates at JWLD within days of translocation. Four sturgeon appeared to remain trapped in the reservoir, and their long-term survival was deemed unlikely. Given our low sample size, and examination of male fish only, we cannot conclude that a trap-and-transport program would ultimately fail to restore spawning above JWLD, but our findings suggest that the risk of adult mortality is nontrivial. Alternatively, we suggest future studies examine the population level trade-offs associated with translocation of adults or consider alternatives such as a head-start program to rear and release juvenile sturgeon above JWLD to study viability of their passage in addition to effects on overall recruitment in the population.     

Retention of loop,monel, and passive integrated transponder tags by wild,free‐ranging Lake Sturgeon (Acipenser fulvescens Rafinesque, 1817)

Tagging or marking of fish is instrumental to fisheries biologists and managers seeking to distinguish groups of fish, track movement or migration patterns, and monitor population characteristics. However, tag loss can inhibit the ability of biologists and managers to reach these objectives. The ability of Lake Sturgeon to live for long periods of time and reach large sizes, in combination with their dynamic spawning activity, requires tags to be retained under a variety of environmental and physically demanding conditions. This study evaluated tag retention of loop, monel, and passive integrated transponder (PIT) tags on wild, free‐ranging Lake Sturgeon in Lake Huron and the St. Clair – Detroit River System. Lake Sturgeon in this study were double‐tagged with both a PIT tag under one of the three anterior‐most dorsal scutes and an external tag (loop or monel) at the base of the dorsal fin. Fish were at large for up to 16 years. Overall, tag loss for PIT tags was 1% followed by monel tags at 12% and loop tags at 36%. Tag loss for loop tags was higher when the initial length of Lake Sturgeon tagged was smaller. Tag loss for monel tags increased with time at large but was not related to length at initial tagging. Monel tags left behind abrasion marks when attached to smaller Lake Sturgeon. PIT tag retention was higher than reported in previous studies that tagged other sturgeon species in different body locations. Monel tag retention was higher than other external tag types evaluated in previous studies while loop tags had similar retention rates to external tag types. Most previous studies on tag retention of sturgeon species were of shorter duration and conducted in laboratory settings, therefore loop tags may have performed more favorably during studies under short term laboratory settings. Results of this study suggest that PIT tags inserted below dorsal scutes represent a viable option for long‐term tracking of Lake Sturgeon. Monel tags attached at the base of the dorsal fin also seem to be a viable option relative to other external tag types, but should be limited to larger sturgeon as they can leave behind abrasion marks.     

Incidence of lamprey marks on Lake Sturgeon (Acipenser fulvescens Rafinesque, 1817) in the St. Clair – Detroit River System: Implications for Sea Lamprey (Petromyzon marinus Linnaeus, 1758) effects

Laurentian Great Lakes Lake Sturgeon (Acipenser fulvescens) are hosts to lamprey species, including native Silver Lamprey (Ichthyomyzon unicuspis) and invasive Sea Lamprey (Petromyzon marinus). Silver Lamprey coevolved with Lake Sturgeon and cause negligible mortality, but Sea Lamprey can negatively affect Lake Sturgeon populations. Sea Lamprey abundance in Lake Erie has been above targets set by resource managers, with the St. Clair – Detroit River System (SCDRS) suspected as a source of Sea Lamprey production into Lake Erie. This study summarizes lamprey marking on Lake Sturgeon captured during agency assessment surveys in the SCDRS since 1996 and provides insight on the potential for Sea Lamprey to negatively affect Lake Sturgeon in the SCDRS. Lamprey marks (any lamprey species) were noted on 48.2% of Lake Sturgeon (2.5 marks/fish) and 3.3% of Lake Sturgeon assumed to be susceptible to mortality by Sea Lamprey (<760 mm TL; 0.06 marks/fish). Silver Lamprey were the only lamprey species found attached to Lake Sturgeon and there was no difference between oral disc diameters of Silver Lamprey and marks measured on Lake Sturgeon in Lake St. Clair and the lower St. Clair River (p = .45). Based on logistic regression, probability of at least one lamprey mark increased with Lake Sturgeon total length and was highest in Lake St. Clair. The probability of observing at least one lamprey mark on a 760 mm Lake Sturgeon was 8.1% or less for each sampling location in the SCDRS aside from Lake St. Clair (28.1%). Results suggest that parasitism of Lake Sturgeon by Sea Lamprey in the SCDRS is rare, particularly for Lake Sturgeon <760 mm TL. Low incidence of lamprey marks on Lake Sturgeon assumed to be susceptible to mortality from Sea Lamprey parasitism and zero occurrence of Sea Lamprey being observed attached to a Lake Sturgeon suggest Sea Lamprey at their current abundance likely have little effect on the Lake Sturgeon population in the SCDRS. Caution should be taken when using mark size to assign marks to lamprey species as there is substantial overlap among species oral disc diameters, potentially inflating the perceived impact of Sea Lamprey on Lake Sturgeon in areas with native lampreys.     

Movement and habitat use of juvenile Lake Sturgeon (Acipenser fulvescens,Rafinesque, 1817) in a large hydroelectric reservoir (Nelson River,Canada)

Movement and habitat utilization of juvenile Lake Sturgeon (Acipenser fulvescens) were examined in Stephens Lake, a large hydroelectric reservoir on the Nelson River, Manitoba, Canada, between 21 June 2011, and 15 October 2012. Stephens Lake is defined by a sharp hydraulic gradient at the upstream end (Gull Rapids) and a pronounced reservoir transition zone (RTZ), characterized by a change in substrate composition from coarse to fine. Twenty juvenile Lake Sturgeon <600 mm fork length were captured in the RTZ, implanted with acoustic transmitters, and tracked using stationary receivers. Our primary hypothesis considered that, if foraging behaviour was contingent on sand substrate, these fish would spend the majority of the open‐water season foraging in the relatively small area where hydraulic gradients dictate sand deposition. Data indicated that tracked individuals were highly bottom oriented, and utilized deeper thalweg habitats exclusively during the first open‐water season. On average, juveniles spent only 22% of their open‐water time in the RTZ (river kilometer [rkm] 4.5–7.0). Most fish spent more time upstream as opposed to downstream, but a few individuals did utilize backwatered thalweg areas, suggesting that silt‐overlay habitats may be suitable for foraging. A seasonal spatial shift in distribution was also observed. Juveniles vacated the RTZ as winter progressed, moving further downstream and occasionally laterally into backwatered shallows, potentially avoiding extreme ice conditions and a large hanging ice dam that develops downstream of Gull Rapids. After ice break‐up, most individuals with active tags returned to the upstream end of Stephens Lake. The results add to the growing body of evidence that suggests factors other than habitat suitability influence Lake Sturgeon movement and utilization patterns, raising questions about the mechanisms for core‐area affinity in this species.     

Assessment of lake sturgeon spawning stocks using fixed-location, split-beam sonar technology

Fixed‐location, split‐beam sonar technology was used successfully to identify adult lake sturgeon Acipenser fulvescens as they moved upstream and downstream for spawning in the Sturgeon River, Michigan, May–June 2004. A Hydroacoustic Technology Inc. Model 241 Split‐Beam Echo Sounder operating at 200 kHz and a single 4 × 10° elliptical‐beam transducer with a near field range of 1.7 m set perpendicular to the river flow was used. Data collected from migrating lake sturgeon included direction of movement, swimming speed, range from transducer, time and date of passage, and target strength. The spawning population of lake sturgeon was estimated to be at 350–400 fish, with almost equal numbers of fish seen moving upstream as downstream. Most fish were recorded moving within the mid‐section of the river, 1.5–1.65 m deep, and swimming speeds upstream were slower than those for downstream moving fish. These results show that spilt‐beam sonar can be applied to lake sturgeon assessments, without the stress of actually handling these large, pre‐spawning fish.     

Ontogenetic behavior of Kootenai River White Sturgeon, <Emphasis Type="Italic">Acipenser transmontanus</Emphasis>, with a note on body color: A laboratory study

Laboratory studies indicated the following ontogenetic behavior and body color of wild Kootenai River White Sturgeon, Acipenser transmontanus, (hereafter, Kootenai Sturgeon), a landlocked population in the Kootenai River, a major tributary of the Columbia River (United States) and Kootenay Lake (Canada). Hatchling free embryos (hereafter, embryos) are photonegative and hide under cover at a spawning site, and have a grey body. Late-embryos are photopositive and weakly prefer white substrate, use cover less with age, and develop a black tail. Day 13 larvae forage in the day on the open bottom, use cover less with age, prefer bright habitat, have a light-grey body and black tail, and initiate a mostly nocturnal dispersal for about 21 days, and then, continue a weaker dispersal. As they age, the entire body and tail of larvae is a dark-grey color when they develop into juveniles (about 66 days). The common body and tail color of larvae from the Kootenai, Columbia, and Sacramento rivers indicate a common adaptation to signal conspecifics or avoid predators. Juveniles are variable for foraging height, do not hide in bottom cover, and continue a weak nocturnal downstream movement. Movement of larvae and juveniles in the artificial stream suggests wild Kootenai Sturgeon have a long slow dispersal style (disperse for months). The long dispersal style of young Kootenai Sturgeon may adapt larvae to dispersing all summer in a 100–200 km long reach with a low abundance of food. The final destination of Kootenai Sturgeon during their first rearing season is unknown, but the long dispersal suggests fish could easily move to the lower river or to Kootenay Lake. Ontogenetic behavior of Kootenai Sturgeon is slightly different from Columbia River White Sturgeon, which has a weak embryo dispersal, but both populations have a similar major dispersal by larvae. However, both of these populations differ qualitatively from Sacramento River White Sturgeon, in which juveniles initiate the major dispersal. Thus, major geographic behavioral variation exists among populations and should be considered in restoration programs.     

The lake sturgeon,Acipenser fulvescens,in the Menominee River,Wisconsin-Michigan

Synopsis The Menominee River is the boundary between the Upper Peninsula of Michigan and northeastern Wisconsin. It contains one of the few fishable lake sturgeon populations remaining in either state. Two sections of the river harbor naturally reproducing sturgeon stocks. Dams at either end of these sections curtail upstream movement of fish; however, some sturgeon move downstream over the dams. Surveys conducted in 1969 and 1970 and again in 1978 and 1979 intensively studied the sturgeons in a 42 km stretch of the river. Mark and recapture estimates of the number of sturgeon longer than 25 cm in this section ranged from 1884 to 2865. The number of fish longer than 107 cm ranged from 185 to 243. An estimate of 2834 sturgeon between 25 and 165 cm was made on a different 34 km stretch in 1979. Censuses on the hook and line fishery in 1969 and 1970 estimated sturgeon harvests of 59 and 48 fish, respectively. In 1974, the size limit was raised from 107 to 127 cm. Harvests of these larger fish were estimated at 19 and 41 in 1981 and 1982, respectively. Some other characteristics of the sturgeons and the fishery on the Menominee are also included.     

Genetic diversity and population structure of a cyprinid fish (Ancherythroculter nigrocauda) in a highly fragmented river

In this study, samples of Ancherythroculter nigrocauda, an endemic fish in the upper Yangtze River, were collected above and below dams in the Longxi River, a tributary of the upper Yangtze River, China, to investigate the genetic impacts of dams. The mitochondrial cytochrome b gene (cyt b) and 13 microsatellite (SSR) loci were used to analyze whether dams have resulted in loss of genetic diversity of the two fragmented populations or caused genetic differentiation between them. The results showed that the haplotype diversity (0.488; 0.486), nucleotide diversity (0.084%; 0.082%) and average expected heterozygosity (0.652; 0.676) of the two populations were all at a low level, and recent bottlenecks were detected. However, there was no genetic differentiation detected by the low genetic differentiation index (F_st, cyt b: ?0.1677, p = 0.99707; SSR: 0.00259, p = 0.81427). Besides, 11 pairs of half‐sibling relationship were found between the two populations indicating that there were individual movements and gene flow between them. This could be the larvae moving from upstream to downstream when water spilled over dams in flooding season. Therefore, our analysis showed that the dams have caused a loss of genetic diversity of the populations of A. nigrocauda in the Longxi River, blocked the active upstream movement but allowing passive downstream drift of larvae.     

Long‐term studies on restoration of Connecticut River anadromous sea lamprey,Petromyzon marinus Linnaeus 1758: Trend in annual adult runs,abundance cycle,and nesting

This study is the first to evaluate the results of 60 years of restoring anadromous sea lamprey, Petromyzon marinus, to historical spawning and rearing habitats using fish passage at barrier dams in the Connecticut River, USA. We obtained counts of pre‐spawning adult P. marinus annually passed upstream at Holyoke Dam (river km 140), Connecticut River, MA, during 37 years (1978–2014), and we counted P. marinus nests during 25 years (1986–2010) in the Fort River, a tributary upstream of Holyoke Dam. These two data sets were used to study relationships between adult passage and subsequent nesting and to study nesting timing and ecology. During the 37 years, annual adult P. marinus abundance at Holyoke Dam ranged from 15,000 to 95,000, but regression analysis found no trend (p = .50) for increasing annual adult abundance with years. However, during the 37 years, adults gained access via fish passage at dams to an estimated double the amount of spawning and rearing habitat upstream compared to the 1970s. The lack of a trend for increased adult abundance is consistent with a hypothesis of non‐natal river homing by adults. However, the lack of a trend in adult abundance, when many more larvae are likely present in the watershed compared to the 1970s, is inconsistent with the hypothesis that greater numbers of larvae (and greater concentration of larval pheromone) results in greater number of adults attracted to a river. Instead of an abundance trend of adults with years, we found a rare life history phenomenon occurs in anadromous adult P. marinus ─ an abundance cycle with peaks at 6 year intervals (autocorrelation analysis, p = .04). Comparison of passage timing with nesting timing found passage did not affect nesting initiation or duration (p = .61). Annual date of nesting initiation strongly affected the duration of nesting with earlier nesting resulting in longer nesting (p = .001). Time series comparing Fort River temperature and discharge with nesting found these factors did not clearly predict annual initiation of nesting. This suggests a role for day length (photoperiod) as the trigger for nesting initiation (most nesting began annually during 1–14 June, all year mean, 7 June), regardless of river conditions. However, river discharge may affect nesting ecology because most nesting occurred during decreasing discharge when variability in daily discharge was small (stable discharge). The present research on passage and nesting contributes to a new Connecticut River restoration program for P. marinus, which is a keystone fish species in the watershed.     

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.     

Projected lake sturgeon recovery in the Milwaukee River,Wisconsin, USA

Using a forward projecting population model, the timing and scale of Lake Sturgeon Acipenser fulvescens (LS) recovery and natural reproduction was estimated for the Milwaukee River in Wisconsin, USA for the years 2004–2048. LS, a widely distributed potamodromous Acipenseridae species in North America, have, like other sturgeon species, suffered population declines due to overharvest and other factors since the 1800s. LS recovery efforts were initiated in the early 1980s following the successful development of LS propagation techniques and stocking has become a widely utilized tool in LS recovery programs. Sturgeon recovery programs are long-term endeavors, with annual stocking usually planned for 25 plus years, along with population assessments and habitat improvements conducted over decades before project success can be verified. LS recovery activities on the Milwaukee River, a Lake Michigan tributary in southeast Wisconsin, began with barrier mitigation in 1997 and stocking in 2003. The modest size of the Milwaukee River, along with habitat improvements, and 17 years of stocking, provided an opportunity to model and predict LS population recovery trends. A forward projecting population model with an imbedded stochastic Ricker stock-recruitment relationship, adjusted for the estimated LS productive capacity of the Milwaukee River, estimated the timing and scale of LS recovery during 2004–2048 predicting a 18.3% annual average population rate of increase during the stocking period, and a 5.7% post-stocking rate. Accelerated LS growth and maturity due to goby consumption built into the model resulted in projections of gravid adult males present in 2012 and spawning adult females and natural recruitment in 2019. During the 2020 spawning period a group of adult-sized LS were observed in the Milwaukee River below an intermittent barrier on the river. The model predicted adult LS densities to grow from 22 in 2012 to 8088 in 2048, and annual natural recruitment to grow steadily from 76 yearlings in 2019 to an average of 4000 yearlings/year during 2039-2049. The estimated number of yearling recruits per spawning (gravid) female dropped steadily from 21 in 2019 to 8 recruits per gravid female in 2048 as a function of the Ricker stock-recruitment relationship built into the model. The model results indicate that significant LS recovery can possibly begin in a shorter time frame on Great Lakes tributaries than originally expected through systematic resolution of habitat issues and LS stocking. Results can also be used to help anticipate recovery trends, fine tune stocking and habitat strategies, and plan LS recovery assessments.     

Is Niagara Falls a barrier to gene flow in riverine fishes? A test using genome‐wide SNP data from seven native species

Since the early Holocene, fish population genetics in the Laurentian Great Lakes have been shaped by the dual influences of habitat structure and post‐glacial dispersal. Riverscape genetics theory predicts that longitudinal habitat corridors and unidirectional downstream water‐flow drive the downstream accumulation of genetic diversity, whereas post‐glacial dispersal theory predicts that fish genetic diversity should decrease with increasing distance from glacial refugia. This study examines populations of seven native fish species codistributed above and below the 58 m high Niagara Falls – a hypothesized barrier to gene flow in aquatic species. A better understanding of Niagara Falls’ role as a barrier to gene flow and dispersal is needed to identify drivers of Great Lakes genetic diversity and guide strategies to limit exotic species invasions. We used genome‐wide SNPs and coalescent models to test whether populations are: (a) genetically distinct, consistent with the Niagara Falls barrier hypothesis; (b) more genetically diverse upstream, consistent with post‐glacial expansion theory, or downstream, consistent with the riverscape habitat theory; and (c) have migrated either upstream or downstream past Niagara Falls. We found that genetic diversity is consistently greater below Niagara Falls and the falls are an effective barrier to migration, but two species have probably dispersed upstream past the falls after glacial retreat yet before opening of the Welland Canal. Models restricting migration to after opening of the Welland Canal were generally rejected. These results help explain how river habitat features affect aquatic species’ genetic diversity and highlight the need to better understand post‐glacial dispersal pathways.     

Evidence of autumn spawning in Suwannee River Gulf sturgeon,Acipenser oxyrinchus desotoi (Vladykov, 1955)

Evidence of autumn spawning of Gulf sturgeon Acipenser oxyrinchus desotoi in the Suwannee River, Florida, was compiled from multiple investigations between 1986 and 2008. Gulf sturgeon are known from egg collections to spawn in the springtime months following immigration into rivers. Evidence of autumn spawning includes multiple captures of sturgeon in September through early November that were ripe (late‐development ova; motile sperm) or exhibited just‐spawned characteristics, telemetry of fish that made >175 river kilometer upstream excursions to the spawning grounds in September–October, and the capture of a 9.3 cm TL age‐0 Gulf sturgeon on 29 November 2000 (which would have been spawned in late September 2000). Analysis of age‐at‐length data indicates that ca. 20% of the Suwannee River Gulf sturgeon population may be attributable to autumn spawning. However, with the very low sampling effort expended, eggs or early life stages have not yet been captured in the autumn, which would be the conclusive proof of autumn spawning. More sampling, and sampling at previously unknown sites frequented by acoustic telemetry fish, would be required to find eggs.     

Spatial and Temporal Relations between Fluvial and Allacustrine Yellowstone Cutthroat Trout, Oncorhynchus clarki bouvieri, Spawning in the Yellowstone River, Outlet Stream of Yellowstone Lake

Yellowstone cutthroat trout (YCT), Oncorhynchus clarki bouvieri, that spawn in the outlet of Yellowstone Lake show two potamodromous migration patterns, fluvial and allacustrine. The main purpose of this study was to determine whether those fluvial and allacustrine YCT represent reproductively isolated stocks. Redd surveys indicated spawning occurred during about 5 consecutive weeks between late May and mid-July 1993–1995. Lake fish (N=6), defined as radiotagged YCT that entered Yellowstone Lake after the spawning period (i.e. allacustrine pattern), were found in the river between the lake outlet (river kilometer [Rkm] 0) and Rkm 20.0 during spawning. Probable lake fish (N=28; tagged YCT that were last detected near the lake outlet) were found between Rkm 0 and Rkm 22.5 during spawning. River fish (N=4; tagged YCT that remained in the river when annual tracking concluded in fall, i.e. fluvial pattern) were found between Rkm 1.1 and Rkm 18.0 during spawning. Fidelity to spawning areas used between consecutive years was suggested by one of five lake fish and the single river fish for which data were available. Spatial overlap in spawning and a lack of temporal separation between the life-history types during spawning suggested that fluvial and allacustrine YCT were not reproductively isolated. Radiotagging, as well as visual observations made annually from boats during April and May, indicated fluvial YCT overwintered downstream from Rkm 14 and were few, probably on the order of 10% of all YCT that spawned in the Yellowstone River.     

The effect of cascaded huge dams on the downstream movement of <Emphasis Type="Italic">Coreius guichenoti</Emphasis> (Sauvage &amp; Dabry de Thiersant, 1874) in the upper Yangtze River

Huge dams (installed capacity > 100 MKW) are generally built on large rivers that display high biodiversity and include major migration routes for many diadromous and potamodromous fish. As a result, these dams lead to severe ecological impacts and receive more attention than smaller dams. Most previous work on the impact of huge dams on fish downstream movement has focused on a single dam and reservoir, so little is known about the effect of cascaded huge dams and reservoirs on downstream movement. During the period 2012–2014, two huge dams (the Xiangjiaba and the Xiluodu) were constructed on the upper Yangtze River and the reservoir impoundments began, respectively in October 2012 and May 2013. These cascaded hydroelectric projects could have a large adverse effect on the downstream movement of an important potamodromous fish species, Coreius guichenoti. To study the effect of cascaded impact of sequential huge dams and reservoirs on passive and active downstream movement of C. guichenoti, eggs and larvae were collected in the Yibin section during 2012–2014 and fish were collected monthly at a site in the Hejiang section from June 2012 to July 2014. Our results showed that, compared to one huge reservoir and dam, cascaded dams exert a more serious effect and obstruct downstream movement of eggs, larvae and young fish (particularly the yearlings and two-year-olds) of C. guichenoti. Individual C. guichenoti were able to pass with relative ease through one reservoir and dam, but passing through both reservoirs and dams was very difficult. To allow access to the spawning grounds upstream, a fish passage should be built on the Xiluodu dam. However, due to the hydropower development in the whole upper basin, captive breeding and maintaining at least 60 km of riverine habitat upstream of a spawning ground could be a more cost-effective approach to maintaining C. guichenoti populations in the Upper Yangtze River.