Fish Passage Research Articles

Safe passage for fish: The case for in-stream turbines

Current hydro power turbine technology is reviewed regarding safe fish passage. Much of the literature has focused on dam turbine configurations, while some have recommended to develop in-stream turbines as an alternative and potentially more environmentally- and ecologically-friendly option. Throughout the literature, several key design parameters and considerations appear consistently, with which future turbines are recommended to be designed if installed near fish habitats or migratory routes. Design parameters and recommendations are compiled and examined here, and conclusions about future developments of hydro turbines are provided. Additionally, the criteria and considerations are applied to an in-stream turbine design, which is shown to exhibit fish-friendly operation over a wide range of riverine velocities with mitigated fish access to the tip region. It is shown that in-stream turbines are a future-friendly (environmentally, ecologically and socially) technology that can provide sustainable energy generation to people throughout the world as an alternative to dams, including those in off-grid communities with little-to-no access to electricity.

Assessment of the effectiveness of fish passage through the vertical-slot fishway at the main dam on the longest Baltic River

Abstract In 1969, a dam was constructed on the Vistula River, the longest in the Baltic drainage basin, 276 km from its estuary, which resulted in the collapse of migratory fish populations throughout the drainage basin. The existing, ineffective fishway was rebuilt in 2014. Monitoring at the new fishway indicated that 24 fish species used it, and over a period of five years the number of individuals fluctuated from nearly 4,000 to over 23,000. Radio frequency identification (RFID) technology was used to investigate 12 fish species, mostly vimba and bream, passing through the fishway. Fish caught in a trap in the upper part of the fishway were tagged with PITs and then released below the dam. Returning fish were recorded by four loop antennas. Of the 877 fish that were tagged, 13% returned to the fishway mostly on the first and fourth days following release. Of the fish that returned, 30% turned back while passing through the fishway: 30% vimba and 52% bream. The transit time of vimba and bream did not depend on fish size or the time after which they returned. Overall, estimated fishway efficiency based on the migration of the tagged fish was high.

A framework for functional fish passage decision-making

A framework for functional fish passage decision-making

Fishway performance of adult Chinook salmon completing one of the world's longest inland salmon migrations to the upper Yukon River

Fishways can restore functional connectivity within rivers for migratory fish where barriers compromise connectivity. Providing fish passage is particularly important for semelparous, anadromous species that require access to upriver habitats to successfully reproduce. From 2017 to 2020, we used a combination of acoustic and radio telemetry to investigate the passage success of Chinook salmon (Oncorhynchus tshawytscha) in the upper Yukon River through the wooden Whitehorse Rapids Fishway and compared this to the migration of salmon in the nearby free-flowing Takhini River. The upper Yukon River population of Chinook salmon studied here is highly unique, completing a 2800 km inland migration to Whitehorse, YT, before attempting to pass the Whitehorse Hydro Plant (WHP) to reach spawning sites upstream. We found that upstream passage success was variable across four years of study (0%–66%), was low overall at 31%, and was considerably lower for female salmon. In contrast, salmon migrating up the free-flowing Takhini River had high migration success to spawning grounds and had many times faster migration rates. Attraction (86%), entrance (77%), and passage efficiency (36%) were less than that reported for Chinook salmon at other fishways. Within the fishway, a disproportionately high number of salmon returned downstream upon reaching a daytime-operated viewing chamber (fish trap) located ∼115 m up the fishway. Upon passing the fishway, salmon had high migration success to spawning grounds, though 12% of salmon returned downstream of the WHP, typically after multiple days and after traveling dozens of kilometres upstream of the facility. Findings from this study reveal opportunities to improve fishway performance and thus connectivity for one of the world's most impressive animal migrations.

Destruction and reconstruction: is freshwater offsetting achieving No Net Loss?

ABSTRACT ‘Biodiversity offsetting’ aims to address the residual negative environmental impacts of human development. It’s goal is to achieve No Net Loss (NNL), and preferably a Net Gain of biodiversity attributes between development and offset sites. Biodiversity offsetting suffers from a lack of ‘proof of concept’, as little work has been done to critically test the ecological efficacy of offsetting practices. Offsetting of stream ecological functions was investigated in Aotearoa New Zealand, via review of regulatory documentation for 76 offsets and field assessments of 14 offset sites. Field results indicated some functions, including fish passage connectivity, respond rapidly post-offset enhancement whilst others exhibit recovery lags (such as fish spawning habitat provision). However, most offsets were considered unlikely to achieve NNL due to failures in practice and process. Factors included inadequate consideration for restoration uncertainty, inappropriate monitoring conditions, insufficient data and inequivalent trade-offs. Key linkages between aquatic and terrestrial ecosystems, particularly regarding floodplain connectivity, were poorly considered in offset implementation. Thus, development may be occurring based on inadequate regulatory processes, and poorly evaluated ecological outcomes. These findings join a growing body of evidence which indicates that, in the context of a global biodiversity crisis, offsetting is not achieving its purpose of NNL.

Optimization schemes to significantly improve the upstream migration of fish: A case study in the lower Yangtze River basin

The construction of fish passage facilities is a promising approach to balance fish conservation and water resource development by improving river connectivity. To be successful, a fishway must efficiently attract fish to its entrance and have an interior flow field that meets the needs of migrating fish. In this paper, methods were explored to improve the efficiency of fishway passages by optimizing the flow field near the fishway entrance and the flow field inside the fishway in a case study at the Niuling Reservoir in the lower Yangtze River basin. The original engineering design was modified by designing a unique retaining wall with a notch to shape the appropriate flow pattern near the fishway entrance. The new wall optimized the flow field near the fish entrance to meet the hydraulic demands of target fishes under three operating scenarios, and the area of the suitable velocity zone increased by 48.79%, 12.71%, and 4.06% in scenarios 1 through 3, respectively. The original vertical slot fishway was modified with a sill to optimize the flow field inside the fishway. The optimized fishway design scheme reduced the main flow velocity inside the fishway to a large extent, and the maximum flow velocity in the upstream and downstream sections of the fishway decreased by 0.10 m/s and 0.09 m/s on average, respectively. The results of this study provide an important reference for the construction and optimization of fish passage facilities and have great significance for fish migration and biological diversity.

Safe passage of American Eels through a novel hydropower turbine

AbstractObjectiveStudy the effects of downstream passage through a novel turbine designed for fish safety, the Restoration Hydro Turbine (RHT), on American Eels Anguilla rostrata in a recirculating turbine test facility.MethodsA 55‐cm‐diameter RHT was operated under 10 m of hydraulic head and 667 revolutions/min. In total, 131 eels were passed through the turbine and 43 eels were used as experimental controls (length = 33.9–65.5 cm). High‐speed video of passage through the runner region was captured for 89% of turbine‐passed eels, and injury and behavioral effects were recorded immediately before and after passage, as well as after a 48‐h holding period. A subset of 37 eels was additionally examined with X‐ray imaging for internal injuries.ResultThe 48‐h survival rate for both treatment and control groups was 100%, with no major internal or external injuries present after the holding period.ConclusionThis is a substantial improvement over eel survival rates through conventional Kaplan and Francis turbines, which may range from 40% to 95%, and suggests that hydropower turbines designed for safe downstream fish passage could be implemented without major impacts to eels.

Uphill Flow Rock Ramps. How the Design Impacts Their Functionality

Enhancing river passability is considered a central part of the efforts to maintain fish population and achieving good ecological status, according to the EU Water Framework Directive (WFD). One commonly proposed approach to achieving this aim involves the creation of fishways. However, recent studies have shown that many of these fish passes are often lacking an optimal design with far reaching consequences for fish migration. Several promising new designs such as nature-like rock ramps, with uphill flow, have been recently developed. Such studies attempt to address these drawbacks by adapting several structure-related features (i.e., boulder size and shape and friction walls). In this study, we used a 2D computational fluid dynamic model to assess how the key hydraulic variables (water depth, velocity and turbulent kinetic energy) were impacted by different design elements of uphill flow fishways with different configurations. With regard to the standard boulder shapes and sizes, our results reveal that: (1) doubling the boulder sizes results in a decrease of turbulent kinetic energy in resting corridors of up to 33%; (2) the inclusion of small friction-walls in the ramp design increases uphill velocity in the intermediate corridors by up to 49%; (3) the trapezoidal shape of the boulder leads to the largest decreases in maximum velocity in the gaps (16%) and the largest increases in the uphill velocity in the resting corridors (180%). These results may allow us to optimise the uphill flow rock-ramp design to improve the passability of this type of fishways.

Machine learning based assessment of small-bodied fish tracking to evaluate spoiler baffle fish passage design

Fish passage research is important to mitigate the adverse effects of fragmented river habitats caused by waterway structures. The scale at which this research is undertaken varies from small-scale laboratory prototype studies to in-situ observations at various fish passage structures and bottlenecks. Using DeepLabCut, we introduce and evaluate a machine learning based workflow to track small-bodied fish in order to facilitate improved fish passage management. We specifically studied the behaviour and kinematics of Galaxias maculatus, a widespread diadromous Southern Hemisphere fish species. Upstream fish passage was studied in the presence of three different patches of spoiler baffles at an average water velocity of 0.4 m/s. In semi-supervised mode, the fish locations were extracted, and fish behaviour, such as swimming pathways and resting locations, was analysed based on extracted positions and recorded kinematic parameters. Individual fish behaviour and kinematic parameters were then used to assess the suitability of the three different spoiler baffle designs for enhancing fish passage. Using this technique, we were able to demonstrate where different spoiler baffle configurations resulted in significant differences in fish passage success and behaviour. For example, medium-spaced smaller baffles provided more accessible and uniform resting locations, which were required for efficient upstream passage. Results are discussed in relation to fish passage management at small instream structures.

The Kinematics and Dynamics of Schizopygopsis malacanthus Swimming during Ucrit Testing.

Simple SummaryThe swimming ability of fish plays a guiding role in the construction of fish passing facilities. However, it is not enough to know the swimming ability of fish. We need to know the behavior and energy consumption of fish in the process of movement, so as to determine the activity area and migration suitable for fish. Therefore, a video tracking program was used to record and analyze the motion of five test fish in a Brett-type flume during each velocity step. The results obtained the kinematic and dynamic parameters of fish. Secondly, we found that steady fish swimming is not entirely steady in flowing water, with swimming speed varying by 2.2% to 8.4% and increasing with flow velocity. Further, because energy expenditure increases with the cube of swimming speed, a slight excess in fish passage flow velocity could result in a disproportionately large decrease in the rate of passage success. Therefore, we recommend guarding against an excessive flow velocity in the main velocity zone and ensuring that resting pools along the passageway are adequate. Our characterization of the kinematics and dynamics of fish swimming provides important new information to consider when indices of swimming ability from controlled tank testing are applied to fish passage design.The swimming kinematics (how fish move) and dynamics (how forces effect movement) of Schizopygopsis malacanthus were investigated during the determination of by stepped velocity testing. A video tracking program was used to record and analyze the motion of five test fish in a Brett-type flume during each velocity step. The findings fell into three groups: (1) Even when flow was uniform, fish did not swim steadily, with speeds fluctuating by 2.2% to 8.4% during steady swimming. The proportion of unsteady swimming time increased with water velocity, and defining steady and unsteady swimming statistically, in terms of the definition of standard deviation of instantaneous displacements, may have higher accuracy. (2) In steady swimming, the forward velocity and acceleration of fish were correlated with body length (p < 0.05), but in unsteady swimming the correlations were not significant. The maximum swimming speed (1.504 m/s) and acceleration (16.54 m/s2) occurred during unsteady swimming, but these measurements may not be definitive because of tank space constraints on fish movement and the passive behavior of the test fish with respect to acceleration. (3) Burst-coast swimming in still water, investigated by previous scholars as an energy conserving behavior, is not the same as the gait transition from steady to unsteady swimming in flowing water. In this study, the axial force of fish swimming in the unsteady mode was significantly higher (×1.2~1.6) than in the steady mode, as was the energy consumed (×1.27~3.33). Thus, gait transition increases, rather than decreases, energy consumption. Our characterization of the kinematics and dynamics of fish swimming provides important new information to consider when indices of swimming ability from controlled tank testing are applied to fish passage design.

Numerical study of the effect of a V‐shaped weir on turbulence characteristics and velocity in V‐weir fishways

AbstractFishways are used to allow fish to migrate around water infrastructure, whether movements are in the upstream or downstream direction. Hydrodynamic conditions within various fishways, including turbulence levels, are important for successful fish passage. A numerical hydrodynamic study was conducted for V‐weir fishways, which assist species migrating upstream. The variables investigated included, fishway slopes of 4%, 7%, and 10%, relative spacing between weirs of 1.3, 2.6, and 4 (D/L, ratio of distance between weirs to pool width), and weir angles of 22.5°, 40°, and 60°. Turbulence characteristics, including turbulent kinetic energy (TKE), Reynolds shear stress (RSS), turbulent intensity (TI), turbulent dissipation (TD), as well as maximum water velocity, were examined by CFD (computational fluid dynamics) simulation using a re‐normalized group or RNG turbulence model. The CFD was calibrated with flow measurements made in a physical model of the V‐weir fishway in laboratory control experiments. Based on inferences from fish passage literature, the results showed that, in the range of parameters studied, a weir angle of 22.5°, a slope of 10% and relative spacing between weirs of 1.3 and 2.6 were assessed as offering the best potential performance for several species and sizes of fish, while the latter has lower cost as it requires fewer weirs. The V‐weir fishway may be adaptable for smaller fish.

Ranking of Downstream Fish Passage Designs for a Hydroelectric Project under Spherical Fuzzy Bipolar Soft Framework

Nowadays, several real-world decision-making problems concerning falling economies, power crises, depleting resources, etc., require efficient decision-making. To solve such problems, researchers proposed several hybrid models by combining the spherical fuzzy sets with other theories, such as spherical fuzzy soft sets, which is an efficient tool to deal with the uncertainties concerning positive, neutral, and negative memberships in the soft environment. However, all the existing hybridizations of spherical fuzzy sets fail to deal with information symmetrically in a bipolar soft environment. Accordingly, this paper presents a novel hybrid model called spherical fuzzy bipolar soft sets (SFBSSs) by fusing spherical fuzzy sets and bipolar soft sets, considering the opposite sets of parameters in symmetry. An example considering the selection of a chief management officer (CMO) for a multi-national company illustrates the proposed model in detail. In addition, some symmetric properties and algebraic operations of the initiated model, including subset, complement, relative null SFBSSs, relative absolute SFBSSs, extended union, extended intersection, restricted union, restricted intersection, AND, and OR operations, are discussed and illustrated via numerical examples. Further, some fundamental results, namely, commutativity, associativity, distribution, and De Morgan’s laws are presented for SFBSSs. Moreover, by considering the massive impact of hydropower in combating the energy crisis and possible dangers to fish migration, a multi-attribute decision-making problem concerning the ranking of downstream fish passage designs for a hydroelectric project is modeled and solved under the developed algorithm based on SFBSSs. Finally, a comparative analysis discusses the supremacy of the initiated model over its building blocks.

Seasonal migration and habitat use of adult barbel (Barbus barbus) and nase (Chondrostoma nasus) along a river stretch of the Austrian Danube River

Migration patterns and habitat use of adult barbel (Barbus barbus) and nase (Chondrostoma nasus) were monitored by radio telemetry over a period of 13 months along a 58-km-long section of the Austrian part of the Danube River. The study site is confined upstream and downstream by two hydropower plants, and contains a larger tributary, the Pielach River. Telemetry transmitters were implanted into fish caught in this tributary after spawning in June (25 individuals per species). The results show that both species use the entire available width and depth spectrum of the Danube along the full migratable river length. Nase had an average home range of 22.4 km, while that of barbel was 34.4 km. The habitat use of the two species differs significantly. While the nase was primarily encountered in the free-flowing section, barbel mainly used deep areas of the impoundment during the year. Nase showed a distinct site fidelity to certain areas in the free-flowing reach which were periodically revisited. During the spawning season, distinct homing behavior was observed in both species. All seven nase that could still be detected during the spawning season returned to the tributary (homing rate 100%). Six homing nase migrated up to the first migration barrier in the tributary but did not pass the existing fish passage facility. In contrast, only nine barbel returned to spawn in the tributary (homing rate 50%), while nine barbel were most likely using a spawning location in the head of impoundment section. Homing fish entered the tributary during darkness.

Cost and relative effectiveness of Lake Sturgeon passage systems in the US and Canada

Sturgeons throughout their circumpolar range comprise one of the most imperiled group of fishes due to their unique life history characteristics, overharvest and habitat changes and loss. Lake Sturgeon, a potamodromous Acipenseridae species, despite showing recent significant signs of recovery in the US and Canada, continue to be impacted in systems throughout its range in part because of dams on rivers in which critical sturgeon spawning and nursery habitat is not available in sufficient quantity and/or quality to maintain adequate long-term recruitment in the population due to fragmentation. There are a small number of fish passage structures and methodologies specifically designed and implemented for Lake Sturgeon in North America, but there has been no general evaluation of their installation and operation costs nor their relative effectiveness. Data were collected on seven different fish passage systems representing the range of options currently being utilized for Lake Sturgeon passage within the species’ North American range. Costs per meter of head were $206 060 for nature-like by-pass channel fishways, $305 579 for an upstream projecting pool-weir fishway, $1.1 million for a vertical slot fishway, and $1.6 million for a fish elevator. A capture and transfer operation cost an estimated $11 284 to transfer an average of 110 adult Lake Sturgeon annually. Projected costs per sturgeon passed after 40 years of operation were estimated at $12/fish for the upstream projecting pool-weir fishway, $85/fish for the nature-like by-pass channel fishways, $132 for capture and transfer, $1659 for the vertical slot fishway, and $1680 for the fish elevator. Before Lake Sturgeon passage is pursued, it is important to first determine whether passage would be necessary for restoring, maintaining, or sustaining a LS population and its genetic integrity within the system (above and below the dam); followed then, if passage is warranted, by determining what type of passage methodology might be the most effective, and the most effectively and practically funded, operated, and maintained at the dam in question.

Sea Lamprey Alarm Cue Comprises Water- and Chloroform- Soluble Components

A diversity of aquatic organisms manage predation risk by avoiding waters activated with conspecific alarm cues, a chemical mixture released from injuries. The sea lamprey (Petromyzon marinus) is a nocturnal migratory species that relies on its alarm cue to navigate around areas of predation risk when moving through river channels. Identification of the cue’s chemistry would allow managers to harness this innate behavioral response to guide migrating sea lamprey to traps (invasive population in the Laurentian Great Lakes) or to fish passage devices where dams block migrations in their native range. We pursued isolation of the sea lamprey alarm cue through behaviorally guided fractionation, fractionating the alarm cue into water-soluble and chloroform-soluble fractions, each of which elicited a substantial avoidance response. Recombining the two fractions restored full reactivity, suggesting the alarm cue mixture contains components that exhibit high solubility in water (e.g., nitrogenous compounds), chloroform (e.g., lipids), or perhaps materials that dissolve readily in either solvent. We further screened 13 individual compounds or pure isolates and 6 sub-fractions from the water-soluble fraction and found one of the pure isolates, isoleucine, evoked an avoidance response on its own, but not consistently when found in other mixtures. In a third experiment, we observed no behavioral response after recombining 32 compounds isolated and identified from the water-soluble fraction. These results confirm other suggestions that the process of elucidating alarm cue constituents is challenging. However, we suggest the pursuit is worthwhile given the strong evidence for the utility of alarm cues for use in the conservation and management of fishes and other aquatic organisms.

Optimizing fish-friendly flow pattern in vertical slot fishway based on fish swimming capability validation

Vertical slot fishways are increasingly common structures for the passage of a wide variety of migratory fish species, which account for 59.2% of all fish passage facilities in China. The slot structures and redirected flow in the pools are of great importance to guide the fish to swim through the fishway. To optimize the design of vertical slot fishways, physical experiments and numerical simulations were used to study the hydraulic characteristics for 10 different geometric configurations (referred to as designs D1-D10), combining with the study of fish behavior in the model. A comparative study on vertical slot fishway with L-shaped baffle (D1) and with a new vertical baffle (D2) was conducted. Flow patterns, velocity fields and other key hydraulic parameters including the maximum velocities and turbulence kinetic energy in the pools were analyzed. No obvious differences in all key hydraulic between designs D1 and D2 present in the study, which suggests an upgrade of the L-shaped baffle to the new vertical baffle with the advantage of being more compact and economical. When modifying design D2 in the baffle thickness and length (D3-D10), three typical flow patterns with the different area ratios of left recirculation region to right recirculation region (Scl/Scr) were observed in the pools, which were denoted as flow pattern 1 (FP1), flow pattern 2 (FP2) and flow pattern 3 (FP3). An analysis of the behavior of grass carp (Ctenopharyngodon Idella) in the scaled model clearly showed that the flow and structure in FP2, with the principal flow in the center position and suitable velocity gradient and recirculation area ratio, were more friendly to fish with better migrating efficiency, which present sufficient velocity gradient and area of cushion zone for migratory fish to pass with less energy consumption.

A national fish passage barrier inventory to support fish passage policy implementation and estimate river connectivity in New Zealand

Fragmentation of river systems is a key driver of freshwater biodiversity loss. Instream structure inventories are an essential component of delivering a strategic approach to restoring river connectivity. We used a range of data sources to collate the best available information on fish passage barriers and instream structures across the whole of New Zealand. To support implementation of new national policy objectives to maintain and improve river connectivity, the structure inventory has been integrated with a new fish passage assessment mobile application that allows river managers and asset owners to ground-truth, update and add records to the database. The data are provided open access and are being used for environmental reporting and to support strategic prioritisation of barrier mitigation actions. We used these data to quantify river connectivity at regional and national scales and demonstrated that at least half of New Zealand's river network is upstream of fish migration barriers. Limitations to our calculation of upstream river connectivity included the unknown influence of unrecorded structures, particularly on private land, missing information within recorded data, the large number of instream structures that have been identified but not yet assessed for their risk to fish passage, and spatial errors in the automated pairing of structures to a digital river network. A critical challenge for river managers is understanding the consequences of these uncertainties in the data for prioritisation and decision-making regarding barrier removal or mitigation.

The Status Quo and Future of Hydropower in Slovenia

Slovenia is a Central European country with a long history of hydropower. This paper gives a brief introduction to the current status of hydropower utilization and informs about some selected successful examples of hydropower plant operation. One such example is fish passage and flood risk reduction on the lower reaches of the Sava River at the Brežice hydroelectric power plant, taking into account a complex of morphological, hydrological, hydraulic, and anthropogenic factors. Future development is considered against the background of the National Energy and Climate Plan, which does not envisage any significant expansion in the capacity or function of the hydropower sector. The envisaged capacity increase is from the current 4430 GWh to around 4580 GWh by 2030. It is shown that the current energy storage capacity of Slovenia’s only pumped storage plant will be sufficient to offset the introduction of new non-dispatchable renewable energy sources by 2030. By around 2028, the country will have a need for electrical energy storage from renewable energy sources, reaching a modest total of only 6140 MWh per year. However, by sticking to the unambitious National Energy and Climate Plan, Slovenia will miss the opportunity to remain self-sufficient in electricity generation and significantly increase its share of renewable energy sources. The National Energy and Climate Plan aims to increase the share of renewable energy in total energy generation from 25% in 2020 to 27% by 2030.

Collaboration between fish passage scientists and engineers: Insights from an international questionnaire

Fish passage science and practice seeks to facilitate the movement of fish around obstacles in their habitat, primarily through the construction of fishways and culverts. Successful implementation of fish passage requires collaboration between groups with very different backgrounds and expertise, including knowledge-producers (scientists who study fish passage and related topics such as fish swimming ability) and knowledge users (engineers who apply that knowledge to design fish passage solutions). To investigate the nature of collaboration between these groups, we surveyed fish passage scientists and engineers from around the world. Respondents were asked about the importance of collaboration, mechanisms of collaboration, potential barriers to collaboration, and how collaboration can be improved. Both fish passage scientists and engineers reported high importance of collaboration and that they collaborated frequently with the other group. Respondents reported that consultation with other professionals (of their discipline and the other) was the most important means of obtaining and sharing information related to fish passage science and engineering. Both groups also tended to over-estimate their knowledge and use of the other's discipline. While respondents reported high engagement in collaboration, key themes emerged with respect to barriers to collaboration and means of improving collaboration. These included lacking a shared understanding of both disciplines, professional differences, insufficient institutional support, and inadequate sharing of knowledge (e.g., reporting and publishing). Opportunities for improving collaboration identified by respondents included 1) more interdisciplinary opportunities that facilitate interaction (particularly conferences and workshops); 2) promoting collaborative projects and interactions between fish passage scientists and engineers on project teams; and 3) ensuring that information is shared between groups (e.g., through accessible publications). Findings from this research have the potential to enhance collaboration between scientists and engineers, to the benefit of fish passage and fish populations.

Environmental DNA reveals anadromous river herring habitat use and recolonization after restoration of aquatic connectivity

AbstractRestoring and sustaining anadromous fish populations is a grand challenge in the conservation of freshwater ecosystems. Alewife Alosa pseudoharengus and Blueback Herring Alosa aestivalis are two closely related anadromous fish for which responses to fish passage restoration are poorly known. We used a targeted environmental DNA method to sample 17 major tributaries of the Chesapeake Bay, USA, to determine the spatial extent of habitat use during the spawning season and the associations of habitat use with watershed characteristics. To help understand responses to restoration of fish passage, the study included portions of eight watersheds where dam removals, fish ladders, or dam failures provided access to habitats that were previously inaccessible to anadromous fish. Overall, 27.5% of samples showed positive detections for river herring, including at least one sample in 16 of 17 tributaries sampled. The presence of both species combined was negatively associated with elevation (relative influence [RI] = 29.8%) and positively associated with a watershed area (RI = 14.5%) and percent cropland (13.6%) of the watershed. In species‐specific analyses, both Alewife (RI = 24.8%) and Blueback Herring (26.2%) presence was negatively associated with elevation, Alewife were more common in watersheds with high percentages of cropland (RI = 19.1%), whereas Blueback Herring were more common in streams with large watersheds (23.2%). The use of formerly blocked spawning habitats was generally greater upstream of sites where dams were removed (2%–100% of habitat used) compared with sites where fish ladders were installed (0%–66.8% of habitat used). The broad‐scale sampling enabled by eDNA methods made it possible to identify habitat use patterns that can be applied to prioritize future restoration efforts and predict species‐specific responses.