Fish Passage Research Articles

Fish passage across Southeast Asia: key informant insights into motivations and triggers for water resource planning and policymaking

Southeast Asia’s inland fisheries are under threat from extensive irrigation and hydropower development. Of particular concern is the Mekong River, which is currently home to the world’s most productive inland fishery. To minimise these negative impacts, fish-friendly infrastructure can be incorporated into these barriers to facilitate fish migration or “passage.” However, historically fishways have often been overlooked given cost considerations and a lack of awareness of benefits across irrigation managers and funding bodies. This study aimed to understand fishway implementation decision-making across the Mekong. Findings from 19 interviews showed that funding was central to decision-making and acted as an ability and a trigger, to both positively and negatively influence stakeholders’ motivations for fishway implementation. Masterclasses were viewed as key vehicles for building technical capacity and consent within the region through the creation of networks and social capital between irrigation engineers and fisheries staff, as well as the development of “champions.”

PIT tagging systems are suitable for assessing cumulative impacts of Mekong River hydropower plants on (upstream) fish migrations in Lao PDR

The Mekong River has the largest inland fishery in the world but is threatened by planned mainstem hydropower developments that could impact fish migration patterns. Here we provide direct evidence of long-distance migrations along the Mekong mainstem by Hypsibarbus malcomi, one of the 10 most important species in the Mekong River fishery. The migration included a 354 km passage through one current and three proposed hydropower developments. We tagged and released 233 wild fish, including 77 H. malcomi with Passive Integrated Transponder (PIT tags) near the capital Vientiane in April of 2022. Five PIT tagged H. malcolmi were detected at the top of the Xayaburi hydropower plant fish ladder within 15 months of being released at Vientiane. Our findings empirically demonstrate that this species undertakes long distance migrations in the main stem of the Mekong and therefore that suitable fish passage facilities will need to be incorporated in mainstem hydropower developments to meet their migratory needs. The findings also demonstrate the value of including a fishery independent and efficient monitoring technology, such as PIT tagging, to assess the migration patterns of fish in the face of upcoming developments in the Lower Mekong Basin.

Adfluvial migration and passage of Steelhead before and after dam removal at a major Great Lakes tributary

Despite the importance of Great Lakes fisheries and the increasing popularity of dam removal as a method to restore river connectivity and increase fish passage, the adfluvial migration of Steelhead (Oncorhynchus mykiss) has been drastically understudied and only relatively few published studies have examined the impacts of dam removal on fish movement and timing. To help fill these knowledge gaps, spawning migrations of Great Lakes adfluvial Steelhead tagged in spring and fall were monitored for two years before and three years after removal of a dam that partially blocked upstream movement for 100 years. Removal of the dam not only reduced downstream delay and increased passage at the site of the dam removal itself, but increased travel speeds and increased passage at remaining upstream dams for both spring and fall run fish, underscoring the cumulative impact of successive dam passage on fish migration. Fall fish were most impacted by the dam removal and were able to pass not only the dam footprint, but, for the first time, were also able to pass both remaining upstream dams, allowing them to overwinter at locations closer to known spawning areas. For these fish, delay at the footprint was reduced from the order of 100+ days to < 1 hour and the number of days passage occurred compared to the number of days fish were present and blocked increased from 2% to 82%. The benefits of dam removal should ultimately equate to increases in fish production, as more critical habitat becomes reconnected and more fish are able to pass dams and arrive at spawning beds. The observation that fish are likely able to remain in better condition and retain more energy for continued migration and reproduction warrants further study.

Direct male development in chromosomally ZZ zebrafish.

The genetics of sex determination varies across taxa, sometimes even within a species. Major domesticated strains of zebrafish (Danio rerio), including AB and TU, lack a strong genetic sex determining locus, but strains more recently derived from nature, like Nadia (NA), possess a ZZ male/ZW female chromosomal sex-determination system. AB fish pass through a juvenile ovary stage, forming oocytes that survive in fish that become females but die in fish that become males. To understand mechanisms of gonad development in NA zebrafish, we studied histology and single cell transcriptomics in developing ZZ and ZW fish. ZW fish developed oocytes by 22 days post-fertilization (dpf) but ZZ fish directly formed testes, avoiding a juvenile ovary phase. Gonads of some ZW and WW fish, however, developed oocytes that died as the gonad became a testis, mimicking AB fish, suggesting that the gynogenetically derived AB strain is chromosomally WW. Single-cell RNA-seq of 19dpf gonads showed similar cell types in ZZ and ZW fish, including germ cells, precursors of gonadal support cells, steroidogenic cells, interstitial/stromal cells, and immune cells, consistent with a bipotential juvenile gonad. In contrast, scRNA-seq of 30dpf gonads revealed that cells in ZZ gonads had transcriptomes characteristic of testicular Sertoli, Leydig, and germ cells while ZW gonads had granulosa cells, theca cells, and developing oocytes. Hematopoietic and vascular cells were similar in both sex genotypes. These results show that juvenile NA zebrafish initially develop a bipotential gonad; that a factor on the NA W chromosome, or fewer than two Z chromosomes, is essential to initiate oocyte development; and without the W factor, or with two Z doses, NA gonads develop directly into testes without passing through the juvenile ovary stage. Sex determination in AB and TU strains mimics NA ZW and WW zebrafish, suggesting loss of the Z chromosome during domestication. Genetic analysis of the NA strain will facilitate our understanding of the evolution of sex determination mechanisms.

Sensor Fish Deployments at the Xayaburi Hydropower Plant: Measurements and Simulations

Fish protection is a priority in regions with ongoing and planned development of hydropower production, like the Mekong River system. The evaluation of the effects of turbine passage on the survival of migratory fish is a primary task for informing hydropower plant operators and authorities about the environmental performance of plant operations. The present work characterizes low pressures and collision rates through the Kaplan-type runners of the Xayaburi hydropower station. Both an experimental method based on the deployment of Sensor Fish and a numerical strategy based on flow and passage simulations were implemented on the analysis of two release elevations at one operating point. Nadir pressures and pressure drops through the runner were very sensitive to release elevation, but collision rates on the runner were not. The latter showed a frequency of occurrence of 8.2–9.3%. Measured magnitudes validated the corresponding simulation outcomes in regard to the averaged magnitudes as well as to the variability. Central to this study is that simulations were conducted based on current industry practices for designing turbines. Therefore, the reported agreement helps turbine engineers gain certainty about the prediction power of flow and trajectory simulations for fish passage assessments. This can accelerate the development of environmentally enhanced technology with minimum impact on natural resources.

Deciphering the effect of variation in slope on flow characteristics in a vertical slot fishway

The effects of a vertical slot fishway slope (with slope values from 1.5% to 6%) on a flow field are numerically investigated, using a re-normalization group k – ε model. The distribution of the velocity, turbulence kinetic energy (TKE), average energy dissipation rate per unit volume (E), and vorticity for different slopes are systemically explored. The results indicate that, with an increase in slope, the appearance of downward flow in conjunction with an increase in vertical velocity results in three-dimensional flow characteristics. The recirculation region in Hs, at a 6.0% slope, was 20.9% less than that at a 1.5% slope. Meanwhile, the flow velocity in the vertical slot region grew with increasing slope, which would limit the passage of fish with burst speed lower than the velocity in the vertical slot region. The TKE and E may locally exceed the threshold at larger slopes. Furthermore, vorticity distribution shows little variability with increasing slope, but may interfere with the equilibrium of the fish in the vertical slot region. In addition, the change in water level has little effect on the flow field, which is changed by the increase in slope. These findings can aid vertical slot fishway designs especially in terms of the efficiency of fish passage.

Feasibility study for test rig assessments of fish passage conditions in a Kaplan turbine

The assessment of fish passage conditions in hydroelectric turbines consists of identifying and quantifying physical magnitudes leading to increased risks of injury of fish passing through turbines in operation. Such assessments are usually carried out either with the use of computer-based methods during design or with field testing of live fish and sensors passing through prototypes. A method in between consists of test rig experimentation, which is critical for testing fish-focused design concepts and offers the opportunity for implementing the most effective design measures for improved fish survivability. However, fish-related assessments in test rigs are not sufficiently documented for industrial applications. This work presents the main findings of an experimental campaign to quantify fish-related hydraulic magnitudes in a physical model of a Kaplan turbine in a commercial test rig. Two operating conditions were tested by releasing miniaturized autonomous sensor devices (termed Sensor Fish Mini) at the turbine intake flow, passing them through the runner in motion and recovering them at the draft tube exit. During passage, time series of acceleration, absolute pressure and rotational velocity were recorded. The recordings were then interpreted to determine the magnitude and likely location of hydraulic stressors hazardous to fish. The statistical tests on the reported measurements indicated that low pressure, collisions on the runner and rotations in the draft tube were not different between the two tested operating points. On the other hand, pressure drop and collision rates on the distributor differed considerably as a function of net head. The outcomes of this investigation showed that test rig evaluations of fish-related properties with Sensor Fish Mini can contribute to an evidence-based development of turbine geometries designed for providing safer passage conditions. Future work will investigate the scaling of test rig measurements to hydraulically equivalent magnitudes in the prototype and their biological consequences.

Integrating fish swimming abilities into rapid road crossing barrier assessment: Case studies in the southeastern United States.

Many aquatic networks are fragmented by road crossing structures; remediating these barriers to allow fish passage is critical to restoring connectivity. Maximizing connectivity requires effective barrier identification and prioritization, but many barrier prioritization efforts do not consider swimming capabilities of target species. Given the many potential barriers within watersheds, inventory efforts integrating species-specific swimming speeds into rapid assessment protocols may allow for more accurate barrier removal prioritization. In this study, we demonstrate an approach for integrating fish swimming speeds into rapid barrier assessment and illustrate its utility via two case studies. We measured critical swimming speeds (Ucrit) of two stream-resident fish species with very different swimming modes: Yoknapatawpha Darter (Etheostoma faulkneri), an at-risk species whose current distribution is restricted to highly degraded habitat, and Bluehead Chub (Nocomis leptocephalus), an important host species for the federally endangered Carolina Heelsplitter mussel (Lasmigona decorata). We assessed potential barriers for Yoknapatawpha Darters in the Mississippi-Yocona River watershed, and Bluehead Chubs in the Stevens Creek watershed, South Carolina, USA. We integrated Ucrit into the Southeast Aquatic Resources Partnership (SARP) barrier assessment protocol by estimating the proportion of individuals per species swimming at least as fast as the current through the assessed structures. Integrating Ucrit estimates into the SARP protocol considerably increased barrier severity estimates and rankings only for Yoknapatawpha Darters in the Yocona River watershed. These results indicate the importance of including species-specific swimming abilities in rapid barrier assessments and the importance of species-watershed contexts in estimating where swimming speed information might be most important. Our method has broad application for those working to identify barriers more realistically to improve species-specific fish passage. This work represents a next step in improving rapid barrier assessments and could be improved by investigating how results change with different measurements of swimming abilities and structure characteristics.

Swimming behaviour of Atlantic salmon kelts migrating past a hydropower plant dam: Effects of hydraulics and dam operations

Hydropower plants commonly impede the downstream migration of Atlantic salmon (Salmo salar) kelts. Thus, understanding the effects of hydraulic conditions on kelt behaviour and passage performance at dams is crucial for developing effective mitigation measures. In this study, we investigated the influence of hydraulic conditions on kelt passage performance and swimming behaviour at a Norwegian hydropower plant. We combined biological data from 48 kelts collected via acoustic telemetry with hydraulic data modelled using computational fluid dynamics. We assessed kelt passage performance using metrics such as time-to-pass, total number of detections, and total number of detections per day. Additionally, we analysed swimming depths and speeds in relation to the hydraulic conditions created by different dam operating conditions. We found that the dam operation schedule impacted the kelts' ability to find a route past the dam. Though kelts could have passed the dam throughout the study period via a submerged pipe at the dam (which had seemingly sufficient discharge for the kelts to find), 98 % of the kelts instead waited for a spill gate to open partway through the study period. The swimming depth analysis indicated diel variation, with kelts swimming nearer to the water surface during the night. We found that swimming speed increased with increasing kelt body length, particularly under high turbulence kinetic energy and during the day. Furthermore, kelts swam faster as water velocity increased, but slowed down again as turbulence intensity increased. Our findings reveal the effects of hydraulic conditions and dam operations on the migration behaviour of Atlantic salmon kelts. This provides valuable insights for developing strategies to optimise dam operations and improve fish passage performance, including the need to spill enough water to increase passage success and will contribute to sustainable management of Atlantic salmon populations in regulated rivers.

Hedonic analysis of willingness to pay for dam removal: evidence from Plymouth, Massachusetts

This study investigates how the removal of small dams, and subsequent river restoration investments, affect residential housing prices in Plymouth, Massachusetts, selected for its unusually high density of dam removals. From 2002 to 2019, 5 dams within 1.05 miles of each other have been removed. Following removal, riverine restoration was completed, leading to improved public safety for downstream infrastructure, fish passage, and better water quality. Yet, contention exists in local government decision-making from fear of loss of property values of homes that are no longer classified as “waterfront.” To examine that charge, we evaluate sale prices of 9,710 single-family homes from 1997 to 2021, in a comparative estimation of a hedonic regression and a difference in differences model. The results indicate that the price of homes sold in Plymouth after dam removal increased, with no impact on property values of waterfront and properties within 1 mile of riverine restoration locations. For properties within a 5-mile radius of a former dam, we observe comparative difference in price of a fraction of a percent for lots around 2 of the dams, in relation to all home lots in Plymouth. We explain that this difference is due to property-specific features, rather than the impact of dam removal. The implications are that dam removal would not negatively impact waterfront property values, while it may translate into an overall environmental price premium for the community at large.

Survival implications of diversion entrainment for out‐migrating juvenile Chinook Salmon and steelhead

AbstractObjectiveEfforts to ameliorate the negative effects of diversion dams on aquatic species of concern are important in rivers where water withdrawal supports agricultural economies, and they are likely to become increasingly important with impending climate change. A multi‐year study was conducted to evaluate the survival consequences of diversion dam passage for juvenile Chinook Salmon Oncorhynchus tshawytscha and steelhead O. mykiss in the highly managed Yakima River, Washington. Canal entrainment and passage were evaluated at four diversion dams in the lower Yakima River where seaward‐migrating juvenile salmon and steelhead either pass dams via spill bays or enter canals with downstream fish‐screening facilities designed to collect entrained fish and return them to the main‐stem river.MethodsWe used acoustic telemetry to monitor the downstream migration behavior of 4403 juvenile Chinook Salmon and steelhead during 2018–2021.ResultPercent entrainment into canals was substantial (6–59%) at three of the four diversion dams studied, and entrainment probability was positively associated with the proportion of streamflow diverted into canals. Survival probability estimates for groups of tagged fish that were entrained into canals were lower than survival probability estimates for tagged fish that passed through spill bays on the dams. Absolute differences in survival probabilities between routes ranged from 0.099 to 0.369, demonstrating that canal entrainment reduced the survival of outmigrating juvenile Chinook Salmon and steelhead. We also found that entrainment resulted in migration delays, which could further affect survival because fish are increasingly exposed to predation and decreased water quality as water temperature increases throughout the migration season.ConclusionCanal entrainment resulted in increased mortality and travel time through the study area, highlighting the need to limit the entrainment of juvenile salmon and steelhead at diversion dams in rivers where salmon recovery is important.

When unlocking rivers results in building more infrastructure: A group mental model shares lessons from weir remediation in the United Kingdom

AbstractThe last several decades have seen a rise in efforts to remove weirs, but there is little research investigating how projects are carried out, potential areas for improvement, or sharing of lessons to facilitate reconnection of more rivers. The aim of the study presented here was to explore how people involved in weir remediation perceive project processes, factors that facilitate or hinder action implementation, and possible ways processes could be improved to reconnect more rivers. We carried out semi‐structured interviews with people (n = 11) who had been actively involved in weir remediation processes in the Severn River Catchment, United Kingdom, and used their responses to create a group mental model. The group mental model was created to support learning and communication about weir remediation projects between individuals and groups. We found broad agreement from those involved in creating the group mental model about weir remediation project processes and potential areas for improvement. One of the only points of divergence within the group mental model was associated with the impact of different weir remediation actions, particularly weir removal. Based on the group mental model, we set out three calls to action to reconnect more rivers in the UK. First, move beyond opportunistic projects and establish national goals and catchment‐scale plans for weir remediation. Second, reform fish passage legislation and legislate weir ownership. Doing so would support more effective remediation solutions by recognizing the diversity of fish species that reside in UK rivers and help mitigate risks from hazardous weirs through owner accountability. Third, build cross‐sector and public partnerships to encourage removal or improved fish pass designs. We direct the three calls to action to policy makers and anyone already engaged in or envisioning weir remediation projects in the UK. The calls also have potential implications and relevance to people in other countries in Europe and beyond.

Vegetation responses to large dam removal on the Elwha River, Washington, USA

Large dam removal can trigger changes to physical and biological processes that influence vegetation dynamics in former reservoirs, along river corridors downstream of former dams, and at a river’s terminus in deltas and estuaries. We present the first comprehensive review of vegetation response to major fluvial disturbance caused by the world’s largest dam removal. After being in place for nearly a century, two large dams were removed along the Elwha River, Washington, USA, between 2011 and 2014. The exposure, erosion, transport, and deposition of large volumes of sediment and large wood that were impounded behind the dams created new fluvial surfaces where plant colonization and growth have occurred. In the former reservoirs, dam removal exposed ~290 ha of unvegetated sediment distributed on three main landforms: valley walls, high terraces, and dynamic floodplains. In addition to natural revegetation in the former reservoirs, weed control and seeding and planting of desirable plants influenced vegetation trajectories. In early years following dam removal, ~20.5 Mt of trapped sediment were eroded from the former reservoirs and transported downstream. This sediment pulse, in combination with transport of large wood, led to channel widening, an increase in gravel bars, and floodplain deposition. The primary vegetation responses along the river corridor were a reduction in vegetated area associated with channel widening, plant establishment on new gravel bars, increased hydrochory, and altered plant community composition on gravel bars and floodplains. Plant species diversity increased in some river segments. In the delta, sediment deposition led to the creation of ~26.8 ha of new land surfaces and altered the distribution and dynamics of intertidal water bodies. Vegetation colonized ~16.4 ha of new surfaces: mixed pioneer vegetation colonized supratidal beach, river bars, and river mouth bars, and emergent marsh vegetation colonized intertidal aquatic habitats. In addition to the sediment-dominated processes that have created opportunities for plant colonization and growth, biological processes such as restored hydrochory and anadromous fish passage with associated delivery of marine-derived nutrients may influence vegetation dynamics over time. Rapid changes to landforms and vegetation growth were related to the large sediment pulse in the early years following dam removal, and the rate of change is expected to attenuate as the system adjusts to natural flow and sediment regimes.

Upstream experience and experimental translocation of invasive bigheaded carps results in increased upstream passage success at a navigation lock in a large river

AbstractFish movements in regulated rivers can be challenging to study because anthropogenic modifications, such as locks and dams, can influence animal behavior. Upper Mississippi River Lock and Dam 19 (LD 19), for example, is an invasive carp movement bottleneck due to an impassable dam. Upstream fish passage at LD19 is restricted to the lock chamber, making it an optimal location to test invasive fish deterrents that could limit further range expansion. Evaluating the effectiveness of experimental deterrents requires baseline knowledge of fish movements and suitable sample sizes of fish encountering the deterrents to ensure adequate statistical power. Some evidence indicates fish with prior upstream experience may return upstream or challenge potential deterrents at a higher rate than fish without such experience. To test how previous upstream experience could increase the rate at which fish moved upstream through a navigation lock chamber, we compared upstream passage through LD 19 using bigheaded carp captured below the dam (downstream‐origin) and two groups of bigheaded carp captured upstream from the dam: those that swam downstream on their own volition (upstream‐origin fish) and those that were captured upstream and translocated downstream of LD 19 (translocated upstream‐origin). Translocated upstream‐origin carp demonstrated the highest rate of upstream passage, with 59% of the fish detected downstream from LD 19 passing upstream during our study. In contrast, downstream‐origin carp made no upstream passages over 2 years. Fish origin was shown to influence upstream passage success. This may be an important consideration for fish passage studies and deterrent evaluations.

Vortex identification based on the Liutex method and its effect on fish passage upstream

Vortex identification based on the Liutex method and its effect on fish passage upstream

A watershed-scale multi-approach assessment of design flood discharge estimates used in hydrologic risk analyses for forest road stream crossings and culverts

Flood peak magnitudes and frequency estimates are key components of any effective nationwide flood risk management and flood damage abatement program. In this study, we evaluated normalized peak design discharges (Qp) for 1,387 hydrologic unit code 16 to 20 (HUC16-20) watersheds in the White Mountain National Forest (WMNF), New Hampshire and in five Experimental Forest (EF) regions across the United States managed by USDA Forest Service (USDA-FS). Nonstationary regional frequency analysis (RFA) and single site frequency analysis (FA) with long-term high-resolution observed streamflow data along with the deterministic Rational Method (RM) and semi-empirical United States Geological Survey regional regression equation (USGS-RRE) were used. Additionally, a hydrologic vulnerability assessment was performed for 194 road culverts as a result of extreme precipitation-induced flooding on gauged and ungauged watersheds in the Hubbard Brook EF (HBR) within the WMNF. The RM outperformed the USGS-RRE in predicting Qp in the gauged and ungauged HUC16-20 watersheds of WMNF and in three other small, high-relief forest headwater watersheds—Coweeta Hydrologic Lab EF’s watershed-14, and watershed-27 in North Carolina and HJ Andrews EF’s watershed 8 in Oregon. However, the USGS-RRE performed better for larger watersheds, such as the Fraser EF’s St. Louis watershed in Colorado and the Santee EF’s watershed 80 in South Carolina. About 31 %, 26 %, and 56 % of the culverts at the HBR site could not accommodate the 100-yr Qp estimated by RFA, RM and USGS-RRE, respectively. Based on the chosen RIs and techniques, it is determined that except for one culvert with diameter = 0.91 m (36 in.), none of the culverts with diameter of 0.75 m (30 in.) or larger are hydrologically vulnerable. Our results suggest that the observation based RFA works best where multiple gauges are available to extrapolate information for ungauged watersheds, otherwise, RM is best-suited for smaller headwater watersheds and USGS-RRE for larger watersheds. Results from the hydrologic vulnerability analysis revealed that replacing undersized culverts with new culverts of diameter ≥ 0.75-m will improve flood resiliency, provided that the structure is geomorphologically safe (with minimal effects of debris flow, erosion, and sedimentation) and allows for both bank-full discharge and necessary fish passage within that design limit. This study has implications in managing road culverts and crossings at Forest Service and other forested lands for their resiliency to extreme precipitation and flooding hazards induced by climate change.

The effect of in-flume habituation time and fish behaviour on estimated swimming performance

Swimming performance is important for fish migration, habitat selection, and predator-prey interaction, as well as for fish passage design. Procedural choices made when experimentally estimating it may influence the results. Systematic experiments were conducted to study the effect of different in-flume habituation times, habituation behaviour, and the use of external encouragement on burst swimming performance of Rutilus aula, a small-sized cyprinid, in a fixed velocity testing protocol. Increasing habituation times from 30 s to 5 or 20 min substantially increased the success proportion of swimming trials and estimated fish swimming performance, with no difference between the latter two habituation times. Fish resting on the downstream grid before the start of testing velocity outperformed those who swam during habituation and transition periods. Fish swimming volitionally in response to flow at testing velocity showed a significantly improved performance compared to fish motivated by external poking. The results of this study highlight that in-flume habituation time is important, and fish behaviour before actual testing may influence the outcomes of swimming performance results.

Diadromous fish run assessment: a double-observer model using acoustic cameras to correct imperfect detection and improve population abundance estimates

IntroductionDiadromous fish populations have strongly declined over decades, and many species are protected through national and international regulations. They account for less than 1% of fish biodiversity worldwide, but they are among the most perceptible linkages between freshwater and marine ecosystems. During their migration back and forth, diadromous fish species are subjected to many anthropogenic threats, among which river damming can severely limit access to vital freshwater habitats and jeopardize population sustainability. Here, we developed a method based on a double-observer modeling approach for estimating the abundance of diadromous fish during their migration in rivers.MethodsThe method relies on two independent and synchronous records of fish counts that were analyzed jointly thanks to a hierarchical Bayesian model to estimate detection efficiencies and daily fish passage. We used simulated data to test model robustness and identify conditions under which the developed approach can be used. The approach was then applied to empirical data to estimate the annual silver eel run in the Touques River, France.ResultsThe analysis of simulated datasets and the study case gives evidence that the model can provide robust,accurate, and precise estimates of detection probabilities and total fish abundance in a set of conditions dependent on the information provided in the data (annual distribution of fish passage, annual number of observation, pairing period, etc.).DiscussionThen, the method can be applied to various species and counting systems, including nomad acoustic camera devices. We discuss its relevance for programs on river continuity restoration, notably to quantify population restoration associated with dam removals.

Relationships among the fish passage efficiency, fish swimming behavior, and hydraulic properties in a vertical‐slot fishway

AbstractThree‐dimensional hydrodynamic numerical simulation and laboratory fish‐passage experiment with juvenile grass carp (Ctenopharyngodon idella) were used to systematically estimate hydrodynamic parameters, fish passage efficiency, fish swimming behavior, and fish swimming trajectories in a vertical‐slot fishway (VSF) under five different flow discharges. The spatial‐mean time‐averaged velocity magnitude along the slot section determined passage success rate. The absolute value of the time‐averaged Reynolds‐shear‐stress component in the X–Y direction played a decisive role in selecting the swimming path. In addition, fish energy expenditure along the mean‐50% swimming route indicated that higher mainstream velocity did not necessarily signify larger energy consumption, because successfully‐migrating fish selected the relatively lower velocity zone during the migration process. Our findings for fish swimming behavior in response to velocity and turbulence can be used to develop high‐efficiency fishways for grass carps or other species with similar traits.

High water temperature significantly influences swimming performance of New Zealand migratory species.

Anthropogenic structures in freshwater systems pose a significant threat by fragmenting habitats. Effective fish passage solutions must consider how environmental changes introduce variability into swimming performance. As temperature is considered the most important external factor influencing fish physiology, it is especially important to consider its effects on fish swimming performance. Even minor alterations in water properties, such as temperature and velocity, can profoundly affect fish metabolic demands, foraging behaviours, fitness and, consequently, swimming performance and passage success. In this study, we investigated the impact of varying water temperatures on the critical swimming speeds of four migratory New Zealand species. Our findings revealed a significant reduction in critical swimming speeds at higher water temperatures (26°C) compared to lower ones (8 and 15°C) for three out of four species (Galaxias maculatus, Galaxias brevipinnis and Gobiomorphus cotidianus). In contrast, Galaxias fasciatus exhibited no significant temperature-related changes in swimming performance, suggesting species-specific responses to temperature. The cold temperature treatment did not impact swimming performance for any of the studied species. As high water temperatures significantly reduce fish swimming performance, it is important to ensure that fish passage solutions are designed to accommodate a range of temperature changes, including spatial and temporal changes, ranging from diel to decadal fluctuations. Our research underscores the importance of incorporating temperature effects into fish passage models for habitat restoration, connectivity initiatives, and freshwater fish conservation. The influence of temperature on fish swimming performance can alter migration patterns and population dynamics, highlighting the need for adaptive conservation strategies. To ensure the resilience of freshwater ecosystems it is important to account for the impact of temperature on fish swimming performance, particularly in the context of a changing climate.