Search

Throughout the world, emerging barriers in river systems block longitudinal connectivity for migrating fish, causing significant impacts by precluding them from carrying out vital life cycle activities. Fishways are still the main mitigation solution implemented, where barrier removal is not feasible. Within the multiple technical fish passage devices, the vertical slot fishway (VSF) is considered the most reliable. Early design guidelines, established for cyprinids, indicate that the volumetric dissipation power (Pv) in the pools should be Pv < 150 Wm−3, while most frequent slope values range from 10 to 12%. In this study, an experimental approach was conducted to question and debate the validity of these recommendations. For this, the Iberian barbel (Luciobarbus bocagei, Steindachner, 1864) passage performance was assessed in a full-scale fishway that exceeded Pv design guidelines, under different configurations. These varied in discharge (Q) and mean pool water depth (hm): VSF1 (Q = 81 Ls−1; hm = 0.55 m); VSF2 (Q = 110 Ls−1; hm = 0.80 m); and the same design was equipped with a deep notch: DN1 (Q = 71 Ls−1; hm = 0.55 m); DN2 (Q = 99 Ls−1; hm = 0.80 m). The slope was set to 15.2% while the head drop per pool was Δh = 0.28 m, which generated a Pv that ranged from 222 in VSF1 to 187 Wm−3 in DN2. Passage behaviour was assessed using PIT telemetry and time-to-event analysis to evaluate the barbel upstream passage using standardized metrics: (i) motivation (ii) ascend success, and (iii) transit time. The hydrodynamic scenarios experienced by fish were characterized through a numerical model using computational fluid dynamics (CFD). The results, contrary to what was expected, showed a higher performance in VSF1 confirmed by the ascent analysis and transit time. Although no differences were found in fish motivation, the results indicate that larger fish displayed lower times to perform the first passage attempt. The CFD results show that, although maximum velocities and turbulence (turbulent kinetic energy (TKE) and Reynolds shear stress (RSS)) do not change significantly between configurations, their distribution in the pools is quite different. Regarding TKE, larger volumes with magnitudes higher than 0.05 m2s−2 were notorious in both DN1 and DN2 configurations compared to VSF1, influencing passage efficiency which is in line with the ascent and transit time metrics results. Overall, the present research undeniably debates the literature design guidelines and reinforces the need to jointly assess species-specific fish passage criteria and fishway hydrodynamics, whereas precaution should be taken when using very general recommendations.

Hydropower can have significant impacts on riverine ecosystems due to hydropeaking (i.e., artificial rapid and short-term fluctuations in water flow and water levels downstream and upstream of hydropower stations) that negatively affect downstream fish. However, when it comes to analyzing species habitat use and availability above and below small hydropower plants (SHPPs), studies conducted at the microhabitat scale are scarcer, particularly in Mediterranean rivers. The goal of this study is to assess the seasonal (early and late summer) and size-related (juveniles and adults) microhabitat use by native fish above and below SHPPs. Fish were sampled by a modified point electrofishing procedure, and a multivariate approach was used to analyze microhabitat use and availability data from sites located upstream (reference) and downstream (disturbed) from two SHPPs in northeast Portugal. Cover and water depth were the most influential variables in the use of microhabitat for all species at both the reference and disturbed sites, although some differences in the variable rankings were found. Leuciscids exhibited similar patterns of non-random (i.e., selective) microhabitat use between the reference and the disturbed sites. Overall, the seasonal and size-related patterns in species microhabitat use were similar, with the majority of species displaying seasonal patterns in microhabitat use from early summer to late summer. This study showed that differences in fish microhabitat use between downstream SHPP and upstream reference sites were negligible. Cover might have had a significant role in tempering the effects of detrimental environmental conditions, namely, peaking flows, by providing hydraulic shelter, highlighting the need to maintain riparian vegetation strips and mosaics of submerged aquatic macrophytes, as well as the provision of coarse substrata that can be critical for fish. Future studies are needed to better clarify how different size classes of fish select microhabitats when facing past and present hydropeaking conditions.

Requalification of low-head ramped weirs through the addition of substrates (retrofitting) has attracted attention in recent years. However, few studies are available on how this measure affects the negotiation of ramped weirs by fish. This study aimed to assess the performance of an experimental ramped weir (3.00 m long with 10% slope; 0.30 m head-drop) to enhance the passage of a potamodromous cyprinid species, the Iberian barbel (Luciobarbus bocagei). Attention was given to testing the effects of the addition of a substrate, in this case cobbles, to the ramp (Nature) vs. a smooth bottom (Control), and discharge (Q; 55 L·s−1 and 110 L·s−1 (or specific discharge per unit width, q = 92 L·s−1·m−1 and 183 L·s−1·m−1)) on fish passage performance. Fish physiological responses to stress and fatigue, measured by glucose and lactate concentrations in blood samples, were also analysed. Results showed that the Nature design generally increased fish movements and successful upstream passages, and enhanced fish passage performance by enabling faster negotiations. Fish movements were also affected by increasing discharge, registering reductions with 110 L·s−1. Results of the physiological parameters indicate that both glucose and lactate concentrations were also influenced by discharge. The outcomes from this study present important information about fish passage performance across low-head ramped weirs and could provide data needed to help biologists and engineers to develop more effective structures to alleviate small instream obstacles.

In this study, the 3D numerical modelling of flow in a pool-type fishway with bottom orifices was performed using computational fluid dynamics (CFD) software (FLOW-3D®). Numerical results were compared with experimental data obtained from acoustic Doppler velocimetry (ADV) and particle image velocimetry (PIV) measurements. Several hydrodynamic variables that influence fishways efficiencies, such as flow depths, flow patterns, water velocity, turbulent kinetic energy, Reynolds normal stresses, and Reynolds shear stress parallel to the bottom component, were qualitatively and quantitatively compared. The numerical model accurately reproduced the complex flow field, showing an overall good agreement between the numerical model predictions and the experimental data for the analysed variables. The importance of performing a numerical model validation for all the parameters under analyses was highlighted. Additionally, scaling effects were analysed by running an upscaled numerical model of the prototype fishway. The model performed with similar accuracy for both physical model and prototype dimensions with no evidence of scale effects. The present study concludes that CFD models (namely FLOW-3D®) may be used as an adequate and efficient design and analysis tool for new pool-type fishways geometries, reducing and complementing physical model testing.

Pool-type fishways have been increasingly tested to improve fish passage performance and minimize migration delays. Designing cost-effective fishways is essential for a trade-off between water uses and successful longitudinal connectivity restoration. The multislot fishway (MSF) concept, which operates with 30–50% lower discharge than a vertical slot fishway (VSF), was recently developed. This study assessed and compared the entrance performance (entrance time; entry efficiency) and transit times of two cyprinids for VSFs and MSFs. Four configurations, with the same structural characteristics (slope; water depth; head drop; pool width and depth), operating with different discharges (Q), were tested (VSF 1: Q = 112 L·s−1; VSF 2: Q = 80 L·s−1; MSF 1: Q = 58 L·s−1; MSF 2: Q = 37 L·s−1). Hydrodynamics characterization was performed using a numerical model. Results showed that entry efficiency was higher in MSFs than in VSFs, while entrance time and transit time were overall lower. Numerical modelling revealed that velocities were around 30% lower in MSFs, and turbulence could reach a difference of around 70% in Reynolds shear stress and 50% in turbulent kinetic energy. Overall, MSFs can be considered as a cost-effective fishway solution that can balance the trade-offs between divergent interests in water uses.

The rapid river flow variations due to hydropower production during peak demand periods, known as hydropeaking, causes several ecological impacts. In this study, we assessed the potential of an overhead cover and velocity-refuge structure in an indoor flume as an indirect hydropeaking mitigation measure for the cyprinid species Iberian barbel (Luciobarbus bocagei). We designed a lab-scale LUNKERS-type structure using two different materials which were used separately: Wood (opaque) and acrylic (transparent), tested under hydropeaking and base-flow events. Physiological (glucose and lactate) and behavioral (structure use) responses were quantified. The structure use (inside and in the vicinity) and the individual and schooling behavior was assessed. Although there was no evidence that the hydropeaking event triggered a physiological response, the wood structure use was significantly higher than the acrylic one, where the metrics of use increased in the hydropeaking event. Differences between individual and group behavior were only higher for the entrances in the wood structure. The higher frequency of wood structure use under hydropeaking conditions suggests that the visual stimulus conferred by this shaded refuge enables fish to easily find it. The results suggest that the use of overhead and velocity-refuge structures may act as an effective hydropeaking flow-refuge mitigation measure.

The addition of substrates to small instream obstacles, like low-head ramped weirs, has been considered a useful management solution to retrofit those structures and enhance fish passage. Substrate dimensions and spatial arrangement, together with discharge, and consequently water depths, appear as important factors for the creation of hydrodynamic conditions that may facilitate the successful passage of fish, though related studies are scarce to support decision-making. This study assessed the influence of discharge (Q) and different retrofitting designs (RD) on the upstream passage performance of a potamodromous cyprinid, the Iberian barbel (Luciobarbus bocagei). Different substrates (small boulders, large boulders, cobbles) and spatial arrangements (aligned, offset) were tested. Numerical modelling was performed to characterize the hydrodynamics. Results indicate that Q and RD influenced the upstream negotiation of ramped weirs. Cobbles randomly distributed along the ramp (Nature design) was the most successful configuration, recording the highest number of upstream passages. Low velocities along the ramp, and low turbulence downstream, were registered in this configuration, indicating that the use of natural substrate may help to increase the permeability of ramped weirs to fish movements. The outcomes from this work can help engineers and biologists to design more appropriate passage structures for low-head instream obstacles.

Most fishway studies are conducted during the reproductive period, yet uncertainty remains on whether results may be biased if the same studies were performed outside of the migration season. The present study assessed fish passage performance of a potamodromous cyprinid, the Iberian barbel (Luciobarbus bocagei), in an experimental full-scale vertical slot fishway during spring (reproductive season) and early-autumn (non-reproductive season). Results revealed that no significant differences were detected on passage performance metrics, except for entry efficiency. However, differences between seasons were noted in the plasma lactate concentration (higher in early-autumn), used as a proxy for muscular fatigue after the fishway navigation. This suggests that, for potamodromous cyprinids, the evaluation of passage performance in fishways does not need to be restricted to the reproductive season and can be extended to early-autumn, when movements associated with shifts in home range may occur. The increased effort during the non-reproductive period suggests that adapting the operational regime of fishways, at biologically meaningful seasons in a year, should be assessed by considering the physiological state of the target species.