The artificial pulsed-flows impact associated with hydropower production on the downstream biological and physical processes has been extensively addressed, showing that it may cause fish drift while changing fish habitat selection toward lower water velocity patches, acting as refuge areas. We aimed to evaluate the attraction efficiency of two flow-refuges differing in their approaching angles, for Luciobarbus bocagei at an indoor experimental flume. We tested two flow-refuge insertion angles (45° and 70°), and two flow events (base 7 L/s and pulsed-flow 60 L/s) for each. To analyze flow-refuge efficiency, we quantified fish individual and group patterns, flow-refuge use, and permanence time, while measuring glucose and lactate responses. The results showed that the individual frequency of use during simulated pulsed-flow was higher, especially on the 45° flow-refuge. The simulated pulsed-flow condition did not disrupt group behavior for both approaching angles. Glucose and lactate analysis did not trigger physiological responses in the tested trials. The 45° flow-refuge was used more often than the 70°. The narrowing of the channel caused by the 45° flow-refuge, and the consequent higher flow homogeneity increased the attractiveness of the flow-refuge. Therefore, the flow-refuge insertion angle should be a decisive criterion in the construction and installation of flow-refuges for cyprinids, and be considered a potential indirect mitigation measure.

Artificial sub-daily flow fluctuations caused by peak-operating hydropower plants are considered one of the most significant impacts on riverine ecosystems downstream of dams. These rivers have, therefore, been subject to numerous studies in recent decades. However, cyprinid fish, in contrast to salmonids, have hardly been addressed in hydropeaking studies yet, and extensive knowledge gaps remain. Therefore, our experimental study aims to assess the effects of rapid flow reductions on the early life stages of two European cyprinid indicator species, the common barbel (Barbus barbus L.) and the common nase (Chondrostoma nasus L.).We conducted mesocosm experiments (2.25×2 m) under semi-natural conditions at an outdoor experimental facility (http://hydropeaking.boku.ac.at), simulating different hydropeaking scenarios with varying down-ramping rates during day and night. At each trial, 100 fish from one species (body length <20 mm) were stocked at peak flow (80 L.s-1). After an acclimation time (15 min.), the flow rate was reduced with variable ramping rates (0.3–1.8 cm.min-1) to constant low flow conditions (10 L.s-1). As a response parameter, larval stranding on a gently sloped shoreline mimicking typical nursery habitats was quantified during day and night.The results reveal distinct diurnal patterns for both species, with increased stranding rates at night for all experimental scenarios. In addition, the data indicate differences between the tested down-ramping rates and show interaction effects between both parameters. The difference between species may result from water temperature and ecological factors. The study outcomes will benefit the ongoing discussion on species-specific hydropeaking mitigation by providing first insights on the direct effects of artificial flow down-ramping on early life stages of cyprinid fish.

Research on how intermittent water releases from hydropower plants affect the early life stages of fish has advanced in the last years, focusing not only on the direct impacts of rapid flow changes (hydropeaking), but also on the short-term fluctuations in water temperature (thermopeaking). Flow and thermal fluctuations caused by hydropeaking may affect fish movement patterns and migration at critical stages of a species’ life cycle, e.g., by inducing passive downstream drift. Using two experimental outdoor channels, we investigated how nase ( Chondrostoma nasus , Cypriniformes) larvae respond to a rapid drop in water temperature during hydropeaking (simulating a cold thermopeaking event), reaching on average 5.5 °C under peak flow (maximum discharge) conditions, in comparison with a hydropeaking treatment with a constant water temperature regime. Responses of fish larvae were analyzed during acclimation, up-ramping (increase in discharge), peak flow and down-ramping (decrease in discharge) phases. Fish drift increased during peak flow in the cold thermopeaking treatment compared to hydropeaking. Higher drift rates were also negatively associated with pronounced water temperature drops during peak flow conditions. In addition, the starting temperature of the experiment influenced drift during up-ramping. Overall, the results suggest that cold thermopeaking may increase drift in the early life stages of cypriniform fish compared with hydropeaking with stable water temperature. Hence, monitoring and active water temperature adjustments following hydropower releases should be adopted as strategies to mitigate power plant-related impacts on aquatic organisms.

Downstream displacement and stranding of fish are key impacts in hydropeaked rivers, adversely affecting the organisms and, subsequently, the population. While these phenomena have been extensively studied for salmonid fish, few studies have been conducted on cyprinids, particularly at heterogeneous river banks that provide nursery areas for the earliest life cycle stages. This study examines the effects of rapid flow decreases (i.e., down-ramping) on downstream displacement and stranding of common nase, Chondrostoma nasus, larvae at two river bank morphologies (a flat gravel bar and a heterogeneous river bank consisting of a sill and ditch) and daytime periods (day and night) in a mesocosm facility. For a subset of the sill and ditch experiments, stranding location was noted to quantify the trapping effect of the structure. The results show that nase larvae's downstream displacement and stranding rates were higher at the sill and ditch than at the flat gravel bar. In addition, the effects of down-ramping were more visible at night than during the day. The displacement and stranding rates were the highest for the sill and ditch structure at night. In this case, the stranding rate was four times higher in the ditch area compared to other available areas of the mesocosm. Riverbank depressions can increase the stranding risk when disconnected and dewatered from the main channel after down-ramping, constituting potential ecological traps for fish larvae. These findings highlight the need to integrate riverbank morphology and flow schemes in hydropeaking mitigation frameworks to reduce fish displacement and stranding risks.

River ecosystems are exposed to a multitude of stressors, including increasing pesticide run-off driven by precipitation and irrigation. Pyrethroids are the fourth major group of insecticides in use worldwide and have extremely negative effects on aquatic fauna. In this study, we aimed to assess the effects of an acute 2 h sub-lethal exposure to different levels of the pyrethroid esfenvalerate on the swimming behaviour of two Cypriniformes species: the native Iberian barbel (Luciobarbus bocagei) and the non-native invasive bleak (Alburnus alburnus). The experimental set-up consisted of previous exposure to three esfenvalerate concentrations (control, 1.2 (low), and 2.0 (high) μg/L) before being stocked in a three-artificial-flume-channel mesocosm for behavioural trials through direct observation. Monitored behaviours included (i) routine activity, (ii) shoal cohesion, and iii) boldness. Significant differences in fish behaviour were detected for the native species (barbel), as individuals spent significantly more time holding position (i.e., resting) in the control (44.9%) than in the high esfenvalerate concentration (25.2%). Concordantly, control barbels were also found to perform more directional changes than the ones exposed to high esfenvalerate concentrations. Behavioural changes were also found for boldness, measured by the proportion of fish attempts to negotiate the upstream ramp, which were significantly higher in the control (37.4%) and in the high concentration (41.5%) compared to the low one (21.1%). Finally, regarding shoal cohesion of the barbel, it was tighter in the control (81.3%) than in the low- (70.5%) and high- (71.1%) esfenvalerate treatments. For the invasive bleak, there were no significant differences in any of the behavioural traits upon previous exposure to an increasing esfenvalerate concentration. This experimental study demonstrated that even short-term exposure to the pyrethroid esfenvalerate was sufficient to alter the behaviour of a native Cypriniformes fish species while not affecting the non-native species. This may confer greater competitive advantages to non-native fish species in the context of global changes.

The artificial pulsed flows occurring downstream of hydropower plants due to electricity demand, i.e. hydropeaking, affect habitat selection by fish. This effect is particularly unknown for cyprinids, which are the most representative freshwater fish family in European rivers. This study aimed to evaluate the utility of two types of flow-refuge by Iberian barbel (Luciobarbus bocagei) at an indoor flume (6.5m x 0.7m x 0.8m) as a potential solution to mitigate the effects of pulsed flows associated to hydropower production. Based on previous comprehensive research conducted on cyprinids and with the results of this study, the best type of flow-refuge was selected, up-scaled and implemented downstream of small hydropower plants. Two different approach angles with the flume wall - 45⁰ and 70⁰ - were tested to assess the effectiveness of the created hydraulic conditions on attracting fish to the flow-refuge. For each type we tested a base flow event (7 l.s-1), simulating river natural conditions, and a peak flow event (60 l.s-1), simulating pulsed flows. For each setting, two flow-refuges (downstream and upstream) were installed in the flume and tested with a school of five Iberian barbels. The utility of the flow-refuges was assessed by the frequency and time of use by fish at two distinct flow-refuge locations i.e., downstream (area between the flume and the adjacent flow-refuge walls), and inside (the effective covered area of the flow-refuge). Blood glucose and lactate levels were quantified to identify potential physiological adjustments associated with the pulsed flows and the flow-refuge type. Preliminary results indicate that fish behavior differs according to flow event and the type of flow-refuge. The frequency of a single fish using the flow-refuge was higher in the 45⁰ refuge during pulsed flows than in the 70⁰. Overall, the average time spent inside the flow-refuges was higher during pulsed flows for both types and higher in the 45⁰ refuge. After the 60 l.s-1 events, the blood glucose and lactate levels were higher than in the 7 l.s-1 events. In addition, lactate levels for the 45⁰ flow-refuge during the 60 l.s-1 events, were the highest when compared to 7 l.s-1 events. These results may be explained by the higher velocities created in the presence of the 45⁰ flow-refuge, shown by ADV results, that favoured individual use and rheotactic behaviour, setting off physiological adjustments, increasing residency time and the efficiency to use the flow-refuge.

Iberian rivers often face annual periods of flow intermittence during the dry season, when habitat availability for freshwater organisms is drastically reduced. Climate change and the presence of small barriers such as weirs may further exacerbate this lack of suitable habitat, particularly for freshwater and migratory fish that perform seasonal movements to complete their life cycle, by narrowing the submersed area and their possibilities of overcoming these barriers. Using the River2D model, we investigated how the current released flows from a nearby large hydropower plant in the Tagus River affect the passability of native fish species at a downstream rock weir (Pego), equipped with a nature-like fish ramp. Using mean daily flow data from gauging stations, we compared the passability of six fish species under low flow conditions (Q90), considering a historical period (1991–2005), and two flow datasets based on climate change projections until the end of the century (2071–2100) for the Tagus River Basin (“moderate” RCP 4.5 and “extreme” RCP 8.5). Target species included three migratory guilds: (i) anadromous—Allis shad (Alosa alosa), twaite shad (Alosa fallax) and sea lamprey (Petromyzon marinus); (ii) catadromous—European eel (Anguilla anguilla) and thlinlip grey mullet (Chelon ramada); and (iii) potamodromous species—Iberian barbel (Luciobarbus bocagei). Overall, our results show that the passability of all fish species may only be ensured with a minimum flow of 3 m s−1 and by using a fish ramp. Furthermore, the passability for all species was found to be significantly lower in the RCP scenarios when compared to the historical period. Our study suggests that climate change is expected to reduce the passability of native fish species in weirs in Iberian rivers, highlighting the importance of considering future flow conditions for a proper management of fish populations in the presence of weirs and other barriers.

To fight against global warming, we have to change our ways of consuming energy. Due to its low carbon impact and strong dispatchability, hydroelectric production will be one of the bases of this transition. However, peak electricity demand produces rapid and artificial flow fluctuations in tailwaters, i.e., hydropeaking, which has negative effects on fish biota. Thus, developing effective mitigation measures against hydropeaking is an urgent matter. The present study aims to limit the impact of this flow fluctuation on an Iberian cyprinid fish: the Iberian barbel (Luciobarbus bocagei). We experimentally tested different angles of flow refuge entrances (45° and 70°) in an indoor flume (6.5 m × 0.7 m × 0.8 m) to determine if this would affect the behavior of the fish. For each angle configuration, two refuges were installed and distanced 2.30 m from each other on the same side of the flume. Three possible resting locations were defined: downstream, inside, or upstream of each structure. Both angles were tested at 7 L/s (base flow), simulating the normal conditions of the river, and 60 L/s (peak flow), simulating a hydropeaking event. Each replicate comprised a group of five fish. For each, the frequency and residence time were quantified. The preliminary results indicated that the fish movement patterns changed when peak flow occurred. The downstream refuge was more frequently used in both configurations during peak flow. Additionally, the inside parts of the refuges were more frequently used, instead of the parts immediately downstream or upstream, and the time spent inside the refuge at peak flow was higher when compared to base flow. Additionally, hydraulic experiments were carried out at each configuration to determine the velocity field using ADV (Vectrino) technology. For the base flow, mean water depth and average velocity were 8 cm and 12 cm/s, respectively, increasing to 24 cm and 39 cm/s during peak flow. Measurements showed that velocity was equal to 74 cm/s in the narrowed area due to the refuge location, and velocity was null inside and directly downstream of the refuge. The results from this study will allow the development of guidelines for designing flow refuges for cyprinid fish, and hence mitigate the impact of hydropeaking.

Rapid water level decreases due to hydropeaking are known to negatively affect riverine biota, mainly due to the stranding of organisms in the river bank area that becomes regularly dewatered. Even though studies of the last decades have focused on salmonid fish, also cyprinids may be affected. However, limited knowledge is available of this fish family. Therefore, we conducted mesocosm experiments under semi-natural conditions, simulating single hydropeaking events at two different lateral bank slopes (2% and 5%) with varying down-ramping rates (0.7–3.0 cm min−1) during day and night. As a response parameter, we quantified stranding rates of different larval stages (III-IV and V) of common nase (Chondrostoma nasus L.). The experiments revealed that lower sloped banks exhibited distinctly higher stranding rates than steeper ones. Daytime revealed a similar pattern, with more fish becoming stranded at night than during the day, and this was consistent for all down-ramping rates. The data also indicate increased stranding with higher down-ramping rates, particularly at low sloped riverbanks, and interaction effects between the tested parameters. Overall, this study, for the first time, quantifies the consequences of flow down-ramping on nase larvae, also revealing differences between larval stages. The gained information will, therefore, advance the ongoing discussion on hydropeaking mitigation by providing a deeper understanding of the effects of artificial sub-daily flow fluctuations on the early life stages of cyprinid fish. Our results can inform management and policy to sharpen existing mitigation concepts and fine-tune hydropower operations to reduce negative effects on riverine ecosystems.

Climate change represents a major challenge for the management of native fish communities in Mediterranean rivers, as reductions in discharge may lead to a decrease in passability through small barriers such as weirs, both in temporary and perennial rivers. Through hydraulic modelling, we investigated how discharges from a large hydropower plant in the Tagus River are expected to affect the passability of native freshwater fish species through a rock weir (Pego, Portugal), equipped with a nature-like fish ramp. We considered not only mean daily discharge values retrieved from nearby gauging stations (1991–2005) for our flow datasets, but also predicted discharge values based on climatic projections (RCP) until the end of the century (2071–2100) for the Tagus River. Results showed that a minimum flow of 3 m3 s−1 may be required to ensure the passability of all species through the ramp and that passability was significantly lower in the RCP scenarios than in the historical scenario. This study suggests that climate change may reduce the passability of native fish species in weirs, meaning that the construction of small barriers in rivers should consider the decreases in discharge predicted from global change scenarios for the suitable management of fish populations.

River fragmentation by large hydropower plants (LHP) has been recognized as a major threat for potamodromous fish. Fishways have thus been built to partially restore connectivity, with fish lifts representing the most cost-effective type at high head obstacles. This study assessed the effectiveness with which a fish lift in a LHP on the River Lima (Touvedo, Portugal), allows potamodromous fish—Iberian barbel (Luciobarbus bocagei), Northern straight-mouth nase (Pseudochondrostoma duriense) and brown trout (Salmo trutta fario)- to migrate upstream. Most fish (79.5%) used the lift between summer and early-fall. Water temperature was the most significant predictor of both cyprinids’ movements, whereas mean daily flow was more important for trout. Movements differed according to peak-flow magnitude: nase (67.8%) made broader use of the lift in the absence of turbined flow, whereas a relevant proportion of barbel (44.8%) and trout (44.2%) passed when the powerhouse was operating at half (50 m3s−1) and full-load (100 m3s−1), respectively. Size-selectivity found for barbel and trout could reflect electrofishing bias towards smaller sizes. The comparison of daily abundance patterns in the river with fish lift records allowed the assessment of the lift’s efficacy, although biological requirements of target species must be considered. Results are discussed in the context of management strategies, with recommendations for future studies.

Heatwaves, which can be defined as increases of at least 5 °C in air temperature for more than five consecutive days for a specified reference period, are expected to become more frequent under the ongoing climate change, with freshwater organisms being particularly vulnerable to high temperature fluctuations. In Mediterranean-climate areas, depending on the extent of summer droughts and loss of longitudinal connectivity, river segments may become isolated, maintaining fish populations confined to a series of disconnected pools, with no possibility to move to thermal refugia and thus becoming more prone to thermal stress. In this study, we evaluated the effect of a simulated heatwave on the swimming behaviour of juvenile stages of a potamodromous native cyprinid fish, the Iberian barbel Luciobarbus bocagei, under experimental mesocosm conditions. Behavioural traits included fish activity, boldness and shoal cohesion and were continuously measured at a constant flow velocity of 18 cm s-1, which is typical of riffle habitats. Overall, results show that the behaviour of juvenile Iberian barbel is likely to be affected by heatwaves, with fish displaying lower activity and boldness, while no clear difference was observed in shoal cohesion. This study highlights the importance of managing thermal refugia that are crucial for fish to persist in intermittent rivers. Future studies should focus on the interaction of heatwaves with other stressors, such as oxygen depletion, for a broader understanding of the perturbation affecting freshwater fishes under a changing climate.

Rivers are dynamic systems where flow is constantly changing, making early fish life stages with lower swimming abilities potentially vulnerable to rapid changes in water velocity. In this study, we evaluated the response of critical young life stages of three Iberian leuciscids – Achondrostoma occidentale, Iberochondrostoma lusitanicum, and Iberochondrostoma almacai – to increasing water velocities, by determining their critical swimming speed (Ucrit) and its relation with morphometric traits potentially associated with better swimmers. Results suggest a positive relation between both fish length and body depth with Ucrit and species-specific differences in swimming performance. Moreover, most fish could not withstand the maximum velocity tested (20 cm s−1), which can be surpassed in the rivers they occur. This study highlights the importance of considering individual traits for future management of fish stocks in rivers.

When different genotypes choose different habitats to better match their phenotypes, genetic differentiation within a population may be promoted. Mating within those habitats may subsequently contribute to reproductive isolation. In cichlid fish, visual adaptation to alternative visual environments is hypothesized to contribute to speciation. Here, we investigated whether variation in visual sensitivity causes different visual habitat preferences, using two closely related cichlid species that occur at different but overlapping water depths in Lake Victoria and that differ in visual perception (Pundamilia spp.). In addition to species differences, we explored potential effects of visual plasticity, by rearing fish in two different light conditions: broad-spectrum (mimicking shallow water) and red-shifted (mimicking deeper waters). Contrary to expectations, fish did not prefer the light environment that mimicked their typical natural habitat. Instead, we found an overall preference for the broad-spectrum environment. We also found a transient influence of the rearing condition, indicating that the assessment of microhabitat preference requires repeated testing to control for familiarity effects. Together, our results show that cichlid fish exert visual habitat preference but do not support straightforward visual habitat matching.

Declines in freshwater fish populations are occurring at a fast rate, increasing the importance of ex-situ conservation programs supported by sound knowledge of population dynamics and life-history traits of the target species. We analysed the growth and age structure of wild and captive stocks of the western ruivaco Achondrostoma occidentale, a Portuguese endangered cyprinid, targeted for captive breeding and restocking since 2007. Specifically, we compared maximum size, longevity, and length-at-age among captive and wild populations, restocked and non-restocked We found considerable variation in length-at-age and longevity between captive-bred and wild fish, with the former generally growing faster and living longer. Analysis of length-age distributions among wild populations suggested a positive effect of restocking actions in fish abundance. Results highlight the value of captive breeding for the conservation of endangered cyprinids, and reinforce the need for detailed data on life-history trait variation between captive and wild stocks to assess the efficiency of ex-situ conservation programs.

Cyprinid fauna endemic to the Iberian Peninsula is currently facing several human-caused threats which enhance their risk of extinction. The populations of the critically endangered Achondrostoma occidentale, occurring only in three small and intermittent Portuguese river basins, have been reinforced with fish bred ex-situ in order to minimize their risk of extinction. This study aimed to describe the restocking actions conducted and to identify and characterize the summer refugia that should become targets for restoration and further restocking. The results evidence population increments and a high representativeness of younger age classes in the restocked populations. The importance of annual monitoring of fish abundance to assess how the species is coping with the stochastic and deterministic stressors that may eventually compromise its survival is highlighted.

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