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• Smolts show a great shift in their activity level during the migration season. • Their behaviour concomitantly changes from a reluctance to an attraction to light. • Accounting for fish developmental level is essential when designing fishways. The development of passage systems for migratory fish is crucial to mitigate the impact of river fragmentation. Concerning downstream migration of juvenile salmon (smolts), understanding their behaviour is a key to improving the efficiency of bypass systems. Among devices to improve efficiency, artificial lighting has proved effective in certain situations. Based on (1) recent observations of early migrating smolts where migration was delayed in the Poutès dam reservoir (Allier River, France) and (2) the fact that the implementation of bypass lighting devices was based on experiments involving later-season migrants, the present study assessed the effect of a lighting device on wild early-migrating smolts. One hundred wild smolts were tagged with acoustic transmitters and their behaviour near the bypass entrance under lit or dark conditions was assessed using 2D acoustic telemetry. A very abrupt change in behaviour around mid-April was observed, which directly affected their response to light. In the first phase of the downstream migration season (before mid-April), lighting significantly reduced the attractiveness of the bypass, while this surprisingly seemed to favour passage: smolts less frequently approached the bypass entry zone but passed through it more frequently. However, in the second phase (after mid-April), lighting attracted and kept the smolts close to the bypass entrance and significantly increased passage, corroborating previous experiments. The present study demonstrated an interaction between the development of migratory fish and their behaviour under lit or dark conditions. It also highlighted the importance of taking account of such behavioural change during the migration season when designing fish passage systems. [ABSTRACT FROM AUTHOR]

Abstract Several routes are available for the downstream migration of silver eels in the river Rhine system. Very different effects on migration success can result from this choice, such as speed and migration duration or escapement rate. We studied the downstream migration of silver eels in a river section with two different routes. The first route is the initial and old riverbed, with two dams equipped with two rather small or medium sized hydropower plants (HPPs) located at the beginning and at the exit of the bypass stretch. Both HPPs have small bar spacing (10 mm and 20 mm, respectively) and the second HPP has two downstream bypasses. The second route is a power canal, supplying four major HPPs (maximum discharge capacity = 1400 m3 s−1) and a nuclear power plant with cooling water. Firstly, this study focused on highlighting the factors influencing route choice. Secondly, we focused on the consequences of this choice. We demonstrate that water current management in the old riverbed at the study site had a 40% higher negative effect on eel survival than that by a consecutive passage in four turbines. [ABSTRACT FROM AUTHOR]

Connectivity in aquatic systems is often related to abundance and permeability of physical barriers, such as dams, which delay or impede movements of biota with important consequences for aquatic biodiversity. Water quality may, however, also control connectivity between essential habitats. In macrotidal estuaries, Estuarine Turbidity Maxima (ETM) have a strong impact on water quality because of the low oxygen concentration occurring as a response to the related high bacterial and low photosynthetic activities. In this study, we assess Allis shad estuarine spawning migration in 2011 and 2012 in the Loire River (France) where the ETM occurs at spring and summer. Using an acoustic telemetry array, we show that trans-estuarine migration is inhibited during hypoxic episodes in the middle part of the estuary. Shad tends to stay in downstream areas, and even at sea, where oxygen conditions are more suitable. Trans-estuarine migration occurs hastily during neap tide when the ETM decreases, both in terms of spatial extent and intensity, inducing a shift in a set of covariates including dissolved oxygen, which increases, and suspended matter, which decreases. In the context of climate warming, ETM are expected to increase with probable adverse implications for shad migration success and doubtless other diadromous populations. [ABSTRACT FROM AUTHOR]

In the global context of river fragmentation, predicting fish migration is urgent to implement management actions aimed at protecting and promoting the free movement of diadromous fish. However, large-scale applicability of conservation measures requires transferable models that enable prediction of migration even in data-poor regions. Here, we surveyed 12 contrasted European river sites to predict the activity peaks of silver eels (Anguilla anguilla) during river migration towards spawning areas through an ensemble modelling approach. Site-specific Boosted Regression Tree (BRT) models were adjusted using standardized hydrological variables to predict migration probability, which were aggregated in consensus predictions. Results of independent cross-validations demonstrated that silver eel migration runs were accurately predicted in response to changes in river discharge. Transferability and predictive performance were improved by considering catchment-size dissimilarity between river sites (85 to 109,930 km2) when combining the site-specific predictions. Nevertheless, we provided two examples for which the effects of human actions on flow conditions were so high that they prevented reliable predictions of migration runs. Further contributions should thus take advantage of the flexibility of our approach for updating model collection with new sites to extend the predictive performance under a larger range of ecological conditions. Our transferable hydrological-based modelling framework offers an opportunity to implement large-scale management strategies for eel conservation, even in rivers where eel monitoring data lack. The BRT models and prediction functions were compiled in an R package named 'silvRpeak' to facilitate operational implementation by end-user managers, which can determine when mitigation measures should be implemented to improve river continuity (e.g. turbine shutdown and sluice gate opening) and balance their economic activity towards eel conservation. The only input required is discharge records that are widely available across European hydrological stations. Unlabelled Image • Transferable tools for predicting migration are required to protect diadromous fish. • A generic hydrological-based model was developed to predict silver eel migration. • European silver eel migration was accurately predicted in response to river discharge. • Cross-validations demonstrated the model transferability in 10 river sites in Europe • This study offers an opportunity to implement large-scale conservation measures [ABSTRACT FROM AUTHOR]

Hydropower plants are commonly reported as a major cause of the worldwide decline of freshwater eels (Anguillidae), so that management solutions are urgently needed to mitigate their impacts. Where downstream passage solutions are complex to develop, turbine shutdown appears as an effective management solution to protect silver eels during their river migration toward spawning areas. However, the definition of operational decision rules for turbine shutdown is challenging due to the duality between the benefit for eel conservation and the concomitant cost in term of hydropower production. Here, we proposed a decision framework for turbine shutdown based on simple hydrological criteria to guide negotiations between stakeholders toward a trade-off between silver eel escapement and hydropower generation. Eel migration was assumed to be triggered by a minimum river flow associated with a minimum discharge pulse, so that threshold values can be directly implemented as decision rules for turbine shutdown. To estimate relevant thresholds, a generic methodological framework was developed to generate alternative decision rules from data collected at hydropower plants, which can include telemetry surveys and estimates of eel abundance. A multiple-criteria decision analysis was then conducted to rank alternatives and to determine the best compromise between promoting silver eel escapement and limiting turbine shutdown duration. Graphic outputs can help stakeholders to understand the competitive interests between eel conservation and hydropower production, while visually identifying a range of consensual alternatives to support negotiations in the choice of operational thresholds. The method was illustrated for three river systems in Europe featured by distinct hydrological conditions and can be applied in other areas, providing that eel monitoring surveys and flow data are available. • Turbine shutdown can reduce silver eels mortalities when crossing hydropower plants. • A trade-off between silver eel conservation and hydropower production is required. • Stakeholders need simple decision rules to maximise eel escapement. • Compromise alternatives are identified with multiple-criteria decision analysis. • Outputs provide quantitative elements to guide negotiations between stakeholders. [ABSTRACT FROM AUTHOR]

In freshwater ecosystems, water temperature and discharge are two intrinsically associated triggers of key events in the life cycle of aquatic organisms such as the migration of diadromous fishes. However, global changes have already profoundly altered the thermal and hydrological regimes of rivers, affecting the timing of fish migration as well as the environmental conditions under which it occurs. In this study, we focused on Atlantic salmon (Salmo salar), an iconic diadromous species whose individuals migrate between marine nursery areas and continental spawning grounds. An innovative multivariate method was developed to analyse long-term datasets of daily water temperature, discharge and both salmon juvenile downstream and adult upstream migrations in three French rivers (the Bresle, Oir and Nivelle rivers). While all three rivers have gradually warmed over the last 35 years, changes in discharge have been very heterogeneous. Juveniles more frequently used warmer temperatures to migrate. Adults migrating a few weeks before spawning more frequently used warm temperatures associated with high discharges. This has already led to modifications in preferential niches of both life stages and suggests a potential mismatch between these populations' ecological preference and changes in their local environment due to global change. [ABSTRACT FROM AUTHOR]

Climate change is modifying the hydrological and thermal regimes of rivers worldwide, threatening the triggering of organisms' key life-cycle processes. European eel (Anguilla anguilla) is a critically endangered fish species that migrates over several thousand kilometres between its rearing habitats in continental waters of Europe and North Africa and its spawning area in the Sargasso Sea. Downstream migration of adult eels occurs during periods of decreasing river water temperature associated with high discharge but changes in these environmental cues may affected eel migratory conditions. An innovative multivariate method was developed to analyse long-term datasets of daily water temperature, discharge and eel passage in two European rivers. Over the past 50 years, water temperature and discharge increased in both rivers during the downstream migration period from August to November. Silver eels preferentially migrated at temperatures between 10 and 20 °C combined with high discharge. Environmental changes have resulted in the migration of silver eels under warmer water temperatures. This example illustrates how the changes in environmental cues have led to a growing mismatch between the migratory conditions preferentially selected and those actually used, which may threaten the completion of the eel's life cycle and ultimately the persistence of this already critically endangered species. [ABSTRACT FROM AUTHOR]

Temperate eels are three panmictic catadromous species with a long period of oceanic passive larval drift and large distribution areas in contrasting environments. Spatial patterns of life history traits have been observed, and are correlated with environmental gradients, and may arise from both adaptive phenotypic plasticity and genetic polymorphism. This raises the question of the effect of spatially heterogeneous anthropogenic pressures on these populations. In this context, we used Geneveel, an individual-based optimization model that includes both phenotypic plasticity and genetic polymorphism, to explore the effects of different kinds of anthropogenic pressures: glass eel and silver eel fisheries, obstacles to upstream migration, and turbine mortality. More specifically, we analyzed the effects of these pressures on five output variables: the number of escapees, the proportion of females, the proportion of slow growers, the mean length-at-silvering and the resulting egg production. Our results suggest that phenotypic plasticity could act as a compensatory mechanism that mitigates the effects of some pressures (glass eel fishery and obstacles to upstream migration) and could be a source of resilience for the population, while other pressures did not show any compensatory effect (silver eel fishery and turbine mortality). Therefore, global impacts are very hard to assess, and the pressure that kills the most individuals does not necessarily have the biggest impact on the spawning biomass. [ABSTRACT FROM AUTHOR]

Most key life-events of organisms are synchronized by complex interactions of several environmental cues to ensure optimal survival and growth of individuals and their offspring. However, global change is known to affect multiple components of ecosystems and cues at the same time. Therefore, detecting joint trends in covariate time series is a crucial challenge in global change ecology that has rarely been addressed so far. In this context, we designed an innovative combination of kernel density estimations and Mann-Kendall trend tests to detect joint temporal trends in a pair of environmental variables. This methodological framework was tested on >30 years (1976–2019) of water temperature and discharge data for 6 large French rivers (the Garonne, Dordogne, Rhône, Rhine, Loire and Vienne rivers). The implications of such trends in both temperature and discharge for diadromous species key life-cycle processes were then explored by checking if significant bivariate environmental changes occurred during seasons of upstream and downstream migration, and reproductive activities. Results were contrasted between rivers and seasons: many rivers displayed an increase in the number of days with high water temperature and low river discharge, but local discharge regulation measures could have mitigated the trend in discharge. Our findings showed that species migrating or spawning in spring were likely to be strongly impacted by the new environmental conditions in the Garonne, Loire and Rhône rivers, given the marked changes in water temperature and discharge associations detected by our new method. Conditions experienced by fall-running and spawning species have been strongly affected in all the rivers studied. This innovative methodology was implemented in a new R package, ChocR , for application to other environments and ecosystems. Unlabelled Image • Organism life-cycle events are synchronized and triggered by abiotic factors. • Global changes are concurrently modifying many of these environmental factors. • We developed a method to detect joint temporal trends in abiotic factor time series. • We related the joint abiotic trends to diadromous fish ecological preferences. • Changes in thermal and hydrological regimes threatened many migratory processes. [ABSTRACT FROM AUTHOR]

Abstract Restoring the longitudinal connectivity of rivers is becoming a conservation priority in countries with high hydroelectric plant (HEP) development. Newly designed downstream passage solutions for fish are being installed in small and medium-sized HEPs in France, and an accurate evaluation of their functionality is needed. Here we addressed the efficiency of protection systems for the downstream migration of Atlantic salmon smolts at four HEPs (three 26° horizontally inclined racks and one 15° oriented to the flow rack in the bank alignment, all with 20 mm spaced bars). Between 239 and 300 hatchery-reared salmon smolts were PIT-tagged and released in 5–6 groups 100 m upstream of each studied HEP. Their passages through the HEPs were detected with radio frequency identification (RFID) antenna in the bypasses for downstream migration and the fish passes for upstream migration. On average between 82.8% and 92.3% of released smolts successfully passed the HEP through one of the two non-turbine routes. Resulting mean bypass passage efficiency ranged from 80.9 to 87.5% and all fish groups reached over 70% passage efficiency. Excepting one site, 50% of smolts passed through the bypass in less than 23 min after release, and 75% of them in less than 2 h 15 min. Combining our findings with previously estimated fish entrainment rates into the intake channel and turbine-related mortality rates, we assessed the overall fish survivals at the studied dam/HEPs which are between 98.24% and near 100%. Our results confirm recommended design criteria for inclined and oriented racks and the interest of the tested devices for the protection of downstream migrating salmon smolts. [ABSTRACT FROM AUTHOR]

The Atlantic salmon (Salmo salar) is one of the world’s most emblematic freshwater fish. Despite conservation and rehabilitation plans, populations of this species are dramatically declining due to human impacts such as habitat fragmentation, overfishing and water pollution. Owing to their large body size, anadromous adults were historically invulnerable to fish predation during their spawning period migration. This invulnerability has disappeared in Western Europe with the introduction of a new freshwater predator, the European catfish (Silurus glanis). Here we report how adults of Atlantic salmon are predated in the fishway of a large river of SW France, where the delayed and narrow passage created by the structure increases the probability of predator-prey encounter. We assessed predation risk by monitoring salmon and catfish in one fishway of the River Garonne, using video fish-counting from 1993 to 2016. We analysed the predation strategy of catfish using observations made with acoustic camera and RFID telemetry in 2016. Our results demonstrate a high predation rate (35%—14/39 ind.) on salmon inside the fishway during the 2016 spawning period migration. Our results suggest that a few specialized catfish individuals adapted their hunting behaviour to such prey, including their presence synchronized with that of salmon (i.e, more occurrences by the end of the day). Such results suggest that the spread of European catfish will potentially impact migration of anadromous species through anthropized systems. [ABSTRACT FROM AUTHOR]

The sustained development of hydropower energy in the last century has caused important ecological impacts, promoting recent advances in efficient mitigation measures to be implemented in existing and future hydropower plants. Although upstream fish migration has been largely addressed with the development of fish-pass infrastructures, downstream passage solutions are often missing or inefficient, strengthening the need for their improvement and efficiency assessment. The efficiency of horizontally inclined (26°) low bar spacing racks associated to a bypass was assessed using salmon smolts radiotelemetry along three successive hydropower plants (HPP) in the Ariège River (southern France). In average, nearly 90% of the smolts were successfully protected by the racks and rapidly guided to the bypass, within few minutes in most cases. Furthermore, we detected a significant positive influence of the bypass discharge (Q bp% expressed as the proportion of concurrent HPP discharge) on the probability of successful bypass passage, reaching 85% of successful passage with a Q bp% of only 3%, and more than 92% when the Q bp% exceeded 5%. The probability of bypass passage without hesitation (e.g. passage within the first 5 min) also increased with Q bp% , and reached 90% with 5% of Q bp%. Passage without hesitation was especially detected on the site having larger bypass entrances and transversal currents, providing better guidance into the bypass. High-efficiency results of inclined racks yielded with reduced Q bp% confirmed their relevance to mitigate some of the HPP ecological impacts, re-establishing safe downstream salmon migration with lower impact on energy production than older less efficient solutions. • Efficient fish protection devices should be implemented at hydropower plants. • ~90% of salmon smolts are protected with horizontally inclined low bar spacing racks. • Low bypass discharge is needed to yield high efficiency levels. • No significant migration delay is added. • This passage solution re-establishes safe downstream salmon migration. [ABSTRACT FROM AUTHOR]