Roles of discharge and temperature in recruitment of a cold-water fish, the European grayling Thymallus thymallus, near its southern range limit

Recruitment of salmonids is a result of density-dependent factors, specifically egg production in the previous year, and density-independent environmental processes driven by discharge and temperature. With the plethora of knowledge on major drivers of Atlantic salmon Salmo salar and brown trout Salmo trutta recruitment, there is a requirement to explore less known species, such as European grayling Thymallus thymallus, whose postemergence time coincides with period of increasing temperature and low discharge. This study assessed drivers of grayling recruitment in a southern English chalk stream, a system vulnerable to discharge and temperature alterations under future climate change predictions. The analyses explored age 0+ grayling survival in relation to conspecific and heterospecific densities and discharge- and temperature-derived factors. The final mixed-effects model revealed a positive relationship between age 0+ grayling survival and incubation temperature anomaly and age 0+ trout abundance. Similarly, postincubation temperature anomaly had a positive effect on 0+ grayling survival, but only up to a threshold temperature of 13.5°C, beyond which it had a negative effect. In contrast, increasing number of days with low discharge postincubation negatively influenced age 0+ grayling survival, with no evidence of an effect of elevated discharges following spawning. Our results emphasise the importance of maintaining natural discharge regimes in salmonid rivers by tackling multiple stressors operating at the catchment scale, including land and water use to mitigate for predicted climate driven changes. In addition, further research on recruitment drivers in less stable, rain-fed systems, is required

Rôle de l'hétérogénéité de l'habitat dans le succès des tactiques alternatives de reproduction du saumon atlantique (Salmo salar L.)

Diplôme : Dr. d'Universit

Rôle de l hétérogénéité de l habitat sur les tactiques alternatives de reproduction du saumon atlantiques (Salmo salar L.).

La complexité de l habitat sur les sites de frai pourrait favoriser le succès reproducteur des mâles précoces furtifs de saumon atlantique. Cette hypothèse a été testée, grâce à une approche multi-disciplinaire, en milieu naturel et en conditions expérimentales, où le comportement ainsi que la contribution des géniteurs (anadromes comme tacons précoces) ont été étudiés en relation avec la structure de l habitat. La diversité en abris sur les frayères semblerait jouer un rôle important dans la compétition intrasexuelle qui existe entre mâles précoces plutôt que sur la compétition avec les mâles anadromes, permettant à un plus grand nombre de géniteurs précoces de féconder les œufs. L effet de la complexité de l habitat est discuté en relation avec le rôle des mâles précoces dans les petites populations de saumons atlantiques et le maintien d une diversité génétique élevée.The habitat complexity in the spawning environment may favour the reproductive success of sneaking precocious males of Atlantic salmon. This hypothesis was tested with a multidisciplinary approach, both in the natural environment and in experimental conditions. The behaviour as well as reproductive success of breeders (anadromous and precocious male parr) were studied in relation to the structure of the spawning habitat. The diversity in shelters around redds has an important role in the intrasexual competition among mature male parr, rather than in the competition with anadromous males. It allows precocious breeders to be more numerous to fertilize eggs. The effect of habitat complexity has to be considered in relation to the role of precocious males in small populations and the maintenance of high genetic variability.PAU-BU Sciences (644452103) / SudocSudocFranceF

Drawdown flushing of a hydroelectric reservoir on the Rhône river ::impacts on the fish community and implications for the sediment management

Sediment flushings of hydropower reservoirs are commonly performed to maintain water resource uses and ecosystem services, but may have strong impacts on fish communities. Despite the worldwide scope of this issue, very few studies report quantitative in situ evaluations of these impacts. In June 2012, the drawdown flushing of the Verbois reservoir (Rhône River) was performed and subsequent impacts on the fish community were assessed, both inside the reservoir (fish densities by hydroacoustic surveys) and downstream (short-term movement and survival of radio tracked adult fish). Results showed that after the flushing fish acoustic density decreased by 57% in the reservoir, and no recolonization process was observed over the following 16 months. Downstream of the dam, the global apparent survival of fish to the flushing was estimated at 74%, but differed between species. The nine-year delay from the previous flushing and thus the amount of sediments to remove were too stressful for the low-resilience fish community of the Rhône River. Alternative flushing schedules are discussed to reduce these impacts

Drawdown flushing of a hydroelectric reservoir on the Rhône River: Impacts on the fish community and implications for the sediment management

Sediment flushings of hydropower reservoirs are commonly performed to maintain water resource uses and ecosystem services, but may have strong impacts on fish communities. Despite the worldwide scope of this issue, very few studies report quantitative in situ evaluations of these impacts. In June 2012, the drawdown flushing of the Verbois reservoir (Rhone River) was performed and subsequent impacts on the fish community were assessed, both inside the reservoir (fish densities by hydroacoustic surveys) and downstream (short-term movement and survival of radio tracked adult fish). Results showed that after the flushing fish acoustic density decreased by 57% in the reservoir, and no recolonization process was observed over the following 16 months. Downstream of the dam, the global apparent survival of fish to the flushing was estimated at 74%, but differed between species. The nine-year delay from the previous flushing and thus the amount of sediments to remove were too stressful for the low-resilience fish community of the Rhone River. Alternative flushing schedules are discussed to reduce these impacts

The importance of habitat diversity on the reproductive success of precociously mature male parr of Atlantic salmon

International audienc

Mitigation of ecological impacts on fish of large reservoir sediment management through controlled flushing ::the case of the Verbois dam (Rhône River, Switzerland)

Sediment trapping within reservoirs is a worldwide phenomenon which impairs the ecological functioning of upstream and downstream ecosystems. It also reduces reservoir water storage volume, which lessens the services dams provide such as hydropower production or flood control and questions their sustainability. Hydraulic flushing is a widely used operation to recover the reservoir volume, but ecological impacts are massive. Recently, environmental awareness led dam operators to modify their management practices: ‘Controlled Sediment Flushing Operations’ (CSFOs) include environmental objectives in their implementation and are designed to be less harmful for aquatic ecosystems by controlling the flow and Suspended Sediment Concentration (SSC) downstream. However, CSFOs are not yet widespread, their ecological impacts are poorly documented, and comparisons with ‘classical’ flushing operations are unreported. Here, we analysed impacts on fish of the first CSFO of the Verbois reservoir in 2016, both upstream and downstream of the dam, and compared these with those from the empty flushing of 2012 using the same methodology (Grimardias et al., 2017). Time-series of hydroacoustics surveys enabled us to estimate the fish abundance in the reservoir, while radiotelemetry measured movements and apparent survival below the dam for four representative species. The 2016 CSFO lasted 10 days, and released a mean Suspended Sediment Concentration (SSC) of 3.47 g·L−1 (peak = 11.98 g·L−1). The fish density as assessed by the mean acoustic scattering strength SA in the reservoir did not change significantly pre- and post-CSFO, and SA seasonal estimates of year 2016 did not differ from those of 2015 and 2017. The apparent survival estimated from capture-recapture survey of marked fish (N = 118) decreased significantly during the CSFO for all species and differed across species, while the distances moved downstream increased. By comparison with the 2012 empty flushing, the 2016 CSFO allowed fish to remain in the reservoir, while impacts below the dam were mostly behavioural rather than lethal. Overall, despite significant impacts, the CSFO advantageously replaced ‘classical’ flushing from an ecological viewpoint. Provided that an acceptable balance between economical, ecological and technical aspects is found, CSFO can be considered for many reservoirs while accounting for their biological and physical site-specificity

Spatial distribution of European grayling (Thymallus thymallus) post-emergent fry and habitat sub-units preferences : a management-oriented approach

Résumé communication oraleInternational audienc

Ecological connectivity of the upper Rhône River ::upstream fish passage at two successive large hydroelectric dams for partially migratory species

The restoration of migration pathways for fish is crucial for the maintenance of populations in a worldwide context of increasing river fragmentation due to the development of dams. Studies aiming at quantifying fish passage at dams through fishway efficiency estimates concern few species, mostly diadromous. Data are lacking regarding most potamodromous species, despite their need to move to complete their life cycles. This study aimed to evaluate the efficiency of two vertical slot fishways for upstream fish migration on two consecutive run-of-the-river hydropower dams (Verbois and Chancy-Pougny) on the Rhône River. Radiotelemetry and PIT-tagging were used together (1) to describe the behaviour of fish under the dams and (2) to quantify entrance probability and passage efficiency of the fishways, for three potamodromous, partially migratory species, comprising of two cyprinids (Barbus barbus and Squalius cephalus) and one salmonid (Salmo trutta). The study revealed that fish preferentially moved upstream of the dam beside the gates, independently of their opening, rather than in the main flow of the turbines. While the passability of fishways did not differ among species (68% for all species), the attractiveness was dependent on the dam/fishway, the species and some individual characteristics such as fish body length and condition. A few fish remained below the gates or near the fishway for a long time, up to several months, without being able to detect the entrance. Overall, results revealed a lower efficiency of the fishways for trout than for the two cyprinids. Attraction was pointed out as the major limiting factor to the efficiency of the Verbois fishway for barbel and trout. Overall, results suggested to better account for fish motivation (migratory behaviour) to assess fishway efficiency for these species

Discrimination between schools and submerged trees in reservoirs: a preliminary approach using narrowband and broadband acoustics

International audienceWith a growing demand for hydroelectric energy, the number of reservoirs is dramatically increasing worldwide. These new water bodies also present an opportunity for the development of fishing activities. However, these reservoirs are commonly impounded on uncut forests, resulting in many immersed trees. These trees hinder fish assessments by disrupting both gill-netting and acoustic sampling. Immersed trees can easily be confused with fish schools on echograms. To overcome this issue, we developed a method to discriminate fish schools from immersed trees. A random forest algorithm was used to classify echo-traces at 120 and 200 kHz, recorded by an EK80 (SIMRAD) in narrowband (continuous wave) and in broadband mode (frequency modulated). We obtained a good discrimination rate between trees and schools, especially in broadband (90% ratio of good classification). We demonstrate that it is possible to discriminate fish schools from immersed trees and thus facilitate the use of fisheries acoustics in reservoirs