Differentiable production and condition indices of premigrant eels (Anguilla anguilla) in two Atlantic coastal catchments of France

This paper assesses potential production of premigrant European eels Anguilla anguilla based on analysis of sedentary eel populations in two small river systems in western France that are in close proximity. Abundance and biological characteristics were evaluated from electrofishing surveys conducted in three years in September and October, before the catadromous migration of silver eels. Mean density and biomass density of the eel population differed greatly between the systems (39 ± 6 ind.100 m ± 2 and 1352 ± 171 g.100 m ± 2 in the Frémur River and 3 ± 0.32 ind.100 m ± 2 and 385 ± 42 g.100 m ± 2 in the Oir River). Premigrants were dominated by males in the Frémur (85.8%) and by females in the Oir (79.0%). Estimated premigrant biomass density was 4.5-fold higher in the Frémur (254.5 g.100 m ± 2.year ± 1) than in the Oir (56.0 g.100 m ± 2.year ± 1). Mean Fulton’s K condition factor was significantly higher for both sexes in the Oir (0.20 ± 0.004 and 0.20 ± 0.003 for males and females, respectively) than in the Frémur (0.17 ± 0.002 and 0.17 ± 0.004, respectively). The large differences in densities and biological characteristics of eels from neighboring catchments suggest that huge variability of both quantity and quality of silver eel production can be expected at the scale of the European stock

European eel (Anguilla anguilla): prediction of spawner escapement from continental population parameters

This paper describes the assessment of silver European eel (Anguilla anguilla) escapement based on a “sedentary” population fraction analysis in a 60-km2 watershed of northern Brittany (France). Downstream migration fluxes were monitored using eel traps and related to environmental factors. Intensive electrofishing and fyke-net fishing were conducted to assess eel biomass, densities, and population structure. A total of 564 eels, including 81 silver eels, were PIT tagged. In 1996, 616 eels were caught in the catchment including 68 silver eels (11%). During the following downstream migration period, 12 of the PIT-tagged silver eels, among a total of 678, were recaptured in the downstream traps. Seven were recaptured in the catchment in 1997. It was shown that (i) only about 20% of the silver eels present in the catchment emigrated during the following year, (ii) 12% stayed in the catchment including two (3.4%) that recovered yellow eel characteristics, and (iii) the other eels either died or settled in the catchment but were not recaptured. It was also estimated that 2% (650 eels) of the population (34 000 eels) among 3000 silver eels considered “emigration candidates” emigrated each year

Spatial distribution of an eel population (Anguilla anguilla L.) in a small coastal catchments of northern Brittany (France). Consequences hydraulic works.

The Frémur is a 60 km2 catchment of Brittany where many hydraulic works (dams, weirs, gauging structures, etc.) have reduced the recruitment by elvers and glass eels since at least 50 years. In 1992, an eel lift was built on the main dam (14 m high) and in 1996, two other dams (5 m high) located downstream were equipped with eel passes to improve upstream migration. In 1995, before the construction of the passes, eel population parameters (density, biomass and size distribution) were assessed at the scale of the whole watershed by electrofishing (removal method) in 33 stations. The average biomass was high (19 g/m2 ) despite the numerous physical obstructions. However, there was an accumulation of eels downstream many hydraulic works and very low densities immediately upstream. Moreover, downstream the first obstruction, at 2 km from the sea, the population is dominated by boot lace eels and elvers (< 100 mm), whereas this size class is absent in upstream reaches. These results suggest that the standing capacity of the watershed is not reached and that the population could be enhanced in the next years by the recently installed eel passes

Towards a standardized characterization of the potentially migrating silver European eel (Anguilla anguilla L.)

We defined a standardized method for discriminating candidate silver eels that may undergo catadromous migration in the following season from the sedentary fraction of a population. A combination of two qualitative criteria (state of differentiation of the lateral line and colour contrast) and one quantitative criterion (Ocular Index OI) was used to determine the development toward silvering. In the non-migratory phase, we found a gradient of the three criteria between yellow (0 criterion), presilver (1 to 2 criteria) and silver (3 criteria) eels. In the migrant phase, silver eels had ended their metamorphosis process and were characterized at the same time by the presence of the 3 silvering criteria. A mark-recapture survey using PIT-tags provided evidence that only identified silver eels (3 silvering criteria present) in the catchment actually emigrated the following season. Moreover, the use of a single criterion of silvering among the three generated large variation in the estimated proportion of candidates for emigration which varied between –22% and +63 %. Such a result confirmed that a multicriteria approach is needed to characterize in a standard way the potentially migrating silver eel

Isotopic niches of diadromous fishes inform on interspecific competition in an obstructed catchment

Competition arises when species share a limited resource, but this can be avoided through niche partitioning. Despite the large body of literature on diadromous fishes, very few studies have focused on niche partitioning when competing for resources. Diadromous fishes are suffering a global decline throughout their range in part due to their peculiar life history traits as they migrate from the sea to freshwater to spawn or the reverse. They are particularly sensitive to river fragmentation induced by barriers. Dams for instance are expected to alter the spatial distribution and resource exploitation of diadromous fishes as well as other organisms. Here, we studied the ecological niche of six taxa of diadromous fishes, temporally co-occurring in the same land–sea continuum obstructed by two river dams. We used Bayesian mixing models run on C and N isotopes to infer the various habitats used and the origin of the trophic carbon in muscle and scale tissues of diadromous fishes. Results showed that the sub-adults of Anguilla anguilla, juveniles of Petromyzon marinus, and Salmo trutta exploit mainly the freshwater, or marine part for adults of Lampetra fluviatilis, whereas juveniles of Platichthys flesus and Mugilidae use all the habitats of the land–sea continuum. In the freshwater habitat, the microphytobenthos and biofilm are the main sources of carbon used, while in the marine habitat, the basal carbon sources are more diverse. The analyses of niche overlaps between diadromous fishes demonstrate that the widest isotopic niches, observed for A. anguilla and P. flesus, also have more impact within the community. Results are discussed in terms of intra- and interspecific competition between these diadromous fishes and their generalist/specialist strategies in an obstructed catchment

Towards a standardized characterization of the potentially migrating silver

Abstract: We defined a standardized method for discriminating candidate silver eels that may undergo catadromous migration in the following season from the sedentary fraction of a population. A combination of two qualitative criteria (state of differentiation of the lateral line and colour contrast) and one quantitative criterion (Ocular Index OI) was used to determine the development toward silvering. In the non-migratory phase, we found a gradient of the three criteria between yellow (0 criterion), presilver (1 to 2 criteria) and silver (3 criteria) eels. In the migrant phase, silver eels had ended their metamorphosis process and were characterized at the same time by the presence of the 3 silvering criteria. A mark-recapture survey using PIT-tags provided evidence that only identified silver eels (3 silvering criteria present) in the catchment actually emigrated the following season. Moreover, the use of a single criterion of silvering among the three generated large variation in the estimated proportion of candidates for emigration which varied between -22 % and + 63 %. Such a result confirmed that a multicriteria approach is needed to characterize in a standard way the potentially migrating silver eel

Développement méthodologique du programme de surveillance des élasmobranches du descripteur 1 Biodiversité Poissons & Céphalopodes

Il n’existe pas à l’heure actuelle de surveillance dédiée pour les élasmobranches (raies et requins) dans le PdS DCSMM. Avant d’envisager la création d’un suivi spécifique il semble essentiel d’exploiter les données existantes pour notamment répondre à i) l’OE scientifique D01-PC-OE02 « Favoriser la restauration desélasmobranches en danger critique d’extinction selon la liste rouge des espèces menacées pour les façades NAMO, SA et MED » et ii) à la sous-action 1 « Cartographier la présence et la sensibilité des différentes espèces d’élasmobranches réglementées » du programme de mesures dont PatriNat a la charge.Pour cela, un jeu de données unique concernant toutes les espèces d’élasmobranches (40 raies, 59 requins mais aussi 7 chimères) résidentes de manière régulière ou occasionnelle en France métropolitaine a été compilé sur la période 2003-2021. Les données proviennent des suivis de la pêche professionnelle (ObsMer, EOS, CF-DCF/DACOR), des campagnes scientifiques (DATRAS, Med/CorseaCan, PELAGIS, etc.) et des sciences participatives (Ailerons, APECS, BioObs).Ce document rend avant tout compte de l’avancement de la sous-action 1 de cartographie du programme de mesures et de son apport vis-à-vis de l’OE D01-PC-OE02. Un examen préliminaire du jeu de données confirme également qu’il pourrait potentiellement venir compléter les évaluations quantitatives (critères D1C2 et D1C4) de certaines espèces dans certaines sous-régions marines de France métropolitaine. Mais pour cela des développements méthodologiques sont nécessaires et à prévoir en 2025

Bases biologiques d'un modèle pour estimer la biomasse féconde de l'anguille européenne en fonction des recrues fluviales et du contexte de croissance (approche comparative à l'échelle de petits bassins versants)

Nous avons examiné sur une période de 8 années (1996-2003) la dynamique continentale d'une sous population d'anguilles européennes Anguilla anguilla d'un fleuve côtier breton aménagé, le Frémur. Sur cette période, les trois écophases (Recrutement, Stock et Dévalaison) ont été quantifiées et caractérisées (stade, longueur et âge). Le taux de disparition estimé est important (53,2%) probablement en raison des abondances éleve es (0,40 anguilles.m-2) et des habitats dégradés. Chaque année, une prédiction du nombre potentiel de futurs reproducteurs à entreprendre leur migration à partir de la connaissance de la structure des anguilles sédentaires, est réalisée. Le nombre effectif variant en fonction de la gestion hydraulique du système. Finalement, l'influence du contexte de croissance sur la production du nombre et de la qualité des futurs reproducteurs, et les implications de cette étude en matière de conservation de cette espèce à l'échelle européenne, sont discutées.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF

Suivi de la migration d'avalaison des anguilles par une conduite de débit réservé

International audienceThe survey of the catadromous migration of eels through a major dam was operated on a pipe insuring a minimal legal discharge of the dam of Bois Joli on the Frémur River (Northern Brittany, France : height, 13.7m ; Volume, 3 Millions of m3). Part of the silver eels used this pipe to emigrate, at the beginning of the downstream run when it represented the only possible downstream passage, but also later when the dam spill enabled a second emigration pathway. Because of the low discharge and of the design of the pipe, it is assumed that eels look for a possible seawards passage. Downstream passage through the pipe was estimated to represent 12% of the downstream run and thus contributes significantly to the transit of silver eels. The contribution of the migration pathways to the migration runs are thought to depend uponthe water discharge. Despite its modification to facilitate emigration, the discharge pipe induced the mortality of 11% of migrating silver eels. Mortality was mainly due to abrasion. In many cases, passage through discharge facilities probably induces high or even total mortality depending upon management options, characteristics of the dam and design of the discharge facility. This induced mortality is likely to reduce significantly the production of breeders by river systems. Marked eels that passed the dam before the emigration peak stopped for a few weeks to one year. This observation confirms that reservoirs decreases migrations speed and efficiency. The dams and other works should be managed in order to facilitate passage and reduce mortality of emigrating silver eels

Catadromous migration dynamics of European eel (Anguilla anguilla, L.) in a dammed catchment.

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