344 research outputs found
Differentiable production and condition indices of premigrant eels (Anguilla anguilla) in two Atlantic coastal catchments of France
No full textThis 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
The yellow European eel (Anguilla anguilla L.) may adopt a sedentary lifestyle in inland freshwaters
No full textWe analysed the movements of the growing yellow phase using a long-term mark–recapture programme on European eels in a small catchment (the Frémur, France). The results showed that of the yellow eels (>200 mm) recaptured, more than 90% were recaptured at the original marking site over a long period before the silvering metamorphosis and downstream migration. We conclude that yellow European eels >200 mm may adopt a sedentary lifestyle in freshwater area, especially in small catchment
European eel (Anguilla anguilla): prediction of spawner escapement from continental population parameters
No full textThis 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
Migration pattern of silver eel (Anguilla anguilla, L.) in an obstructed river system.
No full textThe downstream migration of European silver eels in the River Frémur was examined to determine the potential effects of the numerous migration barriers that exist on waterways in western France. The Frémur has a 14 m high dam which creates a 3 · 106 m3 water supply reservoir, 6 km from tidal limit. Based on 8-year records (1996–2004) of migrant silver eels captured in a Wolf trap located about 1 km below the dam, the influence of this dam on the migration was examined. These records indicate that 91% of silver eels were captured when the dam was overtopped (i.e. when the water reservoir was full). The timings of the overflow periods varied greatly between years mainly because they are function of the duration of the filling period which in turn is dependent on the level of summer water abstraction and annual hydrological conditions. Consequently, migration periods occurred at variable dates (between November and April) which is late in comparison with nonobstructed European river systems (generally between August and December). During overflow periods the migration of silver eels increased markedly during rain events (increasing river flow). This indicates that flow flushes, are essential so that settled silver eels can orientate themselves and pass over the dam. The hypothesis of a ‘dam effect’ that could stop temporarily or permanently some of silver eels in the reservoir is discussed. Finally, the influence of the migration delays on the condition of silver eels is considered
Environmental determinism of year to year recruitment variability of European eel in a small coastal catchment, the Frémur river, north-west France
No full textThe influence of environmental factors (mainly the river flow) on the year-to-year variability of European eel Anguilla anguilla fluvial recruitment in a small coastal River (north-west France) was examined. A comprehensive survey of catches from fixed traps at two weirs located at 4·5 km (Pont es Omnes Dam) and 6·0 km (Bois Joli Dam) above the river mouth was carried out between 1997 and 2004. Young pigmented elvers (mean ± s.d. total length, 133·7 ± 29·6 mm) were recruited in eel-passes from February to October, but the main runs followed a modal curve from April to September with a peak centred in May to June. Catches varied greatly between years, from 381 to 26 765 elvers. For each trap, a positive linear relationship between monthly mean river flow that preceded the maximal intensity of captures and annual total catches was observed. These relationships explained 73·1% (P <0.01)
and 89·0% (P <0.001) of the year-to-year variability of the recruitment observed in the Pont es Omnes and Bois Joli traps respectively. A significant increase in river flow at the beginning of the migration peak would thus trigger a greater proportion of A. anguilla settled in the estuary
and in the downstream zone of the Fremur River to begin their freshwater colonization. The physicochemical roles of changes in river discharge in stimulating upstream migration are discussed. It is concluded that fluvial recruitment in the Fremur River is mainly determined by environmental factors
Patterns of silver eel (Anguilla anguilla L.) sex ratio in a catchment
No full textChanges in the numbers and size-class structure of European silver eels, Anguilla anguilla, in the River Fre´mur (France) were examined over a 9-year period after installation of downstream eel passes. The number of silver eels migrating downstream peaked in 1999, then decreased strongly and steadily after 2000, reaching relatively low levels. At the same time, a gradual shift in the silver eel sex ratio from a dominance of males (size from 270 to 442 mm, age from 3 to 6 years) to females (size from 366 to 1112 mm, age from 4 to 9 years) was recorded. Possible explanations for the escapement patterns observed are environmental sex determination and the installation of eel passes on the main hydraulic engineering structures in 1992 and 1996
Towards a standardized characterization of the potentially migrating silver European eel (Anguilla anguilla L.)
No full textWe 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
Factors determining survival of European eels in two unexploited sub-populations
No full textBoulenger, Clarisse Acou, Anthony Gimenez, Olivier Charrier, Fabien Tremblay, Julien Feunteun, EricRegion Bretagne; MAVA Foundation; Bretagne Grands MigrateursThis study was funded by the 'Region Bretagne', the MAVA Foundation, and 'Bretagne Grands Migrateurs'. The surveys and sampling were organised and operated by the company FISH PASS and the association 'Coeur Emeraude' in the Fremur River, and the INRA-Rennes and ONEMA in the Oir River. We are especially grateful to Frederic Marchand (INRA), Julien Tremblay (INRA), Richard Delanoe (ONEMA), Virgile Mazel (FISH PASS), Sebastien Quinot (FISH PASS) and all the people that helped with sampling and data gathering. We are also grateful to ORE PFC for the salmonid data.Wiley-blackwellHobokenEstimating accurate age-specific survival probabilities and understanding the processes (density dependent or independent) that regulate this demographic parameter are fundamental to propose sustainable management options for the endangered European eel (Anguilla anguilla). In the present study, we used extensive mark-recapture data sets (13 and 17years) on eel >150mm to estimate accurate natural survival probabilities of two eel sub-populations, from the Oir and Fremur rivers, western France, and then we analysed survival probabilities in relation to density-independent (temperature) and density-dependent factors to identify those causing survival variability. The Fremur and Oir rivers are two small (<100km(2)) river systems in close proximity (65km apart). The Fremur River is a small river obstructed by dams with medium water quality, high eel recruitment and density (0.37eelm(-2)), and a male dominant population; whereas, in the Oir River, low densities of eels were observed (0.04eelm(-2)) and the sex ratio was skewed towards females. Furthermore, previous research suggested that the lotic habitats of the Fremur River have reached carrying capacity, whereas habitats in the Oir River are below habitat saturation. In the Fremur River, there were significant spatial and temporal variations in the survival probabilities. However, survival probabilities observed in the Oir River were stable over time and space. The results highlight that the differences in the characteristics of the two systems and the two sub-populations prompt different responses to regulatory processes. The contrasting pressures applied on these two sub-populations impact survival, which possibly lead to different life history strategies such as sex differentiation
Spatial distribution and impact of the gill-parasitic Mazocraes alosae (Monogenea Polyopisthocotylea) on Alosa alosa and A. fallax (Actinopterygii, Clupeidae)
No full textAre the distribution of Mazocraes alosae and its impact on the host similar between Alosa alosa and A. fallax according to their resemblances? Parasites were numbered on each gill of shads sampled in North-East Atlantic coastal waters and connected rivers. Their impact on host condition was measured using girth, gonado-somatic ratio, C/N ratio, and Fulton’s K. Prevalence and mean intensity of M. alosae were significantly higher for A. alosa than for A. fallax, including in sympatric conditions. The mean intensity varied among sites whatever fish species; it was higher in coastal–estuarine versus fresh waters only for A. fallax. The distribution of M. alosae was aggregated in the host population whatever species. At the host individual level, some gills (second and third for A. alosa, second for A. fallax) were significantly more inhabited than others, probably in relation with larger water volumes flowing on these gills and mazocraeid sedentary lifestyle. Despite high prevalence and intensity, no negative impact of M. alosae was demonstrated on the host condition whatever the index considered. Our study underlines the major occurrence of M. alosae on shads and the potential use of such benign parasite as biological tag to discriminate closely related host species. © 2015, Springer International Publishing Switzerland
Biogeographic and temporal analyzes of the variability of silver eel (Anguilla anguilla) life traits in response to environmental changes
No full textDepuis 60 millions d’années, le genre Anguilla a montré un succès évolutif indéniable et pourtant, aujourd’hui, selon l’UICN l’anguille européenne est en danger critique d’extinction. L’étude de la valeur reproductive des organismes constitue un enjeu majeur pour comprendre ce déclin en lien avec les changements environnementaux imputables aux activités humaines. Chez l’anguille, les coûts énergétiques liés à la survie et la reproduction dans un environnement en évolution sont optimisés par l’expression d’un large éventail de traits d’histoire de vie (THV) qui constituent des indicateurs de la valeur reproductive. L’hypothèse de ce doctorat est que les changements environnementaux d’origines anthropiques perturbent le contrôle biogéographique naturel des THV et compromettent la résilience et la persistance de l’espèce. Parmi ces changements, la pollution et le parasitisme par le nématode non-indigène Anguillicola crassus constitueraient des risques sous-évalués pour la valeur reproductive des anguilles. Leurs effets sur les THV des anguilles matures n’ont cependant jamais été testés puisque le stade argenté immature constitue le dernier stade accessible avant la migration transocéanique entre l’Europe et la zone de reproduction océanique. L’objectif de ce doctorat est d’étudier la variation des THV au stade argenté pour mieux comprendre la persistance de l’anguille européenne i) dans l’espace sous l’influence à large échelle de la contamination chimique et du parasitisme et ii) dans le temps à l’échelle du fleuve Loire. Un indicateur a été développé pour standardiser le suivi de la qualité intrinsèque des anguilles argentées sur plusieurs bassins versants européens. Plus des deux tiers des anguilles échantillonnées étaient infectées par A. crassus et les polluants organiques montraient de fortes prévalences et contrôlaient la variabilité de l’indicateur. Les investigations sur les THV des anguilles femelles ont montré que l’effet local des bassins versants expliquait davantage la variation des THV que la biogéographie. Cet effet local est en partie lié aux contaminants inorganiques retrouvés dans les gonades. Les contaminants étaient positivement corrélés avec les traits associés à l’argenture et à la maturation d’une part, et négativement reliés avec les traits de fécondité, d’adaptabilité et de migration de reproduction d’autre part. Des simulations ont montré que la réduction de la pollution était une mesure efficace dans la restauration de la fécondité. Sur le fleuve Loire, les proportions de grandes femelles et de petits mâles argentés ont diminué entre 2001 et 2020. La taille des femelles a également diminué alors qu’elles restaient plus longtemps dans le bassin versant. L’abondance en anguilles argentées a été divisée par trois. Cette chute serait due au déclin progressif du recrutement en civelles à l’échelle européenne mais également à un épisode sévère de sécheresse estivale en 2003 et à la réduction ces dernières années du débit à l’étiage. Les résultats suggèrent que le taux de mortalité sur la Loire est en augmentation, probablement à cause d’un effet de densité dépendance et/ou de la contamination chimique des anguilles subadultes. Dans un milieu contaminé, la détoxification imposerait un coût énergétique supplémentaire réduisant l’investissement pour la fécondité et la production de biomasse. La pollution constitue un risque majeur de réduction de la valeur reproductive et serait un frein dans la reconstitution du stock et la persistance de l’espèce. La connaissance des seuils sublétaux de contamination n’est pas un prérequis pour mettre en œuvre des mesures environnementales puisque toute réduction de la pollution est bénéfique sur les THV. La considération de la qualité de l’anguille dans les plans de gestion constitue un effort nécessaire pour la restauration de la valeur reproductive de l’anguille.For more than 60 million years, the genus Anguilla has shown clear evolutionary success but today, according to IUCN, the European eel is critically endangered. The study of the fitness of organisms is a key issue in understanding this decline in relation to environmental changes caused by human activities. The energy costs in eels associated with survival and reproduction in a changing environment are optimized by expressing a wide range of life history traits (LHT) that are indicators of fitness. The hypothesis is that anthropogenic environmental changes disrupt the natural biogeographic control of LHT and compromise the resilience and persistence of the species. Among these changes, pollution and parasitism by the non-nature Anguillicola crassus nematode would constitute underestimated risks for the fitness of the eel. However, their effects on LHT of mature eels have never been tested because the immature silver stage is the last available stage to be sampled before transoceanic migration between Europe and the oceanic spawning area. The objective of this thesis is to better understand the persistence of the European eel by studying LHT i) at the scale of its distribution area (Europe) to study the influence of these two factors (pollution and parasitism) on reproductive capacity, and ii) long-term on the largest river in France (La Loire). First, we develop an indicator to standardize the monitoring of the intrinsic quality of silver eels in several European catchments. More than two-thirds of the investigated eels were infected by A. crassus and organic pollutants showed high prevalence and controlled the variability of the quality. Investigations on LHT of female eels showed that the catchment effect better explained the LHT variation than biogeography. This local effect is partly linked to inorganic contaminants found in gonads. Contaminants were positively linked with traits associated with migratory readiness on the one hand, and negatively related with fecundity, adaptability and reproductive migration traits on the other hand. Simulations have shown that reducing pollution could be an efficient way to restore fertility. On the Loire River, the proportions of large females and large silver males decreased between 2001 and 2020. The abundance of silver eels, estimated by the capture-mark-recapture method, was divided by three. This decline is likely due to the stepwise decline in recruitment of glass eels across Europe, to a high summer drought in 2003 and to the reduction in low water flow in recent years. The results suggest that the mortality rate on the Loire is increasing, probably due to a density dependence effect and / or chemical contamination of subadult eels. In a contaminated environment, detoxification would lead to an additional energy cost reducing the investment for fecundity and biomass production. The pollution constitutes a major risk of fitness collapse that prevent the stock recovery and the persistence of the freshwater eels. The definition of the sublethal contamination thresholds is not a mandatory prerequisite for implementing environmental measures since any reduction in pollution is beneficial for LHT. Incorporating eel quality into conservation strategies is a necessary effort to restore the fitness value of the eel
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