Assessing the freshwater distribution of yellow eel

In the global context of the decline in wild species, modeling the distribution of populations is a crucial aspect of ecological management. This can be a major challenge, especially for species, such as the European eel, that have complex life cycles, exhibit cryptic behavior, or migrate over long distances. A review of the literature suggests that eel size data could be used to assess and analyze freshwater distribution of eel. We argue that analyses based on small yellow eels (≤ 300 mm) along the longitudinal course of rivers could provide a valuable tool for population monitoring. We propose a standardized catchment recruitment index and a colonization index based on the probability of occurrence (presence/absence data) using logistic models for different size classes. The model developed here provides a convenient guide for assessing yellow eel stages in freshwater areas, and should have concrete applications for management of the species

Strategy of recreational anglers and estimated eel (Anguilla anguilla) catches in a large European catchment basin, the Loire (France)

The European Council Regulation (18 September 2007) for the recovery of eel stocks required European states to report on eel stocks and anthropic mortalities. The data sources for France reveal gaps in our knowledge about angling in the public domain, and catches from privately-owned rivers. We tested a simple method, using a survey, for estimating the strategy employed and the catches landed by the 350 000 anglers fishing in the Loire catchment area. The first findings show that the answers of the fishermen varied between 1 and 2% in the departments with the most eels, and between 0.1 and 0.2% in the departments located further upstream. The interest in eel fishing, the average weight of the eels caught by the fishermen, and the total catch all decreased from downstream to upstream. Finally, we estimated the annual average weight of eel caught by anglers, which ranged from 0.8 to 5.3 kg per year depending on the department and the local interest in eel fishing. This preliminary study shows that it is possible to estimate the different fishing strategies used and the quantities caught by means of a survey

Relation entre l’ichtyofaune et les marais salés macrotidaux : l’exemple de la baie du Mont Saint-Michel

Près de 100 espèces de poissons sont connues pour être présentes dans le domaine intertidal (estuaires, marais salés et vasières) de la baie du Mont Saint-Michel (France). Entre mars 1996 et avril 1999, 120 marées ont été échantillonnées dans les chenaux des marais salés macrotidaux. Au total, 31 espèces ont été capturées. Ce peuplement est largement dominé par les mulets (Liza ramada représentant 87% de la biomasse totale) et les gobies (Pomatoschistus minutus et P. lozanoi représentant 82% des effectifs totaux). Ces espèces ainsi que Gasterosteus aculeatus, Syngnathus rostellatus, Dicentrarchus labrax, Mugil spp., Liza aurata et Sprattus sprattus sont les plus fréquentes. En Europe, les marais salés et leurs chenaux ne sont inondés que lors des marées de grandes amplitudes. Par conséquent, les poissons ne peuvent y pénétrer que lors de courtes périodes d’immersion et aucune espèce ne peut être considérée comme résidante des marais salés

Patterns in fish assemblages in the Loire floodplain: the role of hydrological connectivity and implications for conservation

Hydrological connectivity is known to determine biodiversity patterns across large river floodplains, but it is often greatly altered by human activities. Indicators and predictors of the response of river alteration or restoration are therefore needed. Recent papers suggested that fish environmental guilds – based on species flow preferences – could be used as a tool to assess ecological status of rivers. In the Loire floodplain, we described fish assemblages across the floodplain at the onset of the dry season and we determined whether observed spatial patterns could be related to environmental variables, especially connectivity. Based on specific composition of 46 electrofished waterbodies, a hierarchical typology of the Loire floodplain assemblages was built using self-organizing maps. Each assemblage of the typology was characterized by a set of species using the indicator value method. These species sets and the composition of the assemblages revealed a gradient of flow preferences in the different assemblages identified. A stepwise discriminant analysis showed that the most important variable determining assemblage composition was the hydrological connectivity. Finally, the conclusion was made that a high connectivity level is needed to conserve native fish diversity in the Loire floodplain, notably because the number of protected and native species increased with connectivity, and because the number of exotic species increased with isolation

Temporal patterns in the upstream migration of European eels (Anguilla anguilla) at the Couesnon estuarine dam

The temporal migration patterns of European glass eel Anguilla anguilla at the Couesnon estuarine dam (Mont-Saint-Michel Bay, France)were examined in winter and spring 2004 and 2005. The dam which is located close to the river mouth constitutes a major obstacle for upstreammigrating glass eels. The migration was observed at different temporal scales, from within individual tides to complete tidal cycles between successive spring tides. The maximum number of glass eels arrived downstream of the dam at the beginning and in the middle of the flood tide. Glass eels migrated through the dam openings preferentially from the middle of the flood to the beginning of the ebb tide. Eel densities were highest during the second tide of each tide cycle that arrived at the estuarine dam and when the difference in water level between upstream and downstream of the dam was greatest, particularly at the end of the flood. Analysis of the influence of each environmental factor provided a good prediction of the glass eel recruitment peaks and, therefore, of the most favourable temporal windows for their migration. The water level and temperature were the most important environmental factors. These results provide the information needed for a dam-management program that is compatible with glass eel migration

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

Un outil d’aide à la gestion de l’anguille : le tableau de bord anguille du bassin Loire

L’anguille européenne (Anguilla anguilla, L.) est longtemps apparue comme une espèce commune, représentant une composante majeure des milieux littoraux et des eaux continentales. La prise de conscience de la diminution de son abondance est réelle et engendre l’émergence de processus de gestion. Ce constat situe l’enjeu du Tableau de Bord Anguille du Bassin Loire mis en place en avril 2002 sous l’égide du COmité de GEstion des POissons MIgrateurs du Bassin de la Loire, des Côtiers Vendéens et de la Sèvre Niortaise. Son objectif in fine est de contribuer à une gestion et à un développement durable de l’espèce et de ses habitats. Il vise à surveiller les caractéristiques des stocks locaux présents et des habitats disponibles au moyen d’une collecte régulière et objective de données synthétisées sous forme d’indicateurs de population et de milieu. Le concept, le fonctionnement actuel et les applications de ce tableau de bord sont ici présentés

Is scuba sampling a relevant method to study microhabitat in lakes? Examples and comparisons for three European species

We compared fish microhabitat use patterns in the littoral zone of a lake using a new direct method (i.e. Point Abundance Sampling by Scuba, PASS) and the widely used Point Abundance Sampling by Electrofishing technique (PASE). We collected microhabitat data for age 0+ roach (Rutilus rutilus L.), perch (Perca fluviatilis L.), and pike (Esox lucius L.). The two methods yelded different results for fish assemblage structure and microhabitat patterns. Using PASE, fish were mainly found in "shelter habitats" such as shallow waters and dense vegetation. It is likely that this behaviour is caused by the disturbance of the observer stamping around. Using PASS, fish escapement behaviour was rarely observed. Therefore, we concluded that this direct and non-destructive sampling technique is able to provide an accurate microhabitat estimation of a fish community and is assumed to be more suitable than PASE for fish habitat studies

Are young sea bass, Dicentrarchus labrax L. (Teleostei : Pisces) adapted to mussel cultures?

In Mont Saint-Michel Bay, one of the most abundant food items in young sea bass diets was Mytilus edulis. This invertebrate is an abundant cultured bivalve in many coastal systems and it is known to be in direct trophic competition with the natural food items of sea bass. This study shows that mussel culture can play the role of a nursery area, providing food for young opportunistic fish. Quantification of these interactions appears to be of a major importance in order to enable sustainable management of coastal areas