No abstract

Freshwater reclaimed marshes along the European Atlantic coast are highly suitable for European eels (Anguilla anguilla). However, European eel stocks have declined, and the coastal marshes have been subjected to major disturbances. The objective of our study was to analyze the processes governing patterns of European eel microhabitat distribution of four eel size classes (from Ͻ160 mm to Ͼ360 mm) in a reclaimed marsh (France). Analyses were conducted using artificial neural network (ANN) techniques and ecological profiles. Our ANN results showed that eel densities were significantly related to three major influencing variables: the width of ditch section, the silt depth, and the density of emergent plants. Such ecological profiles were significantly different between small (Ͻ240 mm) and large eels (Ͼ360 mm): small eels were more widespread than large eels. Large eels were absent or at low densities in shallow ditches with a high aquatic plant cover obstructing the water column and a large quantity of silt. These characteristics seem to define the ditches not directly connected with the main river where dredging operations were rare. Management of regular dredging operations in the channels by maintaining a mosaic of permanent aquatic habitats and avoiding the heavy silt loads in most ditches should be promoted. This dredging operation was probably one of the most promising ways for restoring inland eel stocks.

Visible implant elastomer (VIE) tagging showed no significant effect on survival of either 230 single-tagged or 60 multiple-tagged small European eels Anguilla anguilla. Mean tag retention was 98Á7% during the 5 month laboratory experiments. Multiple VIE tags had no observed effect on European eel locomotor behaviour. VIE appears a reliable method for individually tagging small European eels, and could be useful in capture-recapture field studies.

1. When entering freshwater, young European eels either become resident or disperse to settle upstream. While this process shapes the whole population structure, little is known about how the 'decision' is made. Here, we investigated the possible role of body size. 2. A 1-year capture-recapture programme was carried out on 1505 European eels (length-range 62-633 mm) at two freshwater sites on the Saucats River, France. Eels were separated into two groups based on body length and this determined the tagging method; 1350 smaller eels (<240 mm) were tagged with visible implant elastomer tags, and 155 larger eels (>239 mm) were tagged individually with a passive integrated transponder. 3. Migration was detected only in eels <240 mm and decreased with increasing body length within this group. The probability of migration was higher in the summer, suggesting an environmental window for upriver dispersal. During this period, the mean probability of migration was estimated at 0.79 for 62-mm-long eels, 0.5 for 121 mm-long eels and 0.05 for 223 mm-long eels. 4. This is the first study to demonstrate, using capture-recapture models, the existence of differing eel behavioural tactics (residence and migration) based on body size. Such models are useful tools for analysing site fidelity and dispersal. If we assume that a migration probability >0.05 means that such eels contribute substantially to upriver colonisation, this corresponds to eels <220 mm in the Saucats River, although the threshold body size may be highly dependent on the river system.

The decline of the European eel (Anguilla anguilla) stock has led the European Commission to enforce a regulation (Council Regulation N°1 100/2007), in which each member state was required to establish an eel management plan. Various measures in the French plan aim at restoring river connectivity by mitigating the impact of obstacles on the colonization of continental water by eels. Consequently, many obstacles are going to be equipped with elver ladder in the near future. In this context, a method to assess the passability of an obstacle seems essential. In this study, we developed a tag-recapture method, appropriate to glass eels and elvers, and an associated multi-state mark-recapture model (i) to assess the passability of a ladder and (ii) to quantify the effect of various environmental factors on this passability. An application to a specific obstacle is applied as an illustrative example that demonstrates the relevance of the assessment method, and how the results can be used to propose technical solution to improve the efficiency of the ladder. Nine tag-recapture campaigns were carried on this obstacle, and about 4400 young eels were tagged. The model demonstrates that the efficiency of the ladder was rather low, especially during low river flow periods, mainly because of accessibility problems. The model also demonstrates the major influence of the river flow on the probability for an eel to pass the ladder; consequently, managing river flow during the migration period can be a relevant measure to improve river connectivity and facilitate colonization of the watershed.