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.

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–  The spatial distribution of yellow European eel (Anguilla anguilla) smaller than 300 mm was analysed during the upstream colonisation process. A 9‐year electric‐fishing programme in the Gironde catchment (France) provided eel occurrence data in 256 sites and eel abundance data in 23 sites. Generalized linear models showed that small eel spatial distribution decreased with river slope, dam number and with downstream‐upstream distance, estimated using either the distance from the tidal limit, called ‘tidal distance’, or the ‘relative distance’, calculated as the fish’s position relative to the total distance between tidal limit and river source. This new descriptor should be considered in future eel distribution studies as it reveals fractal dimension in eel spatial distribution and may provide a standardised method to compare directly freshwater eel assessment between streams and catchments of different lengths. If the relevancy of this descriptor is subsequently confirmed, it may have important implications for the management of eel population conservation.

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