Resident and migratory tactics in freshwater European eels are size-dependent

P>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.