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Habitat use patterns of 3 species of temperate eels, Anguilla anguilla, A. japonica and A. rostrata, were investigated using otolith strontium:calcium ratio life history transects. Published and unpublished data from 6 sites (Canada, United States, Sweden, France, Taiwan and Japan) sampled across the geographical range of each eel species were compiled. Sr:Ca patterns indicated that the 3 species displayed similar patterns of habitat use. In all sites, patterns of habitat use consisted of either residency in one habitat (fresh, brackish, or marine) or movements between habitats. One movement pattern consisted of either a single change or 2 changes of habitat from fresh to brackish waters, or from brackish water to freshwater. Seasonal movements between fresh and brackish waters were observed for all 3 species. When only a single habitat switch event was detected, it occurred between 3 and 5 yr of age. Occurrence of eels with no freshwater experience was demonstrated, but such eels accounted for a smaller proportion of the overall sample than eels with some (even brief) freshwater experience. Contrary to the common convention that these are obligate catadromous species, we must now consider them as facultative catadromous, with far more flexibility in habitat use. The most variable parameter among study sites was the relative proportion, rather than the diversity, of lifetime spent in the various habitat use patterns. Eels found at higher latitudes exhibited a greater probability of remaining in the lower reaches of watersheds in brackish water. Diversity of habitat use appears to be a common strategy of temperate eel species, and, as a life history tactic, is under environmental control.

Deux phases principales ont été préalablement identifiées lors de la colonisation des bassins versants par les migrateurs amphihalins thalassotoques. Ces espèces utilisent le transport tidal sélectif (TTS) lors de la première phase dans l’estuaire tidal, puis une nage active à contre-courant lors de la seconde phase dans les rivières non tidales. Les thalassotoques doivent donc réaliser une bascule comportementale entre ces deux comportements. Cette bascule n’a pas encore été observée directement. Sa localisation ainsi que sa période restent inconnues encore à ce jour. Notre protocole expérimental visait à reproduire en mésocosme la traversée de la limite tidale, lors d’une expérience longue de 3 semaines. Des individus de deux espèces thalassotoques, le mulet porc et l’anguille européenne, ont été initialement soumis à une renverse de courant toutes les 6,2 heures durant une semaine. Puis une déformation progressive de l’onde de marée a été effectuée lors de la seconde semaine. Enfin, lors de la 3e semaine, les poissons n’ont été soumis qu’à un seul courant unidirectionnel. Nos résultats suggèrent que les thalassotoques utilisent le TTS aussi loin que possible, jusqu’à la fin de la limite tidale. Arrivés à ce point, dans notre étude expérimentale, aucune juvénile de l’année n’a effectué la bascule comportementale pour commencer à nager à contre-courant. Cela tend à confirmer que cette bascule a lieu plus tard, et que donc la seconde phase de colonisation est effectuée uniquement par des poissons plus âgés et plus grands, et non par des juvéniles de l’année. / During watershed colonisation by catadromous species, two main phases have been identified: tidal estuary crossing and non-tidal river colonisation. Fishes use selective tidal-stream transport (STST) during the first phase of this colonisation, and counter-current swimming during the second phase. Therefore, catadromous species have to achieve a behavioural shift, from STST to constant counter-current swimming. This has not yet been observed, and the location and period of this shift is still unknown. Our experimental protocol aimed to mimic the spatial progression of crossing the tidal limit within a 3-week experiment. Two catadromous fishes, thinlip mullets and European eels, were initially subjected to current reversal every 6.2 h during the first week. A gradual tidal distortion was performed during the second week, and fishes were submitted to a unidirectional water current during the third week. Our results reveal that all catadromous species use STST as far as possible within the tidal limit. At this point, in this experimental study, no young-of-the-year (YOY) fishes shifted from STST to constant counter-current swimming. This confirms that the behavioural shift occurs later, and that the second part of the upstream migration, counter-current progression, is performed by larger, older fishes and not YOY fishes.