In many rheophilic fish species, young stages with low swimming capacities are confined to lateral habitats protected from the main current. Among these lateral habitats, we defined „dead zones” as small bays with shallow water and slow water current caused by physical structures (obstacles or curves of the bank), but without a clear frontier with the main channel. Nevertheless, a study using two and three dimensional hydraulic models revealed the existence of a transiton zone characterized by a strong velocity gradient between these dead zones and the adjacent channel. Young stages of grayling constitute a good model for the study of the different aspects of the use of these lateral habitats. An approach based on direct observation in the field and on experimentations in an artificial channel allowed a precise description of young grayling sizedependent distribution patterns. The grayling undergoes an ontogenetic shift between larval and juvenile habitats. Larval stages are only found in dead zones: the smallest (15-20 mm) are found very close to the river bank and, with increasing age and size (20-40 mm), they begin to get closer to the transition zone. From a size of 40 mm, an increasing number of individuals is observed in the river channel, holding a benthic feeding station. A diel habitat shift also occurs between feeding habitats of larvae and juveniles and dead zones, where the fish are observed resting on the bottom in very shallow water at night. This points out the importance of lateral habitats as (1) exclusive nursery areas for larvae that use them both by day and by night and (2) resting habitats for juveniles at night.
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