An approach based on direct observation from the riverbank allowed a precise description of the size-dependent distribution patterns of young grayling. The smallest larvae (15–20 mm) were mainly observed in the upper layers of the water column and very close to the riverbank. With increasing age and size (20–35 mm), old larvae and young parr began to hold positions closer to the bottom and to the edge of the main channel. From a size of 35–40 mm. an increasing number of juveniles were observed in the river channel, holding a benthic position. A diel habitat shift was also shown for individuals of all stages, between their respective daytime habitats and dead zones, where they were observed resting on the bottom in very shallow water at night. The benefits of this shift might be expressed in terms of energy conservation and (or) decreased predation risk. This study points out the importance of marginal habitats in the first weeks of grayling life as (i) exclusive nursery areas for larvae that use them both by day and by night and (ii) resting habitats for juveniles at night.
In an experimental stream containing available refuge zones, moderate and instantaneous flow variations did not cause high mortality or substantial increase in downstream migration of 2‐month‐old grayling, Thymallus thymallus L. Fish were able to react rapidly to spate simulations by a change in space utilization determined by changes in water velocity patterns. Reaction of fish to these experimental flow variations corresponded to a refuge strategy, i.e. seeking shelter and minimizing the distance to find it.
Grayling spawning sites were investigated in two French rivers, the Pollon (1 year) and the Suran (2 years) and described by current velocity, water depth, and substrate composition, completed by an assessment of bottom shear stress with FST-hemispheres. A comparison was made between used and available habitats, the latter being characterized by random sampling of 300 m long (Pollon) and 510 m long (Suran) river sections, both including three riffle/pool sequences. Mean velocities observed on spawning sites did not differ significantly between rivers or years (overall mean 48·9 cm s 1 , ..=11·9, range 25·8-91·7 cm s 1 , n=150). Most water depths ranged from 10 to 40 cm in both rivers, but mean depths were significantly different (P<0·05). Substratum of spawning grounds was dominated by gravel and pebbles (2-64 mm) in both rivers. Most spawning sites (99%) were characterized by a narrow range of hemispheres (nos 9-13), i.e. a range of shear stress of about 5-16 dyn m 2 . In the Pollon, spawners between spawning acts were found in a resting pool located immediately downstream from the spawning area and characterized by slow-flowing water (mainly <20 cm s 1 ) and great depth (mainly >60 cm), with cover provided by overhanging branches and tree roots.1995 The Fisheries Society of the British Isles
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