HABITAT SELECTION BY GRAYLING .1. SPAWNING HABITATS

Grayling spawning sites were investigated in two French rivers, the Po llon (1 year) and the Suran (2 years) and described by current velocit y, water depth, and substrate composition, completed by an assessment of bottom shear stress with FST-hemispheres. A comparison was made bet ween used and available habitats, the latter being characterized by ra ndom sampling of 300 m long (Pollon) and 510 m long (Suran) river sect ions, both including three riffle/pool sequences. Mean velocities obse rved on spawning sites did not differ significantly between rivers or years (overall mean 48.9 cm s(-1), S.D. = 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 bot h rivers. Most spawning sites (99%) were characterized by a narrow ran ge of hemispheres (nos 9-13), i.e. a range of shear stress of about 5- 16 dyn m(-2). In the Pollen, spawners between spawning acts were found in a resting pool located immediately downstream from the spawning ar ea and characterized by slow-flowing water (mainly <20 cm s(-1)) and g reat depth (mainly >60 cm), with cover provided by overhanging branche s and tree roots. (C) 1995 The Fisheries Society of the British Isles