EXPERIMENTAL-STUDY OF YOUNG GRAYLING (THYMALLUS-THYMALLUS) PHYSICAL HABITAT SELECTION FACTORS IN AN ARTIFICIAL STREAM

We investigated the importance of the shape (slope, depth) of dead zon es for their selection by young grayling (15-40 mm). Our hypothesis, d rawn from field observations, was that (1) larval grayling are sensiti ve to the shape of the dead zone they occupy and (2) they search for d ifferent habitat conditions by day and by night. Dead zones of the sam e dimensions (50 x 50 cm) but with different slopes and depths were pl aced along one bank of a 1-m wide experimental stream. Four shapes of dead zone were proposed to young grayling, from the combination of slo pe (gentle or steep) and water depth (shallow or deep). Habitat select ion was studied over three weeks, both by day and by night. Results sh owed that by day, young grayling preferentially selected dead zones wi th the greatest depth, while by night they were mainly observed in dea d zones having a flat bottom. The use of a 3D hydraulic model corrobor ated the following biological explanation: by day, while feeding, gray ing select dead zones with the maximum density of drifting prey, while by night, they select dead zones with low bottom velocity. This study indicates the importance of the structure and thus the functioning of dead zones for young stages of grayling.