Diel and seasonal patterns of habitat use by fish in a natural salmonid brook : an approach to the functional role of the riffle-pool sequence

The spatio-temporal use of the riffle-pool sequence in a natural salmonid brook was studied by day and night for one year on brown trout (Salmo trutta L.), bullhead (Cottus gobio L.), European minnow (Phoxinus phoxinus L.) and stone loach (Barbatula barbatula L.). Young-of-the-year brown trouts were more numerous in riffles in spring, whereas older trouts chose pools. A diel pattern of habitat use between riffle and pool was observed for one-year-old individuals, who moved towards riffle during daylight in summer and early fall. Seasonal variations in trout densities were related to movements at a large spatial scale before and after the spawning period. Bullhead preferred riffles to pools, especially young-of-the-year individuals, even if this preference decreased during ontogenesis. On the contrary, minnow chose pools throughout the year and seemed to leave the riffle-pool sequence for spawning, whereas no preference was observed for loach except for riffle during the spawning period. Results and an approach to the functional role of the riffle-pool sequence in lowland salmonid brooks are discussed.