Microhabitats of brown trout when feeding on drift and when resting in a lowland salmonid brook: effects on Weighted Usable Area

Summer habitat preferences by brown trout (10-20 cm fork length) were deter mined by daylight and night dive sampling in the Saint Sauveur brook, a tri butary of the River Scorff (Brittany, France). Habitat change was analyzed from night and day sampling. At the geomorphological unit scale, fish prefe rred riffles during daylight hours and pools at night. According to behavio ural data, night and day locations in the channel depended on the two activ ities: resting (night samples) and feeding on drift (daylight samples). Sui tability curves drawn at the microhabitat scale showed differences between day and night, in terms of water depth (25-45 cm by daylight, 40-55 cm at n ight), velocity (15-45 cm/s by daylight, 0-15 cm/s at night) and substrate particle size (gravel, cobble and stones by daylight, silt and sand at nigh t). Based on these habitat suitability curves, Weighted Usable Area was com puted by EVHA, the French version far the Physical Habitat Simulation syste m. Results show that WUA varied in magnitude from 1 to 9 between night and day, depending on stream discharge. In addition, the need to modify the "St andard" Habitat Suitability Indices (HSI) is emphazised, owing to the diffe rences in the shape of the suitability curves between Standard HSI and Feed ing HSI according to water velocity.