Impact of hydropeaking on the abundance of juvenile brown trout in a Pyrenean stream

Many French rivers are concerned by hydropeaking management. Sudden flow va riations cause disturbances of the aquatic environment, fish habitat being particularly affected. The abundance of the most sensitive life stages of b rown trout (age groups 0+ and 1+) in the River Oriege (Pyrenees, France) we re studied 330 m upstream, and 700 and 3,000 m downstream of a hydroelectri c power plant with hypolimnetic releases from a nearby high-altitude reserv oir. The natural flow of the river was preserved when the plant was inopera tive. Hydropeaking strongly reduced densities of young trout: and disturbed their choice of mesohabitat 700 m below the plant, but such effects were a ttenuated 3,000 m downstream, mainly due to reduction of the slope. These r esults provide elements to assess the capacity of a regulated stream to sup port the early life stages of trout, and therefore the whole population.