Reproduction de la truite (Salmo trutta L.) dans le torrent de Chevenne, haute-savoie. Un fonctionnement original?

Abstract

The spawning of a mainly endemic Mediterranean brown trout (Salmo trutta L.) population is described in Chevenne stream, a high-gradient (10%) boulder bed torrent, obstructed by small waterfalls, with scarce spawning substrate located in Alps of Savoy, France. The spawners (residents in the stream or migrants from the main river) differed in size and sex-ratio. A high percentage (58%) of already ovulated migrant females entered the stream. There is a great downstream-upstream decreasing gradient of spawner's and redd's number. A total of 157 redds was decribed showing a great diversity in the microhabitat (5 main types) used for spawning, particularly with a high percentage (65%) of redds built in protected zones with a low water velocity (< 20cm/s). Spawning first occurred in the standard riffle microhabitat, but also extended to other microhabitats before the end of spawning in riffles. An evaluation of the availability and the use of the adequate spawning substrate (size of 1 to 3 cm) in the different spawning microhabitats was performed at a later date to 1995 spawning. The unusual "side pool" spawning microhabitat was as much used as the classical riffle microhabitat. The lens of sheltered adequate spawning substrate located near the banks or in currents were prefered. On the contrary the lens of unsheltered adequate spawning substrate were avoided. A spate at the end of the spawning season totally destroyed 36% of the redds with great differences according to the kind of microhabitat. The redds digged in side pool position or in sheltered lens of gravel near a bank were less destroyed (15-17% of total destruction) than the redds built in other microhabitats (50 to 67% of total destruction). Then, the diversification of spawning sites might be an important factor for the population to survive in such an unstable stream.