• Trout;
  • Salmo trutta, fario;
  • Habitat;
  • Regulated streams


The aim of this paper is to illuminate any environmental preference of brown trout (Salmo trutta, fario) in streams of the South of France and, in this case, to evaluate the reliability of the Instream Flow Incremental Method, suggested by Bovee (1974) for American streams.

At various randomly distributed points on five tributaries of the Tarn and Ariège rivers the occurrence of fishes at different stages of development and selected morphodynamic characteristics (velocity, depth, substratum) were observed simultaneously. The presence of fish was checked by using two special electric fishing techniques: (1) casting the anode ahead of the operator for a sudden immobilization of fish; and (2) connecting previously immersed electrodes on the riverbed.

Curves have been plotted for velocity and depth parameters in each river at each season by analysing the frequency of appearance of fish at different stages of development in the different habitats. These curves do not vary greatly from one river to the next and the differences between seasons are not very significant. Single curves have thus been plotted by grouping the data from the different series of measurements.

The curves obtained for the velocity parameter are very close to those proposed by Bovee and reveal that the fishes have a preference for low velocities, regardless of their stage of development. As shown by the Bovee curves, the preference obtained for the depth parameter is related to the stage of development: shallow waters are much preferred by fry and juveniles but are not greatly frequented by adults. This study seems to confirm the existence of a real environmental preference among brown trout, indicating that the Instream Flow Incremental Method can be used for French 1st category streams.