A STUDY OF BROWN TROUT (SALMO-TRUTTA L) S PAWNING MACROHABITAT IN A FRENCH MOUNTAIN STREAM - EVALUATION OF A PHYSICAL HABITAT POTENTIAL FORSPAWNING

Macrohabitat used by brown trout (Salmo trutta L.) for spawning was st udied in the Neste du Louron river, in 61 morphodynamic units (pool, r iffle, ....) distributed in 8 reaches. These units have been character ized by physical variables (surface and bottom current velocity, depth , substrat size, water gradient, discharge) and by biological variable s (trout density and biomass, trout redds density). Results showed tha t, at a morphodynamic unit scale, there are no correlation between red ds density and (1) trout density and biomass in this morphodynamic uni t, (2) depth variables, (3) width of unit. There are high correlations between redds density and (1) the percentage of bottom area with grav el suitable for spawning (0,2-5 cm), (2) substrat size, (3) current ve locity variables, (4) flow/width variable, (6) water gradient. At a re ach scale, there are significant correlations between redds density an d trout density and biomass. A statistical relation has been assessed from multiparametric regression, and from a sample of 41 morphodynamic units randomly chosen. It uses 4 variables and explains 87 % of the r edds density. This relation has then been validated on another sample of 20 morphodynamic units. Some applications from these relations are briefly discussed.