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

Macrohabitat used by brown trout (Salmo trutta L.) for spawning was studied in the Neste du Louron river, in 61 morphodynamic units (pool, riffle, ....) distributed in 8 reaches. These units have been characterized by physical variables (surface and bottom current velocity, depth, substrat size, water gradient, discharge) and by biological variables (trout density and biomass, trout redds density). Results showed that, at a morphodynamic unit scale, there are no correlation between redds density and (1) trout density and biomass in this morphodynamic unit, (2) depth variables, (3) width of unit. There are high correlations between redds density and (1) the percentage of bottom area with gravel suitable for spawning (0,2-5 cm), (2) substrat size, (3) current velocity variables, (4) flow/width variable, (6) water gradient. At a reach scale, there are significant correlations between redds density and 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 redds density. This relation has then been validated on another sample of 20 morphodynamic units. Some applications from these relations are briefly discussed.