Ability of instream flow incremental methodology to predict brown trout spawning habitat.

The Instream Flow Incremental Method was applied to brown trout reproduction, in 205 morphodynamic units of 6 Pyrenees mountain streams. The correlation between Weighted Usable Area (WUA) and redd density was higher when using a local habitat suitability curve, instead of the standard curve of BOVEE (1978). However, WUA was high in some units with low redd density. This was due to an over-estimation of the WUA in some types of unit where the gravel taken into account in WUA calculation was scattered between coarse material and thus unusable for trout spawning. This study underlines the importance of using an aggregation criterion when describing the substratum for spawning salmonids.