A NEW TOOL TO INTERPRET MAGNITUDE AND DURATION OF FISH HABITAT VARIATIONS

The main result of habitat simulation procedures is a static relations hip between an index of potential habitat, e.g. weighted usable area ( WUA), versus discharge in a study reach representative of a stream. A new methodology was developed to analyse the timing and magnitude of p hysical habitat variations. Three options are presented: (i) the habit at time series; (ii) the habitat duration curves; and (iii) the contin uous under threshold habitat duration curves. The last option is a new procedure to interpret habitat chronicles. It determines continuous d urations during which the total WUA in a study reach was lower than a given threshold. The assumption according to which some durations/thre shold values could represent limiting events for fish population dynam ics is illustrated with surveys of two wild brown trout populations. T he relationship between spawning habitat conditions and the relative d ensity of 0+ the year after was studied. A continuous duration of more than 20 days with spawning habitat conditions lower than 80% of the o ptimum conditions seemed to limit the number of 0+ trout. This procedu re is presented as a tool to interpret natural discharge time series f or management.