A new tool to interpret magnitude and duration of fish habitat variations

The main result of habitat simulation procedures is a static relationship 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 physical habitat variations. Three options are presented: (i) the habitat time series; (ii) the habitat duration curves; and (iii) the continuous under threshold habitat duration curves. The last option is a new procedure to interpret habitat chronicles. It determines continuous durations during which the total WUA in a study reach was lower than a given threshold. The assumption according to which some durations/threshold values could represent limiting events for fish population dynamics is illustrated with surveys of two wild brown trout populations. The relationship between spawning habitat conditions and the relative density of 0 + the year after was studied. A continuous duration of more than 20 days with spawning habitat conditions lower than 80% of the optimum conditions seemed to limit the number of 0 + trout. This procedure is presented as a tool to interpret natural discharge time series for management.