Study of Austropotamobius pallipes (Crustacea, Astacidae) populations in aforest brook in Normandy. II. Distribution in relation to habitats structures: stability and variability during five years.

Austropotamobius pallipes populations were studied in two sites located dow nstream and upstream in a forest brook in Normandy (Western part of France) during five years (1991-1995). Every year and for each site, crayfishes we re sampled at the beginning of October month by hand nets with small mesh s ize (3 mm) in main facies sequences. Sequences in downstream site are homog eneous with gravels, pebbles and some boulders, shallow running waters. Seq uences in upstream site are more heterogeneous, some of then are rheophilic with gravels, pebbles and scarce boulders with Fontinalis clumps, other on es are lennitic with sand, silt, mud and organic detritus. In downstream site annual crayfish densities are quite similar whatever the sequence and annual variability is high. In upstream site crayfish densiti es change in space and time and are correlated to facies substrate heteroge neity. Spatial segregation between size classes is clearer upstream than do wnstream. Largest adults have a more homogeneous spreading between facies t han smallest individuals. Sex-ratio is stable between sequences and annual variations are weak. In the two sites order of sequences chosen by crayfish adults is stable dur ing years independently of density variations. Variability in choice is hig her in the youngs of the year. Variations of crayfish density between downstream and upstream sites are in dependent. Density is correlated with pebbly area in the youngs of the year and with boulders quantity. Adults are more ubiquitous in their choice and their density less little correlated with boulders quantity. Accidental variations in substratum structure, like packs of branches and o f dead leaves, increase density by creating new shelters. But sand accumula tion above this obstacle is unfavourable for crayfishes especially for smal lest ones. Likewise forestry activities increase the quantity of deposed si lt and consequently crayfish populations density is reduced in upstream sit e during 1994 and 1995. This study shows that crayfish population density is quite stable during th e study period in spite of local variability in space and time. Differences of density between both sites can also partly result in differences in gro wth rates compared to previous data. All these results can be used to manage brook in order to improve favourabl e habitat for crayfishes and to detect deleterious effects of forestry acti vities.