Abstract

The area of distribution of the two native crayfish in France: Austropotamobius pallipes Lere. and Astacus astacus L. found to overlap in the Morvan. Formerly the two species could be found in the same stream of this granitic country. Today such a finding is rare, but we found such a biotope. We tried to characterize and compare the biotic and abiotic parameters in the area of each species, using simple technics in order to set up, simultaneously, a method for the quantitative description of the crayfish's habi­tats in a small brook. A. pallipes settle upstream and A. astacus downstream. Despite the proximity of each habitat, the two species never cohabit, in addition, between each species' habitat there is a 'no mans land'. The brook is narrower, smaller and deeper embanked in the A. pallipes region where there are many excavations; on the other hand the speed of the current near the bottom and the slope are of the same order of magnitude in each zone. The bed is made mainly of blocks in the A. pallipes sector and of stones and gravel where A. astacus is living. Mosses are more frequent in the A pallipes area. The study of daily fluctuations of pH and dissolved oxygen in the hot season does not show many differences between the two zones, but one can only observe time changes for the apparition of the upper and lower values according to the exposure of the site to the sun. On the contrary, water is always colder in summer time in the A. pallipes area, the gradient between day and night reaches 8°C during a very hot day. A small quantity of Salmo fario L. is the only fish which lives with crayfish. Quantitative sampling of bottom fauna shows differences between the two species: Plecoptera is the most abundant invertebrate in A. pallipes area an Gammarus pulex L. is the most important animal among the A. astacus sector. Many old observations show that A. astacus can replace A. pallipes in brooks, but when the two species are present in the same stream they do not live in sympatry; A. pallipes chooses the zone with the more accentuated torrent character, generaly upstream. It would have been helpful to continue studies in our brook focusing on the behaviour of the two species when they come in contact. However the cutting down of a deciduous forest, on the left bank of the brook and timbering brought sand in the A. astacus area and impoverished the biotope. Fir trees reforestation made on the same deciduous forest will certainly bring unfavourable changes in this brook already at the lowest limit of oligocalcy for crayfish.

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