Habitat use of the crayfish Austropotamobius torrentium in small brooks and in Lake Constance, Southern Germany

Habitat use of Austropotamobius torrentium was studied in two small brook systems winding through woods and open grassland and in Lake Constance at the outflow of the river Rhine. Flow velocities in the brooks ranged from less than 1 cm/s to 66 cm/s and resulted in average flow discharges between 1.8 l/s and 27.4 l/s. Flow velocities in the Rhine were between 30 and 62 cm/s. All 5 sites inhabited by crayfish had a slightly basic pH, high Ca2+-concentrations and a Saprobial index of about 2 indicating moderate pollution. Two brook stretches were not occupied by crayfish. In one case an eutrophic lake influenced the downstream brook stretch causing an increase in the Saprobial index to 2.5. Further downstream a pipe inhibited the upstream-invasion of crayfish. The second stretch was an artificial and monotonous channel with high flow velocities. Food selection experiments with submersed flora and fauna collected in the habitat showed that the stone crayfish fed on macroinvertebrates as well as on small fish, submersed macrophytes and dead leaves. At low temperatures (7°C) their food was dominated by plants while at higher temperatures (14°C) they consumed more macroinvertebrates than plants. It is concluded that stone crayfish inhabit a variety of running waters and that a main threat to this species is channelling and obstructions of flow ways.