Survival of natural populations of Austropotamobius pallipes in rivers in Bizkaia, Basque Country (North of Iberian Peninsula)

Some relict populations of the native crayfish Austropotamobius pallipes have been located in rivers in Bizkaia, (Basque Country, Spain), and its population numbers and dynamics, and habitat conditions have been studied for three years. The first descriptive results are given in this paper. The native crayfish populations must be considered residual because of the disrupted area distribution and highly fluctuating demography of the species. Up to now, the species has been located in more than thirty fluvial areas of relatively high slope and shallow and good quality water. Population characteristics (sex ratio, length and weight relations and length frequency classes) are studied in nineteen cases. Maximal relative population numbers are about 100 captures per hour ; these values are correlated to variables of conductivity, hardness, and concentrations of nitrates, nitrites, magnesium, potassium and ammonium. The degree of mineralization must reach a minimum level and, within the values found in the studied rivers, its increase favours the population of crayfish. Management measures to conserve native crayfish must include the protection and improvement of their habitat, prevention of access to it and to the commencement of a genetic study to palliate the phenomenon of endemism. It would also be highly recommendable to begin experimental restocking of riverbeds now without crayfish fauna and with apparently optimum conditions for the establishment of populations of autochthonous crayfish