Cambios en las condiciones físicas, químicas y faunísticas de un sistema fluvial (río Oma, Bizkaia), y desaparición de una población de cangrejo autóctono (Austropotamobius pallipes) : ¿causa y/o efecto?

The Oma (Oka basin, province of Biscay) is a relatively little perturbed river, which runs through places where the main activities are forestry, agriculture and cattle raising. The native crayfish Austropotamobius pallipes was well established on it but in the summer of 1995 this population had disappeared. Crayfish mass mortalities that could have been due to the crayfish plague were not detected. The main objective of this work is to study the changes of abiotic and biotic summer conditions of the Oma River from 1994 to 2001. The Oma River conditions are compared with the general conditions of the close fluvial reaches inhabited by native crayfish. In addition to the disappearance of the crayfish population, changes in water chemistry and faunal composition of the benthic community were detected. Temporal variations are described in this work and the possible causes are also discussed, especially those related with the extinction of the protected species (predators' influence, decrease of the habitat quality by leaches, works on the riverbed). We have been able to detect the main cause for the mortality and/or disappearance of crayfish in other close fluvial zones, but not in the Oma River. In this stream it could have been due to the synergic interaction of several factors. However, the temporal evolution of the river system in the last years seems to show a trend to recover the conditions of the river on both physicochemical quality and faunal composition, that had in 1994.