The distribution and abundance of many of the 23 native fish species of the Guadiana River basin in Portugal have declined markedly in the past 20 years. This is because of increased pressure to exploit the water resources of the catchment for domestic supply, agriculture and recreation, but also as a result of general habitat degradation and the introduction of exotic species. Thirteen reservoirs have already been constructed in the region and a further 25 proposals have been put forward, mainly to supply the tourist sector in the south of the country. As a result of these activities, one species of fish, Anaecypris hispanica (Steindachner), is critically endangered. The present paper examines the status of the populations of A. hispanica in the Guadiana in Portugal, assesses the impact of water resource schemes on the stocks and general biodiversity, and suggests management options for conserving and enhancing the already degraded stocks. [ABSTRACT FROM AUTHOR]