Abstract

1. The Biobío River basin of south-central Chile exhibits the greatest species richness of all rivers in Chile, where it is one of the most important rivers for human use. Use for provision of drinking water, irrigation, sewage effluents, hydropower generation and industry has increased dramatically during the last decade. To help understand the effects of human activities on the Biobío River, we document recent changes in the fish community. 2. In this study, current patterns of distribution and abundance of fishes were compared with the expected longitudinal pattern, and to historical data from studies conducted before the rapid development of the last decade. Fish distribution, biomass, abundance and diversity were studied at eight sampling stations in the middle and lower zones of the river in both high and low flow seasons. 3. Contrary to the pattern observed in less impacted river systems, species richness (S), diversity (H′) and abundance [calculated catch per unit effort (CPUE)] all tended to decrease downstream from the uppermost sampling locations. Mean S decreased from 7.9 to 5.4, mean H′ decreased from 0.7 to 0.4, and mean CPUE decreased from 111 to 43 from hyporithral to potamal locations. 4. Comparison with previous records indicates loss or reduction in distribution of native species, and a concurrent expansion in distribution and abundance of tolerant introduced species (e.g. Gambusia holbrooki, and Cyprinus carpio) over the last 10-15 years. These comparisons suggest a large-scale and long-term effect of recent human impacts on the river. © 2005 Blackwell Publishing Ltd.