Seasonal changes in fish community structure of intermittent streams in the middle reaches of the Guadiana basin, Portugal

Seven sites on three tributaries with intermittent flow regimes in the midd le reaches of the Guadiana, i.e. Xevora, Caia and Degebe, were sampled appr oximately every 3 months from April 1995 to April 1997. The density and bio mass of fish were greater in downstream rivers and sites. Along the Xevora River, a series of point abundance samples showed evidence of fish zonation along the river. Canonical correspondence analysis (CCA) suggested a simpl e relationship between the different species and their preferred habitat. T he small-sized Rutilus alburnoides and Leucisus pyrenaicus used most or all of the habitats with some cover and flowing water. Greater concentrations of larger fish were found in downstream reaches which generally had greater depths as stream width increased: large Barbus microcephalus, B. steindach neri and B. comiza preferred such sites on the Degebe River. Droughts in se asonal Mediterranean streams such as the Guadiana River are a predictable n atural disturbance. In the summer, flow ceases and some stretches of river consist of isolated pools. This results in intense aggregations of fish and possible competition for food and/or space. Although some species are well adapted to natural droughts, habitat degradation and possibly the introduc tion of exotic species contribute to marked variability in species composit ion. (C) 1999 The Fisheries Society of the British Isles.