Fish zonation in a Mediterranean stream: Effects of human disturbances

Abstract.

The fish assemblage and limnological features along twelve sites from the Terri River basin (Catalonia, Spain) were sampled quarterly from August 1999 to May 2000. Twelve fish species were captured, of which four were native and eight exotic. Correspondence analysis revealed that spatial variation accounted for most of the variation (73.7%) in fish species composition. The upstream, urban area was dominated by chub (Leuciscus cephalus) and common carp (Cyprinus carpio) with some exotic species coming from nearby Lake Banyoles, whereas Mediterranean barbel (Barbus meridionalis) predominated in the rest of the Terri River. Along the longitudinal gradient, fish assemblage displayed discontinuous variation and disappearance in more polluted areas. Fish abundance was significantly correlated with summer oxygen concentration and decreased in downstream sampling sites. Species richness was higher in downstream sampling sites due to the proximity of the Ter River. Common carp size increased progressively along the course of the Terri River, whereas the pattern for the eel was the opposite, with the smallest found in downstream sampling sites and a gradual increase in size in the upper reaches. The natural zonation in the fish assemblage of the Terri River is presently altered to a high degree by habitat degradations, pollution, and dispersal of exotic species.