Fish ecology of a seasonal lowland stream in temperate South America

El Pescado (literally `the fish') is a lowland stream in the pampean plain, entering the Rio de La Plata at 35 degrees 55'S, 57 degrees 45'W, 70 km SW of the city of Buenos Aires. The stream develops as a potamon with variabl e flood frequency along its 36 km length. The water is slightly soft, with relatively high conductivity and dominance of HCO3-, Na+ and Cl- ions. Thre e fish families, Characidae (29%), Pimelodidae (14.6%) and Loricariidae (12 .8%) are most abundant among 55 species. Differences in species composition among four stations with different environmental traits were observed. Flo oding slightly affected the fish fauna. In open-water and rocky-bottom stat ions, both species number and number of individuals were correlated with co nductivity, an indication of the decrease of fish density with floods. Spec ies richness was well over worldwide means. Diversity values, higher than i n small tropical streams in Venezuela and southern Brazil, ranged from 0.32 to 2.59, being higher during autumn and spring. The stream has a high pers istence, with the composition of the fish community maintained or recoverin g as soon as flooding ends. One Synbranchus marmoratus specimen was found a mong rocks, the first record of this behaviour for the species.