Abstract

The Nonguén stream (order 3) is a lower zone tributary of the Andalién river coastal mountain basin (Bio-Bío region) which displays a marked difference between its rithral sector (well-conserved and with a secondary Nothofagus forest) and its potamal area (urbanly-used and organically polluted waters). This contrast could imply an alteration in the normal pattern of Chilean ichthyofauna, as described for a variety of fluvial systems in the country: a downstream increase in numbers of species, individuals, and diversity downstream, as well as increased corporal sizes of individuals in the lower zones of the river. Species composition of fishes of the Nonguén stream and their community patterns along the fluvial system within an annual cycle have been studied. The results show a high number of species in the lower parts, i.e. four species in the rithron and ten in the river mouth. The diversity, abundance and distribution of the fishes varied according to the season of the year, reflecting displacements associated with the fauna' s ontogeny, reproduction and feeding. The sampling stations of the middle and lower zones of the river presented an assemblage with smaller individuals than those found at the head of the river, with the consistent dominance of larger salmonidae in the upper zone. Accordingly, it is suggested that: (i) the alteration of the potamal area does not significantly modify the characteristic patterns of ichthyofauna in fluvial systems, and (ii) the Nonguén stream is a reproduction and growth system for juveniles of both native and introduced species which make up permanent associations in the fluvial system.