Effects of land use on fish assemblages in Patagonian low order streams

Abstract

We assessed the effects of land use on the structure of stream fish assemblages in Northwest Patagonia, Argentina. To this end, a total of 18 sites distributed in 15 streams were sampled during the low water period. At each site, 26 regional and local scale variables were used to describe the environmental conditions. The macroinvertebrate density, and the composition and abundance of fish were then recorded. Statistical associations between local assemblage structures and environmental variables were quantified by canonical correspondence analyses. The results revealed that, at a regional scale, the fish assemblages were primarily determined by watershed features like altitude and catchment area, and by land use effects (exotic forest and pasture). Assemblage compositions were also related to some local-scale features such as water velocity, food availability (density of benthic macroinvertebrates), instream characteristic and riparian features. Also, the observed fish species richness was low. Overall, exotic salmonids largely dominated in the samples both in numbers of individuals (98.9%) and in biomass (99.9%), whereas native fish (Hatcheria macraei and Odontesthes hatcheri) were scarcely represented. Rainbow trout (Oncorhynchus mykiss) was the most abundant fish, and was present in all streams. Native species were restricted to five streams located in the ecotone between the Andes and the steppe, where salmonids were also present. Salmonids were the only fish at Andean streams which suggest the existence of a strong geographic segregation between native and exotic species. Some aspects related to the incidence of the introduced species in the structure of fish assemblages are discussed.