Assessing the impact of non-native freshwater fishes on native species using relative weight

The aim of the research was to test relative weight (Wr), a condition index which allows evaluation of fish well-being, as a tool to investigate the impact of the presence of non native species (NNS) on the condition of the key native species (NS) of the Tiber River basin (Italy): Barbustyberinus Bonaparte, Leuciscus cephalus (Linnaeus), Leuciscus lucumonis Bianco, Rutilus rubilio (Bonaparte) and Telestes muticellus (Bonaparte). By means of Canonical Correlation Analysis, data from 130 sampling sites, distributed throughout Tiber River basin, were examined. Wr of NS was related to densities of NNS and to environmental variables. Moreover, the correlation between Wr of NS and density of NNS was investigated through linear regression analysis and covariance analysis. Preliminary results encourage the use of Wr as a tool to assess the relationship between NS and ecological factors (such as the presence of NNS) and to explain the changes that occur along the longitudinal gradient of a river.