The accuracy of bioassessment programmes is highly limited by the precision of the systems used to derive sensitivity-tolerance values for the organisms used as indicators. We provide quantitative support to the objective evaluation of freshwater fish species sensitivity to different sources of disturbance, accounting for co-variation issues not only between perturbations and natural gradients (especially river size), but also between different perturbations. With this aim, we performed two different principal component analyses: (i) on a general environmental matrix to obtain a perturbation gradient independent of river size effects and (ii) on human impairment-related variables to extract independent synthetic perturbation gradients. Then, we checked each species responses to those gradients to assess their sensitivity-tolerance through an available-used chi-squared analysis in the first approach and through a t-test / ancova analysis in the second one. In this way, we obtained sensitivity-tolerance which could be included in future bioassessment tools, enabling effective evaluations.