Fish assemblage changes along a trophic gradient induced by agricultural activities (Santa Lucía, Uruguay)

► The structure of the fish communities reflected the trophic status of the reaches. ► Oligotrophic reaches exhibited a higher evenness and bigger individuals. ► Oligotrophic reaches assemblage presented at least 40% of sensitive species. ► Eutrophic reaches showed the opposite community features. ► The use of fish assemblage composition is valuable for stream ecosystem assessment.