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River damming promotes profound changes in aquatic biodiversity, including the facilitation of biological invasions, with subsequent impacts on native communities. The Eurasian otter (Lutra lutra) is a generalist aquatic predator that often incorporates new prey types when they become available. We studied the short- and long-term effects of river damming on the trophic ecology of the otter across a riverine landscape gradient, from the flooded area to upstream tributaries, focusing on the largest artificial lake of Western Europe. Through the analysis of otter feces (spraints), we describe otter diet composition in 14 sites at different distances from the impounded area in three periods: (i) the pre-filling phase (i.e., lotic conditions); (ii) the immediate filling phase; and (iii) 16 years after the dam closure. The diet of the otter drastically shifted in the short term, from being dominated by native cyprinids to being mainly composed of invasive species of fish and crayfish. In the long term, the number of invasive species consumed by the otter continued to increase while native fish completely disappeared as prey near the reservoir. Presumably, the construction of the dam generated clear spatial gradients of abundances of native and invasive fish species. These patterns arose in the short term but became accentuated in the long term and were mirrored in otter diet composition. Our results demonstrate the potential of otter diet analyses as a surrogate for the impacts of human alterations on aquatic ecosystems, including those of river damming and the associated facilitation of biological invasions.