The construction of dams is a significant environmental issue due to its impacts on native fish populations and riparian animals and habitats. The response of otters Lutra lutra Linnaeus, 1758 to such disturbances was evaluated using the Aguieira reservoir (Central Portugal) and its associated tributaries as a model. Visiting rates suggested the presence of resident otters, both in the reservoir and in two of the tributaries. There was a significant negative influence of water level and precipitation in the number of spraints found around the reservoir. Nevertheless, marking behaviour was positively related to substrate and negatively related to vegetation cover, though these variables explained only a small amount of variation in otter signs. In the tributaries, although precipitation also negatively affected the number of otter signs found, the availability of rocks in the riverbed and the presence of refuge had a positive influence in marking behaviour. Prey composition and abundance differed between reservoir and tributaries, but the main otter prey in both systems was the pumpkinseed, which is very abundant in the reservoir. Results demonstrate that otters regularly use the Aguieira reservoir, which provides a good prey base, and the associated tributaries, which provide shelter.