The seasonal diet and size of fish prey of the otter Lutra lutra were studied in an almost pristine river of North Portugal (Paiva river) through the analysis of spraints collected during 1 year. To identify Iberian freshwater fish species ( Salmo trutta , Micropterus salmoides , Lepomis gibbosus , Luciobarbus bocagei , Cyprinus carpio , Carassius auratus , Pseudochondrostoma polylepis , Achondrostoma arcasii , and Iberocypris alburnoides ) consumed by the otter keys based on scales and vertebrae were first developed and tested. Both keys allowed identification of all fish species, although three Cyprinidae species could not be discriminated ( P. polylepis , A. arcasii , and I. alburnoides ). Regression equations were calculated to estimate the dimensions of preyed fish. Highly significant linear relationships were obtained for the standard length versus vertebrae measurements, for all fish species, except for C. carpio . Fish was the predominant prey category in autumn and winter, but the otter also preyed on amphibians, reptiles, mammals, and birds, particularly during spring and summer. In general, the otter consumed high proportions of small size fish (3–12 cm for the three indiscriminate Cyprinidae, 3–9 cm for C. carpio , and 6–12 cm for S. trutta ).