The freshwater habitats of semi‐aquatic carnivores in Europe have undergone substantial alterations due to regulation and construction of artificial watercourses. We compared seasonal estimates of otter Lutra lutra food composition with concomitantly collected data on fish availability and otter holt location in a strongly human‐modified ecosystem comprising the upper stretches of the Wieprz‐Krzna Canal, the longest artificial waterway in Poland, and an associated regulated river. Concrete lining of the canal, extreme water level variation and regular vegetation clearance to enhance water flow substantially limited otter food and shelter resources, yet the canal offered attractive habitats for some fish and hibernating amphibians, while otter resting sites were available at the closely adjacent river. The taxonomic composition of fish in the otter diet, reconstructed based on faecal analysis, was largely reflected in electrofishing catches. Fish communities in our study area were numerically dominated by small‐sized species and otters fed on very small fish (median seasonal total lengths: 46‐54 mm). Either the smaller fish in the population were taken or prey sizes did not differ from those available. The significant contribution of amphibians and waterbirds to otter diet (seasonally 41‐75% in terms of biomass) indicated poor fish supplies for otters in the canal‐river system. High trophic diversity, compared to other otter studies in temperate climatic conditions, indicates that otters facing unstable food conditions exhibit great flexibility in their diet, which may facilitate colonisation of anthropogenic, depauperate environments. Otter occurrence provides little opportunity for conflict with human economic interests in ecosystems that are highly managed for purposes other than fish production. Since anthropogenic habitats are typically poor in some resources, such as shelter structures, their suitability for otters can be enhanced by ensuring good connectivity with nearby less‐disturbed habitats, where the necessary resources can be supplemented.

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