| Summary: | The diet of otters Lutra lutra was studied by spraint analysis in six river and stream habitats in eastern Poland. In all of them fish were the staple food of otters, constituting from 29 % to 96 % of prey consumed in particular seasons. In upland (river Tanew) and mountain (river San, streams Dwernik and Głęboki) habitats, bullheads (Cottus gobio) were the most frequently eaten fish throughout the year (with maximum contribution of 57 % in the winter diet on river Tanew). In these habitats otters preyed on total of 10 fish species, including: brown trout (Salmo trutta), stone loach (Barbatula barbatula ), brook minnow (Phoxinus phoxinus) and stickleback (Gasterosteus aculeatus). On the lowland rivers, otters relied on more fish species than on the upland and mountain rivers (14 species on river Czarna Hańcza and 12 species on the upper and lower course of river Biebrza). Beside various cyprinids, otters preyed on stickleback, perch (Perca fluviatilis), mud loach (Misgurnus fossilis), burbot (Lota lota) and pike (Esox lucius). On all rivers, except the lower Biebrza, most of fish (from 70% to 88%) captured by otters were less than 10 cm long. Amphibians and crayfish were two other important groups of prey. On small streams in the Bieszczady Mountains, frogs (mainly Rana temporaria) comprised up to 46% of otter prey. Crayfish (Ortonectes limosus) remains were recorded in otter spraints only on the lowland rivers: in summer crayfish comprised 52% of prey on the Czarna Hańcza River and 47% of prey on the upper Biebrza River. In upland and mountain habitats the food niche breadth of otters was lower (from B=2.62 to B=6.12) than on the lowland rivers (from B=3.10 to B=11.09) and did not differ much between the seasons. On the lowland rivers the food niche breadth was high in winter and low in summer, and the seasonal differences in otter diet were much more pronounced as compared to the upland and mountain habitats.
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