Diet of the Great Cormorant (Phalacrocorax carbo) was studied using regurgitated pellets, individual fish bones and fish remains collected from below roosting trees at Želivka and Slapy Reservoirs, Czech Republic, during summer, a warm winter and a cold winter. Both reservoirs have the same trophic status and similar fish assemblages. Using diagnostic bones (os pharyngeum, dentale, praeoperculare) and our own linear regression equations relating dimensions of the diagnostic bone and fish total length (L_T), a total of 2,055 fish of 18 species and four families were identified in the diet of Great Cormorants and their size was reconstructed. Both fish total length and weight differed significantly between seasons being, on average, 12.0 cm and 30 g during summer, 18.3 cm and 109 g during a warm winter and 22.8 cm and 157 g during the cold winter. The average weight of fish taken by Great Cormorants significantly increased with decreasing air and water temperature. The contribution of the dominant “large growing”, torpedo-shaped fish species in the diet of Great Cormorants dramatically increased from summer to the cold winter. In contrast, the contribution of dominant “small growing”, torpedo-shaped species, or humped body-shaped species, showed completely the opposite tendency. Great Cormorants seem to consume all fish of appropriate size that they are able to catch in summer and select for larger fish in winter. Thus, the winter elevation of foraging efficiency described for Great Cormorants in the literature is due to capturing larger fish not to capturing more fish.