According to optimal foraging theory the prey choice strongly affects the benefitcost ratios. Predators search prey giving the highest benefit and costs of all components of predation (i.e. prey search, encounter, pursuit, capture, and handling) may be considerably reduced if the prey is more available. The study on Cormorant diet and the species composition of prey fish assemblages in the Dobczyce Reservoir (area 985 ha, submontane, eutrophic reservoir in Southern Poland) in spring (May-June) and in autumn (OctNov) showed differences in the food composition and the prey size affected by seasonal changes in fish availability. The diet of Cormorant included eleven fish species and the dominant species in the food was roach in spring (72%) and roach and perch in autumn (49% in total). Roach and perch had the highest share in prey assemblages too (56% in spring, and 53% in autumn). Significant preference toward roach in spring was found. The share of roach and perch did not changed seasonally and could not explain the change in the composition of Cormorant diet. The range of the total length (LT) of fish in Cormorant diet was 3.5–35.2 cm. Diet consisted of distinctly smaller fish in autumn. Relative number of small fish was ca 3 times greater in this period compared to spring. Weighted mean of fish TL in prey assemblage was 20.0 cm for roach and 12.5 cm for perch in spring, and 11.8 and 8.1 in autumn, respectively. The proportion of average weight of roach (W = 0.004074 LT³‧³³⁴) to that of perch (W = 0.005779 LT³‧²⁶⁰) was greater in spring (4.1:1) than in autumn (2.9:1). Probably it can explain the diet shift in autumn. The switch to smaller but more abundant fish in autumn was not related to temperature but to fish availability which reduced the cost of searching and the prey may be easily found.
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