The fish catch composition in the reservoir showed seasonal fluctuations. There were seven fish species captured in spring and summer and only four species in winter. Common carp and trout were captured during spring and summer but not during winter. White bream were captured only during summer, whereas largemouth bass were caught only in spring. The pike-perch was not included in the study because only one individual was captured. Barbel, nase, mirror carp and roach were regularly captured and were abundant in the reservoir all year long. Figure 2 shows seasonal values for IRIs for each food category for these four regularly sampled species. Among all native species, the barbel is the one with the greatest trophic diversity (Table 2). This was the only species whose diet showed statistically significant seasonal changes (Kruskal-Wallis test; χ2 = 10,315; P < 0.01). During winter, barbel was mainly a bottom feeder, with chironomid larvae, benthic algae and detritus as principal food categories in its diet. From winter to summer, barbel increased the consumption of planktonic food items and decreased the consumption of benthic prey. In summer, barbel fed mainly on cladocerans. Accordingly, niche width decreased markedly from winter to summer. The nase (the other common native species) had a narrow dietary breadth, feeding almost exclusively on detritus in all seasons. Only during spring (Fig. 2) was the nase diet supplemented with other food categories concurrently sucked from the sediments (e.g. debris, seeds). During the other seasons the nase showed the lowest trophic diversity of less than as measured by Shannon-Weaver (Table 2)