Cyprinid predation on zooplankton along the longitudinal profile of a canyon-shaped reservoir

In the deep, elongated Rimov Reservoir (South Bohemia, Czech Republic), the diet composition of the three most abundant cyprinid species, as well as the fish and zooplankton distribution, were investigated at three sites (dam, middle, tributary) located along the longitudinal transect of the reservoir, and during two periods of the year. The studied fish were roach (Rutilus rutilus) and bream (Abramis brama) of two length classes (10-18 cm SL and >18 cm SL), and bleak (Alburnus alburnus) of one length class (10-18 cm SL). The diets of all cyprinids along the entire longitudinal transect constituted almost exclusively crustacean zooplankton. In May, Daphnia galeata was a highly preferred prey item of all fish, except for those of the 10-18 cm length class in the tributary station, which consumed mainly bosminids. In August, bleak and roach of both length classes preyed on D. galeata and Leptodora kindtii. Larger bream (>18 cm SL) consumed these prey items as well and, moreover, fed significantly on Diaphanosoma brachyurum. Proportions of L. kindtii in the cyprinid diets were particularly conspicuous in the dam and middle sites. In the tributary site, cyclopoid copepods were an important prey item in the diets of both length classes of bream, in contrast to their infrequency in the diets of roach and bleak. Densities of epilimnetic zooplankton declined gradually downstream in the reservoir, they were about two-fold higher in the tributary site compared to the dam site in both months, respectively. A relatively higher total fish catch per unit effort was found in the tributary site in comparison to the dam or middle site, and a decreasing mean size of D. galeata in zooplankton was observed from the dam to the tributary site in both months. Such results indicated stronger fish predation pressure in the upstream part of the reservoir during the two periods of the year. Moreover, lower diet overlaps within the cyprinid assemblage of the tributary site than within the cyprinid assemblages of the dam and middle sites suggested intense competition for food at the upstream end of the reservoir.