Despite the fact the plankton net used was not the most appropriate for rotifers capture, we collected several congeneric species, mainly Brachionus spp., which are organisms of relatively large size. Rotifers are the predominant group of zooplankton found in the majority of reservoirs, constituting more than 60% of the total zooplankton present (Armengol 1982; Marneffe et al. 1998; Rodriguez and Matsumura-Tundisi 2000). However, in our study, they would have been underestimated by the net mesh size used. For this reason, in the Prado Reservoir, rotifers were less abundant than other zooplankton components, representing less than 20% (Fig. 2).