The shift to smaller zooplankton in Lake Victoria cannot be attributed to the ‘sardine’ Rastrineobola argentea (Cyprinidae)

Abstract

After the population increase of introduced Nile perch (Lates niloticus) in Lake Victoria, a decrease in the relative abundance of herbivorous cladocerans and calanoids was reported for the pelagic zone. The shift from large herbivores to small-bodied predatory cyclopoids has been attributed to the increase of the zooplanktivorous cyprinid dagaa (Rastrineobola argentea), which rose concomitantly with the Nile perch. However, we observed that in the Mwanza Gulf of the lake cyclopoids dominated the zooplankton community already before the rise of dagaa. Furthermore, there are indications that dagaa takes fewer calanoids than expected from their representation in the environment, although a slight preference for cladocerans may exist. Calanoids dominated the diet of small Nile perch of about 4 cm length, but juvenile Nile perch did not occur in the study area until after the size shift in zooplankton. The lumped biomass of dagaa and small Nile perch is lower than the biomass of the original haplochromine-dominated community of zooplanktivorous fish. Thus, the decline of large zooplankters in the lake cannot be explained by intensified predation after the replacement of zooplanktivorous haplochromines by dagaa and Nile perch. Increased eutrophication, resulting in a shift in predominance from diatoms to cyanophytes, is suggested as an alternative explanation for the shift in zooplankton composition.