Effects of fish biomass and planktivore type on plankton communities

We performed a field experiment in 15 fish ponds manipulating the size and biomass of the Nile tilapia, Oreochromis niloticus, to test the hypothesis that the effects of planktivorous fish on plankton dynamics and community structure depend on the planktivore type and fish-stocking biomass. Juveniles and adult tilapia were either stocked alone or combined in the same proportion at 20 and 40 g/m(3). Because tilapias show an ontogenetic niche shift from visual predation on zooplankton to filter (suction) feeding on both zooplankton and phytoplankton, we expected differences in the biomass-related effects of juveniles and adults on plankton communities. Our results show that rotifer abundances were higher in treatments stocked with juvenile tilapia whereas cladoceran abundances were higher in treatments stocked with adults. However, total zooplankton and copepod abundances, chlorophyll a concentrations and Secchi depth were not affected by tilapia size, suggesting that variation in the feeding mode of tilapias affects the structure of zooplankton communities but that this does not cascade down to affect phytoplankton biomass and water transparency. Competitive interactions among the fish constrained their potential to depress zooplankton abundances and increase algal biomass as fish-stocking biomass increased. Competition among size classes of tilapias was highly asymmetric and juveniles were better competitors than adults for plankton resources in our experimental ponds.