Filtration Rates of Catfish Pond Phytoplankton by Nile Tilapia Oreochromis niloticus

Filter-feeding fishes have been examined as biological controls of phytoplankton populations. Although increased phytoplankton biomass resulted in approximately 50% of studies and no effect in 25% (Smith 1988), changes in species composition and dominance have been noted. For example, blue tilapia Oreochromis aureus reduce or eliminated blue-green (Cyanobacteria) and green (Chlorophyta) net phytoplankton in fertilized, earthen ponds (Perschbacher 1975) and green and golden-brown (Chrysophyta) net plankton in unfertilized, concrete pools (Drenner et al. 1984). In a 24 h aquarium test fingerling blue tilapia removed 66-1 00% of blue-green and 74-90940 of green net plankton (Perschbacher 1975). Nile tilapia Oreochromis niloticus also has the ability to filter-feed on phytoplankton (Northcutt and Beveridge 1988), as do all tilapiine congeners (Trewavas 1983) and Haplochromis species (Greenwood 1953) that possess pharyngeal jaws. At sizes above 50 mm TL, Nile tilapia in ponds consumed primarily phytoplankton (Yashouv and Chervinski 196 1). Diet-related research indicated Nile tilapia prefer blue-green algae. The diet of Nile tilapia is primarily Microcystis (a blue-green alga) and Botryococcus (a green alga) in the Ethiopian lakes Aswa and Zwai, respectively (Getachew 1987), and Microcystis in Ugandan Lake George (Moriarty and Moriarty 1973). Carbon assimi-