Assessment of types and abundance of live food for fish farming in Makoba Earthen ponds, Zanzibar, Tanzania
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Surveys of naturally-occurring live food for fish in Makoba earthen ponds, Zanzibar were conducted from November 2001 to August 2002. This involved identification of dominant plankton in the water column as well as the composition of algal mats. The effect of nutrients on the composition and abundance of the organisms was also assessed. Protozoa were found to be the dominant zooplankton in the water column, followed by rotifers. The most abundant phytoplankton genera were Prorocentrum (13.4%), followed by Coscinodiscus (10.4%) and Diplopsalopsis (7.5%), while the benthic algal mats were dominated by cyanobacteria, mostly Spirulina (22%) and Oscillatoria (18.4%). To complement the surveys, a time-series experiment was conducted for 21 consecutive days to assess the dynamics of plankton in earthen ponds and simulation containers, using a fertilisation rate of 83 kg dry matter/ha/day. Protozoa were the most dominant zooplankton in the earthen ponds. Fertilisation with chicken manure was effective in increasing the number of rotifers in simulation containers, but not in the earthen ponds; this was probably because, unlike the earthen ponds, the simulation containers were sheltered from external influences such as rain, flooding by tide water, etc. The phytoplankton genera Pyramimonas (16.7%), Biddulphia and Microcystis (8.3%) dominated the fertilised ponds. Chlorophyll-a concentration ranged from 1.8 to 16.9 mg/l, whereas soluble reactive phosphorous (SRP) and ammonia-N reached maximum concentrations of 1.4 mg-at.P/l and 9.7 mg-at.N/l respectively. Overall, salinity and fertilisation played a major role in controlling the abundance of live/natural food in the system.