Journal of Aquaculture Research & Development

Abstract

Evaluation of Plankton Community Structure in Fish Refugia Acting as Oreochromic niloticus Propagation and Nursery Units for Rice/Fish Trials, Uganda

Grace A Ssanyu, Joseph Rasowo, Elmada Auma and Moses Ndunguru

To determine the possible success or failure of the propagation system, plankton species diversity and biomass were investigated for 98 days in relation to fish fingerling numbers produced from the fish refugia along rice paddies. The experiment was laid out in a split-plot design, with a rice variety (Kairo 25) as the main plot and method of rice-fish culture (fish refugia) as the sub-plot. The fish refugia were propagating Tilapia fish (Oreochromis niloticus) and were manure only once at the beginning. The results showed that the level of nutrients (nitrate-nitrogen and orthophosphate) was low during the growing season limiting the phytoplankton wet biomass. However, a diverse phytoplankton community was realised with Euglenophyta having the higher number of species followed by Chlorophyta, Cyanobacteria, Bacillariophyceae, Dinophya and Cryptophyta. Among the zooplankton, rotifers were more common than crustaceans. Bachiomonus sp Keliyota sp and Asplanchina sp were the most dominant rotifers while Moina and Cyclopoids were also the commonest crustaceans in the refugia. A high number of fish fingerlings harvested every two weeks from each refugia. The fish refugia (paddy 2) that recorded higher numbers of large sized phytoplankton (Euglenoids and Dinoflagellates), had a higher number of large sized fingerlings harvested. This was attributed to the selective feeding of the fingerlings for smaller zooplankton leaving large size zooplankton that effectively feeds on smaller phytoplankton. It was realised that fish refugia are favourable for propagating and raising tilapia fry due to the presence of a good plankton community. Regular manuring of the fish refugia is envisaged to maintain better plankton community for higher fingerling yield in the rice paddies.