Occurence of Phytoplankton in Stomach Content and Its Selectivity by Nile Tilapia (Oreochromis niloticus L.) Cultured in Fertilized Earthen Ponds
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Nile tilapia (Oreochromis niloticus L.) was cultured in earthen ponds receiving 0, 20, 40, 60 and 100 kg/feddan/month of inorganic fertilizer (20:20:5 N:P:K). The obtained results showed that the composition percentage of food items in stomachs of Nile tilapia was ranked as phytoplankton> detritus> zooplankton at all fish sizes. Zooplankton did not exceed 1.5% of total stomach's contents. The main algal species found in fish stomach belonged to Cyanobacteria, Chlorophyta, Bacillariophyta and Euglenophyta. Detritus consisted mainly of scraps of macrophytes and mud, and its contribution to gut content decreased for increasing fish size, while phytoplankton contribution increased. Bacillariophyta represented the main phytoplanktonic division at small fish size (30-60 g/fish), while Chlorophyta is the dominant group at large fish size (60-70 g/fish). The most frequently genera represented in fish stomach in all treatments were Anabaena, Merismopedia, Microcystis, Nodularia and Oscillatoria (Cyanobacteria), Cerasterias, Chlorella, Crucigenia, Pediastrum, Scenedesmus and Tetraedron (Chlorophyta), Amphora, Cocconeis, Cymatopleura, Cymbella, Gyrosigma, Melosira, Navicula, Nitzschia, Pinnularia, Serurella and Synedra (Bacillariophyta) and Euglena and Phacus (Euglenophyta). Results revealed that Nile tilapia could select Cyanobacteria during the investigation period and sometimes select Bacillariophyta and Euglenophyta.