Browsing by Author "Tadesse, Zenebe"

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    Food and Feeding Ecology of Tilapia, Oreochromis Niloticus L. And Effects of Diet on the Lipid Quality of Fish in Some Lakes in Ethiopia
    (Addis Ababa University, 1998-12) Tadesse, Zenebe; Ailgren, Gunnel (PhD)
    Oreochromis niloticlls L. is commercially the most important fish in Ethiopia. Despite its importance, little information is available on the biology and ecology of this fish in most water bodies in Ethiopia. This thesis is a contribution to understanding the feeding ecology and importance of diet on the lipid quality of fish with emphasis on O. niioticlIs in Lakes Chamo and Langeno. The natural food of O. niloticlIs were studied, based on monthly samples collected, during December 1995 and December 1996 from both lakes. The diet of Chamo fish was mainly composed of phytoplankton. Cyanobacteria, both filamentous (e.g. Anabaena, Anabaenopsis, Raphidiopsis, Oscillatoria) and coccoid forms (e.g. ChrococclIs and Microcystis), were found to be the most important food items. Diatoms, mainly Navicula, and Nitzschia and green algae (e.g. Cosmarium and Scenedesmus) also constituted the diet. Macrophytes were found in abundance during the rainy season while food of animal origin was only encountered occasionally. Significant seasonal variations (ANaYA p25°C) all year round promotes feeding rate and assimilation efficiency of the diet. Total lipids (1.7-21 % of d.w.) and the fatty acid (1.6-9.3% d.w.) contents of O. niloticus dorsal muscle tissue varied significantly between samples taken from five lakes in Ethiopia. Most fish from Lakes Haiq and Chamo contained higher levels of fat (2::10% d.w.) compared to fish from Lakes Ziway, Langeno and Awassa which contained ~5% d.w. fat. The 003/006 ratios ranged from 1.3-7.6 and O. niloticus from L. Haiq showed the highest ratios, 5.1-7.6, indicating that the fat was of high nutritional quality. The reason for this distinction likely mirrors the varied composition of the diet available to the fish in the various lakes. The extent of variation was more pronounce in the herbivore O. niloticus (17.2-208.2 mg g-! d.w.) than the omnivore Em'bus sp.(24.9-94.9 mg g-! d.w.) or the carnivore Clarias gariepinus (18.7-90.8 mg g-! d.w.). Irrespective of the species, the most important FA were palmitic acid (16:0), docosahexaenoic acid (22:6003), stearic acid (18:0) oleic acid (18:1009) and arachidonic acid (20:4006). Aquaria feeding studies have also shown that the FA composition of the fish can be influenced to some extent by the fatty acid content of the diet. Water temperature appears to show little influence on the FA content of this therrnophillic fish. Rather it promotes feeding rate and conversion efficiency of the diet. In general, the success of O. nilotiClis in most lakes in Ethiopia is very likely attributed to the plasticity of the feeding habit of the fish to utilise available food sources in water bodies.