Seasonal Changes in the Nutrition of Oreochromsniloticus Linn, (Pisces: Cichlidae) in Lake Ziway; Ethiopia

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Addis Ababa University


The quality and digestibility of the natural food of Oreochromis niloticus in Lake ZiHay, Ethiopia, ~/aS studied for a period of one year, October, 1988 to September, 1989. The stomach and rectal contents of 956 fishes I'/ere analyzed for total organic matter, protein, lipid, carbohydrate and energy. Different forms of inorganic nitrogen and or tho-phosphate in the I-later 11ere also determined to assess if there is any relationship I-lith the composition of the diet. Total organic matter (55.7% D.\<I.), protein (4.6% D.W.), lipid (5.3% D.I'1.) and energy (8.7KJ/g D.w.) Here consistently lO~1 for most parts of the year, Hhile carbohydrate (21.5% D.~/') ~ms relatively high. Protein, lipid and carbohydrate accounted for 57.0% of the ash free dry Height (AFDI'Il. Detritus is assumed to contribute most of the unaccountable pOl:tion of the AFDI>I. All nutrients shoHed significant monthly variation (ANOVA, P < 0.0001), hOHever, the variation of nutrients bet~leen sexes Has not significant (ANOVA, P > 0.05). Estimates of assimilation efficiency using ash as a reference marker revealed that 36.7% of the total organic matter I'las absorbed. Protein, lipid, carbohydrate and energy !lad dic;jestibility values of 73.3%, 13.3%, 39.1% and 37.3%, respectively. Assimilation efficiency values ~Iere also estimated for three months using three indigenous markers, ash, hydrolysis resistant ash (HRA), and hydrolysiS resistant organic matter (HROB). Results shol1ed that ash and HRA gave higher values in tHO (August and November) of the three months considered. HROH gave higher values only in one month (Hay). The quality of food expressed as the ratio of digestible protein to Kilo Joule of digestible energy varied significantly over the 12 months considered, and ranged from 6.0 to 117.0. Although the qual! ty of the food appears to be sufficient to support gro~lth during most of the year, 101'1 amount of digestible~ organic matter in the diet seems to restrain the maximum grol1th possible. Condi tiol} of Q. nl.loticus Has variable betl'leen months but not betl'leen seXes. : Honthly food quality and the corresponding condition of fish had no significant correlation (r = 0.012) indicating that fish condition might not be sensitive eno\lgh to detect changes in food q\lality over a short period of time. The overall condition of O. lliloticus in Ln)<e Zi\-my I'las found to be lO~ler than that of Lake lmassa, and this is probably due to inferior quality of the diet and 10l'ler feeding rate. The length-weight relationships of O. niloticus that range from 11. 2 to 31.0 em in size ~Iere fou\\d to be, ~l == 0.028 L2.37I " r2 = 0.906, P < 0.0001 for males and \q = 0.106 L2 •448 , r2 = 0.852, P < 0.0001 for the females. There Has no relationship betl~een the composi tiOll of the diet of O. nilotious of Lake Zi\omy and the nuti:ients (nitrogen and phosphorus) in the lake water.