The Effects Of Stocking Density And Feed On The Growth Performance Of Oreochromis Niloticus In Suspended Cage In Lake Babogaya Ethiopia

The effects of stocking density and supplementary feeding on growth performance of Nile tilapia, O.rnniloticus in cage culture in Lake Babogaya were investigated for five months from February 2007 tornJuly 2007. Juvenile of O. niloticus having a mean body weight of 30 gm and mean length of 12.5 cmrnwere stocked in duplicates of five cages, with stocking densities of 50 (with and with out feed), 75,rn100 and 125 per meter cube. The feed treatments were providing supplementary feeds with arncomposition of 60% sweeping meal, 20% crushed seed cotton and 20% pea flour, with a feeding raternof 2% of the body weight. The feed was divided equally into two and was fed twice a day. Growthrnchange and feed conversion ratio of the stocked fish were studied for the sampling periods.rnThe survival rates of the stocks were high in all treatments and were not affected by difference inrnstocking density. The results of this experiment indicated that significantly higher mean weightrn(MWT) and daily growth rate (DGR) were obtained from the lower stocking densities with feed (50rnfish/m³), than those from the higher stocking densities (125 fish/m³) and the control stock (50 fish/m³)rn(P < 0.05). This result revealed that growth performance was negatively affected by stocking density.rnMoreover; feed had a strong positive effect on growth performance. The gut analysis result indicatesrnthe importance rank of natural feed (phytoplankton: Microcystis aeruginosa, Bacileariacaea andrndinoflagellates) were 1st, 2nd and the 3rd respectively. The biodynamic of the Lake changed after April;rndinoflagellates became more dominant and Microcystis aeruginosa became more decreased, but thernimportance rank remain the same throughout the sampling periods, which indicated O. niloticus isrnselective feeder and Microcystis aeruginosa, and Bacileariacaea are the preferred natural feed in LakernBabogaya.rnThe least growth performance coincided with the decline of primary production, the change inrnbiodynamic of the Lake and the vertical shifting of the natural feed to metalimnetic region. Thernnegative growth of the control stock in May and June indicates that extensive cage culture practice isrnimpossible while the primary production is decreasing and the natural feeds (phytoplankton andrnzooplankton) are shifted to the lower depth (metalimnetic region) from April to September unless thernsize of the cage is big enough to reach to the feed richer site of lower depth, but this has to take thernDO concentration into consideration.rnA nematode parasite worm Contracaecum sp. with length of 2.6 to 4.5 cm and 5 to 7 in number perrnfish were identified in the gill cavity of the stocked fishes with size of 60 –100 g; however, frequentrnlonger sized worms were not common in natural wild fishes of comparable size. But these parasitesrnrarely occurred in bigger sized wild fish. The competition for feed of the wild fishes was very highrnwhich bring stress and competes for feed and space; during sampling and harvesting times betweenrn17 – 89 % extra wild fishes were identified. Relatively improved growth was observed in those cagesrnthat were fixed toward shore side and in shallow water than the front lined cages that were hanged inrndeeper position which indicates the position of the cages has effect on growth performance.rnKey words / phrases: Cage culture, Lake Babogaya, stocking density

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