The Effects of Stocking Density and Feed on the Growth Performance of Oreochromis Niloticus in Suspended Cage in Lake Babogaya, Ethiopia
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Date
2008-06
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Addis Ababa University
Abstract
The effects of stocking density and supplementary feeding on growth performance of Nile tilapia, O.
niloticus in cage culture in Lake Babogaya were investigated for five months from February 2007 to
July 2007. Juvenile of O. niloticus having a mean body weight of 30 gm and mean length of 12.5 cm
were stocked in duplicates of five cages, with stocking densities of 50 (with and with out feed), 75,
100 and 125 per meter cube. The feed treatments were providing supplementary feeds with a
composition of 60% sweeping meal, 20% crushed seed cotton and 20% pea flour, with a feeding rate
of 2% of the body weight. The feed was divided equally into two and was fed twice a day. Growth
change and feed conversion ratio of the stocked fish were studied for the sampling periods.
The survival rates of the stocks were high in all treatments and were not affected by difference in
stocking density. The results of this experiment indicated that significantly higher mean weight
(MWT) and daily growth rate (DGR) were obtained from the lower stocking densities with feed (50
fish/m³), than those from the higher stocking densities (125 fish/m³) and the control stock (50 fish/m³)
(P < 0.05). This result revealed that growth performance was negatively affected by stocking density.
Moreover; feed had a strong positive effect on growth performance. The gut analysis result indicates
the importance rank of natural feed (phytoplankton: Microcystis aeruginosa, Bacileariacaea and
dinoflagellates) were 1st, 2nd and the 3rd respectively. The biodynamic of the Lake changed after April;
dinoflagellates became more dominant and Microcystis aeruginosa became more decreased, but the
importance rank remain the same throughout the sampling periods, which indicated O. niloticus is
selective feeder and Microcystis aeruginosa, and Bacileariacaea are the preferred natural feed in Lake
Babogaya.
The least growth performance coincided with the decline of primary production, the change in
biodynamic of the Lake and the vertical shifting of the natural feed to metalimnetic region. The
negative growth of the control stock in May and June indicates that extensive cage culture practice is
impossible while the primary production is decreasing and the natural feeds (phytoplankton and
zooplankton) are shifted to the lower depth (metalimnetic region) from April to September unless the
size of the cage is big enough to reach to the feed richer site of lower depth, but this has to take the
DO concentration into consideration.
A nematode parasite worm Contracaecum sp. with length of 2.6 to 4.5 cm and 5 to 7 in number per
fish were identified in the gill cavity of the stocked fishes with size of 60 –100 g; however, frequent
longer sized worms were not common in natural wild fishes of comparable size. But these parasites
rarely occurred in bigger sized wild fish. The competition for feed of the wild fishes was very high
which bring stress and competes for feed and space; during sampling and harvesting times between
17 – 89 % extra wild fishes were identified. Relatively improved growth was observed in those cages
that were fixed toward shore side and in shallow water than the front lined cages that were hanged in
deeper position which indicates the position of the cages has effect on growth performance.
Key words / phrases: Cage culture, Lake Babogaya, stocking density
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Keywords
Cage culture, Lake Babogaya, stocking density