Int. J. Biosci.2(12), 105-111, December 2012
The mechanisms by which Nile tilapia (Oreochromis niloticus) grow under Azolla cover (AC) in semi-intensive system was evaluated for 90 days using structural equation modelling and pathway analysis. The following Azolla cover (AC) extensions were applied: 0%, 15%, 30%, 45%, 60%, 75% and 90% of the surface. Fingerlings (initial mean weight: 15.8 ± 0.2 g) were additionally fed with the same diet formulated with local ingredients and containing 20% of Azolla meal. To quantify the relative contribution of AC and artificial feed in growth of fish, two models were defined, in which: (1): Azolla in ponds was considered as direct food for fish and a macrophyte on water surface; (2): Azolla acts as macrophyte only. Pathway analysis was done using standardized fortnight data and a coefficient was calculated for each causal relationship.Survival rate did not show any significant difference and values were higher than 84%. From the two competing models, indirect effects of AC on fish growth via phytoplankton biomass was similar and was the most important in the first month of experiment, the average ratio calculated being -0.68. As fish size increases, phytoplankton becomes limited and feeding on artificial food progressively grows in importance and influences growth. The coefficient of this direct effect on fish growth can reach 1.36, whilst the indirect effects from AC was -0.58 at the same period. The ratio of the relative contribution of the direct effect of feed versus the indirect effect of AC on Nile tilapia growth was therefore 1.36/-0.58. Summary, in ponds covered with Azolla, fingerlings of Nile tilapia fed agricultural by-products feed primarily on phytoplankton at the beginning, and artificial food gains prominence with time, beneficially in tropical aquaculture where there is a need to reduce the feed cost in that enterprise.
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