An improved integration of the aquatic fern Azolla in diets for production of Nile tilapia O. niloticus in rural area was investigated in tanks and in ponds. Six diets (29.2% CP; 16.9 kJ g-1 GE, each) were tested, containing 0% (diet A0), 10% (diet A10), 20% (diet A20), 30% (diet A30), 40% (diet A40) and 50% (diet A50) of Azolla meal (AM) as fishmeal (FM) substitutes. The diet A0 served as a control. Growth parameters decreased, significantly when Azolla level exceeds 10% and 20% in diets, respectively in tanks and ponds. In ponds, the variation of annual fish production follows the trend in growth parameters, with values decreasing from 8220 to 5120 kg ha-1 year-1. Identical profitability index (range: 0.42-0.89) was obtained up to 30% Azolla. Feeding with A20 could be more advantageous by rearing fish in ponds covered with Azolla mat at a maximum of 45% of the water surface. In tanks and in ponds, fish carcass was not significantly affected, except for the total lipid content of fish fed 30, 40 and 50% Azolla in tanks, and those fed 50% Azolla in ponds. From an environmental point of view, phosphorus and nitrogen contents in experimental fish show similar values, both in tanks and ponds, although phosphorus content in diets varied from 1.13% to 0.76% when Azolla level increased from 0% to 50%. In tanks, phosphorus (P) and nitrogen (N) discharged as wastes by fish decreased with increasing Azolla level in diets. Also, P decreased significantly in ponds whereas identical values were found for nitrogen in all treatments. In tanks, fish fatty acids mirror the profile in diets, except for the high arachidonic acid content (ARA) in high AM-fed fish, probably as a result of elongation and desaturation of linoleic acid. This leads to a lower n-3/n-6 ratio (range: 0.08-0.17) in fish fed A40 and A50. Fatty acid composition of fish fed in ponds was different from that in fish fed in tanks, and from diets. The lower resemblance of fish composition was more pronounced in fish fed A30 to A50 in which large amounts of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) were found, reflecting closely the composition in plankton. The reason could be a supplemental consumption of plankton by those fish as a response to the deficiency in diets A30-A50. Then, the n-3/n-6 ratios (range: 0.35-0.53) were higher in fish fed A20 to A50. Growth and fatty acid composition were better in ponds, and nutrients loading into pond water were lower, probably due to the contribution of natural foods. In conclusion, Azolla use leads to reduced feed and total fish production costs, while ensuring a suitable pond trophic level and a good nutritional quality of the fish produced. The study suggests using Azolla in tanks up to 10% in diets, while up to 20-30% can be recommended in ponds. In the latter case, covering the ponds surface with Azolla mat at a maximum of 45% of its surface could lead to produce fish, while ensuring Azolla mass production for diets and other beneficial utilization.
|Date of Award||19 Dec 2007|
|Supervisor||Jean-Claude MICHA (Supervisor), Patrick KESTEMONT (Jury), François Darchambeau (Jury), Emile D. Fiogbé (Jury) & André Buldgen (Jury)|
- Azolla cover
- Oreochromis niloticus