Agri Kultuur December / Desember 2018 - Page 6

The potential of duckweed as an alternative feed source in tilapia production U Henk Stander Aquaculture Division, Department of Animal Sciences, Stellenbosch University sing fish feeds in aquaculture (sometimes referred to as artificial or aqua feeds) generally increases productivity. Feeds usually constitute the most important operating cost for any farm and efficient growth and food conversion are the main keys for profitability. However, to maximize cost-effectiveness, it is particularly useful in small-scale aquaculture to utilize locally available materials, either as ingredients (raw materials) in compound artificial feeds or as sole feedstuffs. There is also a vital need to seek effective ingredients that can either partially or totally replace marine ingredients (fish meal) as protein sources in animal feedstuffs generally, and in particularly in aquaculture feeds. What potential do duckweed have as an alternative feed option for the tilapia farmer? Food and Feeding Habits of Tilapia: The tilapia group exhibit a great variety of feeding habits and though this diversity, these fish have adapted to fill a number of different feeding niches. This extremely successful exploitation of the environment has enabled them to live together and to feed at different, non-competitive trophic levels ranging from AgriKultuur |AgriCulture species feeding on detritus to the swiftly moving carnivorous forms preying on other living organisms like mosquito larvae. Planktonic algae would appear to be the preferred food of Oreochromis mossambicus and Oreochromis niloticus and are removed from the water by extracting the algae from the respiratory water brought in through the mouth to oxygenate the gills. The algal cells, whether single cells, filamentous chains or aggregate clumps, are entangled by copious secretions of mucus produced by cells lining the buccal cavity. This agglutinates the small, numerous cells, which are then raked back into the alimentary canal by the action of the pharyngeal teeth. This mechanism may be supplemented by a second mechanism which collects small particles that may have escaped the initial mucus entanglement. A series of fine protrusions, the micro-gill rakers, situated on the gill arches may act as a sieve. The gill rakers proper are probably too widely spaced to trap any algae but only the very large particles and therefore serve only to prevent damage to the delicate gill lamellae by debris and sand taken in with the respiratory water. The more closely spaced micro-gill rakers may 6