Abstract :
Duckweed (Wolffia arrhiza) is a tiny aquatic plant, abundantin nutrient rich wetlands which contain up to 43% protein bydry weight and could be used as feed for rearing Tilapia(Oreochromis niloticus) fry. Tilapia fingerlings are in high demand in the development of inland fish production in the country. The aim of this study was to evaluate the potential of duckweed as feed in fry rearing of Tilapia and to utilise thisplant in a profitable manner. The trial was carried out at National Aquatic Resources Research Development Agency,Sri Lanka for a period of 41 days. Six cement tanks (0.75 m3)were stocked with Tilapia fry (0.0754±0.0441g) according to the stocking density of 500 fry m-3. Fresh duckweed (Dw) and commercial feed (Cf) were provided daily at a rate of 5% ofbody weight. Fish were sampled weekly and the feed amounts were adjusted according to total biomass in respective tanks,assuming there was no mortality. The pH and temperature were measured daily. Nitrite (NO-2), Nitrate (NO-3) and Total Ammonia Nitrogen (TAN) were measured weekly. Average Daily Growth (ADG), Specific Growth Rate (SGR-W), Condition Factor(CF), Final body weight (BWfinal), Feed Conversion Ratio (FCR),% survival rates of Tilapia and Toxic Ammonia (NH3) were determined. The Mean ADG (0.0261±0.0077, 0.0039 ± 0.0020g day-1), BW final (1.1450±0.7390, 0.2880 ± 0.2070 g) and CF(1.7727±1.18, 2.655±2.06) of Tilapia fed on Cf and Dw were significantly different (p 0.05), while SGR-W (6.7324±1.7879,3.9035±0.4696) and % survival rates (44.6±16.35, 25.5±6.74) were not significant statistically. The mean temperature,pH, (NO-2), (NO-3) and NH3 in Cf and Dw provided tanks were not significantly different (P 0.05). FCR of Cf and Dw were1.003±0.6097 and 4.2066±0.2462. Wolffia arrhiza could be used as feed for Tilapia fry rearing but the amount Dw provided(5% of body weight) is not sufficient. Further research is needed to determine optimal amounts. The use of W. arrhiza as fish feed would reduce its nuisance value considerably.
