Culture of Azolla in a pond, nutrient composition, and use as fish feed

The aquatic fern Azolla can grow without nitrogenous nutrients because of the presence of the N-2-fixing ability of symbiotic Anabaena, Azolla filiculoides Lam, was cultured in a pond and its potential as a fish feed was examined. The extent of growth in the pond (Komoike pond, Sakai) of 1,200 kg/100 m(2) was less than that in a synthetic nutrient solution and in a diluted secondary treated effluent, The gradual decrease of the growth rate in the pond water was mainly due to phosphorus deficiency. The high level of crude protein of Azolla and higher lysine concentration indicated that Azolla plants may be a good source of fish feed. A significantly higher content of dehydro-ascorbic acid (1,909 mg kg(-1) dry weight) than that of L-ascorbic acid (10 mg kg(-1) dry weight) in Azolla grown for 3d was observed. There was a gradual increase in the ratio of linolenic acid with aging in the composition of fatty acids in Azolla, In a feeding experiment with Tilapia nilotica, a diet containing 20.7, 34.4, and 48.2% of the total weight of dried Azolla, given for 3 weeks, led to a decrease of the fish weight gain of 5.2, 16.8, 17.1% compared with the control, respectively.