44 MARCH 2015 • WORLD AQUACULTURE • WWW.WAS.ORG Water Quality Water quality was similar in ponds across treatments, but varied somewhat by location (Table 1). Indicators of algal density (chlorophyll a, total N, total P, Secchi disk visibility) were similar in ponds across treatments. Water temperature was somewhat cooler in Kenya (18-25 C) than in Nepal (21-33 C). Alkalinity of pond water in Kenya (274-329 mg/L) was greater than pond water in Nepal (114-162 mg/L), although alkalinity at both sites was sufficient, indicating no need for liming. Dissolved oxygen concentration closely followed diel changes in temperatures in all ponds with maximum values between 1200 and 1600 h. In Cambodia, dissolved oxygen concentration at 1400 h was greatest in ponds receiving fertilizer and feed (7.9 mg/L) than in feed-only (5.5 mg/L) and fertilized-only ponds (6.9 mg/L). A restricted feeding rate regime (Diana et al. 1994; Yi et al. 2001; Waidbacher et al. 2006), adequate fertilization and low stocking density results in a suitable pond environment for culture of tilapia and requires only the identification of limiting nutrients to avoid wastage. A simple static nutrient bioassay (Fitzsimmons et al. 1999, KnudHansen 2006) can be used to determine exact nutrient requirements for low-cost production of tilapia in earthen ponds. Fish Production The best tilapia growth in all locations was achieved by fertilization with feeding at half the satiation level, with a final average weight of about 150 g in five months in Kenya (Fig. 2a) and Cambodia (Fig. 2b). In Nepal, at a lower stocking density (1/m2), tilapia grew to 250 g over the same period (Fig. 2c). Many growth experiments start with fish of 35 g average weight and finish with an average weight of 250-300 g (table size) after five months. High growth performance and yield can be obtained by appropriate fertilization and feeding regime, combined with a low stocking density (Table 2). In Cambodia, feed conversion ratio was also significantly better in fed-fertilized ponds (0.8) than in fed-only ponds (2.2). Green (1992) also reported that feed conversion ratio was better in ponds receiving fertilizers and feed than those receiving feed only. Utilization of natural food by tilapia may decrease with increasing availability of artificial feed, resulting in higher FCR and therefore higher costs (Yi et al. 2001). In Nepal, production costs ranged from US$ 0.95-1.50/kg and was significantly less with fertilized ponds with feeding at half ration than ponds with feeding at full ration. Good growth and feed conversion of tilapia was attributed to an abundance of natural food and improved water quality. Fertilizer plays two major roles in semiintensive production of tilapia: improving fish nutrition through stimulation of natural food and increasing dissolved oxygen levels through photosynthesis. Natural food contains abundant high-quality protein, 55-60 percent on a dry weight basis (De Silva 1993). A combination of fertilizers supplemented with feed at a restricted ration level made tilapia more efficient at utilizing feed because of the availability of supplemental natural foods (Green 1992; Diana et al. 1994, 1996, Yi et al. 2001). Feeding rate can be reduced to 50 percent of satiation, resulting in a low feed conversion ratio, good growth and yield, high economic returns and potential for growth to a greater size. These pond studies demonstrate that fertilization supplemented with restricted feeding was more productive, efficient and economical than fertilization alone or satiation feeding alone in small-scale Nile tilapia culture. Notes Julius O. Manyala, Department of Fisheries and Aquatic Sciences, University of Eldoret, P. O. Box 1125 30100, Eldoret, Kenya Robert S. Pomeroy, University of Connecticut-Avery Point, Connecticut 06340-6048 USA robert.pomeroy@uconn.edu Phanna Nen, Freshwater Aquaculture Research and Development FIGURE 2. Growth of Nile tilapia cultured under different input regimes in (a) Kenya, (b) Cambodia and (c) Nepal.
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