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ABSTRACT
In semi-intensive aquaculture systems, cultured fish can rely on natural food, produced through fertilization, up to certain size. Beyond this size, supplemental diet becomes imperative to sustain optimum fish growth. The application of supplemental diet to fish ponds should be optimized. In this regard, the present study was carried out to examine the relative importance of top-down (fish predation) and bottom-up (nutrient addition) control to phytoplankton dynamics in earthen fish ponds receiving different rates of supplemental feed for 19 weeks, and polycultured with Nile tilapia, Oreochromis niloticus (L.); common carp, Cyprinus carpio (L.); and silver carp, Hypophthalmichthys molitrix (V.). Ten earthen ponds (0.1 ha each) were used in this study. Each pond was stocked with 1,000 Nile tilapia (13.7±1.2 g), 200 common carp (10.7±1.7 g) and 1,000 silver carp (1.8±0.3 g). The supplemental diet was provided to the ponds at a daily rate of 0.0% (control), 0.5%, 1%, 3%, and 5% of fish biomass. Water quality analyses revealed that water temperature, pH, free ammonia, and water conductivity were not significantly affected by feeding rate, while dissolved oxygen, orthophosphate, and nitrate levels were significantly (P < 0.05) increased with increasing feeding levels. Average Secchi disk reading was significantly (P < 0.05) lower at higher feeding rates. Chlorophyll a content and phytoplankton and zooplankton densities were also directly correlated to feeding rate up to 5%. The maximum growth of Nile tilapia and common carp was obtained at feeding rate of 3% and 5%, respectively, while the maximum growth of silver carp was obtained at 0.5% feeding rate. The best total fish production was obtained at 3% feeding rate. This study suggested that the optimum feeding rate for Nile tilapia, common carp, and silver carp reared in a polyculture system, in earthen ponds is 3% of fish biomass.
