Pd/a Crsp Eighteenth Annual Technical Report

An experiment was conducted to test using effluents from intensive hybrid catfish (Clarias macrocephalus × C. gariepinus) ponds as nutrient inputs for Nile tilapia (Oreochromis niloticus) culture, which would reduce effluent effects from catfish culture, gain extra fish production at low cost, and possibly make aquaculture more profitable to farmers. Three treatments were done with three replicates each in seven 200-m2 earthen ponds at the Asian Institute of Technology (AIT), Thailand, for 87 days. The treatments were A) catfish alone (control); B) catfish and tilapia without artificial water circulation; and C) catfish and tilapia with artificial water circulation. The pond used for control treatments was partitioned by 1.0-cm mesh plastic net into three equal compartments with 67 m2 each for the replicates. The six other ponds were partitioned into two compartments: 1/3 for catfish and 2/3 for tilapia. Sex-reversed all-male Nile tilapia were stocked at 2 fish m-2, and catfish at 25 fish m-2. Catfish were fed twice daily with commercial pelleted feed at rates of 3 to 10% body weight per day. During the first month, tilapia compartments were fertilized weekly using urea and triple superphosphate (TSP) at rates of 28 kg N and 7 kg P ha-1 wk-1. In the artificial water circulation treatment, the water in the catfish compartment was continuously circulated by a submersed pump to the tilapia compartment at a rate of one exchange per week, starting the second month. There were no significant differences in growth performance of catfish among all treatments. Mean catfish weight at harvest ranged from 237.8 to 249.0 g, giving extrapolated net yields of more than 200 t ha-1 yr-1. Final mean tilapia weight was 114.9 g in treatment B and 115.0 g in treatment C without significant differences. Although there was no significant difference in survival rates between treatments B (92%) and C (70%), high mortality of tilapia was observed in two replicates of treatment C due to heavy waste loading in the tilapia compartment by artificial water circulation. Extrapolated net tilapia yields (7.2 ± 1.3 t ha-1 yr-1 in treatment B and 4.9 ± 0.3 t ha-1 yr-1 in treatment C) obtained by using catfish wastes in this study were comparable to those achieved in organically and inorganically fertilized tilapia ponds. Nutrient budgets showed that total nitrogen and total phosphorus contents in pond effluents in treatments B and C were significantly lower than those in treatment A. Nile tilapia recovered 3.30 and 2.12% of total nitrogen and 1.29 and 0.84% of total phosphorus from feed wastes and fertilizer inputs in treatments B and C, respectively. Concentrations of total Kjeldahl nitrogen (TKN), total phosphorus (TP), and soluble reactive phosphorus (SRP) were also significantly lower in treatments B and C than in treatment A. This experiment indicates that Nile tilapia can effectively recover nutrients contained in wastewater of intensive catfish culture and suggests that natural water circulation between catfish and tilapia compartments can reduce nutrient contents in pond effluents and is cost-effective. EIGHTEENTH ANNUAL TECHNICAL REPORT 88 0 50 100 150 200 250 300