Production and economic benefits of reduced feed inputs and addition of Indian carp (ROHU) on Nile tilapia growout in ponds

The aim of these studies was to determine if reductions in feed inputs and introduction of a native Indian carp, Rohu (Labeo rohita), can increase economic benefits of tilapia culture in earthen ponds in Bangladesh. An on-station pond trial was carried out for 150 days at the Fisheries Field Laboratory, Bangladesh Agricultural University, Mymensingh, Bangladesh. Ponds consisting of four treatments (T1, T2, T3, and T4) with four replications each were stocked with sex-reversed Nile tilapia (Oreochromis niloticus, 5 fish/m²) without (T1) or with (T2) addition of Rohu (0.25 fish/m²) and fed a full daily ration of feed (CP - 35% protein; 10-3% body weight/day. Ponds were fertilized weekly (28 kg N and 5.6 kg P ha/week) in the other treatments and tilapia were grown in the absence (T3) or presence of Rohu (T4) at half the daily feed ration as T1 and T2.  Pond water temperature, transparency, dissolved oxygen, nitrate-nitrogen (NO₃-N), nitrite-nitrogen (NO₂-N), ammonia-nitrogen (NH₃-N), phosphate-phosphorus (PO₄-P) and chlorophyll-a did not vary among treatments, while pH was slightly lower in T1 than the other treatments, but well within the suitable range for tilapia growth.  A total of 27 genera of phytoplankton and 12 genera of zooplankton were identified from the pond water samples.  Total phytoplankton levels were highest in the T4 and T3 groups. The survival rates (%) of tilapia were 81.06 ± 1.03, 76.89 ± 1.28, 76.24 ± 2.06 and 75.29 ± 2.45 in T4, T3, T2, and T1 groups, respectively. The specific growth rate (% day^-1) of tilapia was higher in the T3 (1.87 ± 0.00) and T4 (1.85 ± 0.03) than the T2 (1.76 ± 0.05), and T1 (1.71 ± 0.06) groups (p < 0.05). Feed efficiency was significantly better in the T3 and T4 groups relative to those treatment fish fed the full ration (p < 0.05) with feed conversion ratios of 0.49 ± 0.03, 0.47 ± 0.03, 1.13 ± 0.11 and 1.23 ± 0.16_, for the T4, T3, T2, and T1 groups, respectively. Gross production of tilapia was higher in the T4 (5,385.23 ± 276.98^a kg ha^-1) followed by T3 (5,340.62± 156.47^a kg ha^-1), T2 (4440.99± 440.04^b kg ha^-1) and T1 (4089.83± 518.46^b) groups, respectively. Rohu gross production was similar among the T2 and T4 groups.  A significantly higher net return (BDT 743,977 ha^-1 with benefit cost ratio of 2.92) was found in T3 followed by T4 (BDT 673,750 with benefit cost ratio of 2.72), T2 (BDT 286,469 ha^-1 with benefit cost ratio of 1.49) and T1 (BDT 226,675 ha^-1 with benefit cost ratio of 1.37) groups, respectively. Based on the higher net return and benefit-cost ratio, it may be concluded that fertilization with half feeding was substantially more cost effective over standard full feeding for growout of tilapia. Moreover, tilapia growth was little impacted by reducing feed by half.  It would also appear that addition of Rohu had little impact on growth of tilapia, but could provide an additional source of income for tilapia farmers.