INCLUSION OF NILE TILAPIA Oreochromis niloticus AND SAHAR Tor putitora IMPROVES RODUCTIVITY IN CARP-POLYCULTURE SYSTEM
Carp polyculture is commonly practiced in Nepal but improving productivity of this aquaculture system is a major concern. An experiment was conducted at the Fisheries Development Center, Bhairahawa, Nepal in nine earthen ponds of 200 m2 for 240 days (9 August 2014 to 9 May 2015). The experiment was conducted in a completely randomized design with three treatments in triplicate: a) Carps only or control (10000 fish/ha) (T1); b) Carps (10000/ha) + tilapia (3000/ha) (T2); and c) Carps (10000/ha) + tilapia (3000/ha) + sahar (1000/ha) (T3). Silver carp (Hypophthalmichthys molitrix), bighead carp (Aristichthys nobilis), common carp (Cyprinus carpio), grass carp (Ctenopharyngodon idella), rohu (Labeo rohita) and mrigal (Cirrhinus mrigala) of mean stocking size 6.7, 3.8, 7.3, 3.1, 1.9 and 2.0 g, respectively were stocked in all ponds at the ratio of 3.5:1:2.5:0.5:1.5:1. The mean stocking size of Nile tilapia (Oreochromis niloticus) and sahar (Tor putitora) were 4.4 and 7.2 g, respectively. The ponds were fertilized weekly with urea and di-ammonium phosphate at 4 g N and 1 g P m-2 day-1. Fish were fed once daily with locally made pelleted feed (1:1 rice bran and mustard oil cake; 20% CP) at 2% body weight. At harvest, the combined net fish yield was significantly higher in T3 (3.93±0.15 t·ha-1·yr-1) compared to T1 (3.05±0.26 t·ha-1·yr-1) whereas there was no significant difference between T2 and T3 (Table 1). There were no significant differences in survival and water quality among treatments. The gross profit margin was significantly higher in T3 (2357.4±210.8 USD/ha) compared to T1 (1299.5±315.9 USD/ha) without any significant difference between T2 and T3.