EFFECT OF FEED C/N RATIO PROMOTED BIOFLOCS ON WATER QUALITY AND PRODUCTION PERFORMANCE OF BOTTOM AND FILTER FEEDER CARP IN MINIMUM-WATER EXCHANGED PONDPOLYCULTURE SYSTEM
A 120-day experiment was conducted with the aim to examine the effects of feed C/N ratio promoted bioflocs on water quality and production of bottom and filter feeder carp in land-based experimental mesocosm with minimum water exchange system. For the purpose four bioflocs treatments with carbohydrate feed referred as 'C/N 11', 'C/N 15', 'C/N 19' and 'C/N 23', and control 'C/N 7' without carbohydrate addition consisted of three replicate enclosures (7 m × 7 m x 1.5 m) were arranged. Young bottom feeder mirror carp, filter feeders silver carp and bighead carp were stocked altogether in polyculture at densities of 285 g.m−2, 45 g.m−2 and 15 g.m−2, respectively in control and treatments. The results showed a significant (P<0.05) positive correlation between bioflocs volumes and experimental water temperatures. The total ammonia nitrogen (TAN) concentrations in treatments from C/N 11 to C/N 23 were significantly (P<0.05) lower comparing to control (C/N 7). Nitrite nitrogen (NO2-N), nitrate nitrogen (NO3-N), total inorganic nitrogen (TIN), orthophosphate (PO4-P) and total phosphorus (TP) concentrations decreased significantly (P<0.05) with higher C/N ratios. The total production (TP) of bottom and filter feeders fish reached up to 13895 kg. ha-1 in experimental treatments. The specific growth rates (SGR), protein efficiency ratios (PER) and net production (NP) of mirror carp increased gradually with increases of C/N ratios. The feed conversion rates (FCR) of experimental fish declined significantly (P<0.05) with increasing of C/N ratios in all control and treatments. With increasing C/N ratios, the total feed conversion rates (TFCR) of fish decreased significantly (P<0.05), while total protein efficiency ratios (TPER) increased significantly (P<0.05). The bioflocs influenced muscle compositions in terms of crude protein, crude fat and ash of the mirror carp significantly (P<0.05). The present study revealed that improved production performance and feed utilization in mirror, silver and bighead carp could be achieved with improved water quality with increasing feed C/N ratio.