Effect of feed C/N ratio promoted bioflocs on water quality and production performance of bottom and filter feeder carp in minimum-water exchanged pond polyculture system

• It shows a significant positive correlation between biofloc volumes and temperature.
• The bioflocs can influence mirror carp muscle compositions.
• Growth performance of the cultured fish can be improved in the C/N ratios of 15-19.
• Good water quality can be maintained in the C/N ratios of 11-23.

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, Cyprinus carpio, filter feeders silver carp, (Hypophthalmichthys molitrix) and bighead carp (Aristichtys nobilis) with initial mean individual body weight of 194.72 (± 4.42 g), 54.10 (± 0.84 g) and 58.69 (± 0.68 g) 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. During the experiment water temperatures varied from 24.7 to 12.0 °C. 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. There was no significant (P < 0.05) differences in concentrations of Chlorophyll a, chemical oxygen demand (COD) and dissolved organic carbon (DOC) with increasing C/N ratios in treatments. 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.