Evaluation of different water exchange regimes for optimizing growth and production of koi carp, Cyprinus carpio in tanks

The effect of different water exchange regimes on the growth and survival of koi carp, Cyprinus carpio in tanks provided with the supply of exogenous zooplankton as the food was investigated. Fish larvae (0.15±0.012g) were stocked in outdoor concrete tanks at 0.5 fish/l (T1 and T2); 1.0 fish/l (T3 and T4); and 1.5 fish/l (T5 and T6) and cultured for three months. The water exchange rate was 10% once daily in T1, T3 and T5 and twice daily in T2, T4 and T6. Values of dissolved oxygen were highest in T2, followed by T4, T1, T6, T3 and T5. The T5 treatment showed the highest concentrations of conductivity, NH 4 -N, NO 2 -N, NO 3 -N, PO 4 -P, and bicarbonate alkalinity, which were significantly higher than the other treatments. The final body weight of C. carpio ranged from 4.01 to 8.22g in the different treatments. At harvest, maximum weight gain was achieved in the T2, followed by T4, T1, T6, T3 and T5 in descending order. There was a significant difference in the survival of koi carp among the treatments, ranging from 56.43% (T5) to 96.32% (T2). The percentage and number of fish exceeding a total weight of 5g were estimated from the size-frequency distribution at the end of the study and was significantly higher in T6 ( P <0.05) than other treatments. From the present study, a daily water exchange of 20% could support higher stocking densities of koi carp in tanks and result in high productivity, measured in terms of the number of marketable fish.