FLUCTUATIONS OF DISSOLVED OXYGEN IN INTEGRATED FISH CULTURE PONDS

Studies were carried out in highly intensive fish ponds with main cultivated fishes in three combinations: silver carp, bighead and Tilapia; black carp and grass carp; and black carp solely. Diel, vertical, horizontal and seasonal changes of dissolved oxygen were determined in those ponds. It shows that the amount of dissolved oxygen in pond water is chiefly affected by photosynthesis (P) and respiration (R) of aquatic organisms and the photosynthesis of phytoplankton plays the most importantrole in oxygen dynamics. In less cloudy and sunny days, the addition of dissolved oxygen are 86.0 and 95.3% from photosynthesis of phytoplankton, 14.0 and 4.7% from atmosphoric diffusion, respectively; while the consumption of oxygen are 72.0 and 72.6% thorugh "water" respiration, 22.0 and 13.1% through fish, 2.9 and 5.5% through mud, 3.1 and 8.8% due to escaping, respectively, in growing season. According to the analysis of the role of diffusion in the dissolved O2 dynamics in pond water, it is considered that a pond with squarish or slightly rectangular shape, having a water depth of 2.5 meters and an area of 1 hectare (or 1.5 ha), is more suitable for creating high fish output (15000 kg/ha), because a sufficiently large water surface enables the wind working better for water circulation and reaeration so as to maintain the dissolved .oxygen content of about 1.5—2mg/L at the bottom. Studies were also conducted on the relationships between environmental control and fish stocking structure control in the management of fish pond such as dissolved oxygen distribution in relation to fish feeding, the technique of dissolved oxygen forecast, The critical standing capacity (CSC) of fish. The results show that grass carp feeds normally when dissolved oxygen is at a level of 3 mg/L, black carp begins to feed when dissolved oxygen is at 2 mg/L. The dissolved oxygen value when fish begins to breathe at water surface is lower than 1mg/L for Tilapia, 0.8mg/L for Carassius cuvieri, silver carp and bighead, lower than 0.8mg/L for grass carp and Megalobrama amblycephala, lower than 0.7mg/L for black carp, common carp and Carassius auratus; and a large number of fish die if dissolved oxygen is lower than 0.6mg/L. Based on these results, it seems justified that the suitable value of dissolved O2 is 4mg/L, the security value is 3mg/L, the warning value is 1.5mg/L and the critical value is 1mg/L in an integrated fish pond. In order to ensure normal feeding and growth of fish, the average dissolved oxygen value of water column in July, August and September should be over 2mg/L. The. CSC value should be 6000kg/h in ponds cultured mainly with silver carp, bighed carp and Tilapia, and 11250 kg/ha in ponds cultured mainly with black carp.