The effect of paddle wheel and jet aerators on bottom velocity, dissolved oxygen levels and shrimp distribution in aquaculture ponds
R.G. SAMAN WIJESEKARA, NAKAO NOMURA AND MASATOSHI MATSUMURA
The high shrimp biomass and feeding rates of intensive shrimp culture systems require supplemental aeration to maintain adequate dissolved oxygen (DO) levels. Recent studies have shown that paddlewheel aeration is inadequate to maintain required bottom DO levels in intensive shrimp ponds. The use of heavy aeration systems such as the jet type aerators is thus expected to increase. However, the negative impact of bottom velocity created by a jet aerator has not been evaluated yet. In this work, laboratory tank experiments were conducted using an experimental scale jet aerator. Field experiment; were also conducted in shrimp grow-out ponds. The jet aerator was fixed at different depths in the water column and bottom velocity, shrimp distribution, mortality of shrimps, molting and DO distribution were studied. It was observed that normal shrimp distribution pattern was greatly affected by higher bottom velocity and a bottom velocity greater than 5 cms-1 is critical for 10 - 20 g shrimps. Accumulation of shrimps at low velocity areas increased their mortality due to over-stressed condition and cannibalism. Arrangement of aerators to maintain bottom velocity at a level less than 5 cms-1 was considered ideal for shrimp grow-out ponds. Maintaining a homogeneous pond bottom velocity is advisable to overcome over-crowding of shrimps. Additional aeration such as air bubbling in low velocity areas could improve shrimp performances and enhance aerobic conditions. Field experiments showed that higher bottom velocity at the vicinity of the jet aerator could be lowered by lifting the position of jet aerator above pond bottom without adversely affecting bottom DO levels.