The importance of dissolved oxygen in shrimp culture
You know that the shrimp that were in well oxygenated water (DO over 4 ppm) ate the feed very quickly, with only 21.03% left after just 15 minutes. They ate all of the feed within 45 minutes. The group of shrimp in water with DO 2-4 ppm ate more slowly, but still finished all the feed within 45 minutes. The shrimp that were in water with DO of less than 2 ppm ate very little and after 45 minutes, 59.27 % of the feed was still left. Some of them tried to eat feed while it was near the surface of the water, where the oxygen level is higher. In experimental conditions in the laboratory, excess feed that falls to the bottom of the aquarium will be sucked up by the water filter system, but in real farm conditions the left over feed remains in the pond and causes the water quality to drop. Ammonia and organic matter build up in the water and the amount of Vibriobacteria rise. This has a bad effect on shrimp health. When the shrimp are weakened by low oxygen conditions, they tend to have problems molting and many will die while molting or just after molting.
On most shrimp farms, during the day time the DO level is suitable at over 4 ppm. On sunny days, the phytoplankton in the water photosynthesize and give off oxygen. On farms with large populations of phytoplankton in the water, the DO may rise to over 10 ppm in the afternoon. This is often the case on intensive farms with low salinity and good plankton bloom, where the aerators are turned off during the day when it is sunny. However, even on farms where the day time DO is quite high, after about 21:00 at night the oxygen level starts to drop because of the respiration of the plankton, the shrimp and the micro organisms that are decomposing organic matter. The DO drops to very low from about midnight to early morning. In the pre-dawn hours the DO may be down to 3-4 ppm, 2-3 ppm or even less than 2 ppm on some farms. This is also the case on extensive farms with no aerators and low density of shrimp, and on semi-intensive farms with 30-60 shrimp per square meter and about 6-18 hp per hectare of aeration. The DO usually drops to about 2-3 ppm at night, or even lower at times. The shrimp grow slowly, the survival rate is low and the FCR is high. The shrimp grow better on farms where the oxygen level is higher.
Another experiment demonstrated that in addition to influencing shrimp feeding behavior, DO also has an effect on shrimp growth, survival rate and FCR. For this experiment, White Pacific Shrimp with an average weight of 7.6 g each were kept in 500-liter fiberglass tanks filled with water with salinity of 25 ppt and maintained at a constant water temperature of 29+1 °C. There were 30 shrimp per tank, or a population density of 54 shrimp per square meter. Under farm conditions, shrimp normally exhibit problems of slow growth due to low DO after 50-60 days
Paddle wheel aerators are commonly used in semi-intensive shrimp/prawn culture and is one of the major capital cost item in the farm. The Paddlewheel aerators are used to increase contact surface of water with air thereby increasing the area through which oxygen is absorbed by the water and to create a circular movement of the pond water.
As the pond water moves in a circular fashion the pond bottom is cleaned and the waste matter gets accumulated in the center and the corners. By this method most of the pond bottom is kept clean.
It is very important during application of chemicals or medical treatment with respect to distribution of chemicals/medicines throughout the pond and ensure to be careful in proper water exchange to reduce any side effects. Use of Paddle wheel aerators also has the advantage of accelerating the evaporation rate thereby increasing the salinity. This is however, an advantage in the condition of low salinity.
There is no hard and fast rule regarding the number of Paddle wheel aerators to be used in a pond because it is difficult to define the relation between biomass in the pond and the interaction of various water parameters.
The arrangement of Paddle wheel aerators in the pond is done to maximize circulation efficiency and minimize dead corner areas. The arrangement of Paddle wheel aerators should be to ensure anticlockwise movement of water in the pond in the Northern Hemisphere and clockwise movement of water in the pond in the Southern Hemisphere. This ensures that when bottom drainage arrangements are used, the efficiency of water exchange and draining out of waste product is improved since water when existing from the bottom of the reservoir in the northern hemisphere has an anticlockwise vortex, whereas in the southern hemisphere has a clockwise vortex.
Aerators are usually arranged parallel along the banks and about 5 to 10 m distant from the dikes depending upon the pond size. Each Paddle wheel aerator should normally separated by a distance of 30 m and 50 m for optimum efficiency of the Paddle wheel aerator. Avoid relocating paddle wheels during culture.