Is there any area in the pond where plankton carcasses are accumulated, no aeration, and no water currents through it? Beware of the area that would be a dead spot or dead zone!
Dead spots are hypoxia or low oxygen areas. Hypoxia occurs when dissolved oxygen (DO) is at a concentration below 2 ppm. The area is unfavorable for the shrimp life and could cause death (Read the previous article: Oxygen Demand in the Shrimp Farms).
Identifying it is quite easy, for example with the very low water brightness due to the accumulation of algae. The area is also prone to accumulation of silt or moss that grows on the walls of the pond. The area would have very high oxygen demand (BOD).
What causes dead spots to form?
The increase of nutrient content (especially phosphorus and nitrogen) or also known as eutrophication is the main cause of the formation of dead spots. This eutrophication is triggered by the high organic components derived from shrimp feces, uneaten feed, and decomposition of decomposing bacteria in the area. Dead spots are prone to plankton gathering, especially groups of blue-green algae (BGA) (Also read: Blooming Plankton in Shrimp Farms).
Dead spots could occur in a small area with low circulation. In the shrimp ponds, dead spots often occur in the ponds with a square or rectangular shape with the corners of the pond at right angles. Oxygen supply in this area becomes very low. This area is also prone to the accumulation of various organic and carcasses.
What are the consequences of dead spots?
The dead spots are marked by high algae that causes a decrease of sunlight penetration (decreased water brightness), an increase in carbon dioxide (less DO), and high levels of nutrients (eutrophication). The main effect of the dead points is a decrease in DO which causes hypoxia.
Fish or shrimp that lack oxygen supply could become stressed and susceptible to disease or parasites. Low oxygen could also trigger the growth of pathogenic bacteria which are mostly anaerobic bacteria. Ammonia and nitrite could occur in this dead area, even though the two compounds are toxic compounds.
How to deal with dead spots in ponds?
Modify the pond
Modification of the pond is needed to minimize the corner of the pond with a sharp angle. In square or rectangular ponds, it would produce more dead spots, especially at the corners of the pond. Square or rectangular ponds could still be used, but with slight modifications to each corner. That is by making the corners of the pond more rounded or enlarging the corners of the pond.
Another solution is to make a pool with a round or polygonal shape (6-8 corners). You can also use a pool with a raceway pool design. Coupled with a good circulation current it will minimize dead points.
Add aeration
Aeration or the effort of providing oxygen at the dead spots would make the area healthier. This option could be done if the previous solution, namely changing the construction of the corner of the pond is not yet possible to do. Adding the water circulation at the dead spots also needs to be done so that there would be no buildup of organic matter in the area (Also read: The Role of the Wheel to Maintain Dissolved Oxygen). The addition of aeration and circulation could be done by adding an aerator or bubble jet that makes the water circulate.
Proper aeration would also make the circulation of waste or sediment at the dead spots better. Placing bubble jets symmetrically in each corner of the square pond could also minimize the dead spots (Also read: Tips for Determining the Location and Number of Wheels).
The dead spots reduces the effectiveness of aquaculture ponds. Dead spots are not good for shrimp life and cause accumulation of various toxic organic materials. Modifying the construction of the corner of the pond is the main option or it could be by adding aeration to each corner. Minimizing the dead spots would restore the overall effectiveness of the pond area.
(Eng: Danan)
