During summer season due to prevailing high temperature, high density of fish in the tanks, gradual reduction of water level, deficiency of dissolved oxygen and deterioration in the quality of water, etc., the fish are subjected to lot of stress.
When the problem is severe, there are possibilities of mass mortality of fish in the tanks. Besides stress of environmental factors, the fish becomes susceptible to diseases. The following preventive and control measures are suggested to avoid economic loss.
The fishermen must observe water quality, depth and extent of water and movement of fishes every day during summer season. They must conduct sample netting to assess their growth, colour, appearance of tail, fins and gills, secreting mucus, etc and observe differences from normal growth and behavior. In case of differences noticed, the fishermen must approach the concerned District Fisheries Officer and take up preventive and control measures as suggested by the department to avoid economic loss.
If the fishes move on the surface in the morning gulping air, it indicates deficiency of dissolved oxygen and in such cases, they must fill the tank with fresh water. If it is not possible to fill the water, the fishermen must resort to partial harvest. This action reduces the demand for dissolved oxygen. But most of the fishermen on the reasons of not attaining market size, fewer rates in the market, lack of ice, etc. postpone harvest which is not desirable.
When the quality of the water deteriorates, the fishermen shall add lime @ 100 to 250 kgs / ha. which increases dissolved oxygen besides improving the quality of water and regulating pH.
The aquatic weeds present in the tank shall be removed periodically otherwise they release carbon dioxide in the night and may lead to mass mortality due to severe shortage of dissolved oxygen. In such situations, the fishermen suspect the poisoning by enemies. When the mortality occurs due to poisoning, besides carp species, air breathing habit fish species like murrel, cat fishes, etc. and amphibious habit animals like frogs, water snakes also die. They have to be very careful in establishing the reasons in consultation with the Fisheries Department.
When fish die due to diseases, the dead fish shall be removed forthwith from the tank and either they should be buries or burnt far away from the tank and apply lime @ 100 to 250 kgs / ha. Even after application of lime, if the water quality does not change, they must apply sanitizers like Benzal kolium chloride @ liter/ha. by diluting in the water. It not only improves the quality of water but also heals wounds and kills external parasites. If there is no improvement in the situation, the fishermen as a last report, apply antibiotics along with supplementary feed as per the recommended dosage.
When there is no possibility to fill with water, the water of the tank should be recycled by re-circulating with motor pumps. The best method is to resort to partial
