Management of Mass Mortality of Big Size Fishes in Organically Polluted Freshwater Ponds and Lakes Due to Nitrite (No -) Toxicity: A Case Study

Abstract

A study was conducted in the Prakriti Tirtha lake situated at Newtown Megacity, Kolkata, India. The water area of the lake was 45.5 ha with depth ranging from 10 ft to 32 ft. Levels of most of the water parameters were quite normal except nitrite (NO2) levels. The nitrite values were more than 0.2 mg/L and increased with depth. Large scale mass mortality of big size fishes was observed during the investigation. Mortalities were observed mainly for surface feeders like big size Silver carps (Ctenopharyngodon idella), Bighead carp (Hypophthalmichthys nobilis) and Catla (Labeo catla). A total of 30 to 40 gunny bags (30 to 50 kg per bag) of dead fish was collected from the pond every day and buried into soil. To overcome this problem, 3 tons of coarse salt was applied in two split doses by spreading over the water body in early morning hours with the help of boat. Mass mortality of fish was gradually reduced after 3 days of application of salt and totally ceased after ten days of application of salt. The symptom of brown blood was disappeared after ten days. Lethargic fishes became energetic.