Fish mortality due to cyanobacterial bloom in freshwater pond, Cochin, Kerala

Present study revealed the cause of mass mortality of fishes and color variations of micro algal bloom observed in fresh water pond of Udayathumvathil Sreekrishna Swami Temple, Cochin, Kerala. Monitoring of water quality and phytoplankton abundance was carried out from selected stations during the period of study. Water quality parameters such as nitrate, phosphate and ammonia were observed in higher concentration that normal level directs towards phytoplankton bloom formation. Dissolved oxygen level below 4mg/l was observed which are lethal to aquatic fauna. Among the phytoplankton species, Cyanobacterial density dominated over other beneficial microalgae with a maximum cell density of 2.74×10⁵ cells/ml. Two species of Cyanobacteria such as Aphanocapsa sp and Microcystis sp constituted 60.91 and 33.98 per cent of total phytoplankton abundance in the study area. The fish mortality was probably caused either by oxygen deficiency or toxins secreted by Cyanobacteria or by combination of both.