Hydrobiological studies with reference to sudden fish mortality

Summary Hydrobiological investigation of Gandhisagar (Nagpur) was undertaken with a view to finding out the causes of sudden fish mortality. Extensive observations carried out for over a period of several weeks, prior to, during and after mortality, have led us to the conclusion that depletion of dissolved oxygen was the main cause of sudden fish mortality.The depletion of Oxygen was brought about by several factors, viz., the depletion of phytoplankton resulting in the lesser evolution of oxygen, the gradual increase in the zooplankton which reached the maximum on the day of the fish mortality, resulting in the increased consumption of Oxygen for respiratory purposes, and the decomposition of the bottom sludge which also consumed oxygen to a large measure for the purposes of Chemical oxidation.The changes that are normally correlated with the decrease in the phytoplankton, such as, (i) lowering of the pH (from 8.9 to 7.5), (ii) the decrease in the carbonate contents of the water and (iii) the increase in the bottom turbidity were also noticed. Causes of the depletion of phytoplankton have not been ascertained but may be due to the presence ofDaphnia in very large numbers. Other aspect of the problem is under investigation.Published with the permission of the Director.