Environmental diel cycle during mass fish kills and planktonic collapse in an undrainable rural fish pond

Abstract

With a view to find out the possible causes for the sudden occurrence of large scale mortality of fish and fish food organisms in a shallow pond of 1.25 ha on a hot summer day (June 5, 1982), the diurnal variation of physico-chemical factors and plankton density were studied. Water temperature (29.0–37.5°C), free carbon dioxide (0–32.0 ppm) and dissolved oxygen (0.08–11.6 ppm) showed marked fluctuation round the clock. Phytoplankton density was found to be maximum at 5.00 am. and minimum at 9.00 p.m. and during day time was markedly less. Volumetric estimation of plankton revealed a considerable decline in plankton biopmass (71.32%) over a period of 3 days during the hot spell (June 5–8, 1982). The computed loss in density of phytoplankters was 2981 u/1 and that of zooplankters 2354 u/1 during the period. The dead fishes included Cyprinus carpio, Cirrhinus mrigala, Catla catla and Labeo rohita. The lethal conditions for aquatic life occurred with high temperature and complete deple tion of dissolved oxygen in the day time and the sharp rise in free carbon dioxide in the night. The adverse environmental conditions were further aggravated due to very low water level, high concentration of suspensoids of particulate and colloidal matter at the pond bottom and high rate of decomposition of organic matter etc., which seem to have been cumulatively responsible for the sudden mortality of fishes and the collapse of planktonic organisms.