Dead fish floating in Guwahati tank raises concern

Last Updated30/06/2021

At least 400 dead fish were found floating in a Guwahati tank.
The fish mortality was due to a sudden dip in oxygen level and a high organic load in the tank.
After examination, it is found that the death of fish is not due to poisoning but environmental degradation.

Adding fresh water to the tank is a short-term measure with pumps to increase the oxygen level.

The presence of organic and inorganic wastes in water decreases the dissolved oxygen content of the water.
Water having DO content below 8.0 mg/L may be considered as contaminated.
The higher amounts of waste increase the rates of decomposition and O2 consumption thereby decreases the DO content of water.

Water pollution by organic wastes is measured in terms of Biochemical Oxygen Demand (BOD).
BOD is the amount of dissolved oxygen needed by bacteria in decomposing the organic wastes present in water.
It is expressed in milligrams of oxygen per litre of water.
The higher value of BOD indicates low DO content of water.

It occurs with large quantities of organic compounds in water bodies.
During the decomposition process, the dissolved oxygen in the water body would be used up at a greater rate than it can be replenished, causing oxygen depletion.
It contains large quantities of suspended solids which by increasing the turbidity of the water, does not allow the sunlight to pass through the water surface.
This reduces the light available to photosynthetic organisms.
Organic wastes also settle out on the bottom of the stream. This could alter the characteristics of the substratum.