The hot summer weather, the blue-green algae found on lake surfaces and the sewage let into water bodies can be a deadly combination for their fish. Lakes look green mainly due to their blue-green algae that absorb the nitrogen directly from the atmosphere and choose to remain close to their surfaces, preventing sunlight from penetrating them.
The blue-green algae multiply rapidly and their life cycle ends in two to three days, releasing a toxin. But in the meanwhile they use up a lot of the oxygen in the lake water, leaving it with little for the fish, especially if it does not have proper aeration. Several factors are responsible for this phenomenon which the authorities are well aware of. Once the fish die, the decomposition starts, affecting the entire water body.
We cannot stop sewage entering a lake, but what we can do is at least send primary or secondary treated sewage into it. Every lake must have an STP to make this possible. But as mass death of fish is a seasonal and an irregular phenomenon, the authorities are probably not interested in finding a permanent solution.
(The writer is a well-known limnology and plankton ecology expert)