Eutrophication of water bodies leading to killing of fishes is mainly due to non-availability of

During eutrophication of water bodies, the BOD level increases due to rapid growth of microbes. Eutrophication is the natural ageing of a lake by nutrient enrichment of its water. Depending on the climate, the size of the lake and other factors, the natural ageing of a lake may span thousands of years but it is accelerated due to human activties.

The rise in BOD levels due to increased organic matter decreases the oxygen levels beyond a critical concentration where the survival of aqutic species like species becomes difficult.