Eutrophication of water bodies leading to killing of fishes is...

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.