Answers

Bhupendra Sharma
Aug 1, 2020

Fish mortality increases with influx of nutrients in a freshwater body because
(i) Abundant nutrients encourage the growth of free floating algae and plants, thereby increasing organic matter.
(ii) With the increase in organic matter and its deposition, the water body becomes shallows and warmer. Further deteroriation of organic matter makes the  environment toxic and unfit for survival of aquatic life, i.e. fishes.

The influx of nutrients increases the BOD level of water body, as the microbes flourishing in it consume up all the oxygen in degrading the organic matter. As the  Biochemical Oxygen Demand (BOD) increases, the Dissolved Oxygen (DO) decreases, making the water body unfit for aquatic life.