Question

A lake near a village suffered heavy mortality of fishes within a few days. Which of the following statements could be the correct explanation for this ?
(i) Lots of urea and phosphate fertilisers were used in the crops in the nearby fields.
(ii) The croplands of the village were sprayed with DDT.
(iii) The lake water turned green and stinky.
(iv) Phytoplankton populations in the lake declined initially thereby greatly reducing photosynthesis.

• A. (i) and (iii)
• B. (i), (ii) and (iii)
• C. (ii), (iii) and (iv)
• D. (iii) and (iv)

Answer 1

The correct option is B (i), (ii) and (iii)

The most common causes of fish kills are oxygen depletion, algal blooms (could deplete oxygen or be toxic), pesticide toxicity and disease.

Fertilization of surface waters (eutrophication) results in the explosive growth of algae which causes fish kills. The colouration of the lake to green and sticky represents the presence of the algal bloom. Deoxygenation of water, especially after the collapse of algal blooms, usually resulting in fish kills.

DDT is an endocrine disrupter and can cause reproductive and developmental problems this causes fishes whose eggs develop in water.

Phytoplankton is the base of several aquatic food webs. hence it will not cause any fish kills.

So the answer is '(i), (ii) and (iii)'.