There is a mass mortality of fish in a pond. What may be the reasons?

In a pond, a mass mortality of fish may occur due to following seasons.
(i) Passage of pesticide rich water from crop fields.
(ii) Release of toxic industrial waste.
(iii) Pouring of hot water from an industry or thermal power plant.
(iv) Release of waste rich in heavy metals and mercury.
(v) Blockage of gills of fish by some suspended pollutant such as oil.