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From the Wikipedia: Oxygen enters the water through diffusion. The amount of oxygen that can be dissolved in water depends on the atmospheric pressure, the water temperature and whether the water is salty.

Many cold water fish that live in clean cold waters become stressed when oxygen concentrations fall below 8 mg/l while warm water fish generally need at least 5 ppm (5 mg/l) of dissolved oxygen. Fish can endure short periods of reduced oxygen. Depleted oxygen levels are the most common cause of fish kills.

(the mg/l is MICROGRAMS per litre and so 1 microgram per litre is 1 part per million or 1 ppm)

Additional dissolved organic loads are the most common cause of oxygen depletion and such organic loads may come from sewage, farm waste, tip/landfill leachate and many other food-industry effluent sources.

(The additional organic load consumes Dissolved Oxygen for its own Decomposition Process - thereby reducing the DO level in the water. The more wet waste (additional organic load) we throw in the river water or lake water, the more the DO will be reduced - resulting in mass-genocide of fish populations and fish species.) 

( Based on https://en.wikipedia.org/wiki/Fish_kill )