Fish kill
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The term fish kill, known also as fish die-off, refers to a localized die-off of fish populations which may also be associated with more generalized mortality of aquatic life. The most common cause is reduced oxygen in the water, which in turn may be due to factors such as drought, algae bloom, overpopulation, or a sustained increase in water temperature. Infectious diseases and parasites can also lead to fish kill. Toxicity is a real but far less common cause of fish kill.
There are many causes of fish kill, but oxygen depletion is the most common cause.
Dead and dying European carp in Lake Albert. Fish kills are often a sign of environmental stress.
This pond in New Forest, England, has been restored following a viral infection which killed all the fish.
A small algae bloom on River Cam near Trinity College
Oxygen saturation
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Oxygen saturation is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It can be measured with a dissolved oxygen probe such as an oxygen sensor or an optode in liquid media, usually water. The standard unit of oxygen saturation is percent (%).
Measuring the dissolved oxygen through a multi-parameter photometer
Dissolved oxygen levels required by various species in the Chesapeake Bay (US)