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
Hypoxia (environmental)
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Hypoxia refers to low oxygen conditions. For air-breathing organisms, hypoxia is problematic but for many anaerobic organisms, hypoxia is essential. Hypoxia applies to many situations, but usually refers to the atmosphere and natural waters.
Still frame from an underwater video of the sea floor. The floor is covered with crabs, fish, and clams apparently dead or dying from oxygen depletion.
Jubilee photo from Mobile Bay