Fish Kill: Causes and Prevention

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The term “fish kill” is used to define the scenario where sudden, mass mortality of fish population occurs. The mass mortality is usually localized to a particular area, or a body of water (such as lakes or ponds) and there are many probable causes that can be attributed to the scenario. When a localized population of fish suddenly die, it is an indication of potential problems in the environment and these can already be affecting other aquatic organisms. It may also directly impact other activities that require resources from the environment.

Causes of Fish Kill

Fish kills can be attributed to many causes, some man-made and others natural. The following are a few of the major causes responsible for the fish kill:


When a body of water is overly enriched with excess minerals and nutrients, it induces excessive growth of plants and algae (called algal blooms). This can lead to depletion of oxygen levels in the aquatic environment that can cause fatal consequences on the local population of fish and other organisms. Among the other causes on this list, eutrophication is one of the most prevalent causes of fish kills.

Parasites and Diseases

Fish populations are subjected to various foreign pathogens in their aquatic environment. Bacterial, viral and even protozoic infections emerge as a result. This can also be a sign of the sub-optimal quality of the water, pollutants and contaminants, etc. Signs of diseases include missing scales, lack of slimes, very unusual and erratic behaviour, abnormal growths, etc. Parasites and diseases can spread even more rapidly in fish farms, where it can affect and kill all fish in affected ponds.

Spawn-related Deaths

For some species, it is a part of their lifecycle to die after spawning activities such as building nests, laying eggs and fertilizing them. This is partly related to the ordeal that the fish have to endure – their migration from the ocean to freshwater is very taxing and it takes a physical toll on their bodies. And once they complete their spawning activities, most species of fish just die from sheer exhaustion.

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