Fish Kills

What Is a Fish Kill?

All species of aquatic life have a natural lifespan. Death from natural causes is inevitable. A ‘fish kill’ event is the death of a large number of fish, or other marine or freshwater animals, over a short period. Anything less than about 50 small fish or aquatic animals in a single event is unlikely to qualify as a fish kill. A smaller number of large animals may be significant. Discarded fish from legitimate fishing activities should not be regarded as a fish kill. These can often be identified by signs of hook or net injuries.

Natural Causes?

Most fish kills are the result of natural causes. These include disease, seasonal weather changes, the effects of drought, and freshwater runoff into normally salty water as a result of heavy rain. A very common cause is depletion of dissolved oxygen resulting from the periodic decomposition of organic matter in the water. This can be accumulated from natural resources such as dead leaves or grass.

Large-scale seasonal fish kills have been recorded in rivers in northern Australia since before the advent of agriculture or industry.

Human Causes?

Some fish kills result from human intervention in the natural state of water bodies, and some can be traced to pollution. Prompt and factual reporting by members of the public can enable observed fish kill events to be effectively investigated and follow-up action to be taken where appropriate.

The Need for Speed!

A fish kill should be reported as soon as possible because dead fish decay quickly and contaminated water may flow elsewhere. This can make a useful investigation impossible.

What Are the Symptoms?

Fish disease may affect only fish of one species, and over time the numbers affected may build up from a few to a peak. Death from disease is usually slow.

Lack of oxygen usually causes abrupt fish death, often overnight, and may affect larger fishes first. Fish in oxygen-depleted water are often seen gasping at the surface before death. This should not be confused with normal surface feeding behaviour.

Fish kills associated with pollution are often rapid events affecting all species, and the smaller fish die first.