Fish kills.

fish kills

We’ve recently seen fish kills in some of our Wet Tropics rivers. They are the sudden death of large numbers of fish in a lake, river, or ocean. What causes them and how can we prevent them?

Fish kills can result from a number of natural and human-induced causes

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Mass fish kills can’t always be prevented, especially if they’re driven by natural causes. But they are a reminder of how delicately balanced our aquatic ecosystems are and how important it is to maintain this balance to keep them healthy.

There are a number of factors that can cause kish kills, including:

1. Low dissolved oxygen levels

This is one of the main causes of fish kills. It happens when the water becomes oversaturated with nutrients. More nutrients in the water can lead to excess algae growth which consumes more oxygen as it dies and decomposes, leading to low dissolved oxygen levels in the water. This can be especially problematic in shallow, stagnant bodies of water. In the tropics, it can happen in the wet season as early season storms wash organic matter into the waterways, triggering an increase in algae and a drop in dissolved oxygen.

2. Toxic contaminants

Toxic substances such as chemicals, oil and pesticides being introduced into the water can also cause mass poisonings of fish. Whether released intentionally or accidentally, toxic substances can be catastrophic for fish populations and the overall health of a water body.

3. Disease

Fish can also be killed by the spread of disease. This can be caused by a variety of factors including stress, the release of aquarium fish, and poor water quality. Disease outbreaks can lead to rapid declines in fish populations and can have cascading effects on other species in the ecosystem.

4. Temperature fluctuations

Rapid changes in water temperature, especially if the water becomes too warm or too cold, can lead to mass mortality. Sudden changes in temperature can also lead to the mixing of cooler, low oxygen water with warmer surface water in stratified water bodies. Recent studies and modelling has shown climate change is predicted to increase the prevalence of fish kills due to increases in water temperatures globally.

What can we do about it?

Unfortunately, not all fish kills are preventable, especially those driven by natural causes. However, we can reduce the risk of fish kills by reducing excess nutrient pollution and preventing the release of toxic substances into the water. When fish kills do occur it is important to determine what caused the event, to ensure measures are taken to address it, particularly if it was driven by human activity.