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The Guide to Fish Kills: What They Are & 6 Ways to Prevent Them

written by
Linda McDonagh

Have you ever walked along your pond or shoreline on one of spring’s first sunny days or the morning after a particularly strong storm– only to notice scores of dead fish in your pond floating or washed up? Not a pretty sight.

This is what is known as a ‘fish kill’ -an instance of many fish dying in a lake or pond. While it is unfortunately often too late to stop fish kills once you have observed the dead fish, there’s plenty that you can do to prevent future ones from occurring.

Today, we are exploring what a fish kill is, the methods of preventing them, and the overarching benefits of fountains, diffusers, deicers and aeration. Clarke offers rich experience in developing solutions to help prevent fish kills – such as fountains and aerations or aquatic plant management. Contact our team to get started ahead of the winter season.

A fish floating on a lake's surface - an example of a fish kill

What is a Fish Kill?

A fish kill, to put it simply, is a sudden, unexpected death of a large number of fish or other aquatic life over a short time and within a certain area. It is not the natural, expected deaths from old age, injury, pollution or starvation. Deaths from these causes are typically much smaller in quantity and do not occur at the same time.

What Causes a Fish Kill?

Well, there are several reasons, depending on the season. But the primary reason for fish kills are changes in dissolved oxygen levelsa measurement of how much oxygen is dissolved in the water and available to sustain aquatic life within a waterbody – which are vital to sustaining fish populations. Essentially, when dissolved oxygen levels fall too low, fish suffocate.

How much dissolved oxygen do fish need to survive? It depends. Fish in warm water typically need DO levels at least 1.0 mg/L. Cold water fish usually need about 2.5-3.5 mg/L. It should be noted that while these are the minimum dissolved oxygen levels, they are the best for fish and they are more susceptible to sudden swings in DO levels.

Dissolved oxygen can be affected by many factors, which we will walkthrough below.

Summer Fish Kills

Fish kills can occur in the summer due to low levels of dissolved oxygen, and it should be noted that in general warm waters hold less dissolved oxygen than cold. Other aquatic life, such as weeds and algae, also use up dissolved oxygen during the night when they are unable to photosynthesize. Therefore, fish kills in the summer usually occur alongside the following variables:

  • A large algae bloom dying or aquatic weeds and invasive plants using up too much dissolved oxygen during the night
  • A large algae bloom dying or aquatic weeds and invasive plants decaying after being treated
  • Oxygen-poor water from the bottom of a waterbody being pulled upwards due to strong winds or thunderstorms
  • Species of fish that spawn in the late spring or early summer, crowding out spaces and causing competition for oxygen
  • Run-off from livestock, fertilizers and other organics after heavy rains

A thunderstorm over a lake

At Clarke, we recommend the following to prevent summer fish kills:

  • Install surface aeration fountains or sub-surface aerators and water circulators to maintain healthy oxygen levels and flows, as well as to off-set disturbed dissolved oxygen levels during algae and plant treatment
  • Make sure to use herbicides in moderation, treating only those areas in need of it
  • Be cautious when distributing fertilizers or organics susceptible to running off into a pond, as well as keeping livestock downstream to prevent waste run-off
  • Do not overstock fish populations beyond what can be healthily maintained by the waterbody

Spring Fish Kills

Fish kills that occur in the spring can be deceiving – often, these fish actually die off over the colder winter months. When snow and ice cover the surface of your pond or a portion of your lake, sunlight is blocked out. When sunlight is blocked out, aquatic plants die and no longer give off the oxygen they would otherwise produce through photosynthesis. And these dead plants feed bacteria within the pond, which go on to further use up the oxygen in the waterbody.

An example of a summer fish kill with multiple dead fish floating along a shore

As a result, there often is not enough dissolved oxygen within the pond or lake area to maintain fish populations throughout the colder months, even though fish use less oxygen in the winter. Luckily, there are several things you can implement not just to prevent winter fish kills from happening in the first place, but to help promote the overall water quality of your pond.

At Clarke, We Recommend the Following to Prevent Fish Kills

The biggest and most natural way is by installing fountains, aeration system and water circulators, all aimed at exposing the waterbody to air and allowing oxygen to diffuse into its water as well as continuously circulating the oxygen throughout water columns.

  • Diffuser air systems
  • Water Circulators
  • Deicers (learn more about Deicing here)

The number of, strength of, and position of these units may vary depending on your pond size or climate. Likewise, depending on your waterbody, it may be recommended to centralize these around a shoreline for safety rather than in the middle of the waterbody where people or animals may fall through and become trapped. It is always best to work with an expert trained to create systems tailored for each individual waterbody’s characteristics.

A residential lake with two fountains to properly circulate water

Looking to Prevent Fish Kills in Your Pond or Waterbody?

Our experts at Clarke Aquatic Services have years of experience working with a vast variety of ponds, lakes and communities and take away the stress and time dedicated to planning and implement the correct deicing solution. Clarke consultants work to properly assess and install deicing systems that will work with the unique needs of your community – from initial assessment to ongoing maintenance. To learn more, contact our team here, or check out other articles such as: