The Lake Doctors, Inc.


Determining the exact cause of fish mortality in a pond can be challenging without detailed information and on-site investigation. There are several potential reasons why fish in a pond might die. Some of the common causes include:

  1. Oxygen Depletion: Low oxygen levels are the most common cause of fish kills in ponds. Fish require oxygen to survive, and if the pond’s oxygen levels drop too low, it can lead to fish suffocation and mortality. Factors such as excessive algae growth, high temperatures, and overstocking of fish can deplete oxygen levels. The amount of dissolved oxygen is directly related
    to water temperature: warm water holds less dissolved oxygen than cold water, and fish kills are always more common in the summer.
  2. Poor Water Quality: Water quality is crucial for the health of fish. If the pond water becomes polluted due to excessive nutrient levels (eutrophication), low dissolved oxygen, high ammonia or nitrite levels (common with fertilizer runoff), or other contaminants, it can stress and harm the fish.
  3. “Pond Flipping”: In major rain events after periods of drought, the pond can “flip” whereby the low oxygen water at the bottom of the pond is pushed to the top of the water by excessive water runoff. The fish, trying to find higher oxygen levels, rise to the surface (where the oxygen levels are typically higher) and suffocate on the oxygen depleted water.
  4. Disease Outbreak: Fish can be susceptible to various diseases caused by bacteria (particularly e-coli from feces runoff), viruses, parasites, or fungi. If the pond environment becomes stressful, it can weaken the fish’s immune system, making them more susceptible to infections and disease outbreaks.
  5. Toxic Substances: The presence of toxic substances in the water, such as runoff from roads or excessive pesticides, can be lethal to fish and other aquatic organisms. Note that we always apply our herbicides and algaecides within EPA-label rates, which are designed to protect desired wildlife.
  6. Overstocking: If the pond is overcrowded with fish, it can lead to increased competition for food and resources, elevated stress levels, and poor water quality, ultimately resulting in fish mortality.
  7. Predation: Predators such as birds, mammals, or even larger fish in the pond can prey on smaller fish, leading to fish mortality.
  8. Temperature Extremes: Rapid changes in water temperature, especially extreme cold or hot temperatures, can be stressful for fish and cause mortality.
  9. Lack of Food: If the pond’s ecosystem cannot support an adequate food supply for the fish, it can lead to malnutrition and weakened immune systems.
  10. Invasive Species: The introduction of invasive species into the pond can disrupt the balance of the ecosystem and negatively impact native fish populations.

To determine the specific cause of fish mortality in your pond, it is essential to consider the water quality parameters, recent changes in the environment, fish population dynamics, and any potential signs of disease or predation. The extend of the fish kill is also extremely important. A few dead fish may look like a disaster, but remember: most ponds contain 200–300 pounds of fish per surface acre.

If the issue persists or becomes a recurring problem, seek advice from The Lake Doctors. Our professionals can conduct on-site assessments and water testing may be beneficial in identifying the underlying cause and implementing appropriate solutions. Once the solutions have been implemented, we can also assist in re-stocking the pond with the right mix and quantity of fish to maximize your waterway’s functional, recreational, or aesthetic potential.