What Would Cause a Large Number of Fish to Suddenly Die in a Pond?

One day you have a pond full of big, healthy fish and the next day they're gone. When this occurs it is considered a catastrophic fish kill. This happens regularly in farm ponds when all, or most, of the fish die seemingly out of the blue.

Oxygen Depletion

• Fish kills can be the result of oxygen depletion. Low oxygen levels can occur in the autumn when plants in the pond are dying off or on hot, cloudy, summer days and nights. The biological need for oxygen by the fish is exceeded by the amount that is available. Oxygen needs to be replenished in water and when this doesn't happen, all of the fish in your pond can die.

  Fish kills generally occur as the pond is adapting to warmer weather in the early spring. If there is a harsh winter and ice covers the pond, this enables an adequate exchange of gases between the air and water. However, if there is a thick layer of snow on the pond, this prevents the gas exchange and oxygen is not replenished in the water as it needs to be. Plant and leaf litter decay, which consume oxygen, deplete what oxygen there is and the fish don't get enough.

  Snow also makes it difficult for the sun to penetrate the ice and this reduces photosynthesis, which is the process by which plants release oxygen. When photosynthesis doesn't occur as much as it should, this lessens the amount of oxygen in the water and fish can die.

  Too much aquatic vegetation can potentially kill fish. The vegetation should not cover more than half of the pond. Plants don't produce oxygen at night when there is no sunlight. However, at night the fish and insects use oxygen for respiration. If there wasn't enough oxygen produced during the day a fish kill may occur at night, according to Virginia Department of Game and Inland Fisheries. If you experience several cloudy days in a row, this can reduce the oxygen in the pond that it may result in the death of the fish.

Disease

• Disease is sometimes the culprit, but this generally kills only some species and leaves the others unaffected. When a viral or bacterial disease gets into the water, which generally happens in the spring after a mild winter, this can kill some of your fish but probably not all of them. You can put medicated food pellets into the water to help prevent disease, although not all fish respond to the pellets. Usually a disease outbreak in a pond will resolve on its own without totally destroying the fish population.

Contamination

• Water contaminated by pesticide- or fertilizer run-off can kill fish, according to experts at the Ohio Sea Grant College Program.

Prevention

• You can prevent fish kills by limiting the presence of dense vegetation in the water. If you have a very calm, cold winter this leads to heavy ice cover, which can led to fish kills while a calm summer can also increase the likelihood of fish kills, explains William E. Lynch Jr. and Eric R. Norland

  from the School of Natural Resources at Ohio State University. Installing an aeration system in your pond can prevent fish kills. The system will add oxygen, limit ice coverage, maintain oxygen levels and prevent stratification.