What Causes a Large Amount of Fish to Die at the Same Time in a Manmade Pond?

Dead fish in a manmade pond can be attributed to a variety of factors. Determining the cause of the kill requires the process of elimination to determine the specific issue. Culprits range from environmental factors to poison, predators and disease. Discover the issue before stocking new fish in the pond, as the problem may be recurrent.

Environment

• Environmental issues are common fish kill factors in manmade ponds. Suffocation from a lack of oxygen will kill all of the fish at the same time. Fish require oxygen, and a pond without an inlet or some form of aeration is inadequate for survival. Oxygen depletion may also be caused by overgrowth of moss and aquatic plants. In addition to oxygen depletion, water temperature can stress and kill fish. The water must be deep enough to provide several temperature layers for warm- and cold-water species.

Poison

• Another possible cause of a fish kill in manmade ponds is poison, which can be introduced to the water through the ground or more direct sources. Ponds in agricultural areas with heavy pesticide and herbicide use are subject to poison. Household items like bleach and garden products may also lead to poisoning if they are introduced into the pond. Pond owners must monitor the pH level of the water; the addition of too much acid or base can also lead to a fish kill.

Predation

• Predators will kill all of the fish in a small pond in a single day or night. Herons and other birds are common predators, but raccoons and skunks will also prey on the fish. The predators will eat some of the fish and leave the remainder of the population on the bank. Pond owners must build depth and structure into the pond to provide shelter against predators. Shallow ponds without overhanging rocks and natural structure are susceptible to a fish kill.

Disease

• The introduction of disease will kill the fish in a manmade pond. Fish diseases typically develop in close quarter hatcheries and rearing pens. Purchasing new fish and adding them to the existing population may spread the disease and cause a fish kill. Overcrowding the population in your pond can also lead to the development of disease. Pond owners must monitor the fish for parasites and irregularities that require prompt treatment.