Dan Catchings District III Fisheries Supervisor Alabama DNR Eastaboga, Alabama
An occasional problem that confronts pond owners is the loss of part or all of the fish population in their ponds because of a fish kill. Fish kills can eliminate or severely reduce the sport fishing provided by ponds. A basic knowledge of the causes and symptoms of fish kills and possible remedial measures will be advantageous to the pond owner.
One of the most common types of fish kills in ponds is caused by depletion of dissolved oxygen in the water. Oxygen must be present in the water at or above certain minimum concentrations for fish to survive. The symptoms of an oxygen depletion fish kill are as follows--the fish will be swimming near the surface, attempting to gulp air; large fish will be killed first, followed by smaller fish; the kill will occur at night or in the early morning hours; the majority of fish will be killed within a few hours time; the water in the pond will be brown, gray or black; and a foul odor is detectable when the fish kill starts.
Oxygen depletion fish kills are most commonly caused by a die-off of the microscopic green plants (phytoplankton) in the pond, or overturns in which oxygen deficient water from the deeper levels of the pond mixes with water in the upper levels. Phytoplankton imparts a green color to the water in a properly fertilized pond. It is essential as part of the food chain and as a supplier of oxygen. A sudden die-off of these plants may occur, especially during periods of cloudy, overcast weather, when the plants use more oxygen than they supply to the pond. As the phytoplankton die and decompose, the remaining oxygen in the pond is consumed and within a short time, a fish kill occurs due to oxygen depletion.
To prevent oxygen shortages due to phytoplankton die-off, proper fertilization should be practiced. However, ponds should not be fertilized to the point where they become a “pea soup green,” with visibility of only two to three inches instead of the ideal 12 to 18 inches of visibility. A herbicide can be used to thin out phytoplankton growth when it becomes too dense. Caution should be used to prevent a fish kill. If oxygen depletion occurs, apply either triple superphosphate fertilizer at the rate of 17 pounds per acre, or liquid fish pond fertilizer at one gallon per acre to re-establish the phytoplankton.
Overturns can lead to oxygen depletion fish kills. During summer, water in a pond will become stratified (form layers), with warmer water in the upper level and the colder water near the pond bottom. This stratification is particularly severe in ponds with dense growths of blue-green or “scum” algae on the surface. The colder water may become oxygen-deficient due to interaction with certain elements in the pond bottom and lack of photosynthesis by phytoplankton at greater depths.
An overturning or mixing of this colder, oxygen-deficient water may occur after a heavy rainfall. If the rainfall is of sufficient quantity, it produces a mixing of the pond water as the cold rainwater sinks to the pond bottom. It displaces the oxygen-poor water, which mixes with the water in the upper levels of the pond. The result is a fish kill due to insufficient oxygen in the water.
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