The biggest culprit in fish kills is the water itself

Have you ever seen a multitude of dead fish in a pond or lake and wondered what caused it? Most pond owners blame it on turnover or the unruly neighbors poisoning the fish.

However, in most cases, neither turnover nor the neighbors caused the fish kill. In fact, most fish kills are influenced by the carrying capacity of the pond or lake.

Every pond or lake has a limit on how many fish can live in it at one time. When the carrying capacity has been reached, competition for food, oxygen, and cover increases and some fish won't get what they need thrive. It is at this point when stress on fish increases and the pond becomes more susceptible to having a fish kill caused by low oxygen or disease.

These are the two most frequent reasons for fish kills in Southeast Missouri. Although pollution and turnover can cause fish kills as well, they aren't very common.

Fish increase their activity and oxygen consumption during the summer. This is why a good majority of fish kills occur during the months of July and August. When oxygen levels are low, the larger fish in the pond are affected first. The type of fish can also be a factor. Bluegill, bass, crappie and channel catfish are more susceptible to low oxygen levels the bullheads, gar, bowfin, and carp.

The depth of water can play a role with oxygen levels as well. Shallow ponds will heat up quickly and they leave no refuge for fish to find cool water. So shallow ponds are more susceptible to fish kills than deeper ponds. If a pond is less than 6 feet deep, it is just a matter of time before it has a fish kill.

Aquatic vegetation can also play a role in oxygen levels because plants produce oxygen during the day and take oxygen at night. As the amount of sunlight decreases plants produce less oxygen. During extended periods of cloudy weather plants can affect oxygen levels by consuming more oxygen than produced.

Low oxygen levels can be detected when fish are seen on the surface of the pond appearing to gulp air from the surface. You are most likely to see this in the early morning -- that's the time of day when oxygen levels are at their lowest. A quick response is essential for saving the fish in the pond.

Rare in the winter

Winter fish kills are rare and usually occur in shallow, fertile ponds. Contrary to popular belief, ice on a pond doesn't cause fish kills. However, when ice is covered by snow, sunlight will not penetrate through to the water and no oxygen will be produced.

During early spring, fish are more susceptible to diseases. As the water temperature rises in the spring disease causing organisms increase. During this time, some fish's immune systems are weak because of spawning activities and the stresses of winter. These organisms can infect fish and may kill them. Disease related fish kills usually only affect one species of fish. In most cases it is better to let the disease run its course and allow the fish to repopulate on their own rather than treat the pond or diseased fish.

During periods of high rain, runoff from feed lots, crop land and fertilized lawns can lead to fish kills. Rain can wash nutrients and fertilizers into ponds. Proper watershed management can help in these situations. Divert excessive nutrients away from the pond eliminating the sources.

Pesticides and most herbicides directly entering a pond can be highly toxic to fish. These chemicals usually kill all species and sizes of fish. Most of the chemical fish kills can be avoided by reading all the label instructions and using the chemical for its intended purpose. If you suspect a chemical fish kill, look for sources within the watershed.

Every pond will turn over twice a year, spring and fall, when surface water mixes with water from the bottom of the pond. Turnover is usually a gradual process and is not harmful to fish; however, if temperature changes accelerate this process, although rare, fish can die due to exposure to poor water quality.

Fish kills are usually caused naturally and your pond doesn't have to be a casualty. Keep a well vegetated watershed, at least 8 feet of water, and less than 20 percent of aquatic vegetation to reduce the chances of having a fish kill on your pond.

For more information about fish kills contact the Missouri Department of Conservation in Cape Girardeau at (573) 290-5730.

Chris Kennedy is an agent with the Missouri Department of Conservation