County Extension Director
Extreme temperatures can cause fish die-offs in Florida’s ponds, estuaries, lakes, and rivers. In large natural areas, these events usually do not cause permanent damage to existing fish populations. Fish die-offs in smaller ponds, although a natural occurrence, can be of concern to the pond owner.
Extreme temperatures can cause fish kills in your pond by reducing oxygen levels suddenly.
In some cases, winter weather can slow the spread of invasive, non-native fish species because some of these non-native fish, like tilapia, are especially susceptible to cold water. The cold water can kill fish outright or may weaken them so that they become more susceptible to disease. Cold weather can also cause a natural process called lake or pond-turnover. Turnover happens when cooled surface water sinks and mixes with deeper, warmer oxygen-poor water (like pouring cold milk into your hot coffee). This mixing, or turnover, with oxygen poor water can cause low oxygen-related fish kills.
As we head towards summer, water that gets too warm also can cause oxygen problems for fish. Warmer water can hold less oxygen than cooler water. Many oxygen-related fish kills happen in the middle of the summer when temperatures are hottest.
Fish need oxygen just as we do, and they absorb oxygen directly from the water as it passes over their gills. Oxygen enters the water primarily by photosynthesis (from algae and aquatic plants) and the atmosphere (wind mixing). The amount of dissolved oxygen (DO) in water depends on a number of factors, especially water temperature (cold water holds more oxygen than warm water) and the water’s biological productivity. For optimum health, most of Florida’s fish need DO concentrations of at least 5 parts per million (or 5 milligrams per liter or 5 mg/L). Fish can tolerate brief periods of reduced oxygen, but if DO levels drop below 2 mg/L, they can’t always recover; and if concentrations fall below 1 mg/L, fish begin to die. Oxygen-related fish kills usually affect many different sizes and species of fish, whereas cold temperature-related fish kills tend to affect only one or two species. If it is an oxygen-related fish kill, large fish tend to be affected first. Small fish can be seen gulping or gasping for air at the surface.
Fish diseases, parasites, and stresses from poor water quality, overcrowding, and human impacts can also lead to fish kills, but the reduction of dissolved oxygen in a water body is the most common cause of fish kills in Florida throughout the year.
Aerating your pond is one way in which to reduce the chances or severity of a fish kill.
For more information on pond management and fish health please contact your local County Extension office.
The Florida Fish and Wildlife Conservation Commission (FWC) tracks fish kill occurrences in Florida’s natural waters. This helps biologists determine if there are problems that may need further investigation or restorative measures. Residents can report fish kills in natural water bodies to the FWC at http://research.MyFWC.com/fishkill/submit.asp or call the FWC Fish Kill Hotline at 800-636-0511, or visit http://myfwc.com/.