Anyone who has ever had their breath taken away after sitting in a car that’s been parked in the summer sun will tell you that even a momentary drop in oxygen can turn things upside down. The same holds true for fish if the dissolved oxygen in a pond or lake crashes.
According to Eric Brinkman, district fisheries supervisor for the Arkansas Game and Fish Commission in Hope, fish kills associated with low oxygen levels can be common in many small bodies of water during late summer.
“It can happen on any body of water or a section of a body of water, but the most common places we get fish kills like this are smaller ponds on people’s property,” Brinkman said.
Brinkman says the factors leading up to a fish kill because of low oxygen are more complex than the simple fact that hot water holds less oxygen.
“There’s a lot going on in summer,” Brinkman said. “Fish are cold-blooded creatures, so their metabolism rises with the temperature. A higher metabolism means more activity and more oxygen demand.”
The amount of fish in the system also is at its peak during the summer. Fish and insects that hatched all spring add more demand for the dissolved oxygen available.
“Summer also brings an abundance of vegetation, phytoplankton and zooplankton,” Brinkman said. “Which all use oxygen as well.”
Everyone who’s had third-grade science learned plants provide oxygen when they create sugars through photosynthesis, but plants also use part of that oxygen when they burn those sugars to survive.
“An abundance of fish, insects and plankton can consume a lot of dissolved oxygen, especially at night or during prolonged periods of cloudy weather, when photosynthesis slows due to lack of sunlight,” Brinkman said. “Typically fish kills from depleted oxygen will occur in the very early morning, just before sunup oxygen levels are at their lowest.”
Fish kills resulting from low oxygen also can happen during sudden events called “turnover.” During the hottest part of summer, water will separate, or stratify, into two distinct layers. The upper layer will be warmer and contain most of the oxygen. The bottom layer will be cold, and contains little or no oxygen. Most fish will be in the upper layer of the water column, often very close to where the two sections meet – called the thermocline. However, a cool rain or extreme cold front can cool the surface layer rapidly, which causes it to drop to the bottom of the pond, forcing the oxygen-poor layer to the surface zone holding the fish. The rapid drop in oxygen causes large fish kills, including all species present.
No matter the cause of oxygen loss, the best solution is aeration.
“Oxygen diffuses into the water from the surface quicker if there’s a lot of splashing and wave action,” Brinkman said. “On our hatcheries and on many farms with the proper equipment, a paddle-wheel aerator will get oxygen back into the system efficiently. A lower-cost option for ponds and small lakes is an aeration fountain to cause an adequate disturbance.”
Brinkman says once fish begin to go belly up, there’s not much that can be done, but investing in a fountain aerator definitely guards against oxygen loss and helps prevent the water from stratifying.
Another piece of advice for would-be pond owners is to limit the maximum depth of any small pond you build to less than 10 to 12 feet. Shallower systems do not stratify or turnover as easily.
“Many people think you should have some deeper water for fish to use as refuge during the hot summer months, but it’s actually a bad idea when you’re talking about smaller ponds,” Brinkman said.
Anything that adds nutrients to the system during summer also can increase the demand for oxygen. Overabundant fertilization, runoff from agriculture or sewage treatment areas and livestock waste all can increase the fertility of the system too much and eventually lead to a crash.
Telltale indicators that you may have a turnover occurring on a pond are an overnight change in the water color from relatively clear to a “chocolate milk” appearance, a foul, rotting smell and fish opening their mouths, or “piping,” at the surface of the water. These symptoms are much more prevalent in the morning just as the sun rises.
If a person sees a fish kill, they can call their local AGFC office and ask to speak to a fisheries biologist. They can walk you through a series of questions to determine whether the kill is due to oxygen depletion or if other factors are at play. A list of regional offices is available at http://www.agfc.com/aboutagfc/Pages/AboutRegionalOffices.aspx