Ever since I moved to Kansas I’ve heard fishermen talk about lakes, reservoirs and ponds “turning,” sometimes involving a fish kill, but always resulting in that body of water becoming murky and stirred-up for awhile. Sunday at church a friend who has a nice pond told me he’d just had a bad fish kill in his pond. He said that this spring the pond had become completely covered with algae. Last week one morning he noticed the algae had totally disappeared overnight and the next morning lots of fish were jumping and sucking on the surface of the water, feeding on insects he’d hoped. That afternoon however his wife called to tell him there were lots of dead and dying fish. It then became apparent that the jumping fish were not feeding but attempting to get oxygen which was somehow being depleted from the water. Now that all is said and done, he figures he’s lost 300 to 400 fish of multiple species, including channel cats in the twenty inch range and grass carp that were over four feet long. I figured it was now time to research and find out exactly what happens when a pond or lake “turns.”
I spoke with David Breth, Fisheries Program Specialist with the Kansas Department of Wildlife, Parks and Tourism to glean from his wisdom. Breth says most bodies of water turn in the fall, and even though spring events are still called turning, what happens in the spring is most often a little different occurrence. On the bottom of any body of water, especially farm ponds is a layer of decaying matter composed of dead plants and animals. Normally the bottom layer of water containing that matter is colder than the rest and remains at the bottom, and is very low in oxygen because it has used the oxygen to decompose all that dead matter. In the spring, however water coming into the pond in the form of rain and drainage runoff is just as cold as the bottom layer of water and thus stirs up that bottom layer and mixes it with the rest of the water in the pond. The sudden release of nutrients from that decomposed matter fuels an intense algae growth and bloom. When the algae growth becomes more than the pond can healthily support, if begins to die and all the oxygen in the water is used to decompose that dead and dying algae, thus depleting the water of oxygen for the fish which in turn die too. Since this is a natural occurrence, there is no way to stop it from happening, but aerating the pond by pumping air into the water as soon as possible may stop or halt a fish kill. The only other solution to save the fish is to remove them immediately to fresh water until the decomposition process in the pond has stopped using all the oxygen, which can be from a few days to a couple weeks.
During the summer, all bodies of water heat up and “stratify,” or divide into three basic layers of water. The top layer naturally becomes warm, lighter and less dense, the bottom layer stays cold, heavy and low in oxygen, and the middle layer called a “phermocline” is sort of a combination or the other two; colder at the bottom and warmer at the top. Breth says in the summer, fish are often found in this middle layer of water because it contains the most oxygen. In the fall, the top layer of water becomes colder and starts to sink, stirring up the bottom layer and forcing it to the top with all the dead and decaying matter, and suddenly the lake has “turned” or “flipped” as it is also known.
Breth told me our Kansas lakes and reservoirs turn almost every year, resulting in stirred-up and cloudy water, but rarely causing a major fish kill. He says 90% of all fish kills are caused by an intense algae bloom that dies and sucks the oxygen from the water as it decays; just what happened to my friend’s pond. Major fish kills because of algae blooms rarely happen on bigger lakes and reservoirs because there are always areas of the lake not affected to where fish can retreat and still find oxygen. Most large lakes are also stream and river fed, which constantly brings in fresh oxygenated water.
So now you know what happens when your pond turns on you. As a rule of thumb, the bigger the body of water, the less likely it will result in a fish kill, whether in the spring or in the fall. And if you see it suddenly happening in your pond, the only way to save your fish will be to somehow pump air into the pond, adding oxygen to the water, or to temporarily remove the fish; a pain in the neck for sure, but well worth it when the grandkids start haulin’ in jumbo bluegills or fat catfish one after another…Continue to Explore Kansas Outdoors.
Steve can be contacted by email at firstname.lastname@example.org.