Deadly ‘turnover’

first_imgBy Mike IsbellUniversity of GeorgiaThere were dead catfish out there all right. A big one floatednear the overflow pipe at the dam. Jack, who had called meearlier that morning, had already removed several others he’dfound floating in the water and tossed them out on the grass.”What do you think is killing the fish?” Jack asked as we stoodnear the edge of the pond.But before I told him what I suspected, I asked him a fewquestions. His answers gave me clues to what might have happenedin the pond.The first clue was that the dead fish were all large fish — nosmall ones were dying. They had all died within the past twodays. The sky had been overcast for several days. And it hadrained a really cold, hard rain for a long time right before theybegan to die.Forces at workJust looking at that pond from the surface, it’s hard to imaginethe dynamic world below. But in this mysterious, watery world,biological, chemical and physical forces are at work.We all know that fish require oxygen. Oxygen dissolves in wateras it mixes at the surface, and it’s produced in the water byphotosynthesis of aquatic plants.In almost every pond, oxygen levels will change daily. Thehighest level occurs in the mid to late afternoon and the lowestin the hours just before sunrise.A healthy algae bloom and aquatic plant populations will produceenough dissolved oxygen to support life in a pond throughout a24-hour period.Layers of waterThe coolest water in the pond, having the greatest density,remains in a layer near the bottom. The warmer water, being lessdense, moves to the surface. Swim to the bottom and you’ll feelthe temperature difference.Actually, there are three layers of water. Almost no oxygen isproduced in the cold-water layers near the bottom, becausethere’s no light for the aquatic plants.If you’ve stayed with me so far, I’ll tell you what I thinkhappened in Jack’s pond.The overcast days, without the bright sunlight, resulted in lessoxygen produced in the water. But fish don’t die just becauseit’s cloudy. We have cloudy days all the time. Something elsehappened.The culpritRemember, Jack said it rained really hard about two days earlier.And rain water is cold. The surface water, now suddenly cold anddense, begins to sink, which forces the warmer, bottom water tothe surface. And the bottom water is low in oxygen.Ponds can “turn over” during the summer following heavy rains.This “turnover” mixes the water and can cause the entire pond tobecome oxygen-starved. When that happens, fish will sometimesbegin to die, and usually the largest ones die first.This is what I suspect happened.Luckily, Jack has a mechanical aerator in the pond to agitate thewater and mix oxygen back in the water. Running it at night untilthe pond stabilizes will help reduce the severity of the “waterturnover.”(Mike Isbell is the Heard County Extension Coordinator withthe University of Georgia College of Agricultural andEnvironmental Sciences.)last_img