Every year, starting about now and continuing through May, a lot of farm ponds in Mississippi are plagued by “pond scum.” I pass a few farm ponds on my way to work every morning. When I see the ponds developing their annual ring of pond scum, I know to expect a few phone calls.A lot of different algae grow or float on or near the surface of ponds. The “pond scum” I am referring to floats on the surface in large mats. The mats ring the shoreline, often extending 2 to 3 feet from shore, but the mats may grow out farther or they may blanket small pockets or coves in the pond.
Pond scum is filamentous green algae or blue-green algae. It grows on the bottom in shallow water beginning in early spring. The algae multiply and the mat thickens as the days lengthen, the solar energy increases and the water warms.
Through the process of photosynthesis, the filamentous algae, like all green plants, produce oxygen. Much of that oxygen is released into the water and benefits fish and other aquatic life, but some of the oxygen is released under the mat. In time, the amount of oxygen trapped under the mat increases, forms large bubbles, and then floats the mat off the bottom. Literally overnight, a seemingly weed-free pond is ringed with matted algae.
From a fisheries biologist’s perspective, pond scum is not a problem and it is not necessarily a symptom of other problems. I have seen it grow in very clear ponds and those with only 1 foot of visibility. It occurs in nutrient-rich and nutrient-poor ponds. Except for the unusual case where it covers entire pockets of the pond, the pond-scum fringe is usually narrow and does not interfere with fishing. In four to six weeks, it goes away just as quickly as it appeared.
Fisheries problem or not, many pond owners don’t like it and want to get rid of it. That is easily accomplished by stocking plant-eating triploid grass carp, also called triploid white amur, according to Dr. Wes Neal, fisheries extension specialist at Mississippi State University. Do not stock common carp, and do not stock diploid grass carp that can reproduce if they escape.
Now is the time to stock. The water is cool, so the fish transport and handle well, and survival is high. Some pond owners think the pond scum problem is over because they haven’t seen it in 9 or 10 months. If you had pond scum last year, you will have it again this year.
Stock five fish per acre for prevention, 10 per acre if the pond has a recurring pond scum problem. This stocking rate will control pond scum and many other nuisance aquatic plants, Neal said. The recommended stocking rates are most effective if the grass carp can graze the algae while it grows on the bottom and before it becomes a full-blown, floating problem.
Purchase the fish from a reputable private hatchery; go to msucares.com/pubs/publications/p2525.pdf for a directory of fish farms and vendors selling triploid grass carp. Mississippi Department of Wildlife, Fisheries and Parks does not provide or sell triploid grass carp to the public.
Grass carp are a preferred forage of largemouth bass. You can stock fingerling triploid grass carp into a new or recently renovated pond if bass have not yet been stocked. Stock 9- to 12-inch triploid grass carp if largemouth bass have been in the pond for one or more years; even bigger grass carp should be used if the pond has bass larger than 4 pounds.
Pond scum can be controlled with several chemical herbicides licensed for use in aquatic systems, but Neal does not consider their use necessary. The pond scum will die out naturally in only a little longer time than it takes the herbicides to kill the algae, and you will need to apply herbicides every year. I have controlled pond scum in my pond for nine years with a single stocking of triploid grass carp.
One last piece of advice. Grass carp are strong swimmers and generally swim into the current when in flowing water. It is possible for grass carp to escape from your pond if a heavy rain causes water more than several inches deep to flow over the spillway. Some pond owners install a screen across the spillway to prevent fish from escaping.
Although this sounds like a good idea, the screen will clog with floating debris and stop the flow of water. This could cause the water to rise and flow over the dam, which could cause the dam to fail. Don’t jeopardize the dam to save a few fish.
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