ISU Extension and Outreach
Fisheries and Aquaculture Specialist
In the summer, the high temperatures can result in management issues for pond owners with excessive aquatic vegetation being the most problematic. Excessive nutrients (often from the surrounding watershed) and shallow pond depths along with warm water temperatures can create an ideal habitat for aquatic weed growth. Developing an aquatic weed management plan to control excessive plant depends on correctly identifying the problem weed(s) and selecting control methods that are compatible with desired pond use. It is important to remember that some aquatic vegetation is important to a pond fishery.
Aquatic Vegetation Management
Preventive Methods. — It is easier and less costly to prevent weed problems than it is to control them once they develop. Careful pond site selection, proper pond construction, and watershed practices are the first steps in preventing aquatic plant problems.
Biological Control Methods. — The grass carp is a practical and economical way to control certain types of pond plants, i.e., submersed plants with tender, succulent vegetation. It is recommended that these fish only be stocked (1-2 fish/acre) in ponds that have an excessive plant growth and not part of the initial pond stocking plan. Stocking number may need to be increased in ponds that have a rich and productive plant community but too many grass carp can result in ponds with excessive plankton blooms (green coloration) that can have harmful effects on the pond fish community. If the pond already has a large prey base, such as largemouth bass, the landowner should stock 8- inch long or longer grass carp to enable them to escape predation.
Mechanical Methods. — Various types of aquatic weed cutters and harvesters have been developed for canals and large reservoirs. Use of these machines is not practical in small fish ponds. Early manual removal of weeds by seining or raking can prevent some weed problems.
Cultural Control. — Altering the environment, or cultural control, can be used to manage aquatic weeds. Examples of cultural control for aquatic weeds include, in part use of rocks or other riprap along shorelines, covering bottom sediments with black plastic in small ponds, use of nontoxic dyes that inhibit submersed plant growth and aeration that gives fish a refuge for low oxygen conditions while sometimes alleviating plant growth.
Chemical Control Methods. — The first step in successful chemical control is accurately identifying the problem plant. Plant identification assistance is available through offices of Iowa State University Extension and Iowa Department of Natural Resources. A herbicide that is labeled for aquatic use may be selected for efficacy for that specific plant. The user must read and fully understand the herbicide label (noting restrictions) before applying the herbicide. The best time to apply herbicides is often spring when the water temperatures are cool and the plants are actively growing. Chemical controls later in the summer can result in fish kills if too much vegetation is controlled and undergoes decay (takes up oxygen) as warmer the water, less oxygen is available for the fish.
Integrated Plant Management. — Consider herbicides a temporary and often costly control method. To achieve long-term weed control, use a combination of recommended aquatic plant control methods. The best long-term control is to intercept the flow of nutrients into the pond through modifications of land-use practices or through the use of small wetlands to filter runoff.
Along with the excessive plant growth, there may be some fish loss due to poor water quality with low oxygen being the primary cause. The best time of the day to check for possible low oxygen is just before sunrise when oxygen in the water being the lowest; fish can often be observed swimming along the pond surface seeking available oxygen. Ponds that have intense plankton blooms often suffer from low morning oxygen levels.
Fish kills can occur during the summer when ponds undergo turnover. Temperature has an indirect effect on the fish as a result of water stratification. As temperature changes, so does water density. The temperature at which water is at its maximum density is 39.2°F. In early spring, pond water temperature is uniform from surface to bottom. As the days become warmer, the surface water becomes warmer and lighter. By early summer, the pond may become stratified into three layers: (1) the upper oxygen-rich, warmer layer; (2) the transition layer which is characterized by a rapid change in temperature; and (3) the lower, oxygen-poor, cooler layer. The signs of summer turnover are a rapid change in water color to a brown, black, or gray, a putrid odor, and fish gulping at the surface. These symptoms are usually observed after periods of heavy wind and rain, cold rain have higher density that sinks to the pond causing a turnover. Turnover also may occur with the loss of a phytoplankton bloom resulting in increased sunlight penetration, warming the water to greater depth.
To determine the extent of fish loss, the pond owner can purchase a small minnow seine and collect fish from the pond shallows later in the fall or the next spring. Before restocking the pond, the pond owner should first check to see if indeed all the fish died or just certain species.
Additional information can be found in ISU Extension and Outreach publications PM1352A https://store.extension.iastate.edu/Product/4756 Water Quality and PM1352J https://store.extension.iastate.edu/product/4765.