Ponds are a unique and critical part of many farms and acreages across Iowa. Landowners and managers often have questions about how to manage pond water, wildlife, and vegetation. On this page, we try to help with information on the most common challenges and useful resources for pond managers!
Aquatic Vegetation Management
Read more about managing "pond weeds" in this encyclopedia article.
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.
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. Applications later in the fall often have limited control as the aquatic plants are no longer actively growing due to decreasing water temperatures.
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.
Read more about managing water quality in ponds with this encyclopedia article about cloudy or muddy water.
Pond complexity depends on the food webs involving many types of organisms. The simplicity of such systems is based on the limited number of fish species in them.
Fish for Stocking. — The few Iowa ponds that have cool springs (55°F and less) may be adapted for cold-water species such as trout and salmon. Most Iowa ponds are more suited for warm water species such as Largemouth Bass, Bluegill, and Channel Catfish.
Stocking Rates and Times. —In ponds free of all fish, Bluegills are stocked at 300, 3- to 5-inch fingerlings per surface acre of water, and Largemouth Bass are stocked at 100 3- to 6-inch fingerlings per surface acre. Channel Catfish fingerlings are stocked at 100 4- to 6-inch fingerlings per surface acre. Because Channel Catfish generally do not reproduce well in small ponds, they need to be restocked every two to three years.
Harvest Rates. — Mature Iowa ponds contain about 250 pounds of bluegills per surface acre. Bluegill harvest may start the second year after stocking. Limits on bluegill harvest will generally not be needed in most ponds because bluegill are usually plentiful. An excessive bluegill population will exhibit stunted sizes. Bluegill management requires regulation of bass harvest, such as minimum size limits.
Largemouth Bass populations in a balanced pond may approach 50 to 75 pounds per surface acre. Harvest of bass should not begin until three or four years after stocking. A 15-inch size limit is recommended, which means that only fish greater than 15 inches may be removed from the pond. If too many bass are removed and not enough bluegills, a pond with imbalanced populations of both species may result (few large bass and many small bluegills). A pond is “balanced” if the populations of individual fish species are adequate in number and size for suitable fishing. If the landowner is interested primarily in a large-sized bluegill fishery, no harvest may begin three years after initial stocking. Once half of the Channel Catfish are harvested, the pond should be restocked with 8-inch fingerlings.
Other Species. — Beyond the previously described species, pond owners may want to stock other species in their ponds. Walleye, Northern Pike, and Hybrid Striped Bass are highly sought after by anglers but care must be taken in mixing these fish with others. Large predators such as Northern Pike and Hybrid Striped Bass may actually prey upon Largemouth Bass. Crappie stocked into small ponds often result in a large population of small stunted and compete directly with Largemouth Bass. Hybrid sunfish should not be stocked in place of Bluegills as they will not provide the necessary forage for Largemouth Bass (majority being males due to hybridization).
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.
Temperature has an indirect effect on the fish as a result of water stratification. As temperature changes, so does water density with water having the density at 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. Pond turn over occurs when all of the layers mix and oxygen becomes lower in the entire pond. The signs of pond 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.