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66 • Nebraska Game and Parks Commission less-palatable or smaller species. They are not successful in controlling milfoil, coontail, watermeal, duckweed, pond lilies, or most algae species, especially Chara spp. Grass carp are very inefficient in converting food into body tissue. Their excrement is high in nutrients, which promotes the production of more vegetation. Too many grass carp can result in total elimination of the desirable submergent vegetation that was utilizing or tying up available nutrients. This can result in planktonic algae blooms. Grass carp can live for decades. Once they are stocked in a pond, it is virtually impossible to remove them without killing everything in the pond or draining it. Since the pond owner has no control over which plants grass carp eat or how much they eat, they are rarely, if ever, recommended for vegetation control. Spot treatment with chemicals is a better option. If a pond owner insists on stocking grass carp, they should be stocked at a density of no more than 5 per surface acre of vegetation coverage to control plants or 15 per acre to eliminate them. Always start at a low stocking density and add fish as needed. Stocked fish should be at least 10 inches long to prevent immediate predation by largemouth bass and birds. It may take two to three years before plant growth is reduced. Since grass carp require large rivers to spawn successfully, they may need to be restocked at less than 5 per surface acre every five years to compensate for natural mortality and the likelihood some may have been flushed-out during high water flows. Restock only when vegetation is causing a problem again. Barley straw can be used as a biological control for algae. It is normally applied at a rate of 225 pounds per surface acre of water. As the straw decomposes in the pond, it produces a growth-inhibiting chemical that will prevent new algae growth but may not kill off what is already present. For this reason, it should be applied in March or April, before the algae starts to grow. Barley straw doesn't work on all kinds of algae. The first step is to properly identify which species you have. If this isn't possible, experiment with a algae sample taken from your pond. Place it, along with an adequate amount of pond water, in a large tub or tank. Apply barley straw and determine if control occurs. It may take 2 weeks to see any results, longer if the water temperature is below 68 degrees. Depending on the availability and cost of barley straw and the size of your pond, it may be easier to just do the treatment if you had major algae problems in the past, and then monitor results. Keep in mind the decomposition process uses oxygen, which will reduce oxygen levels in the pond and stress or kill fish if the straw is over-applied. Although many algae species are important food items for zooplankton, high density levels of algae are considered undesirable and somewhat difficult to control. If algae blooms keep recurring once control measures have been tried and gizzard shad are present in high numbers, the shad need to be controlled or eliminated. Shad are very efficient filter feeders and can decimate zooplankton populations that naturally feed on various algae species. Pond Liners Covering the pond bottom with perforated plastic sheeting or fine mesh landscaping fabric can effectively prevent rooted vegetation from growing. The fabric also limits nutrient exchange between the pond bottom and the water. Plastic sheeting 4 mm or thicker should be weighted to keep it in place and perforated to allow gases to escape from the pond bottom. Large sections of window screen can also be placed on top of submerged plants and weighted down. This will compress and shade the plants, which should cause them to die in a couple of weeks. The process can then be repeated in another area. Water Level Manipulation Lowering the water level of a pond can be an easy way to control unwanted aquatic Contact UNL Cooperative Extension staff about algae identification and control.