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Depending on the type of biological control that you use, you may need to get a permit from your local department of wildlife or agriculture. Some species that can control lake weeds are considered invasive in particular areas, and you may not be allowed to introduce them. Always learn what your local laws are before introducing a new animal or fish into your lake. Mute swans can be released onto small lakes. As they feed on both submerged plants and algae, they may be ideal to control vegetation. Keep in mind that swans require extra care, husbandry, and protection from predators. Also known as the white amur, grass carp feed on the submersed plants in your lake. They are usually introduced into a lake to control vegetation. Keep in mind that grass carp will eat almost any submerged vegetation, not just invasive species or weeds.   Grass carp will not help much with duckweed or watermeal. It is recommended that you stock 15 to 30 fish per acre of lake. Get carp that are at least ten or twelve inches long.  Grass carp can be invasive. Check your local laws to make sure that you are allowed to use them as a biological control before you release them into your lake. If you want to target a specific weed, you may look into using insects as form of biological control. You will need to find insects that are host-specific; this means you pick a species of insects that will only eat the specific weed that you want to remove. You can research your particular weed to see if there is an insect species that may help. You might also reach out to your local department of wildlife or agriculture, Noxious Weed Control Board, or an ecologist. Do not release non-native insects until you have verified that they will only eat the plant in question and that they will have no other impact on your local ecosystem.
Check your local laws. Introduce swans to the lake. Release grass carp into the lake. Research which insects may be helpful.