5 | Prevention and control

Once a non-native species is established in the Great Lakes, it is nearly impossible to get rid of it. Therefore, it is extremely important to prevent introductions of new species. Because ballast water is the primary pathway of species introduction, efforts have been focused on preventing the introduction of exotics through ballast water treatment. The most common method of treating ballast water is open-ocean exchange, the act of replacing freshwater ballast with seawater during the voyage. Because saltwater-dwelling species generally cannot survive in freshwater, this water exchange can reduce the likelihood of a new exotic species establishing a population in the Great Lakes. Vessels entering the Great Lakes and St. Lawrence Seaway are required to replace their ballast water with seawater during their voyage. If they do not perform open-ocean ballast exchange, they are not permitted to release ballast in the Great Lakes.

Open-ocean ballast exchange, however, is not the ideal solution as it is sometimes unsafe and not completely effective in preventing introductions. Research is underway on different ballast water treatment options to supplement or replace open-ocean ballast exchange. Some of the research includes:

• filtering ballast water to remove nonindigenous organisms
• heating ballast water or using ultraviolet light to kill invasive species
• using a hydrocyclone, a type of centrifuge, to separate organisms and sediment from water to be discharged into the lakes.

Controlling the numbers and distribution of existing nonindigenous species in the Great Lakes is still extremely important in the ongoing battle against invasive species. There are a variety of methods of controlling existing populations. Some examples include:

• Biocides: Chemicals, such as the lampricide TMF (used to control sea lamprey populations) and herbicides on aquatic plants, are sometimes used to reduce or eradicate local populations of exotic species.
• Barrier construction: Barriers use a variety of methods, including sound waves, electrical impulses, and visual and physical deterrents. These barriers can help prevent the spread of exotics in smaller waterways like canals and streams.
• Physical removal: Harvesting small populations of aquatic plants, for instance, can act as a temporary control in smaller inland lakes and waterways.
• Biological control: Very carefully selected non-native species, usually predators, are introduced to control population growth of another invasive species. A good example of this is work done with insects that specialize in eating purple loosestrife.
• Public education: You can prevent the spread of aquatic nuisance species! Click to the next page to find out how.

Graphic: Rusty Crayfish from Lake Superior. Courtesy Jeff Gunderson/Minnesota Sea Grant.

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