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Twenty-seven invasive alien species in rivers and lakes


1.1 Norwegian ecosystems will achieve good status and deliver ecosystem services.


Number of invasive alien organisms in the following major ecosystems: marine and coastal waters, rivers and lakes, wetlands, forest, mountains and cultural landscapes
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Numbers of different groups of invasive alien species in Norway that are associated with rivers and lakes and that the Norwegian Biodiversity Information Centre has placed in the categories ‘severe impact’ or ‘high impact’. In all, 27 freshwater species have been registered in these categories.

Are we moving in the right direction? Published by the Norwegian Environment Agency

Some invasive alien species have more impact than others on the chances of achieving good ecosystem status in rivers and lakes.

A great deal of work is being done to reduce the range of the salmon parasite Gyrodactylus salaris and to prevent its further spread. In total 50 river systems have been infested by the parasite. Now the parasite is either eradicated or in process of being eradicated from 43 of the 50 infested river systems. Thus, good progress is being made towards a situation where this species will no longer affect the chances of achieving good status in rivers and lakes. 

For other invasive alien species in rivers and lakes, we only have sufficient resources at present to take action in the highest-priority areas. Unfortunately, alien fish species are frequently introduced to new rivers and lakes, and there are insufficient resources to follow this up properly. 

Once alien fish species are firmly established in a river system, it is difficult and costly to eradicate them, and it is a complicated process to restore an ecosystem to its natural state. Some people release fish illegally in lakes and rivers where they do not occur naturally in order to stock waters with new and exotic species for recreational fishing. It may be possible to discourage this by providing more and better information on the potential impacts of such introductions on biodiversity. 

Alien water plants such as Canadian pondweed and Nuttall’s pondweed are also a threat to freshwater ecosystems. The two pondweed species have now been found in more than 100 localities. They can form dense stands in lakes, crowding out other species and  impoverishing the natural aquatic vegetation. They can also cause eutrophication, since they take up nutrients from the sediments during growth and release them to the water column during decomposition. The spread of pondweed to new localities is often discovered too late, when control and eradication measures are no longer practical. The most cost-effective measure for dealing with the two pondweed species is to provide information to the general public in order to avoid further spread to new areas.