Elodea canadensis

In the latter half of the 19th century and the first half of the 20th century, this species was extremely problematic to Europe, having spread rapidly throughout and causing major environmental problems (CABI, 2005). This waterweed can have a general negative impact on the functioning of the aquatic ecosystem and it will outcompete native aquatic plants. In addition, it can impede water flow and adversely affect recreation activities (CABI, 2005). Extracts from this species reduce the growth of several aquatic primary producers, among them epiphytic algae and cyanobacteria isolated from different submersed macrophytes (Erhard & Gross, 2006).

From 1977 onwards the Canadian pondweed Elodea canadensis invaded the lake and established dense covers over large parts of the shallow areas. The crayfish were gradually excluded from areas covered by Elodea, while the yield from remaining areas remained relatively constant. Direct observation by test-fishing and diving confirmed the virtual absence of crayfish within the Elodea stands. There are claims for other Norwegian localities that invading waterweed have been detrimental to crayfish populations, yet Lake Steinsfjorden represents the only well-studied case.