Lake Vättern: Effects of Exploitation, Eutrophication, and Introductions on the Salmonid Community

Abstract

Vättern, the second largest lake of Sweden, contains 28 species of fish, of which Arctic char (Salvelinus alpinus) and whitefish (Coregonus spp.) are the most important economically. There is both a commercial fishery and a sport fishery. The former has decreased steadily, while the latter is increasing. A modest eutrophication is occurring, due amongst other factors to an increasing input of phosphorus and resulting, for instance, in a decrease in transparency.The lake is, however, still to be characterized as typically oligotrophic. The input of polluting matters is alarming, especially as regards toxic substances. Exploitation of fish populations has on the whole increased, although the number of commercial fishermen decreases. The fishery on the char population and military activity may in the long run result in over-exploitation. An inverse correlation between the catches of char and whitefish has been explained as a result of interaction between the two species. An obvious increase in catches of the Coregonus species (whitefish and cisco) is taken to be the result of eutrophication. Introductions of "exotic" species have only resulted in one recorded case of reproduction of rainbow trout (Salmo gairdneri). Several promising experiments with the release of hatchery-reared fish, e.g. landlocked salmon (Salmo salar), have been carried out. Growth rate of several fish species in Vättern has appeared to be faster than normal.