The four largest Swedish lakes, Vänern, Vättern, Mälaren, Hjälmaren, host important commercial fisheries for char, salmon, trout, whitefish, vendace (cisco), perch, pike-perch, pike and eel, i.e. highly diverse biological resources. Case studies illustrate physical, chemical and biological impacts on some of these commercial species caused by constructions of dams and ship canals, eutrophication, and overexploitation. Although some original species have been lost and a few new species have been added, the recent human interference has basically caused major shifts in dominance of the fish community structures because of eutrophication, alterations in the abundance of eel or crayfish, and due to overfishing. The latter is in some cases caused by the Great Lake Fishery Paradox—in an environment with several predators and competitors, but with ample food resources, especially salmonid fish but also species like pike-perch may adapt a life history favoring growth over sexual maturation. If harvested at a conventional size these populations will decline rapidly due to too small spawning stocks.
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