Invasion of vendace Coregonus albula in a subarctic watercourse

Vendace Coregonus albula have recently invaded the Pasvik River system, a s ubarctic watercourse on the borderline between Norway and Russia, after hav ing been introduced into the Finnish headwaters in the 1950-1960s. The wate rcourse is characterised by several consecutive lakes and reservoirs. Venda ce were first recorded in the upper part in 1989, and had by 1995 invaded t he whole 120 km long water system. In the upper part, vendace have become t he dominant fish species in the pelagic zone, whereas whitefish Coregonus l avaretus have been relegated to the profundal and littoral zones. In the lo wer part, whitefish were by 1995 still dominant in the pelagic habitat, but vendace constituted 30% of the pelagic catches after being first recorded there in 1993. Vendace have established reproducing populations in the whol e watercourse, with the 1990 and 1991 year-classes being particularly stron g and important for the downward displacement. The new occurrence of vendac e in the Pasvik River system probably represents the world's northernmost d istribution of the species. The invasion appears to have a major ecological impact on the native communities, and represents a threat for both biodive rsity conservation and the commercial fishery in the watercourse. (C) 1999 Elsevier Science Ltd. All rights reserved.