Invasion of vendace Coregonus albula in a subarctic watercourse

Vendace Coregonus albula have recently invaded the Pasvik River system, a subarctic watercourse on the borderline between Norway and Russia, after having been introduced into the Finnish headwaters in the 1950-1960s. The watercourse is characterised by several consecutive lakes and reservoirs. Vendace were first recorded in the upper part in 1989, and had by 1995 invaded the whole 120 km long water system. In the upper part, vendace have become the dominant fish species in the pelagic zone, whereas whitefish Coregonus lavaretus have been relegated to the profundal and littoral zones. In the lower 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 whole watercourse, with the 1990 and 1991 year-classes being particularly strong and important for the downward displacement. The new occurrence of vendace in the Pasvik River system probably represents the world's northernmost distribution of the species. The invasion appears to have a major ecological impact on the native communities, and represents a threat for both biodiversity conservation and the commercial fishery in the watercourse. (C) 1999 Elsevier Science Ltd. All rights reserved.