| Summary: | The River Vuoksi is the largest Finnish-Russian cross-border river connecting Lake Saimaa in Finland and Lake Ladoga in Russia. The valuable salmonid populations in the river were abundant and healthy still a hundred years ago, enabling also recreational and professional fisheries. The populations, however, started to decline due to human influence such as construction of hydropower plants (HPPs) that obstructed free migration of the fish, dredging of rapids that had been important salmonid breeding and nursery areas, and deterioration of the quality of water by industry. The water quality has during the past decades improved and does not prevent salmonid reproduction anymore. However, the four HPPs in the river still hinder fish migration. In addition, most of the historical salmonid reproduction areas are still non-existent, although the restoration actions carried out have gradually started to improve the situation. However, the short-term river flow regulation and hydropeaking for hydropower production risk salmonid reproduction also in the restored areas. Stocking of hatchery fish has been conducted on the Finnish side, but mainly for the fisheries purposes. Currently, the Lake Saimaa landlocked salmon (Salmo salar m. sebago), that is known to have migrated downstream from the lake to breed in the River Vuoksi above Imatrankoski, is critically endangered, and the Lake Ladoga salmon is endangered. Prior to the construction of the HPPs, the Lake Ladoga salmon migrated from the lake to breed, e.g., in rapids as far as right below Imatrankoski. Natural reproduction of brown trout (Salmo trutta) is also very scarce in the River Vuoksi, especially on the Finnish side, and not much is known of European grayling (Thymallus thymallus) populations existing in the main stream and tributaries. In this study, we aimed at gaining more understanding of the genetic characteristics of the River Vuoksi salmonids, and possible genetic differentiation of the populations. This information would be important for any conservation ...
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