Genetic structure of landlocked salmon, brown trout and European grayling in the River Vuoksi catchment (FIN-RUS)

Abstract

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 and management actions and for sustainable use of the populations in the River Vuoksi and tributaries. We analysed the genetic relatedness and diversity of landlocked salmon, brown trout and European grayling populations from the River Vuoksi and its tributaries. For comparative purposes, a number of populations outside our study area were also analysed. Only one salmon sample from the River Vuoksi on the Finnish side was obtained and could thus not be included in the analyses. The results of salmon on the Russian side suggest that the population is genetically distinct, and its genetic diversity is relatively high. The genetic diversity of the brown trout on the Finnish side was also rather high, but the sampled populations were in practice similar to the hatchery stock with no genetic differentiation to be found. Stocking has thus had an influence on their population structure. On the Russian side, the studied trout populations in the River Vuoksi tributaries belonged to so called “Lake Ladoga group” but were still genetically distinct and differentiated from each other and the other trout populations in the same group. Grayling in the River Vuoksi watershed formed two groups, one on the Finnish and one on the Russian side. However, the populations within the groups were genetically highly differentiated from each other, which may be a consequence of the small population sizes and genetic drift. To conclude, there were differences in the level of genetic differentiation and diversity among the River Vuoksi salmonid populations. It seems that on the Russian side, the salmonid populations form more distinct genetic entities. The same holds for grayling on both sides of the border. Stocking on the Finnish side has homogenized the genetic structure of the brown trout populations. In general, however, safeguarding and supporting natural production should be the principle means to preserve the genetic diversity of the salmonid populations on both sides of the border. The study was part of the “River flows – Life goes” RiverGo project (2019–2022) co-funded by the European Union.