Genetic diversity and population structure of sea trout in Gulf of Finland: implications for conservation and management Riho Gross, Marja-Liisa Koljonen, Oksana Burimski, Jarmo Koskiniemi

There are approximately 1000 sea trout populations in the Baltic Sea of which about 500 reproduce naturally in Baltic rivers, including about 100 populations in Gulf of Finland (HELCOM 2011). Based on electrofishing surveys of parr densities, the sea trout populations in the Gulf of Finland are in an adverse state due to excessive fishing pressure, obstacles to migration, habitat degradation and variation in water flow. Stocking of sea trout is widely practiced with the aim of increasing their production. Potential threats: – irreversible changes of the genetic composition of native populations due to direct (stocking for enhancing purposes) or indirect (immigration of hatchery fish from neighboring rivers) impact; – non-natural selective pressures under captive conditions (domestication effect) or loss of genetic variation through genetic drift and inbreeding (that always occur in populations of restricted size such as hatchery stocks) may compromise local adaptations of the native populations and pose threat to the evolutionary potential of the species. In contrast to the Atlantic salmon, the current knowledge of genetic diversity and structure of sea trout populations in the Baltic Sea is very limited. Only local populations in different countries (e.g. Denmark, Sweden, Poland, Lithuania) have been studied by using various genetic markers but information about the populations in the whole Baltic Sea or even within different major subdivisions of it (including the Gulf of Finland) is still missing. Background

Average densities of 0+ trout in Estonian (EE) Finnish (FI) and Russian (RU) rivers in Gulf of Finland (ICES 2012) Sea trout smolt releases (*1000) to the Gulf of Finland by country in

Aims of the study to reveal the genetic structure of sea trout populations within the Gulf of Finland to estimate the level of genetic variation of sea trout populations within the Gulf of Finland to give general recommendations for management

Material Sampled sea trout rivers in the Gulf of Finland (22 populations ) (16 populations ) (12 populations)

Sampled sea trout rivers in the Gulf of Riga and from Estonian islands 7 populations 5 populations

Methods: Genomic DNA was isolated from fin clips or muscle tissue 15 microsatellite loci: Ssosl417, Str60INRA, SSa407, SSosl1311, BS131, SSosl438, Strutta58, SSsp2201, Str15INRA, OneU9, Str73INRA, Ssa85, Str85INRA, SSsp1605, Ssa197 PCR amplification products were separated by capillary electrophoresis on AB3500 (Tartu) and AB3130 (Helsinki) Genetic Analyzers and the sizes of the microsatellite alleles were determined using Genemapper software Allele sizes of microsatellite loci were calibrated and standardized by exchange of reference samples

Data analysis The genotype data were analysed by standard population genetic analysis tools (GENEPOP, ARLEQUIN, FSTAT, POPULATIONS, MEGA) Genetic diversity was estimated as an allelic richness and an average observed/expected heterozygosity of populations The level of population differentiation was esimated by pair-wise F ST values Hierarchical distribution of genetic diversity was estimated by AMOVA method Genetic similarities/dissimilarities between populations were estimated based on pair-wise D A genetic distances and depicted graphically as an unrooted neighbour-joining dendrogram

Results and Discussion

Genetic similarity of sea trout populations: NJ dendrogram (Nei’s Da distance) Population structure

Average genetic diversity of sea trout populations in different geographic regions

Estonia + Luga, Sista FinlandFin/Rus Russian Karelia Genetic diversity of sea trout populations in the Gulf of Finland

Differentiation of sea trout populations global F ST = % - between geographic regions 6.0 % - among populations within the geographic regions 89.2 % - within populations Gulf of Finland: EST_G. FinFIN_G. FinFIN/RUS_G. FinRUS_G. FinEST_G. RigaEST_Islands EST_G. Finland FIN_G. Finland FIN/RUS_G. Finland RUS_G. Finland EST_G. Riga EST_Islands Average from other Average F ST within (along diagonal) and between regions:

Conclusions The average level of genetic variation of sea trout populations in Gulf of Finland was comparable to that in Gulf of Riga but a little bit lower than in Estonian islands. Within countries, the level of genetic variation among populations was relatively similar in Estonia and Russia but more variable in Finland and Finnish/Russian border area. Populations with the highest genetic variation should be considered as a basis for forming hatchery stocks within each region. The overall level of genetic differentiation between studied populations in Gulf of Finland was moderate (F ST = 0.108). The populations were most differentiated in Finland and Finnish/Russian border area and least differentiated in Estonia and Russia. Based on genetic distances, the populations clustered well according to the geographic regions. These 4 geographic groupings should be managed separately. The results of our study allow to propose genetic management and conservation units for sea trout in the whole Gulf of Finland and enable genetic data based planning for hatchery rearing and enhancement releases.