Molecular Ecology (2001) 10, 2877– 2882 © 2001 Blackwell Science Ltd Blackwell Science Ltd SHORT COMMUNICATION FOUNDER EVENT IN ATLANTIC SALMON Identification of the origin of an Atlantic salmon (Salmo salar L.) population in a recently recolonized river in the Baltic Sea A. VASEMÄGI,*† R. GROSS,‡ T. PAAVER,‡ M. KANGUR,§ J. NILSSON* and L.-O. ERIKSSON* *Department of Aquaculture, Swedish University of Agricultural Sciences, S-901 83 Umeå, Sweden, †Institute of Zoology and Hydrobiology, University of Tartu, 46 Vanemuise, 51014 Tartu, Estonia, ‡Department of Fish Farming, Institute of Animal Science, Estonian Agricultural University, 1 Kreutzwaldi, 51014 Tartu, Estonia, §Estonian Marine Institute, University of Tartu, Viljandi mnt. 18B, 11216 Tallinn, Estonia Abstract The founder event in a recently recolonized salmon population in the Baltic Sea (Gulf of Finland) was investigated. To identify the origin of the founders, four wild populations and two hatchery stocks were analysed using six microsatellite loci. The results of assign- ment tests and factorial correspondence analysis suggest that the initial recolonizers of the river Selja originated from the geographically nearest (7 km) wild population (river Kunda) but as the result of stocking activities, interbreeding between recolonizers and hatchery individuals has occurred in subsequent years. Although the hatchery releases are out- numbering the wild salmon recruitment in the Baltic Sea at present, our results suggest that the native populations may still have an important role in colonization processes of the former salmon rivers. Keywords: assignment test, microsatellite DNA, multilocus genotypes, natural recolonization, Salmo salar Received 9 May 2001; revision received 23 August 2001; accepted 23 August 2001 Introduction Understanding the role of gene flow during the evolutionary dynamics of populations is important in conservation programmes both of wild and captive populations. High genetic differentiation among Atlantic salmon populations, described in many studies using different genetic markers (Tessier et al. 1997; Koljonen et al. 1999; Nilsson et al. 2001), indicates restricted gene flow between rivers or even between tributaries or different stretches of a river (Verspoor et al. 1991; Beacham & Dempson 1998; Garant et al. 2000). As a consequence of accurate homing behaviour in salmon, the founding of new populations or reoccupation of areas where former populations have been driven to extinction may be limited ( Young 1999). However, several studies have shown that some release practices, e.g. delayed release (smolts are kept in cages in the sea for some period before they are released), might increase the straying of hatchery fish (reviewed by Quinn 1993). Considering that hatcheries are releasing approximately nine times more salmon smolts to the Baltic Sea than are estimated to come from wild populations (ICES 1998) the reared individuals may have an important effect on the genetic composition of salmon populations in the Baltic Sea. A suitable model by which to study gene flow and the formation of genetic structure as a result of the founder effect and enhancement practices occurred in the river Selja salmon population in the Gulf of Finland in the Baltic Sea (Fig. 1) where the native stock was driven to extinction in the beginning of the 1970s as a result of pollution (Loigu et al. 2001). When conditions improved during the 1990s, spontaneous recolonization of this river took place. In addition, restocking of the river with hatchery fish was started in 1997. Correspondence: Anti Vasemägi. *Present address: Depart- ment of Aquaculture, Swedish University of Agricultural Sciences, S-901 83 Umeå, Sweden. Fax: + 46 90 123729; E-mail: Anti.Vasemagi@vabr.slu.se, vasemagi@eau.ee