2.1 Study area and sampling
The study area is located in the mountainous range of Jämtland County, central Sweden, and includes 27 lakes/tarns located in five protected areas (Figure 1; Table S1). Several of the localities are connected to each other via creeks, and there is a total of seven such “metapopulations” with 2-7 lakes/tarns per system included in this study. Two of these metapopulations (with a total of 7 lakes) are located above the tree line (>700 meters above sea level), while the remaining four are set below the tree line. The samples also include three “independent” lakes (all below the tree line) which are not closely connected to any of the other sampling localities. All sampling sites represent the uppermost parts of water systems draining into either River Ångermanälven or River Indalsälven, two major rivers that drain into the Baltic Sea c. 400 km from the sampling sites (Figure 1).
The sampling was performed in collaboration with the Jämtland County Administrative Board at two points in time, the 1970-80s and 2010s (Table S1), and was often coordinated with their test fishing activities within the regional environmental management. Fish collected in the 1970-80s were from the first studies of genetic variation in natural populations in Sweden (Allendorf et al., 1976; Ryman et al., 1979; Ryman & Ståhl, 1980; Ryman, 1981, 1983), and we included n =1,263 fish from those collections that have been stored in a frozen tissue bank at the Department of Zoology, Stockholm University. Sampling localities to be included in the present study were selected based on possibilities to obtain additional samples in collaboration with local authorities, sport fishing clubs and/or Sámi communities. One of the study areas (system Hotagen 4; Figure 1) is part of a long-term genetic monitoring research effort that we (N.R., L.L.) have run since the 1970s (Jorde & Ryman, 1996; Laikre et al., 1998; Charlier et al., 2012; Palmé et al., 2013; Andersson et al., 2017b). Present day samples (2010s) includedn =1,319 fish collected in 2012, 2014, 2017, and 2018.