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.