4.2 Identifying populations for monitoring
Recognizing genetic diversity occurring both within and between populations is essential for conservation management of structured populations (Caballero et al,. 2010). We used the structure software to identify genetically distinct populations within systems of interconnected lakes, and then we monitored these structures over time. Our findings indicate that this approach was needed to identify and monitor genetically distinct populations. If we had focused on lakes only, the population diversity within them would have gone unnoticed.
To identify metapopulations we primarily used geographic location and knowledge of waterways for potential migration. For instance, metapopulations Skåarnja 1 and 2 were separated into two systems because of waterfalls that most likely prevents migration between the two areas. Our inferred metapopulation structure was supported by the phylogenetic tree (Figure S2).
Life-history diversity of brown trout is complex, and the fish typically spawn in streams and creeks and feed in lakes, but lake spawning is also possible as well as river residency (e.g. Östergren et al., 2012; Ferguson et al., 2019). Ideally, sampling would occur at spawning grounds and during the reproductive period. In practice this is difficult in these systems because they are located in remote areas that are difficult to access, particularly in late fall/early winter when spawning occurs and when weather conditions are typically harsh. We were not able to sample all water bodies in any of the systems studied, and thus cannot rule out that additional subpopulations exist that can exchange migrants with the populations identified and monitored.