Application of method
The results of the present study demonstrate that RNA sequencing of epidermal mucus can generate a wealth of information on molecular differences among wild fish populations without the use of lethal sampling. Although the populations considered in the present study were located in closely-situated, ecologically-similar lakes, our method was still able to clearly separate individual transcriptomic profiles by lake. Sequencing of mucus could be useful in comparative transcriptomic studies of populations residing in ecologically different conditions, such as studying ecologically-divergent populations residing in two different habitat types or comparing a reference population to one at a polluted site. Moreover, using mucus for gene expression analysis can be applied to various freshwater and marine species as mRNA-based mucus analysis has been successful in mahi-mahi (Greer et al., 2019), channel catfish (Ren et al., 2015), largemouth bass (unpublished), and now lake trout. As mucus sampling is nonlethal, transcriptomic studies of mucus would be of most value to studies concerned with threatened and endangered fish species. mRNA-based analysis of mucus can be used in conjunction with other nonlethal sampling methods, such as blood sampling for biomarkers of interest, as well as individual- and population-level metrics (e.g. through eDNA sampling (Adams et al., 2019)) to gain insight on health at multiple levels of biological organization.