Epigenetic mechanisms underlying phenotypic plasticity can be an important factor in the survival of a fish species through a changing climate or in migrating to a new habitat. The threespine stickleback (Gasterosteus aculeatus) is found throughout the northern hemisphere and can adapt via genetic change in relatively few generations to new environments. Epigenetic mechanisms work faster than genetic change, and have the potential to be passed on to future generations, possibly leading to population-wide changes in gene expression and phenotypic variation. To study genes involved in epigenetic mechanisms in stickleback, populations were selected for sampling between Northern Germany and northern Norway. Eleven populations of stickleback were successfully sampled across this latitudinal gradient, and four evenly distributed populations were selected for gene expression analyses in this research. Collected samples were dissected for gonads and pectoral fin muscle (in addition to other organs) and brought to the Alfred Wegener Institute where RNA was extracted. After converting RNA into cDNA, a targeted qPCR approach was performed to test for expression levels of a number of epigenetic actors; DNMT1, DNMT3ab, TET1, TET3, MacroH2A, and Sirtuin2. DNMTs are involved in promoting methylation, TETs actively demethylate cytosine, and MacroH2A and Sirtuin2 are actively involved in cold acclimation. Results from the fieldwork sampling found that stickleback body size (measured as standard length) decreased as latitude increased, in opposition to Bergmann’s rule of species increasing in size toward the poles. Additionally, there was evidence for slight sexual dimorphism in which males were significantly smaller than females across all populations. Furthermore, gravid females were found to be significantly larger than non-gravid females. Results from the target gene qPCR testing found Sirtuin2 to be more expressed in female gonads of northern populations than in southern populations. This is in line with Sirtuin´s role in cold acclimation which would be more beneficial to northern than southern populations. Male gonads showed higher expression of DNMTs and TETs, possibly indicating greater plasticity of epigenetic actors capable of change. This thesis project is the first to study epigenetic differences in fish populations across a latitudinal gradient. Future research could benefit from increasing the sample size (number of individuals and populations) and/or investigating alternative organs that may also show differential gene expression of epigenetic actors. Overall, epigenetic mechanisms are likely to be differentially expressed depending on factors of organ, sex, population, and local environmental factors, all of which can potentially allow greater adaptive potential under climate change.