A subsample of brown trout (Table 1) at each study site was transported alive to the department of Biology and Environmental Sciences at the University of Gothenburg for dissection. Before the dissection, fish were housed in holding tanks (30 L, 300 × 320 × 340 mm), which provided shelter structures (rocks and plastic plants) and fresh 12 °C water from a flow-through filtration system (flow rate: 2 L min-1). Fish were not fed during this period. All fish were dissected within 20 hours after capture and to randomize the time effect, individuals from allopatric and sympatric sites were dissected in alternating order (with even and odd individuals from allopatry and sympatry respectively). Before dissection, fish were euthanized by an overdose of 2-phenoxyethanol (0.5 mL L-1) and body mass and fork length were measured to the nearest 0.1 g and 1.0 mm respectively. Sex of individuals was determined by inspection of the gonads. Heads of fish were removed and fixed in 4% buffered (pH 6.9) paraformaldehyde solution. Brains were then dissected out as described in Gonda et al. (2009) by opening the scull along the anteroposterior axis and removing muscle tissue, nerves and bones until the brain could be lifted up from the skull and stored in 4% buffered paraformaldehyde solution until further procedure. Brains were imaged with a digital camera (EOS 40D with MP-E 65mm lens; Canon Inc., Tokyo, Japan). Images of brains were taken using the dorsal, left lateral and ventral views to calculate the total volume and the volume of cerebellum, optic tectum, telencephalon, olfactory bulb, and hypothalamus. Measurements were completed using ImageJ 1.48 (Schneider et al. 2021) and used to calculate volume with the formulas outlined by Pollen et al. (2007). Brain morphology was assessed by principal component analysis (Appendix S2) yielding two dominant principal components. PC1 was positively related to the volume of olfactory bulb, and hypothalamus and negatively related to cerebellum and optic tectum, while PC2 was positively related to the volume of telencephalon.