Results
Overall brain volume was affected by the interaction between habitat and the fork length (FL) of individuals (F2,49 = 22.61, p < 0.001, model Radj. = 0.947). The significant interaction indicates that while there was a positive link between the overall brain volume and FL in wild populations i.e., lake and stream habitat (F1,37 = 214.28, p < 0.001, model Radj. = 0.947), hatchery fish showed no association between the brain volume and FL (F1,13 = 0.2431, p = 0.6302, model Radj. = 0.000, Fig. 1a). Relative brain volume was larger in fish from lake than from stream habitat (F1,37 = 5.4795, p = 0.0247). The telencephalon volume increased with increasing volume of the whole brain (F1,52 = 109.49, p < 0.001, model Radj. = 0.912, Fig. 1b); however; telencephalon volume also differed significantly between fish from different habitats (F2,52 = 8.164, p = 0.001). Specifically, the telencephala of stream dwelling trout were smaller than those of trout from both lake and hatchery environments (post-hoc p < 0.006), but the telencephala of lake and hatchery trout did not differ from each other (post-hoc p = 0.252). The volume of the optic tectum increased with the increasing volume of the whole brain (F1,52 = 175.51, p < 0.001, model Radj. = 0.931, Fig. 1c), and optic tectum volume also differed significantly between fish from different habitats (F2,52 = 3.723, p = 0.031). The optic tecta of lake dwelling trout were significantly larger than those of stream trout (post-hoc p = 0.027) and tended to be larger than the optic tecta of hatchery trout (post-hoc p = 0.054); but stream and hatchery trout did not differ from each other (post-hoc p = 0.733). Similarly, volumes of the olfactory bulb, cerebellum and hypothalamus increased with the increasing volume of the whole brain (olfactory bulb: F1,52 = 61.42, p < 0.001, model Radj. = 0.808, Fig. 1d; cerebellum: F1,52 = 80.70, p < 0.001, model Radj. = 0.847, Fig. 1e; hypothalamus: F1,52 = 51.65, p < 0.001, model Radj. = 0.651, Fig. 1f), but volumes of these brain regions did not differ among individuals from different habitats (olfactory bulb: F2,52 = 0.639, p = 0.532; cerebellum: F2,52 = 1.421, p = 0.251; hypothalamus: F2,52 = 2.403, p = 0.100).