Effects of diet quality on development of phenotypic traits necessarily depends on temporal scales on which the consumption of low- and high-quality diets occurs (Brett et al. 2009; Murray et al. 2014). For example, a relatively short-term study (i.e., 8 weeks of dietary treatment) by Závorka et al. (2021) on juvenile Atlantic salmon Salmo salar showed a strong shift of DHA content in the brain, but no significant effect on brain size and performance in a cognitive test. On the other hand, a longer study (i.e., 21 weeks of dietary treatment) by Lund et al. (2012) on juvenile pikeperch Sander luciperca demonstrated that dietary n-3 LC-PUFA deprivation induces leads to reductions of DHA content and size of the brain resulting in lower cognitive capacity. The reliance on terrestrial prey in our study was estimated based on bulk d13C values of fin clips, which indicate dietary carbon on the temporal scale of several weeks (Jardine et al. 2005; Layman et al. 2012). However, a previous study has shown that the dietary shift of our focal species, brown trout, in sympatry with invasive brook trout occurs early in ontogeny and remains stable in later life-stages (Cucherousset et al., 2020). The increased reliance on low-quality terrestrial prey in our study did not appear to be influenced by the availability of aquatic prey, indicating that this dietary shift was likely caused by behavioural changes of brown trout in sympatry with the invasive species (Lovén Wallerius et al. 2017; Larranaga et al. 2018; Cucherousset et al., 2020). Thus, the intra-specific dietary differences in our study likely represent a long-term specialization of individuals that might have a permanent effect on supply of n-3 LC-PUFA to their brain.