In this study, we focus on brown trout Salmo trutta, a salmonid native to European streams, which is a generalist consumer displaying a broad range of dietary specialization among populations (Sánchez-Hernández 2020) and individuals (Evangelista et al. 2014). We use a river system where brown trout co-occurs with the invasive brook trout Salvelinus fontinalis, as previous studies have shown that brown trout increases the consumption of terrestrial macroinvertebrates (i.e., low-quality prey deprived of n-3 LC-PUFA) in sympatry with invasive brook trout (Závorka et al. 2017; Cucherousset et al. 2020; but see Horká et al. 2017). Therefore, this study system allows us to test an entirely natural setting how reduced dietary intake of n-3 LC-PUFA influence the physiology and brain development of wild consumers. We explore how dietary reliance on terrestrial prey affects brain size and brain morphology of brown trout in sympatry and allopatry with the invasive brook trout, while accounting for ontogenetic and sex differences among individuals. We predict that; i) higher reliance on terrestrial prey of brown trout reduces dietary intake of EPA; ii) the content of EPA and DHA in brown trout tissues decreases with increasing reliance on terrestrial prey; and iii) lower DHA content in brown trout tissues is associated with smaller brain size or changes in brain morphology (i.e., region specific brain size reduction).