The trade-off between cognitive capacity and developmental costs drive brain size and morphology across fish species, but this pattern is less explored at intraspecific level. Physical habitat complexity has been proposed as a selection pressure on cognitive capacity that shapes brain morphology of fishes, but development of brain is also inherently linked to supply of energy and nutrients, particularly of omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA). In this study, we compared brain morphology of brown trout Salmo trutta from stream, lake, and hatchery environments, which differ in physical complexity and availably of dietary n-3 LC-PUFA ranging from low habitat complexity and high n-3 LC-PUFA availability in hatchery to high habitat complexity and low n-3 LC-PUFA availability in streams. We found that brain size, and size of optic tectum and telencephalon differed across the three habitats, being largest in lake fish. We suggest that these differences appeared to associate with diet quality and habitat specific behavioural adaptations rather than physical habitat complexity.

1. Omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA) are key structural lipids and their dietary intake is essential for brain development of virtually all vertebrates. The importance of n-3 LC-PUFA has been demonstrated in clinical and laboratory studies, but little is known about how differences in availability of n-3 LC-PUFA in natural prey influence brain development of wild consumers. The availability of n-3 LC-PUFA in the prey communities is driven by primary producers and it is therefore distributed heterogeneously, but predictably across ecosystems, being higher in aquatic than in terrestrial food webs. Consequently, the numerous consumers foraging on the interface of aquatic and terrestrial food webs can differ substantially in their intake of n-3 LC-PUFA, which may lead to in brain development, yet, this hypothesis still remains to be tested.2. Here we use the previously demonstrated shift towards higher reliance on n-3 LC-PUFA deprived terrestrial prey of native brown troutSalmo trutta living in sympatry with invasive brook troutSalvelinus fontinalis to explore this hypothesis.3. We found that the content of n-3 LC-PUFA in muscle tissues of brown trout decreased with increasing consumption of n-3 LC-PUFA deprived terrestrial prey. Brain volume was positively related to content of the n-3 LC-PUFA, docosahexaenoic acid, in muscle tissues of brown trout.4. Our study thus suggests that increased reliance on low quality diet of n-3 LC-PUFA deprived subsidies from terrestrial food web can have a significant negative impact on brain development of wild trout. These findings provide the first evidence of an intra-specific link between n-3 LC-PUFA content in natural prey and brain size of wild vertebrate consumers.5. Ongoing global change is predicted to reduce the availability of dietary n-3 LC-PUFA across food webs. Therefore, our findings emphasise the need for further research on how wild consumers adapt to the shortage of dietary n-3 LC-PUFA in order to maintain optimal development and functioning of their brain, which is crucial for their fitness.