Dietary ALA and EPA are utilized by many vertebrates for internal synthesis of DHA (Murray et al. 2014; Twining et al. 2021; Závorka et al. 2021). However, the synthesis of DHA from short-chain ALA requires more conversion steps and thus more energy than the synthesis from long-chain EPA (Pilecky et al. 2021; Twining et al. 2021). This energy trade-off can lead to the lower capacity of consumers feeding on diet deprived of n-3 LC-PUFA to synthetize DHA and retain it in the brain (Závorka et al. 2021; Pilecky et al. 2021). In addition to the diet quality, this energy trade-off can be amplified by intrinsic factors such as sex of individuals, as females may be able to retain less DHA in the brain due to the higher investment of n-3 LC-PUFA to gametes (Maklakov 2008; Hou & Fuiman 2020). The capacity to synthetize and retain DHA has also been shown to decrease over ontogeny in freshwater fishes (Chaguaceda et al. 2020) and mammals (Brenna 2011). A good example of the energetic trade-off linked to the synthesis of n-3 LC-PUFA from dietary sources of different quality are consumers that depend on a mix of prey from aquatic and terrestrial resources (Twining et al., 2019). Aquatic and terrestrial macroinvertebrates are reciprocal sources of energy and macronutrients (Nakano & Mukarami 2001; Sullivan et al. 2014), but aquatic macroinvertebrates provide substantially more long-chain EPA than terrestrial macroinvertebrates, which contain mainly short-chain ALA (Guo et al. 2018; Brett et al. 2019). Predominantly insectivorous freshwater fishes, such as stream salmonids, depend on macroinvertebrates from aquatic and terrestrial subsidies (Nakano & Mukarami 2001; Syrjänen et al. 2011; Sánchez‐Hernández & Cobo 2018). Therefore, intra-specific dietary specialization and intrinsic capacity to synthetize and retain n-3 LC-PUFA in a freshwater fishes could provide a first general insight in how these dietary biomolecules influence brain development of vertebrates in the wild.