Despite the higher reliance on low-quality terrestrial prey and corresponding decrease of EPA in muscle tissues, there was no significant reduction of DHA in muscles of sympatric compared to allopatric brown trout. A potential explanation is that sympatric brown trout has adapted to the lower dietary intake of n-3 LC-PUFA by increasing retention and/or internal synthesis of DHA (Murray et al. 2014). Such adaptation has been observed in populations of three-spined stickleback (Ishikawa et al., 2021) and European perch (Scharnweber et al., 2021) foraging on low-quality prey from littoral lake habitat that is deprived of n-3 LC-PUFA. Similarly, tree swallows have been shown to increase synthesis of DHA when consuming n-3 LC-PUFA deprived terrestrial macroinvertebrates (Twining et al., 2018). However, our results indicated limits of this adaptation as the relative content of DHA in sympatric trout decreased with increasing reliance on terrestrial prey. This suggests that the combination of external stressor (i.e., interaction with an invasive species) and high consumption of n-3 LC-PUFA deprived prey can affect the capacity of wild consumers to maintain an optimal biochemical composition of their tissues. Synthesis of DHA from short-chain precursors, such as ALA, contained in low-quality terrestrial diet comes at substantial energetic costs that can result in reduced somatic growth rate (Murray et al. 2014; Twining et al. 2016; 2018; Závorka et al. 2021). Our findings thus accentuate the suggestion that the trade-off between internal synthesis and dietary acquisition of n-3 LC-PUFA might be an important driver of phenotypic adaptations across vertebrate consumers (Twining et al. 2021). We also found that larger (and presumably older, i.e.,Bowker 1995; Öhlund et al. 2008) individuals across populations relied more on terrestrial prey and had lower EPA and DHA contents in muscles. This age-dependent effect can be explained by the fact that older consumers require less n-3 LC-PUFA than younger consumers (Brenna 2011; Chaguaceda et al. 2020), because the key period of vertebrate brain development occurs during early ontogenetic stages (Innis 2007; Brenna 2011; Lund et al. 2012).