Behavioral variation within a population is generally maintained by frequency dependent selection allowing various personalities to coexist. Bolder individuals usually engage in more risky behaviors that can gain fitness benefits such as growth under certain conditions. Therefore, it has been suggested that there should be a link between personality and dietary niches, but the results so far are inconsistent. In addition, the equilibrium of the distribution of behavioral traits and the trophic niche of native populations may shift following the introduction of an invasive species. Here, using the invasive brook trout (Salvelinus fontinalis) and native brown trout (Salmo trutta) as model species in two different natural streams in Sweden, we aimed to test whether 1) the trophic niche of native brown trout living in allopatry and brown trout living in sympatry with brook trout differ and 2) bolder brown trout individuals utilize a different foraging niche. Our results suggest that there is a dietary niche convergence between brown trout and brook trout, which likely is a result of brook trout invasion, but that the trophic niche of native brown trout varies across streams, possibly due to difference invasion impact (varying ratio of brook trout to brown trout). We also found a strong positive correlation between trophic position and personality of brown trout irrespective of the presence of brook trout.

Access to omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA), and the physical and social complexity of habitat have been proposed to influence brain development and cognitive ability. We aimed to investigate how juvenile brown trout mitigate dietary n-3 LC-PUFA deprivation in complex habitats, typical of natal streams, by testing the effects of n-3 LC-PUFA deficiency in diet and habitat complexity on somatic growth, cognitive performance, encephalisation, and n-3 LC-PUFA biosynthesis and nutrient routing capacity. Brown trout were raised from egg for seven months post-hatch on either a high (8.91%) or low (1.79%) n-3 LC-PUFA diet; for the final three months, trout were further divided into complex (heavily ornamented tanks with small, changing, populations) or simple habitats (bare tanks with many, constant, inhabitants). Cognitive abilities including recognition, memory and inference were tested by comparing the times required to establish stable hierarchical relationships in agonistic dyadic trials featuring naïve trout and trials in which one of the trout had previously observed the other. Gas chromatography and compound-specific stable hydrogen isotope analysis revealed increased biosynthesis and routing of n-3 LC-PUFA to the brain among trout on n-3 LC-PUFA-deficient diets. Fed ad libitum, trout did not sacrifice somatic growth to fuel biosynthesis and routing of n-3 LC-PUFA, but dietary deficiency in n‑3 LC-PUFA did lead to smaller brains, and smaller brains were associated with lower cognitive performance. Complex habitats gave rise to trout showing better cognitive performance, and were associated with lower somatic growth, but habitat complexity played only minor roles in encephalisation and the n-3 LC-PUFA composition of brain lipids. We conclude that brown trout can only partially compensate for the paucity of dietary n-3 LC-PUFA, and we suggest that cognitive divergences may play a role in the diversification of life-history variants among brown trout in the wild.