Influences of Diet Quality, Nursery‐Habitat Complexity and Sex on Brain Development and Cognitive Performance of Brown Trout (Salmo trutta L.)

Access to omega‐3 long‐chain polyunsaturated fatty acids (n‐3 LC‐PUFA) and habitat complexity have been proposed to influence brain development and cognitive ability. We aimed to investigate the physiological and cognitive effects of dietary n‐3 LC‐PUFA deprivation on juvenile brown trout ( Salmo trutta L.) in complex habitats resembling natal stream conditions in which populations have evolved. We tested effects of n‐3 LC‐PUFA deficiency in diet and habitat complexity on somatic growth, cognitive performance, encephalization, n‐3 LC‐PUFA biosynthesis and nutrient routing capacity. Brown trout were raised from egg for 7 months post‐hatch on either a high (8.91%) or low (1.79%) n‐3 LC‐PUFA diet; for the final 3 months, trout were further divided into complex (heavily ornamented tanks with small, dynamic, populations) or simple habitats (bare tanks with many, constant, inhabitants). 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 to satiation, trout did not sacrifice somatic growth to fuel biosynthesis and routing of n‐3 LC‐PUFA. However, dietary deficiency in n‐3 LC‐PUFA did lead to smaller brains, and smaller brains were associated with lower cognitive performance. Complex habitats elicited better cognitive performance, and were associated with lower somatic growth, but habitat complexity played only minor roles in encephalization and the n‐3 LC‐PUFA composition of brain lipids. We conclude that developmental plasticity in response to environment allows brown trout partially to 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.