Omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA) dietary acquisition is beneficial for survival, reproduction, and brain development in many vertebrates including fishes, positively affecting their cognitive abilities. However, how n-3 LC-PUFA impact fish behaviour, and cognition in natural habitats remains unclear. Populations and individuals of the same species often vary in their capacities to synthesize n-3 LC-PUFA. This may affect their sensitivity to dietary intake of these nutrients and, in turn, their cognitive traits and ecological performance. Here, we tested how dietary n-3 LC-PUFA affects behavioural and cognitive traits of brown trout Salmo trutta from two lacustrine and three riverine populations. We combined laboratory behavioural tests with experiments in semi-natural stream mesocosms to see how trout can acquire resources in natural environment (i.e., prey size and taxonomic composition in their stomach contents). Trout raised on a high n-3 LC-PUFA diet showed less bold behavioural types and better cognitive performance in laboratory tests, and capacity to capture and consume larger prey in the stream mesocosm. Additionally, we observed inter-population differences in behaviour and cognition, although these differences were independent from whether fish were from lakes or river.

Animals in aquatic ecosystems impacted by global changes often face reduced availability of vital organic compounds, such as omega-3 polyunsaturated fatty acids (n-3 LC-PUFA), vital for brain development and cognition. Cognitive skills are crucial for buffering the impacts of environmental stress on fitness, yet the link between the quality of diet and fitness-enhancing behaviours of individuals in food webs altered by global change remains unclear. We examined how dietary n-3 LC-PUFA affect brain development, social dominance, and growth in territorial juvenile salmonids in a large-scale model of a natural pre-alpine stream. For this assessment we used wild fish whose diet quality was estimated using stable isotope analysis, and hatchery reared fish exposed to dietary treatments in a common-garden experiment. In both wild and common garden experiment fish, diets low in n-3 LC-PUFA led to a decreased content of n-3 LC-PUFA in brain tissue but did not affect brain size, morphology, or neuron count. Fish with lower brain n-3 LC-PUFA content exhibited reduced competitiveness in social interactions and suboptimal habitat use, resulting in slower somatic growth. Our findings indicate that the limited availability of key organic compounds may impair behavioural flexibility of top aquatic consumers, possibly with negative impacts on intra- and inter-specific diversity of the consumers.