Deficiency of vital organic nutrients in ecosystems limits brain development and fitness in wild fish.

Animals in aquatic ecosystems impacted by global changes often face reduced availability of vital organic compounds, such as long-chain omega-3 polyunsaturated fatty acids (n-3 LC-PUFA), which are essential 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 mass, morphology (i.e. mass of brain regions) or neuron numbers. 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 n-3 LC-PUFA in aquatic food webs may impair the behavioural flexibility of top aquatic consumers, possibly with negative impacts on their capacity to maintain high fitness in ecosystems altered by environmental change.