Climate change‐induced deprivation of dietary essential fatty acids can reduce growth and mitochondrial efficiency of wild juvenile salmon.

Omega‐3 long‐chain polyunsaturated fatty acids (n − 3 LC‐PUFA) are essential micronutrients for optimal functioning of cellular metabolism and for somatic growth of all vertebrates including fishes. In addition, n − 3 LC‐PUFA could also play a key role in response of fishes and other ectothermic vertebrates to changing temperatures.An important, but largely overlooked, consequence of climate change is the reduced availability of dietary n − 3 LC‐PUFA in aquatic food webs. Changes in availability of dietary n − 3 LC‐PUFA have recently been proposed as a major driver of novel adaptations and diversification of consumers. Yet, there is only limited knowledge about how n − 3 LC‐PUFA depletion in aquatic food webs will affect the performance of wild fishes.Here we combine biochemistry and physiology at the cellular level with physiological and cognitive processes at the whole‐animal level to test how ecologically relevant deprivation of n − 3 LC‐PUFA affects performance of wild juvenile Atlantic salmon Salmo salar.We found that juvenile salmon had a limited capacity to maintain the fatty acid profile of both muscle and brain under an n − 3 LC‐PUFA‐deficient diet. Despite these findings, brain tissues showed remarkable functional stability in mitochondrial metabolism, and we found no effect of diet on learning ability. However, we found that mitochondrial efficiency in muscles and the somatic growth were reduced under an n − 3 LC‐PUFA‐deficient diet. Importantly, we discovered that the somatic growth of juvenile salmon within both treatments decreased with increasing rate of DHA synthesis and retention.Since DHA is essential for functioning of cellular metabolism, which together with body size are traits closely related to fitness of wild fishes, we suggest that the trade‐off between growth rate and accumulation of DHA could play a critical role in resilience of juvenile salmon to the ongoing rapid environmental change. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]