Global change may decrease the production of omega-3 long-chain polyunsaturated fatty acids in primary producers. This decline in key nutrients such as docosahexaenoic acid (DHA) at the base of the food chain can reduce DHA availability in higher trophic levels, affecting the DHA content in their cell membranes, including mitochondrial membranes, thus impacting mitochondrial ATP production and individual metabolism. Over the past two decades, multiple sardine populations have shown declining body condition and size-at-age, primarily due to trophic alterations induced by climate change. Our hypothesis is that reduced dietary DHA would affect sardines' cardiac mitochondrial performance, contributing to physiological disturbance. Our experimental approach involves feeding wild adult sardines a control diet or a DHA-deficient diet for 6 months. Our findings indicate that DHA-deficient sardines had lower membrane DHA content in the heart compared to controls and less efficient mitochondria in ATP production (ATP/O ratio), with increased oxygen consumption for proton leakage (LEAK) and decreased oxygen consumption for ATP production (OXPHOS). Further analyses are ongoing to clarify the relationship between fatty acid membrane composition and variation in mitochondrial performance among individuals. Overall, our results demonstrate that dietary DHA limitation reduces mitochondrial ATP production capacity while increasing oxygen consumption in proton leakage. Impaired mitochondrial performance can lead to cardiac dysfunction and reduced individual performance, potentially impacting observed life history trait alterations in sardines, such as growth or condition, with implications for highly productive trophic networks if occurring in the wild.