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• We explored how changes in food resources influence sardines' length in three areas. • Quality and quantity of dietary lipids available to sardines changed across areas. • Sardines maintained their membrane fatty acids to compensate spatial variations. • Sardines' membrane fatty acid requirements changed with size and age. Differences in diet quality and quantity may influence trophodynamic processes in small pelagic fish. Yet, we currently lack direct and comprehensive information on how large-scale areas differ in dietary resources and the degree to which these differences influence fish physiological performances (i.e., growth), ultimately influencing entire fish stocks. Fatty acid composition is one of the bioindicator that can provide insights on how dietary provisions of essential lipids influence the structure of the membrane fatty acids and subsequently fish growth among contrasted habitats. To address this issue, we conducted a large-scale sampling of European sardine (Sardina pilchardus) a species with major socio-economic importance that plays a key role as an energy pathway linking lower and upper trophic levels in pelagic ecosystems. We sampled individuals from the Gulf of Lions (Mediterranean Sea), the Bay of Biscay, and the English Channel (Atlantic Ocean) of age-0 to −3 + and found clear spatial differences in the quantity and quality of dietary lipids. Sardines from the Gulf of Lions fed on trophic food web based on dinoflagellates, with greater proportions of DHA (22:6n-3; docosahexaenoic acid) in reserve lipids. Sardines' reserve lipids had important proportions of zooplankton biomarkers in the English Channel (e.g., 20:1n-9 and 20:1n-11), and diatoms biomarkers such as 16C fatty acids and EPA (20:5n-3; eicosapentaenoic acid) in the Bay of Biscay. The relationship between sardines' membrane fatty acid composition and individuals' length changed progressively with individuals' age, a result consistent across areas, indicating ontogenetic abilities between largest and smallest individuals. Before maturity, largest sardines had higher DHA proportions, followed after maturity by higher proportions of ARA (20:4n-6; arachidonic acid), EPA and DPA (22:5n-3; docosapentaenoic acid). Finally, the study highlights the importance of considering the quality and diversity of dietary resources to better understand how individuals cope with their physiological needs. It is thus important to consider combined aspects (e.g., diet quality and diversity, influence of particular nutrients on length) to better understand the underlying mechanistic processes influencing fish physiology, likely cascading to different expression of their life history traits and affecting fisheries stocks. [ABSTRACT FROM AUTHOR]