Résumé
Natural populations’ characteristics such as growth are strongly influenced by the quantity and quality of food, which is a considerable source of energy. Over the last decade, the size- and weight-at-age of European sardines (Sardina pilchardus) have decreased substantially in the Bay of Biscay (BoB), especially in fish aged 1 or 2 years, most likely due to changes in feeding conditions linked to bottom-up control. Here, we analysed the fatty acid (FA) composition in the neutral (NL) and polar (PL) lipids in sardine muscles collected across the BoB in May 2018 to establish changes in their diet with age. We found that the total FA contents in NL and PL varied with sardines’ sampling locations and age. Indeed, sardines aged 1 and 2 years, living in Southern BoB, had particularly high contents in FA specific to non-diatom phytoplankton, while older sardines, living in Northern BoB, had higher total FA content and more FA specific to copepods. Therefore, prey availability across the BoB and to a lesser extend a change in sardine feeding behaviour with age were linked to feeding variations. In general, FA in NL clearly reflected finest dietary changes than FA of PL that are conserved at longer time scales. Based on the predominantly phytoplanktonic diet of the youngest sardines in spring, their decline in size-at-age could be the result of variations in primary production.