Internship in spatial and temporal variations in the quantity and fatty acid composition of sardine muscle from the Bay of Biscay

Around the world, small pelagic fish are of major socio-economic interest. Indeed, these species are among the most heavily targeted marine resources globally: in 2022, 7 of the 10 most caught fish species worldwide were small pelagic fish (FAO, 2024). These fish also play a vital role in the proper functioning of ecosystems, as they enable energy to be transferred from lower trophic levels (plankton) to higher trophic levels (piscivorous fishes, marine mammals). In the Bay of Biscay, the size and average weight at age of sardines (Sardina pilchardus) have fallen since the early 2000s (Véron et al., 2020; ICES, 2023). This decline particularly concerns age-1 individuals, with a decrease in total length of 29% per year for females and 26% per year for males between 2003 and 2016 (Véron et al., 2020). One of the hypotheses put forward to explain these declines is bottom-up control by food resources. A change in the quantity and/or quality of planktonic prey could have repercussions on the phenotypic characteristics of sardines. While zooplankton biomass has remained relatively stable over the last two decades, the average size of zooplankton has decreased significantly (Grandrémy, 2023). This decline in size, driven by a relative increase in the abundance of small individuals and a decrease in the individual size of several zooplankton organisms, suggests a deterioration in the quality of sardine prey in the Bay of Biscay (Grandrémy, 2023; Menu et al., 2023).

Lipids are essential molecules for fish, enabling them to store energy in the form of reserve lipids, as well as ensuring that cells function properly, since they make up cell membranes. Membrane lipids are enriched in omega 3, or n-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA). In general, marine fish do not synthesise n-3 LC-PUFA de novo, as they are mainly produced by phytoplankton and then acquired by higher trophic levels via the diet. The quantity and composition of n-3 LC-PUFA is therefore a relevant biomarker for studying nutritional stress in fish. In the Bay of Biscay, several studies, limited to one year, have already shown the existence of spatial and age-dependent variations in the quantity and fatty acid composition of sardine muscle (Bertrand et al., 2022; Mathieu-Resuge et al., 2024). In the northern part of the gulf, where larger and older individuals are found, the n-3 LC-PUFA composition indicates a predominant diet on copepods. Sardines from the southern Bay of Biscay, which are smaller and more coastal, seem to be characterised by n-3 LC-PUFA specific to non-diatomic phytoplankton (Bertrand et al., 2022). Furthermore, within the same cohort, disparities in the proportions of n-3 LC-PUFA have been observed between small and large sardines (Mathieu-Resuge et al., 2024). In order to link the decline in sardine growth to the decline in zooplankton size in the Bay of Biscay, it is now relevant to estimate the spatio-temporal variations in n-3 LC-PUFA present in sardine muscle to better understand how they influence their growth.