This dataset comprises energy density and proximal composition (water, ash, lipid and protein contents) for anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) from the Bay of Biscay, the English Channel and the southern North Sea between 2014 and 2017. Fish were sampled throughout various seasons thanks to the PELGAS (May, Bay of Biscay), EVHOE (October-November, Bay of Biscay), CGFS and CAMANOC (September-October, English Channel) surveys conducted by IFREMER on the RV “Thalassa”, to the JUVENA (September-October, Bay of Biscay) survey conducted by AZTI, and from commercial landings within the European Data Collection Framework (DCF) during the CAPTAIN project (France Filière Pêche). During the surveys, pelagic (PELGAS and JUVENA) or demersal (EVHOE and CGFS) trawl hauls are undertaken to identify species and measure individual fish traits. Professional sampling was performed from pelagic trawl or purse-seine catches.

From the various surveys, a sub-sampling of the trawls was performed to cover as much as possible the spatial extent of the surveys along the french coast. From the various selected trawls, a sub-sampling of 5 fish per size class (when possible) was performed to cover the size range of each species, based on the following size classes : sardine (1 : <15 cm ; 2 : 15-20 cm ; 3 : >20cm), anchovy (1 : <10cm ; 2 : 10-14 cm ; 3 : >14cm).

Each fish was individually measured to the nearest tenth of a centimeter and weighted to the nearest tenth of a gram. These measurements were taken either at sea or later in the laboratory. The collected fish were frozen individually at -20°C before laboratory processing. In the laboratory, maturity stages were determined following ICES guidelines (ICES, 2008) based on macroscopic gonads observation and using a six-stage key as follows: stages 1 & 2 indicate immature and developing individuals, stages 3–5 indicate three steps of increasing gonad development and the spawning period (stage 3: partial pre-spawning; stage 4: spawning (hydrated); stage 5: partial post-spawning), and stage 6 features the final post-spawning period. Fish characterised by maturity stages 3, 4 or 5 were considered as being in an active reproductive period as opposed to fish in stages 1, 2 or 6.

Fish were then ground and freeze-dried during at least 48 hours. Water content of the entire fish was determined from dry mass and wet mass ratio. Then, fish were ground again to obtain fine homogeneous dry powder for subsequent analysis. Energy density measurements were performed following the protocols of Gatti et al. (2018). Two subsamples of fish powder were placed in an adiabatic bomb calorimeter (IKA C-4000 adiabatic bomb calorimeter, IKA-WerkeGmbh & co. KG) for energy measurements. The energy density (ED, kJ.g-1 dry mass) was determined by measuring the heat released through the combustion of a small subsample, approximately 200 mg. If the coefficient of variation between the two measurements exceeded 3%, a third measurement was made. Finally, ED subsamples measurements were averaged and assigned to each individual fish. Energy density analyses were conducted on 503 individuals for anchovy and 976 individuals for sardine. Ash content was determined gravimetrically by combusting dried tissue in a muffle furnace at 550°C for six hours. Lipids and proteins were analysed by a certified laboratory (Labocea, Plouzané, France). Protein content was estimated using the Kjeldahl method. It consists in first determining the quantity of nitrogen contained in the sample, and to convert it into protein content using a conversion factor (6.25 here). Lipid content was determined by hydrolysis, using petroleum ether as an organic solvent. Carbohydrates represent less than 1% of fish mass and were thus neglected. Protein, lipid and ash content did not exactly sum to 1 in DW (anchovy: mean = 0.91, sd = 0.04; sardine: mean = 0.90, sd = 0.04). This discrepancy may arise from residual water, measurement uncertainties, or to a lesser extent the exclusion of carbohydrates. Body component contents have been normalised by dividing each component by the sum of lipids, proteins and ash content, to sum to one, enabling comparisons between fishes, assuming proportional errors across the components. A total of 116 and 104 proximate composition analyses were performed for anchovy and sardine, respectively.