The main objective of Ifremer's pelagic sea surveys (Pelgas in the Bay of Biscay and Pelmed in the Gulf of Lion) is to assess the biomass of anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) populations, based on fisheries acoustic data. Complementary data on the whole pelagic ecosystem (hydrology, plankton, fish eggs and larvae, other fish species, seabirds and marine mammals) are also collected during the cruises. This document describes the procedures used to derive stock abundance estimates from acoustic and fishing data collected during sea cruises, the Pelgas sea survey being used as an example.