ABSTRACTThe Atlantic chub mackerel (Scomber colias) became an important resource in purse‐seine fisheries of the Iberian Peninsula in the mid‐2000s. Being data‐limited at the stock level, 60 years of landings and 40 years of bottom‐trawl survey data were available for part of the area. A biomass model fitted to landings and survey data performed well under the assumption of homogeneous dynamics across the stock area. During 1993–2020, relative stock biomass was above potential reference points with wave‐like variation and peaks in 1995 and 2008. Credible intervals of relative fishing mortality indicated that exploitation may have been unsustainable since 2012. Chub mackerel productivity increased from the 1980s to the 1990s, possibly due to decreased sardine predation and competition and increased larval growth. The approach we used demonstrated how information from a portion of a stock's distribution area can be utilised for stock assessment, to potentially prevent neglect of stock data or alteration of management units.