Interdecadal variability of the diet composition of horse mackerel Trachurus trachurus was examined through analysis of the stomach contents of fish collected from the north-west and south of Portugal during 1990–1992 and 2005–2006. These decades correspond to periods of high and low feeding intensity of T. trachurus. Dietary composition of adult T. trachurus was assessed by determining the frequency of occurrence and biovolume of identified prey, and these two variables were combined to estimate an index of relative importance of prey. Dietary composition differed between areas, seasons and, in particular, decades. The most important prey in 1990–1992 were euphausiids while in 2005–2006 the most important prey were fishes. Results show that periods of different feeding intensity for T. trachurus are characterized by diets with different dominant prey items, suggesting that a description of the diet of a fish species is not complete if only based on samples collected in a limited space and time period. This result has implications for the study of marine trophic food webs, since a generalist predator may easily and rapidly change its trophic niche, to adapt to different prey abundance and availability.