Until July, post-smolt salmon Salmo salar (n=337; 129-375mmL T , mean 225mm) in the Bothnian Sea relied largely on surface fauna (mainly terrestrial insects). From August onwards, fish was the principal food type. The smallest piscivorous post-smolts were <200mm, but the main shift to piscivory occurred at sizes of 240-320mm. Piscivory was promoted by a large smolt size. Almost all one-sea-winter (1-SW) salmon (n=316; 278-524mm, mean 397mm) were piscivorous. Over 70% of the post-smolt and 96% of the 1-SW salmon with identifiable fish species in their stomachs had preyed on herring Clupea harengus. Other fish prey included the ten-spined Pungitius pungitius and three-spined sticklebacks Gasterosteus aculeatus but no sprats Sprattus sprattus. The results support earlier observations of a close relationship between recruitment of herring and production of salmon in the Bothnian Sea, and of the crucial role of smolt size in determining the ability of feeding salmon for utilizing the food resources of the area.