Stable isotope measurements and radiocarbon dates on 54 burials from northern Scotland document trends in marine protein consumption from the late Iron Age to the end of the Middle Ages. They illuminate how local environmental and cultural contingencies interrelated with a pan-European trend towards more intensive fishing around the end of the first millennium AD. Little use was made of marine foods in late Iron Age Orkney despite its maritime setting. Significant fish consumption appeared in the Viking Age (ninth to eleventh centuries AD), first in the case of some men buried with grave-goods of Scandinavian style but soon among both sexes in ‘Christian’ burials. There was then a peak in marine protein consumption from approximately the eleventh to the fourteenth centuries AD, particularly among men, after which the importance of fish-eating returned to Viking Age levels. The causes of these developments probably entailed a complex relationship between ethnicity, gender, Christian fasting practices, population growth, long-range fish trade and environmental change.