The distribution of marine predators is tightly coupled with that of their prey, and may also be affected by interactions between competing predators. In order to adopt an ecosystem approach to the management of a species, it is essential to understand these processes. In this study, we examined whether the distributions of 2 marine predators, harbour porpoise Phocoena phocoena and mackerel Scomber scombrus, are related to the distribution of a major prey species, herring Clupea harengus, on a large spatial scale. Porpoise distribution data were obtained from satellite-tracked harbour porpoises (1998 to 2009), while mackerel and herring distribution data were found by the annual ICES acoustic herring surveys providing data with overlapping temporal and spatial scales (2000 to 2009). We found that the 3 species were not evenly distributed within the study area and that harbour porpoise distribution was best explained solely by the distribution of herring, while herring together with depth explained 50% of the mackerel distribution. These results underline the importance of an ecosystem approach in management plans for predatory species by including monitoring and management of main prey species.