Studying various aspects of the biology of a species of conservation concern allows us to improve our understanding of, for example, how the particularities of the landscape matrix influence its occurrence. The Lower Vouga Lagoon in central-western Portugal offers an unusual diversity of natural and humanised biotopes in an agricultural-wetland complex mosaic. This study aimed to identify the factors that may influence the occurrence and abundance of a diurnal raptor, the marsh harrier Circus aeruginosus, in this kind of landscape. The marsh harrier is a top predator that is considered as an umbrella species, i.e. one whose conservation may also help preserve and promote biodiversity in general in its community. This study provides potentially valuable information on management and conservation strategies in both natural and agricultural areas. We found that natural habitats, such as reedbeds, are a key habitat for this predator, providing shelter, food and suitable nesting sites. Despite the general negative effect of human pressure on the occurrence of marsh harriers, the species seems to tolerate and even benefit from humanised environments, such as rice fields, which constituted the preferred foraging habitat during the non-breeding period. Nevertheless, the degree of disturbance should be carefully considered, since road density and agricultural machinery negatively influenced the presence of the species in the landscape, particularly during the breeding period. The preservation of the distinctive features of the studied mosaic is very important. It is also necessary to establish a conservation programme involving the local human community.