Coastal areas currently face a lot of anthropogenic pressures (Erlandsson et al, 2011; Wilcox et al, 2020) which might further increase through climate change (Maltby et al, 2022). This can lead to the disappearance of crucial, habitat engineering species like blue mussels (Tummon Flynn et al, 2020). The lack of those increases the risk of eutrophication and pollution, on the long run having negative consequences not only for biodiversity and ecological factors but also social and economic factors like income through fishing and risking coastal flood protection (Tummon Flynn et al, 2020; Schotanus et al, 2020; Cooley & Doney, 2009). Blue mussels can form biogenic reefs, which are protected as important habitats under the EU Habitats Directive (Council of the European Communities, 1992) as well as the EU Water Framework Directive (European Parliament & council, 2000). Nevertheless, actual implementation of protection and conservation of those reefs is lacking (Rees et al, 2018), which is why many of them are declining drastically (EEA, 2013). One reason for that lack of protection might be the diverse definitions applied to mussel reefs all over Europe (Stounberg, 2021), while another one could be that many of them are not mapped. In the present study, an attempt was made to map a potential blue mussel reef in the Roskilde Fjord in Denmark, which was first discovered by a previous study (Dahl et al, 2019:39) but not investigated to its extend as that study focused on other goals. A ROV was employed to collect video data of the seafloor of the respective fjord area over a period of three months. The recorded video and GPS data was then processed and analysed to produce a map in QGIS, showing if and where an area of, according to the Danish definition for blue mussel reefs, more than 2500m2 with more than 30% mussel cover and 3 year classes of mussels (Miljøstyrelsen, 2018), is present. Three areas in the chosen search area of this study were confirmed to fulfil those requirements, consistent with the previous findings of Dahl et al (2019). The actual blue mussel reef might be even bigger as other areas which have not been analysed in detail contain blue mussels as well. Therefore, protection measures based on best available scientific knowledge, interdisciplinary work and the knowledge of as many stakeholders possible, including social, economic and ecological aspects, should be developed for the blue mussel reef in Roskilde Fjord. One of them could be to make the reef part of the Natura 2000 protection area already existing in the same area.