Blue mussels (genus Mytilus) as a biofilter against sea lice around aquaculture farms

The ectoparasitic copepods of the family Caligidae are well known for their impact on modern sea cage aquaculture of salmonids. A rather overlooked approach to sea lice management is to use the natural filter-feeding ability of bivalves. In this study, on the example of mussels in the genus Mytilus , we explore the feasibility and expected size of a bivalve biofilter against planktonic stages of sea lice. We measured the filtration of planktonic Lepeophtheirus salmonis larvae by blue mussels ( M. edulis ) in laboratory experiments and combined the measured filtration rates with filtration rates calculated from raw data from comparable studies. The combined results suggest that 50 mm mussels filter sea lice nauplii with a mean rate of 1.66 L h − 1 and copepodids with a mean rate of 0.46 L h − 1 . The observed differences in filtration amongst larval stages indicate that a mussel biofilter will be more effective at preventing the release of sea lice larvae from a salmonid farm than at preventing the entry of the infective stage. Upscaling to a typical salmonid aquaculture setting, we estimate that several million blue mussels would be needed to filter 10% of all passing copepodids and 30% of all passing nauplii, corresponding to more than 10 000 m of mussel rope per open-net cage. In smaller settings, around 1 million mussels corresponding to more than 1 000 m of mussel rope would be needed. Our estimates may be used in future modelling and planning of combined salmonid-mussel farms. • Blue mussels were assessed as a biological tool for controlling sea lice. • Filtration experiments showed mussels reduce sea lice larvae. • Controlling sea lice may require mussel farms at commercial scale. • Co-culture of mussels and salmonids could strengthen sea lice mitigation.