Sampling planktonic salmon lice in Norwegian fjords

Risk of mortality of wild salmon caused by salmon lice is used as an index for managing aquaculture production in Norway and is based on monitoring of lice attached to wild salmonids and modelled concentrations of lice larvae in fjords. Direct sampling of lice from Norwegian waters to determine concentrations has never been published scientifically and has been considered non-feasible for monitoring purposes. Here we tested 4 different methods for sampling planktonic salmon lice copepodids. Salmon lice were found using all 4 methods with highly variable concentrations related to volume of filtered water with the different methods and patchy and variable distribution of lice within the fjords. Comparison between modelled and sampled lice concentrations showed variability within the same range and aspects of patchiness, and that the modelled concentrations were mostly lower than observed. We conclude that planktonic salmon lice can be sampled in Norwegian fjords with standard zooplankton sampling methods. Development of monitoring programmes of planktonic lice is possible but will demand a large amount of resources if implemented along the entire coast, because extensive sampling programmes and manpower for analysing samples are required to be able to capture the high spatiotemporal variability and patchy distribution of salmon lice. This calls for further development and use of modelling as a primary tool for national monitoring and management purposes. For further investigations of the effects of infestation pressure on wild salmonids, sampling combined with numerical modelling can provide valuable information.