The low salinity in the Baltic proper means that the mussels are under high physiological stress and results in a slow growth rate. In addition, blue mussels in the Baltic proper contain less meat and have a lower nutrient content compared to mussels living in higher salinity. Together, this leads to a lower nutrient uptake efficiency in Baltic proper blue mussel farms. In addition, the longer time to harvest in the Baltic increases risks for practical farming problems due to ice, storms, epiphytes and eider preda- tion affecting the small and weakly attached Baltic mussels. These factors have generally been underestimated, but should probably rather be regarded as normal for the Baltic proper.

Holistic scientific studies of mussel farming nutrient uptake capacity have been conducted in areas with higher salinity in the Baltic Sea (>14 psu). But there are few data on harvest yield and nutrient content in blue mussel farms from the Baltic proper, which makes it difficult to provide a reliable estimate of the nut- rient abatement potential of mussel farms. Using available data, we estimate that nitrogen uptake efficiency per farming area and time may be 10 times lower in the Baltic proper than in blue mussel farms from areas with higher salinity. The phosphorus uptake efficiency is approximately five times lower than in hig- her salinity. This estimation needs to be refined when more data is available. Although it may be possible to reach higher uptake efficiency, it is clear that previously published estimates of miti- gation capacity of Baltic Sea blue mussel farms are too high.

Mussel farming is regarded as relatively low-impact aquaculture, but blue mussel farms can have negative effect on the local environment, in particular if they are large or dense. For instance, mussel farms lead to local accumulation of nutrients and organic matter, which may result in oxygen deficiency on the seabed and unwanted plankton blooms. There is a need for more field studies and ecosystem modelling before the effects of large-scale farming for nutrient abatement in the Baltic proper can be properly evaluated.

In conclusion, a realistic evaluation of the potential to use blue mussel farming as a nutrient abatement measure in the Baltic Sea needs to account for the low salinity and for the risk for environmental effects of large-scale farming. There is a need for more research supporting that blue mussel farming can provide a cost-efficient nutrient uptake in the Baltic proper, as well as research on the environmental effects of mussel farming, before it can be recommended as a measure to reduce eutrophication.

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NOTE: In an earlier version of the report, we reported cost for nutrient reductions with wrong unit, €/ton instead of €/kg of N and P (page 10). This error has been corrected in the current version.

Report 2/2018 Mussel farming (1184 Kb)