Eutrophication, caused by excess nutrients such as nitrogen and phosphorus in coastal waters, leads to low levels of oxygen and excessive growth of algae. The EU's Marine Strategy Framework Directive1 proposes that Member States should agree on common environmental targets in the different sea regions, and for the Baltic Sea all countries have signed the HELCOM Baltic Sea Action plan2. Each country should further establish a programme of cost-effective measures for how to reach the targets.
The ability of mussels and other bivalve species to remove nutrients from lakes and seas has been long recognised. The study used nutrient load targets set by the Helsinki Convention and estimated the costs of nutrient cleaning by mussel farms to reach these targets. Using a model, mussel farming was compared with other nutrient abatement measures including increased cleaning at sewage plants, buffer strips, wetland construction and the cultivation of catch crops. Four of the seven basins in the Baltic Sea were studied (the Kattegat, the Sound, the North Baltic Proper and the South Baltic Proper) and the value of mussel farming was calculated in four different scenarios: with and without mussel sales options and in terms of low and high cost due to growth rates and nutrient content in mussels.
The results indicate that mussel farming can cut costs for reaching Helsinki Convention targets under all four scenarios. The overall cost savings range from EUR 20 million to EUR 138 million, which produces a value of EUR 0.02 to EUR 0.21 per kg of mussels. It is estimated that the inclusion of mussel farming could decrease the total abatement cost for eutrophication by as much as 5 per cent, although this figure would be lower under unfavourable cost and growth conditions. Mussels farmed this way could also be used as fish meal in livestock feed for poultry, as many would be too small to be consumed as seafood.
The study also showed regional differences in the value of mussel farming and indicated that the value may be affected by the choice of farming technology. This could be an issue since the technology considered in the study (long-line mussel farming) may not be appropriate for open coast zones. Another issue may be that mussel farming could disrupt the ecosystem. This is yet to be investigated properly.
Nevertheless, the research demonstrates that mussel farming is a viable means of removing excess nutrients in coastal areas. The authors suggest its use could be incorporated into policy in two ways: extending EU support systems aimed at reducing nutrient concentration to include mussel farming and introducing a system of trading nutrient reduction requirements between sectors. For example, an existing sewage treatment plant on the West coast of Sweden is allowed to trade nitrogen cleaning with a mussel farm. The authors suggest this nutrient trading could take place on a more international scale.