Balticsea 2020

Farming blue mussel as an environmental measure - final comments

Blue mussels are effective water filters, and mussel farms can contribute to clearer water locally. When the mussels are harvested, nutrients are removed from the sea and can be processed into chicken feed. The aim of the project was to investigate whether mussel farms could be a cost-effective way of accelerating the reduction of nitrogen and phosphorus in Baltic coastal waters. The results show that extensive technological development and a better understanding of what controls settling and growth are needed if mussel farms are to be a cost-effective measure in the Baltic.

In 2009 BalticSea2020 awarded funding of SEK 6 million to associate professor Odd Lindahl from the Royal Swedish Academy of Sciences for the Environmental Mussels project. The aim was to make mussel farming a cost-effective environmental measure to improve coastal water quality in the Baltic. The idea was to measure the mussels’ growth on the farms and estimate the potential uptake of nutrients from the Baltic Sea. Another aim was to adapt the farming techniques and harvesting equipment developed on Sweden’s west coast to the smaller and more delicate mussels of the Baltic, and to develop ways of reusing the nutrients in the mussels on land, for example in the form of chicken feed.

Two full-scale mussel farms were set up, one at Hagby Hamn in southeastern Sweden and one at Hållsviken south of Trosa near Stockholm. Nets with different mesh sizes and ropes of different thicknesses were tested at both sites. Samples were taken from the farms on five occasions to count the number of mussels that had settled per square metre. The four metre deep nets were suspended from 125 metre long floating PVC pipes anchored two by two (A detailed description of the methodology can be found in the final report: “Mussel Farming as an Environmental Measure in the Baltic”, Lindahl 2012,  www.balticsea2020.org).

The project’s aims could not be achieved

Cold winters with thick ice and substantial ice drift during the life of the project (2009-12) disrupted the mussels’ growth and wrecked the farms (buoys and floating pipes). The number of mussels at the farms was therefore not sufficient to enable comparisons of growth using different types of nets, nor for the development of technology for the production of mussel meal from Baltic mussels. The planned development of a prototype harvesting machine was also abandoned as there were too few mussels to harvest.

The following important experience was gained concerning mussel farming projects in the Baltic:

• Farming techniques are currently costly and require improvement and adaptation if they are to work in the Baltic. The farms need to be developed so that they can deal with thick ice and ice drift.
• Maintenance and harvesting require large and specially equipped work boats.
• The settling and growth of mussels in the Baltic are uncertain and unpredictable. The mussels settled well on the project’s farms, but growth varied and on one occasion a large quantity of mussels were lost. Besides variations in the number of mussels, it was seen that barnacles rather than mussels can sometimes become dominant. The project could not explain differences in growth, why the mussels were lost, or why barnacles outcompeted mussels in some places. An analysis of other mussel farming projects, for instance in the St Anna archipelago in southeastern Sweden, found that in some places the quantity of mussels was very good after two years, while in others there were few mussels or even none at all. Further research is required to ascertain why some locations are well-suited to mussel farming and others are not.
• No differences in growth or settling could be established for different mesh sizes or rope thicknesses. To ensure a sufficient flow of water and keep the weight of the nets down, a mesh size of around 150 mm and rope thickness of 10-12 mm are recommended.
• Other research groups  have noted that large amounts of organic material can accumulate on the seabed beneath the farms, leading to a risk of hypoxia.

BalticSea2020 concludes that mussel farms are one of many ways of helping our seas but face numerous and considerable challenges in the conditions prevailing in the Baltic. It is well-established that mussels have the potential to remove nitrogen and phosphorus from seawater, but mussel farming is financially risky and not a cost-effective environmental measure for the Baltic Sea based on current technology and conditions.