Baltic Blue Growth

Baltic Blue Growth established fully operational mussel farms to counteract  eutrophication and create new blue growth opportunities.

Baltic Blue Growth publications


1. What is the main aim of the Baltic Blue Growth (BBG) project and its pilot mussel farms?

The main aim of the project is to implement five pilot mussels farm and look into whether mussel farming in the Baltic Proper is possible and if so; under what biological and financial conditions. Mussel farms can contribute to cleaner water in the Baltic Sea by taking up nutrients. It has been known for a while that mussels have impressive filter capacities; however, it is still a relatively new concept in the Baltic Proper. This is because of low salinity levels resulting in the mussels growing slower. Other challenges besides low salinity are the climate and predators.

The BBG will also investigate what the harvested mussels can be used for and their role in closing the nutrient loop. Blue mussels can be processed into high quality feed for fish and poultry as the meat contains a lot of Omega3. Moreover, the high levels of protein make it a good substitute to traditional protein sources and so can reduce the amount of soybean or fish meal used for feed, alleviating pressures on both the climate and fish stocks.

2. Is the nutrient extraction only addressing the nutrient content or also the ongoing nutrient input?

The pilot mussel farms extract the nutrients already present in the sea – i.e. the nutrient content. The farms act as a complementary measure to those land-based measures aiming to reduce the nutrient inflow into the Baltic Sea. The mussel farms are in no way substituting on-going measures on land, e.g. restricting nitrate on farmlands or modernising wastewater treatment plants. It’s rather alleviating the symptoms of an illness caused by too much nitrogen and phosphorous flowing in the sea. However, without measures to reduce nutrient content, it will take 40 years or more for the Baltic Sea to recover, even if all inflow of nitrogen and phosphorous stopped tomorrow. Nevertheless, since mussel farming results in an increase in water transparency, we can expect a positive impact on the ecosystem shortly after the mussel farms are established.

3. Is large-scale mussel farming in the Baltic Sea even possible?

Currently hundreds of tons of mussels can be harvested in the Baltic Sea, but there is potential for much more. According to a Danish report, there is a  biological potential to produce 300.000 tons of mussels annually in Danish coastal areas, a large part within the western Baltic Sea. Preliminary project results show that there is great biological potential for large-scale mussel farming also in the Baltic Proper, despite lower salinity levels. A total harvest of some ten thousand tons of mussels annually from the Baltic Proper could be realistic in the future.

However, the biological potential of mussel harvest is currently limited by factors that include outdated legislation and an underdeveloped market for the mussels farmed. Large-scale mussel farming also needs infrastructure, such as specially equipped boats and processing plants. Another pressing issue is technology, as most mussel-farming techniques that are used today only work in sheltered waters. In these waters there are typically many conflicting interests. Lack of space has been mentioned as an argument against large-scale mussel farming, but technical development could enable much larger areas in the Baltic Sea for mussel-farming. Hence, large-scale mussel farming in the Baltic Sea is potentially possible, if questions of technology and economy were to be solved.  

To highlight the potential of mussel farming in the Baltic Sea, a pan-Baltic map on viable regions for mussel growth (salinity > 5psu and chlorophyll > 1µg/l) is being developed by BBG project partners will be available in September 2018.

4. How large do mussel farms have to be to efficiently reduce eutrophication?

In the SUBMARINER Compendium, authors estimated that if Sweden harvested 9000 tons of mussels every year, this would contribute to 2-3 % of its total target of nutrient reduction set in the Baltic Sea Action Plan. This might not sound as too much, but if all countries around the Baltic Sea harvested similar amounts, it would ultimately add up.

Large-scale mussel farming also looks promising if we compare it to many suggested land-based measures aimed to reduce nutrient inflow.

Although agricultural measures can be inexpensive, there is often a lack of waste land or buffer zones between arable land and the coastline and/or steams. Buffer zones are important to stop nutrients leaking from the agricultural land into the sea as they act as nutrient traps. If there are few such buffer zones available, the costs for land-based measures will be very high. Larger constructions that such as wetlands are also regulatory complicated and there are often many stakeholders involved.

Industrial measures such as modernizing wastewater treatment plants are easier in terms of regulation and ownership but considerably more expensive per kilo of phosphorous than agricultural measures, particularly when going from 95-96 % to 98-99% uptake of phosphorous from the water.

Measure in the Baltic Sea Region  Reported N removal costs in €/kg N   Reported P removal costs in €/kg P
 Mussel farming without sales  10 – 64  150 – 900
 Agricultural measures  0 – 150  0 – 10200
 Livestock reductions  6 – 842  112 - 5895
 Wastewater treatment upgrades  11 – 136  39 – 600
 Wetlands  2 – 93  396 – 1518

The table above illustrates the cost effectiveness of different methods for reducing nitrogen and phosphorous in the Baltic Sea Region. Looking at this table it becomes clear that mussel farming can under circumstances be much more cost-effective than many other eutrophication counteracting measures. Of course, these figures depend on location and in which country the mussels will be harvested, and thus can vary greatly. The reported cost figures for all measures also vary greatly in the literature, so the listed values can only be seen as a rough estimation. Reliable values for the costs of mussel farming will be generated by the project team.

5. How do you know that mussel farming has a positive environmental impact?

Eutrophication is considered the biggest threat to the integrity of the Baltic Sea ecosystem and is primarily caused by excessive nutrient pollution in the water. The blue mussels filter the water for phytoplankton, and so store nutrients. When the mussels are harvested, the nutrients are removed from the marine environment. Therefore, the mussel farming can be considered as one of the most promising compensatory measures in order to prevent increased eutrophication in the Baltic Sea area. Moreover, mussel farming can greatly increase water transparency locally, which in turn increases light to aquatic organisms. However, it has been suggested that sedimentation from mussel farms may impoverish oxygen conditions locally below farms. The BBG monitoring around different farms in the Baltic Sea area has clearly shown that the farms never induce critically low oxygen conditions.

6. How do you monitor positive and negative environmental impacts and from what other sources is data gathered?

Environmental monitoring is an important part of the BBG project.  Researchers perform rigorous monitoring in and around the mussel farms to ensure a scientifically sound assessment of the positive and negative environmental impacts. Traditional sampling of water quality variables (e.g. water chemistry, phytoplankton, sediment, phytobenthos, benthic invertebrates and birds) is combined data from innovative oceanographic instruments. These oceanographic instruments record short-term variability of water parameters whereas samples collected at farm sites validate the data provided by the oceanographic instruments. Moreover, mussel recruitment and growth at mussel farms are experimentally evaluated in order to link broad patterns of oceanographic conditions to the net increase of the mussel stock. By using all these techniques interactively, the collated datasets make it possible to place the environmental impacts of each farm into broader spatial and finer temporal scales. This, in turn, allows us to assess mussel farm potential and impacts on a pan-Baltic scale.

7. What are the potential negative impacts of mussel farming on the ecosystem in the Baltic Sea?

Mussels function as efficient biological filters, purifying water masses from phytoplankton and other particulate matter. The filtration makes the water clearer and the increase in water transparency and underwater light conditions may in turn promote plant growth on the seabed (benthos). Mussels and their associated fauna that may drop from the farm enhance the food availability for predators and scavengers close to the sea bed.

However, the increased sedimentation of biodeposits (especially faeces) underneath a mussel farm can locally have a negative impact on the benthos if the mussel farm is placed in a coastal area with slow water currents and bad oxygen conditions. The organic enrichment of the underlying seafloor may lead to hypoxic conditions, releasing nutrients from the impacted sediments into the water. Studies have so far shown that any adverse effects have been restricted to the immediate vicinity of the mussel farms.

If we choose a location with well-oxygenated sediments, negative effects of mussel farming can probably be avoided or reduced. The overall environmental impact of mussel farms may differ between sites and change over time during the production cycle. The environmental monitoring of mussel farms within the BBG-project will help to evaluate environmental impact and identify key factors to optimize the positive environmental effects of mussel farming.

Apart from the potential adverse effect on the seabed directly beneath the farm, there are few other possible negative impacts (see question 6). Buoys and other parts of mussel farm equipment can be disconnected and might cause a local litter problem. Any lost equipment is reported to the Work Package leader by the BBG project partners. Moreover, we are in contact with other marine actors to deal with the issue. Marine litter will also be described in the final report on establishing the pilot mussel farms under the headline interactions with other marine users, conflicts and solutions.

The mussel farm equipment such as anchor blocks may have a local effect on the ecosystem. Some experts regard the creation of a new habitat and modification of naturally occurring zoobenthos communities as negative aspect of the mussel farming (locally decreased populations of suspension-feeders and increased deposit-feeders). Nevertheless, the total environmental effect is rather positive because bottom structures such as anchor blocks may provide a surface area for organisms that are normally not found on soft bottoms, where mussel farming often is practiced. Moreover, mussels growing in a suspended culture can create favourable habitats for other invertebrates, fishes and birds. Species diversity and abundance are therefore higher near mussel farms.

Further reading:

  • Fenchel T, Jørgensen BB, Riisgård HU (2017) Fake News Mussel Farming A "New Climate Bomb". Fish Aqua J 8
  • Miller R.G., Hutchison Z.L., Macleod A.K., Burrows M.T., Cook E.J., Last K.S. 2013. Marine renewable energy development: assessing the Benthic Footprint at multiple scales. Front. Ecol. Environ. 11: 433–440.
  • Wilding T., Gill A., Boon A., Sheehan E., Dauvin J., Pezy J., O’Beirn F., 
  • Janas U., Rostin L., De Mesel I. 2017. Turning off the DRIP (‘Data-rich, 
  • information-poor’) – rationalizing monitoring with a focus on marine renewable energy developments and the benthos. Renewable and Sustainable Energy Reviews 74: 848–859

8. Can the extraction of nutrients lead to a decline in certain fish stocks?

It is highly unlikely that the extraction of nutrients through mussel farming will lead to any detectable decline in fish stocks. The environmental goal of mussel farming is to achieve a local decrease in the potential for algal growth in areas of the Baltic with excessive nutrients. Decreasing nutrient concentrations in targeted priority areas should lead to local improvements in water quality with a minimum of undesirable effects. While there is a theoretical possibility that nutrient extraction associated with mussel farms could negatively affect fish stocks, the total size of farms needed to have a detectable effect on fish stocks far exceeds the size of current or potential mussel farms in the Baltic.

9. Is it true that methane emissions from mussels contribute to climate change?

A recent study claims that mussel farming could add a significant amount of methane to atmosphere. The statement was based on extrapolations from laboratory experiments conducted on sea worms and the Baltic Clam (Limecola balthica), living in carbon-rich Baltic Sea sediments. These ocean critters’ combined methane emissions were calculated to be the equivalent of 20.000 cows. Although this might seem like a lot, you have to look at the bigger picture. There are 23.5 million dairy cows in Europe, and approximately 265 million globally. Moreover, there is no reason to expect that blue mussels will produce the same amount of methane as clams. In fact, comparing clams living in sediment mud to mussels growing in ropes near the surface would be similar to comparing methane production in two species of mammals, say squirrels and humans!

As a reply to the study, a team of researchers claim that this potential methane production is negligible compared to the total methane production in the sediment. The methane released is taken care of by bacteria and do not reach the atmosphere. To read the full statement from BBG on this topic, click here.

10. What criteria are used for site selection?

The selection of the spatial area designated for aquaculture development and careful selection of farm sites are essential first steps to ensure the success and sustainability of aquaculture.

In selecting a suitable site for aquaculture several factors are to be considered, including:

  • physico-chemical and biological conditions of the environment (e.g. Salinity, Chlorophyll, Oxygen, Renewal time, occurrence of drift-ice, current, bed-structure, wave height, nitrogen, phosporus);
  • socio-economic aspects of aquaculture, (e.g. easy access to ports and to other coastal facilities, potential revenue visual impact, cultural heritage, tourism).

The Aquabest project has developed a road map/manual for localizing sustainable aquaculture farms in the Baltic Sea Region. The suitability maps for establishment of mussel farms were produced from the GIS analysis, and 12 favorable mussel farming sites were identified. The conclusion was that farming blue mussels for fish feed might be possible, but questions regarding production capacity and economic viability, among others, remain to be answered. For mussel cultivation in the Baltic Sea, salinity and water dynamics are major challenges. Moving from the west to the east Baltic, favorable natural conditions decrease: with decreasing salinity, the size of products decrease and the time of growth increases. These parameters are greatly influential to attract investors and final consumers.

A poor location of an aquaculture site results in poor production and might create environmental problems; it may also generate a broader impact on environmental, social and economic aspects, such as conflicts with other human activities. Interactions between aquaculture and other activities can have synergistic effect (resulting in benefits for one or both activities) or antagonistic effects (resulting in costs or negative consequences for some of the activities). However, these effects are not always predictable and may have cumulative effects that need to be taken into account in the forward planning and decision-making process. The Aquabest identified a number of potential conflicting interests:

  • Infrastructure: shipping routes and ferry routes, underwater cabling and pipes, fishing, harbours, slipways anchorage points, private water, water scooter permit;
  • Nature protection: shoreline protection, bathing places, valuable landscape, animal and plant protection areas, the bird directive, the locality directive, national parks, nature reserves; antibiotic resistance
  • National interests: fishing, outdoor life, culture, nature, Natura 2000, wind, municipal detailed plan of coastal water.

Further reading:



11. Doesn’t the extraction of nutrients from the water body give polluters arguments against stricter legislation and additional measures, especially against measures to improve the environmental status of upstream water bodies?

This is a hotly debated topic and one of the more contentious policy issues concerning sea-based measures such as mussel farming. Of course, there is a risk that polluters will view sea-based measures as an opportunity to argue against stricter legislation, therefore it is important to point out that mussel farming is a complementary measure to land-based measures. Particularly in regions where there is no realistic potential to reach “good environmental status” by only implementing land-based measures, mussel farming could be a suitable complementary measure.

The Swedish programme of measures to implement the Marine Strategy Framework Directive acknowledges that nutrient reduction through current land-based measures is insufficient to reach the national targets set out in the Baltic Sea Action Plan. Moreover, even if Sweden fully implemented all relevant land-based measures, it would still be 190 tons of phosphorous and 1800 tons of nitrogen away from reaching its HELCOM nutrient reduction target. Therefore, Sweden now wants to investigate whether complimentary measures to reduce internal nutrient load, i.e. the nutrients already present in the Baltic Sea.

You can also turn the argument around - legislators can use mussel farming as a nutrient reduction measure to argue in favour for stricter legislation in order to improve the environmental status of coastal waters.

The Swedish programme of measures illustrates how important it is to have complementary sea-based measures to ensure that our waters reach good environmental standard, and simultaneously enforce strict regulation on land-based pollution sources. There is no doubt that improving the environmental status of upstream water bodies is crucial to preserve fragile ecosystems and measures and regulations in place should be carefully monitored to ensure compliance. However, we also need to realise the environmental and cost benefits of setting up mussel farms to take up the pollution that reaches the sea despite of land-based efforts.

12. Can you legally justify mussel farming as a measure to fight eutrophication?

The current EU framework directives on water protection, specifically the Water Framework Directive (WFD) and the Marine Strategy Framework Directive (MSFD) have driven member states to take far-reaching actions to improve water quality. These actions have also led to a reduction in the inflow of nitrogen and phosphorous into the Baltic Sea. However, these measures are all land-based, focusing on reducing nutrient inflow rather than nutrient content. Neither the WFD nor the MSFD mention mussel farming in any context, which makes it difficult to figure out where EU legislation stand on this issue. 

As frameworks, the WFD and MSFD set up a legal structure for member states to implement national law. The frameworks represent a lowest common denominator, but each state has to decide what measures to put in place and can make their own national legislation more stringent than set out in the directives. Hence, as the WFD make no reference to mussel farming, each member state has to decide whether national law justifies mussel farming as a measure to fight eutrophication. However, national strategies in for example Sweden and Germany explicitly mention shell fish farming as a possible mitigation measure and the guidance document the “EU Commission Staff Document on the application of WFD and MSFD in relation to Aquaculture” regards mussel cultivation as a possible mitigation measure that can also lead to improvement of water clarity.

In Denmark, an empowerment law on compensatory instruments to decrease nutrient pollution from marine aquaculture was adopted in June 2017. The ministry can issue a statutory order whereby a license for a marine fish farm is issued as long as compensation measures are taken. These measures can entail mussel or algae farming that should be located in the same area as the fish farm and neutralise the nutrient input from the fish farm to the coastal or sea areas affected. However, this law is controversial and as of January 2018 the statutory order has not yet been issued.

13. What are the technological challenges for mussel farming in the BSR?

Mussel farming has been practiced in sheltered areas without any major problems caused e.g. by weather influences or ice cover. Most issues that faced and addressed in the Baltic Blue Growth project occur because we test mussel farming in more exposed locations. There, storms, currents, waves, ice and weather can cause material to wear or even get ripped off resulting in new requirements for the construction designs and materials used.

One of the biggest challenges is the regularly occurring ice-cover which can even hit the far south of the Baltic Sea. Ice-covers are not per se a problem, but in combination with currents can represent a major challenge because when the ice melts and breaks up, it can easily rip out the anchoring and rip off the buoys and lines. The most practical solution to this challenge is to build submerged farms. Such farms are also an opportunity to dramatically increase the size of potential mussel farming sites, as the mussel farms then face less competition for space and cause less aesthetic disturbances. Baltic Blue Growth aims to find solutions to the challenges that come with submerged farms such as setting drill-anchors, calculating the necessary farm specifications and the general lack of experience with the novel techniques.

Another problem of mussel farming in some parts of the Baltic Sea is the periodic predation from eider ducks. Different avoidance strategies besides moving the farm to a place where no ducks are, like different technical solutions are currently being tested.

By testing different farming techniques, different materials, set-ups and locations, the project provides valuable input in establishing good practices for mussel farming in the Baltic Sea. However, many processes still need to be optimized to increase the biomass production and eutrophication counteracting capacity of mussel farming in the Baltic Sea.

14. Is sufficient equipment for mussel farming in the Baltic Sea Proper available?

Mussels in the Baltic Sea are either grown on longlines or on nets (smartunits), which for both technology and equipment is available.

Concerning harvesting, it gets a bit more challenging. Especially the harvesting of mussels on nets that in most cases would be the more efficient farming technique. There are mussel farmers with own longline-harvesters in the Baltic Sea Region but no harvesters exist for net units. During the Aquabest project, a harvester was brought all the way from Norway to Åland. Since this is not a practical solution for self-sustaining businesses, farmers in the BSR will have to buy equipment. Since the machines are expensive, most mussel farmers will depend on cooperatives sharing the needed equipment. This is still the case on the Swedish west coast (two big mussel producers own one harvester each, several smaller businesses depend on renting them). Sharing equipment can result in a shortage of machines when more than one farmer wants to harvest at the same time.

In addition, the packaging of the harvested mussels is currently seen as one of the bottlenecks for blue mussel farming in Sweden. The places where mussels are packed need to fulfill legal requirements. Swedish farmers again depend on cooperatives or even sending mussels to the Netherlands for this processing step.

Mussel Farming in the Baltic Sea - BBG project

Operational Decision Support System (ODSS) - Plan your own mussel farm in the Baltic Sea


Basic Project Information

May 2016 - 36 months

Total budget
€4.6 million

EU financial support
€3.6 million


SUBMARINER Network for Blue Growth EEIG

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