The energy the mussel would normally use to grow must instead be expended on regulating the salt levels in its cells


Mussel farming an uncertain measure against eutrophication

The cultivation of blue mussels may have many positive effects; however, as a measure against eutrophication it remains an all too uncertain method. This is the conclusion of a new research report from Stockholm University’s Baltic Sea Centre.

Text: Henrik Hamrén

Mussel farming has been the topic of much discussion over recent years. Many research projects are underway in the Baltic Sea, including in Kalmar Sound, and media reporting has primarily dealt with the potential of these farms to reduce eutrophication and that, in time, blue mussels may replace fishmeal for uses such as fish feed.

“We believe that this reporting has been far too unnuanced. Although mussel farming is often presented as a largely unproblematic, win-win solution for the Baltic Sea, the reality is far from that simple,” says Nils Hedberg, researcher at the Baltic Sea Centre and principal author of the new report Limitations for using blue mussel farms as nutrient reduction measure in the Baltic Sea

The conclusion of the report is that the cultivation of blue mussels remains an extremely uncertain measure for reducing eutrophication in the Baltic Sea – both in terms of efficiency and attendant risk.

“Perhaps the most important factor is the Baltic Sea’s low salinity, which means that blue mussels grow significantly more slowly in the Baltic than in more saline waters. A mussel will grow approximately 10 times more slowly in the Baltic proper than on the Swedish west coast.”

Why is this?

“The energy the mussel would normally use in order to grow must instead be expended on regulating the salt levels in its cells. A new study demonstrates that shell-building also demands more energy in less saline waters,” explains Nils Hedberg.

Mussels cultivated in the Baltic Sea are harvested after approximately two and a half years, when they are approximately three centimetres in length. On the Swedish west coast, harvesting takes place after one and a half years, by which time the mussels are five centimetres in length and weigh an average of 13 grams. Illustration: Azote.

Why is the growth of mussels so important?

“Aside from the fact that Baltic mussels grow more slowly and are smaller, they contain less meat and therefore less nitrogen than, for example, west coast mussels. The sample calculations for nitrogen uptake used until now to justify mussel farming in the Baltic have been overestimated.”

From this year, the Swedish Government’s new ordinance regulating support to local water conservation projects, known as LOVA grants, makes it possible to apply for a grant to begin cultivating mussels as a measure against eutrophication. From this perspective, Nils Hedberg believes that growth levels and nitrogen uptake are of particular relevance. “This is a matter of money that has been ring-fenced for combating eutrophication. If we are to invest state funds in mussel farming as a specific nutrient reduction measure, it is vital that we get the numbers right so we know what we are doing,” he says.

In the report, you also speak of the risks. What do you mean by this?

“One potential risk factor with large-scale mussel farming in the Baltic Sea is that it will have the opposite effect and will actually increase the concentrations of nutrients around coast, rather than reduce them.”

How does this work?

“Most of the nutrients eaten by mussels are excreted again and are sedimented on the sea floor during cultivation. The Swedish coastline is a net importer of nutrients from the open sea via tidal waters and currents. This is known as a coastal filter, a mussel farm can collect nutrients from the open sea that would otherwise have passed by in the currents, thus diluting the filtering effect even more at a local level,” says Nils Hedberg.

Can this be avoided?

“To a certain extent, yes, even if it does bring with it other problems. The movement of the water is necessary to provide new food for the mussels and to transport sediment away from the sea floor. This is why it is advantageous to locate mussel farms in strong currents, such as those of Kalmar Sound. However, this also increases the risk of disruption, for example in the form of storms and ice, that tear the mussels from the cultivation rope or causes the entire farm to drift away with the ice flow,” explains Nils Hedberg.

But irrespective of this, harvesting mussels does take up nutrients from the sea. Isn’t this good?

“Mussel farming will never make a decisive difference to eutrophication and the creep of anoxic bottom waters for the Baltic as a whole; cultivation on such a scale would destroy coastal environments. It may however have local effects in terms of taking up diffuse emissions, for example from agriculture. At the same time, as I said earlier, there are risks and side-effects.”

Many current test farms in the Baltic Sea are partly financed by the EU’s Baltic Blue Growth project. Nils Hedberg welcomes the project and is convinced that their test results will provide a much better basis for estimating nitrogen uptake, efficiency and, in the final analysis, even costs.

“We should definitely await these results before we begin to invest public funds in mussel farming as a solution to the Baltic’s eutrophication problem – because we still don’t know if it is a solution.”