Last year was a real mussel year, with up to 1000 times more mussels than usual in some areas along the West Coast. Scientists are not sure why this increase is happening, but it's a good opportunity to find out more about the species' conditions.
One thing is clear - to hang out seems to be a recipe for survival.
A cold wind is blowing from the Kosterfjorden, and the sun is hidden behind grey clouds. In just over six months, the beach at Bofors Camping on Tjärnö will be full of people enjoying the summer, but on this January day, they are nowhere to be seen. Instead, research on blue mussels is being conducted at one of the piers, where Filip Leander is out checking the results of an experiment.
He is studying for an advanced degree in marine biology at the Department of Marine Sciences. For the past six months he has been at the Tjärnö Marine Laboratory, assisting Professor Mats Lindegart in a multi-year research project on blue mussels (Mytilus edulis) and their habitat.
- Where the mussels settle has a great impact on their survival. It's a phenomenon that hasn't been much researched, and that's what I'm investigating, says Filip Leander.
Where is the best habitat får mussels?
At several piers on Tjärnö, he has lowered stones into the water, a total of 40 pieces. Half are on the bottom, and the rest are hanging a bit above. Each stone had 30 mussels attached to it in the laboratory. Now, a few weeks later, it's time to pull up the stones to see where the mussels have fared best.
Over several decades, the mussel population along the West Coast has decreased. This has been reported from several sources, although there is no clear scientific data to support this trend. However, 2024 turned out to be a real mussel year. Several media outlets have reported on a mussel explosion, and the researchers at Tjärnö also found a significant increase in the population when they conducted sampling along the West Coast during the autumn and winter.
- There have been a lot more, compared to previous years, of the blue mussels that are less than a year old. I would estimate it to be from 100 to up to 1000 times more in some areas. So it's a huge increase, says Mats Lindegart.
But there is no clear answer as to why the mussels thrived last year.
- It's not that the situation has radically improved for the mussels, more likely something temporary has happened. It could be that the weather was favourable, that they were drawn to the shore just when they settled, or that the temperature was good. There could be many reasons, but we don't know exactly what caused the increase, says Mats Lindegart and continues:
- And just because we see many more mussels, doesn't mean that the marine environment has improved, everyone understands that. But at least it shows that it's not impossible for the mussels to recover.
Mussels clean the water
Blue mussels play an important role in the marine ecosystem. They act as water purifiers, build banks that are important for plant and animal life, and are food for humans and several other species living in the sea. Something that starfish seem to have taken advantage of when the mussel population increased dramatically in 2024.
Images from the publication Deep Sea Reporter show how large numbers of common starfish (Asterias rubens) feast on the mussel banks, and this is also something that researchers from the University of Gothenburg have noticed.
- What we see is that in some environments, there are a lot of predators, including starfish and eiders, eating the blue mussels. It´s not certain that they will exploit everything, but this could leave a mark on the population in the future, says Mats Lindegart, who expects to have more answers about the blue mussels' habitat to present in a few years when Cruise Speck, the project's doctoral student, presents his dissertation.
The starfish are feasting
At least it seems clear that blue mussels like to hang out. At the pier on Tjärnö, Filip Leander pulls up stone after stone, and the mussels that were lowered into the water on hanging stones are almost all still there. But on the stones that were placed on the bottom, the mussels have either moved or fallen victim to predators. Like starfish.
One is even still there, munching on the mussels, when Filip pulls up a stone.
- It's clear that the mussels on hanging stones survive to a much greater extent than those on stones that were on the bottom. And it seems like a big reason for that is predation; those on the bottom are eaten by, among others, starfish, while those hanging fare much better, Filip Leander concludes.
Text: Mikael Andersson
