More crabs wear on mussels and clams
Over the past 30 years, there has been a significant decline in mussels and clams along the Swedish west coast. This is the result of the most comprehensive survey to date, based on samples from over 800 sites.
The research points to a number of causes, with crabs and cod appearing to be part of the explanation.
There is widespread evidence that mussels and clams have declined in numbers over time, and that it can now be difficult to find bait for children's crab fishing in the summer. At the same time, the invasive species Pacific oyster has spread at breakneck speed. In less than 20 years, the sharp oysters have become a common sight along the shores of Bohuslän.
"Something is happening to the bivalves in this area. But nobody really seemed to know what and why. I wanted to find out more about it,” says Youk Greeve, a recent PhD at the Department of Marine Sciences, University of Gothenburg.
During his research, Youk Greeve has analysed samples from over 800 sites along the Swedish west coast, many of which he collected himself. He has also compared with studies from the 1990s onwards, and revisited over 70 sites where samples were previously collected.
Pacific oyster better prepared for crab attacks
Analyses indicate that the total biomass of some of the most common clams in the area, common cockles and it´s close relatives that live within the sediment on the seabed, has declined by 50 to 60%. There are also strong indications that mussels have declined, both in numbers and size.
At the same time, the Pacific oyster has taken over as the dominant species. An estimate based on research by Youk Greeve suggests that the Pacific oyster now account for two-thirds of the biomass of all bivalves in Bohuslän.
Increased water temperature, eutrophication and climate change are some of the factors that may be behind the changes.
In his doctoral thesis, Youk Greeve also discusses cascade effects, created by changes in the marine ecosystem, as a likely impact on mussels and clams in particular. Overfishing has led to a decline in cod, which in turn has led to an increase in the number of crabs, as cod eat crabs. Crabs, in turn, eat the bivalves.
An experiment by Youk Greeve shows that mussels and clams survive to a much greater extent when there are no crabs around.
"The pressure from predators is definitely strong enough to make it impossible for bivalves to settle. When all the adult mussels and clams have disappeared and there is a lot of predators - crabs - around, it becomes difficult for them to survive,” says Youk Greeve.
Pacific oysters, on their hand, seem to cope much better with crab attacks, thanks to a harder shell and stronger attachment to the substrate.
Many questions still to be answered
But the picture of the decline of bivalves is complex. In 2024, there was instead something of a blue mussel explosion along the west coast. The number of mussels tends to vary greatly over time, and there has been no regular monitoring of the bivalve population in Sweden, so there is a lack of historical data.
"Since we don't know whether our measurements and those of others were taken at the top of the mussel population or at the bottom, it is difficult to say with certainty how and why the population has changed,” says Youk Greeve.
His research is the most comprehensive review to date of the distribution and change in the mussel, clam and oyster population in the area. Youk Greeve says an important part of it is that it can be used as a basis for future research on changes in the ecosystem along the Swedish west coast.
"If I had the data I have now, but from the 1990s, my results would have been much more reliable,” says Youk Greeve.
Read the whole thesis here: "Spatial distributions and temporal changes of coastal bivalve populations"
Text: Mikael Andersson
All of these are bivalve molluscs that filter their food from the water.
Blue mussel (Mytilus edulis)
Has an elongated, teardrop-shaped shell that is usually blue-black to brown-black in colour. The shell is relatively thin. Very common along the entire Swedish coast, and thrives in both salt and brackish water. Blue mussels are also farmed commercially. Attach themselves to hard surfaces such as rocks, pilings and other mussels using strong, thread-like bristles. They live in shallow to medium-depth water areas.
Common cockle, one species of clam (Cerastoderma edule)
Characterised by its rounded, heart-shaped shell with distinct ribs. The shell colour varies from light brown to yellow and is relatively thick. Common along the Swedish coast, but favours sandy and muddy bottoms in shallow bays and areas. Lives buried in the sediment, making it an infaunal species. It burrows into sand and mud.
Oysters (Various species, e.g. Ostrea edulis, Magallana gigas)
Has an irregularly shaped shell that can be flat or bowl-shaped. The colour is usually greyish or whitish, and the shell is often rough and uneven. Naturally occurring along the Swedish west coast, although not as common as blue mussels. From 2007 onwards, Pacific oysters have taken over as the dominant species in the area. Oysters are also farmed commercially. They attach themselves to hard surfaces, often in the intertidal zone or just below the water surface. They can live both alone and in groups.
