The restoration of mussel beds in the Wadden Sea or delta goes is a lot more successful when young mussels are helped a little with low, protective fences on the bottom. This is according to the research carried out by marine biologist Jildou Schotanus. Jildou did her research at the HZ (in the Added Value with Mussels project) and at the Royal Netherlands Institute for Sea Research (NIOZ) in Yerseke.
On Monday, she defended her thesis 'Restoring mussel beds on highly dynamic mudflats' at the University of Groningen. Mussels are so-called biobuilders. In large shell banks, mussels shape the environment to their liking. They make it easier for their congeners to settle and help other animals find a safe place on the seabed. Moreover, mussel beds trap silt and sand and slow the energy of waves during storms. "For this reason, there are lots of initiatives to restore disappeared mussel beds," Jildou says in a press release from NIOZ. "But that is often still not easy!"
One of the problems in restoring mussel beds is the type of mussel that is used, Jildou explains. "In the past, adult mussels that were not used to dry conditions of intertidal mud flats were often used for this purpose. But if you suddenly try to get these mussels to settle on a mudflat that is exposed to air during low tide, where they also have to endure much more wave energy, they turn out to be unaccustomed to these conditions. Experiments we have done with very small mussels, the so-called mussel seed, show that these are still flexible enough to adapt to the rougher conditions on the tidal flats."
In another experiment, Jildou looked at how to give mussels the best headstart on the barren banks of the Oosterschelde (Eastern Scheldt). She experimented with breakwaters, with netting to keep hungry crabs away and with coir mats to help the shellfish adhere to the bottom. "All of these methods do work to some extent, but they are also very labor intensive," Schotanus says. “Relatively simple fencing that creates eddies in the currents, and can thus create lee for young mussels, seemed the most efficient in that sense."
One of the reasons for restoring mussel beds is to boost biodiversity. That is why Schotanus used camera traps to study how birds like curlews and oystercatchers cope with those strange fences on the drying ground. "These appear to get used to it well after some time," she says.
The main lesson from her research, is that when restoring mussel beds, you need to make use of the beneficial interactions between the shellfish themselves. "Mussels find strength and security in the large numbers. All our tools have their drawbacks. The simpler we can make it, the better."