Fencing young mussels for ecosystem restoration

Ecosystem engineers

Mussels are so-called ecosystem engineers. On large banks, mussels shape the environment to their liking. In this way, they facilitate other mussels to settle, meanwhile helping other animals and plants to find a safe place on the sea floor as well. Moreover, mussel beds retain silt and sand and dampen the energy of waves during storms. "For this reason, there are lots of initiatives to restore mussel beds", Schotanus knows. "But it is, usually, a daunting task!"

Rough conditions

One of the problems in restoring mussel beds is the type of mussel that is used, Schotanus explains. "In the past, adult mussels were often used for this purpose. But they were not accustomed to dry conditions of intertidal mud flats. When they suddenly have to settle on a mudflat that is exposed to air during low tide and where there is a lot of wave energy, they cannot adjust 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."

Headstart

In another experiment, Schotanus 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 labour 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."

Unaccustomed birds

One of the reasons for restoring mussel beds is to boost biodiversity. That is why Schotanus also used cameras to see how birds such as curlew and oystercatcher reacted to these strange structures on the tidal flats. “They appear to get used to them after a while", the researcher observed. "Gulls found the larger quantities of shellfish near the fences very quickly anyway. Oystercatchers needed some time and curlews also took a while to discover that they could find many small crabs near the fences, which is their favorite food."

Positive interactions

The most important lesson from her research, Schotanus thinks, is that in restoring mussel beds, you must take advantage of the positive interactions between the shellfish themselves. "Mussels simply find strength and safety in numbers. All our artificial tools have their drawbacks, so the simpler we can make it for the mussels, the better."

More information

• Jildou Schotanus defended her thesis Restoring mussel beds on highly dynamic mudflats at the University of Groningen on Monday February 5th, 2024.

Text: NIOZ
Photos: Edwin Paree (leadfoto: veldexperiment in de Oosterschelde); Jildou Schotanus