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Shellfish beds or reefs, formed by mussels and oysters, have declined worldwide. Sterre Witte, who conducted her Ph.D. research at the NIOZ Coastal Systems department, has investigated how we can counteract this decline.

"I investigated the rules of the game that need to be followed if you want to stimulate the settlement of mussels and oysters in the channels of the Wadden Sea that are submerged even at low tide," says Witte.

Large-scale field experiment

Most research in the Wadden Sea is conducted on mudflats that are exposed at low tide. Here, previous research has shown that providing a hard substrate enhances the settlement of shellfish. Witte wondered whether this would also be the case in the deeper parts of the Wadden Sea. In a large-scale field experiment, the Ph.D. researcher placed sixty cages of one square meter filled with different materials in ten subtidal channels in the Wadden Sea. She compared four natural and two artificial materials.

Granite boulders, pebbles, empty cockle shells and pieces of bog wood—saturated with water so that they would sink—were the natural materials. She also used reef blocks made of biodegradable plastic entwined with coconut fibers and reef blocks that mimic oysters.

Just one oyster

"After a year and a half, it turned out that it hardly mattered what material I had used," Witte says. "At different, intertidal locations, other researchers found settlement of oysters and mussels within six months. But in the deeper parts, I only found one oyster. And in our case, we also did not see the formation of a mussel bed on the hard substrates."

However, biodiversity had increased compared to the sandy seafloor surrounding the test sites: Anemones, tube worms, fish, pebble crabs and sea slugs had settled there among other species.

Witte explains, "We even found a tiny sea slug that was thought to be locally extinct. The food web on the hard substrates was also more complex than in the surrounding sandy bottom. So, in a year and a half, a hard-substrate community had already been established in the experiment."

A tasty buffet for predators

However, Witte was notably interested in the settlement of mussels and oysters. Why did they not settle on the offered substrates?

"Remains of the valves of Pacific oysters on the substrates proved that there had been settlement of shellfish. And there were plenty of starfish, shrimp and crabs, which all eat shellfish. So instead of a successful nursery for shellfish, we had created a tasty buffet for their predators," Witte observes.

Placing cages works

Witte came up with an idea: In a different experiment, she placed small cages around the substrates.

"They had a mesh size of one millimeter. The predators couldn't get through, but the settling shellfish could," she explains.

It turned out to work very well. Outside the cages there were barely any shellfish, but their densities were high inside the cage refuge. So, Witte was right: Predation pressure plays a critical role in shaping the settlement of shellfish, especially in the deeper parts of the Wadden Sea. That said, she doesn't envisage covering the seafloor of the Wadden Sea with cages: "That's impractical and also undesirable in a natural area."

The real solution

Ultimately, Witte found the solution in introducing substrates with a high surface complexity.

"I compare it to a smooth sheet of paper that you crumple," explains Witte. "Juvenile shellfish can find shelter from predation on such a surface."

She tested this concept in a final field experiment using 3D-printed piramids with different levels of complexity. Settlement of both mussels as well as flat oysters, which are functionally extinct in the Netherlands, can be enhanced when offering a more complex settlement surface.

Boulders into the sea

However, covering the Wadden Sea with these pyramids is also not something Witte would recommend.

"But if nature managers would want to restore shellfish reefs, we now know what is key. They could, for example, consider using substrates with a complex surface to enhance shellfish settlement."

Witte currently works as a postdoctoral researcher in the section for coastal ecology at the Danish Technical University.

"It would be hard to imagine in the Netherlands, but Denmark is actively pursuing large-scale nature restoration; for example, by building substantial boulder reefs and mussel beds."

Witte's thesis title is "Solid foundations—The interactive role of hard substrate and predation in shellfish reef restoration across tidal gradients."