Inter-tidal areas like Wadden Sea important for sharks, rays: Dutch researchers

Intertidal areas like the Wadden Sea play a significant role in the life cycle of sharks and rays, researchers from the University of Groningen and the Netherlands Institute for Sea Research concluded in a study published in the scientific journal Fish and Fisheries this week. Previous studies of these areas focused mostly on birds, but the Groningen researchers found that smaller sharks and rays also count on these areas as feeding grounds.

At low tide, birds settle on the tidal flats. At high tide, sharks and rays seek refuge between the flooded sea banks, researcher and shark specialist Guido Leurs explained to Trouw. “Just like birds, sharks and rays play an important ecological role in the intertidal areas,” he said. “Birds, sharks, and rays may compete when hunting the same food, but we think they need each other in some way.”

Rays, for example, can significantly change the landscape of intertidal areas by digging pits in search of food. Other species can then use those pits during low tide. “Sharks and rays are quite large animals, but still find a habitat in shallow water. We underestimated that for a long time.”

The researchers mainly looked at tropical intertidal areas, which still have more biodiversity. “The more diversity in the food web, the more species - including larger predators. Although, we’re not talking about the top of the food chain. In the intertidal areas, we mainly see smaller species and juveniles of larger species. It is an area for growing up, which is sometimes dangerous because mud flats dry up or because predators lurk outside the area.”

The Wadden Sea is the largest intertidal area in the world, but there are few sharks and rays to be found there now. “The Zuiderzee was probably a breeding ground for stingrays, but its closure has removed that ecological role. Moreover, the water has become more turbid since the construction of the Afsluitdijk, the eelgrass has disappeared, and fishing has increased,” Leurs said.

Overfishing also affects sharks and rays. A third of the 1,200 shark and ray species are threatened with extinction, including half of the species known to use the intertidal areas. The loss of these predators could disrupt the ecological balance in the intertidal areas, the researchers warned.

But the intertidal areas themselves are also at risk. In the past 40 years, an estimated 15 percent of intertidal areas have been lost worldwide. “Intertidal areas are important economically, culturally, and ecologically,” Leurs said to the newspaper. “But they are also important for fisheries and coastal development, for example. To better protect these areas, cooperation with the local population and fishermen is crucial.”