Sonar reveals: artificial tree reefs in the Wadden Sea provide habitat for fish
NIOZ Royal Netherlands Institute for Sea ResearchThe researchers built 32 artificial reefs from discarded fruit trees, each measuring 3 cubic meters. In the spring of 2022, they placed them in groups of eight at four locations in the Wadden Sea. Using bottom nets (kubben), they found six times more bottom-dwelling, ‘benthic’ fish, of much more diverse species and also with larger individuals near the tree reefs. The reefs also appear to serve as a breeding ground for cuttlefish and several fish species. In addition, the activity of crabs, which eat shellfish and other species, was three-quarters lower around the reefs than in areas without artificial reefs.
Recreational sonar
Dickson adapted a recreational fishing sonar (a so-called ‘fish-finder’) for longer-term use on a floating platform next to the artificial reefs. These fish-finders are widely used by anglers, but had never been used for scientific research before. This technique is also able to detect pelagic fish, higher up in the water column. Dickson: “In various size classes, we saw a total of more than 2 to even 3.5 times more fish, swimming around the reefs than in control waters 200 to 300 meters away from a reef.” In a total of one hundred hours of sonar imaging, no less than 92.000 individual fish were counted near the reefs.
According to Dickson, the reefs were very attractive to fish, therefore he does not rule out the possibility that the effect on fish is still measurable even at more than 300 meters from the reef. “In a subsequent experiment, we will also have to choose control sites at greater distances.”
Shark in sight
The sonar is not capable of identifying small fish species. For larger species, however, it is possible to determine at least the species group. “On one sonar image, we even saw a 1.5-meter shark swimming above a reef, possibly a smooth-hound”, says Dickson. During an ‘unofficial inventory’ using old-fashioned fishing rods, Dickson and his colleagues saw that sea bass in particular were attracted to the reefs.
Shipworms and fish food
According to Dickson, the presence of many different, larger fish around the artificial reefs shows that these structures really meet a need in the Wadden Sea. "In the past, the Zuiderzee and the Wadden Sea were full of wood that came from the rivers. There are even descriptions from Roman times, of ships ‘sailing around islands of floating driftwood on the Zuiderzee’. In many cases, that wood sank to the bottom of the Wadden Sea over time. There it played a crucial role in the ecosystem. The wood was eaten by shipworms, which in turn served as food for fish. In addition, the wood was an important hiding place and breeding ground for fish and other animals, as well as an anchor for mussels and other shellfish."
Filling a gap
Nowadays, the supply of wood has been completely cut off by our control of rivers. Reefs made of wood, but also of boulders or shellfish, could fill that gap again, says Dickson. “If we place clusters of these artificial reefs in various locations in the Wadden Sea, a natural system with nurseries and hiding places for fish will be created again. That would make the Wadden Sea a more complete ecosystem again.”
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Text: Royal Netherlands Institute for Sea Research (NIOZ)
Images: Oscar Francken, NIOZ