Nearly 20 tonnes of rock armour carrying thousands of young European flat oysters have been deployed on two cable crossings in the North Sea as part of a pilot project led by TenneT and eight other partners.
The project is testing whether the oysters can survive, grow and reproduce on the rock armour and whether the method can eventually be incorporated into the construction and maintenance of maritime infrastructure.
TenneT uses rock armour to protect subsea electricity cables where they cross existing infrastructure. Rock is also used extensively during the construction and maintenance of offshore wind farms.
The consortium is investigating whether these rocks can be seeded with oysters before being deployed offshore, creating so-called “oyster stones” that could combine cable protection with nature restoration.
For the trial, the partners converted shipping containers into mobile hatchery units and used a method known as “remote setting”. Oyster larvae were transferred from a hatchery to a location close to the sea, where millions of larvae attached themselves to rocks inside the seawater-filled containers.
The oysters then grew to several millimetres in size over a period of several months before the rocks were transported offshore and placed on the two cable crossings.
The deployment marks the first test of the method in open sea conditions. A first trial was carried out last year in the Port of Rotterdam, where researchers studied the survival and growth of young oysters and identified improvements for the latest test.
Among the changes, the temperature and food supply in the mobile hatcheries were kept more stable, resulting in significantly more oyster spat settling on each rock.
The survival of the oysters will now be monitored throughout the different stages of the process, including transport by land, transport by sea and placement on the cable crossings. Monitoring over the coming year will determine whether the young oysters survive and grow under open sea conditions.
Flat oyster reefs were once widespread across the North Sea but have largely disappeared due to overfishing, pollution and disease, according to the project partners, which say they hope that reintroducing large numbers of native oysters could help stimulate the development of new reefs.
“TenneT will realise many new offshore wind farm connections in the coming years. As we develop infrastructure in the North Sea, it is important to seize opportunities that also support the conservation and restoration of marine ecosystems,” said Marcel van Veldhuisen, Spatial and Environmental Strategist at TenneT. “This experiment provides valuable insights into how such measures can contribute to large-scale nature restoration in the North Sea.”
The project involves Wageningen Marine Research, Wageningen University, Stichting Zeeschelp, ARK Rewilding Netherlands, TenneT, Van Oord Ocean Health, The Rich North Sea, Waardenburg Ecology and the Port of Rotterdam Authority.
Van Oord has previously been involved in several offshore oyster restoration initiatives in the Netherlands.
At the Borssele V offshore wind farm, Van Oord installed oyster reef structures as part of a research project examining different approaches to establishing European flat oysters on offshore scour protection. The company has also worked with Eneco on oyster restoration at the Luchterduinen offshore wind farm.
Last year, Van Oord and Ecowende announced plans to install Oyster Hubs at the 760 MW Hollandse Kust West VI offshore wind farm. The hubs are planned to provide a source of oyster larvae that can spread to surrounding areas and support the development of new oyster populations.
Van Oord and Ørsted have also tested droppable oyster structures at the Borssele 1&2 offshore wind farm, with the structures designed to facilitate oyster restoration using relatively small vessels and potentially make the method easier to integrate into offshore maintenance activities.