© Orsted

BioReef: marine restoration in Danish waters

Establishing biogenic reefs in Danish waters
WWF and Ørsted are using European flat oysters and horse mussels to restore biogenic reefs in the North Sea, helping to create new habitats, increase biodiversity, and improve water quality – in collaboration with DTU Aqua.
The BioReef project is part of the global partnership between the renewable energy company, Ørsted, and WWF to advance offshore wind energy deployment that enhances ocean biodiversity. The marine restoration project aims to develop methods to establish viable biogenic reefs in the Danish part of the North Sea.

By restoring the European flat oyster and horse mussel populations – two important habitat-building bivalve species – in this greatly degraded ecosystem, we hope to bring the environmental benefits of biogenic reefs to the area. The new methods we develop could be used to inform best practices for bivalve reef restoration projects around the world.
What are biogenic reefs?

BioReef rel= © Orsted

Biogenic reefs are reefs formed by living organisms. Most people are familiar with the colourful coral reefs of the tropics, but different kinds of biogenic reef can be found in different ocean environments – and all of them play a crucial role in the ecosystems they support.

Once established, biogenic reefs provide food and shelter for all sorts of species. They also provide surfaces for macroalgae, corals, sea sponges, and other organisms that grow on hard substrates. They’re a crucial part of healthy and thriving marine ecosystems.

Why are we restoring biogenic reefs?

The last century has seen a significant decline in the number and quality of biogenic reefs in the North Sea, with different species affected by disease, overfishing, climate change, or water quality.

The BioReef project focuses on two species: the European flat oyster (Ostrea edulis) and the horse mussel (Modiolus modiolus). These ‘ecosystem engineers’ once formed the basis for many of the biogenic reefs found in the Danish part of the North Sea. These reefs consist of both living organisms and the shells they leave behind when they die.

While the European flat oyster and horse mussel were once both abundant here, their populations are now greatly diminished. Both species are on both the OSPAR and HELCOM marine environment protection commissions’ Red Lists of species threatened by extinction in the North Sea and the Baltic region.

How are we restoring biogenic reefs?

BioReef aims to establish one or more sustainable, robust biogenic reefs of European flat oysters and horse mussels in Danish waters. This involves several steps:
  • Finding suitable sites to deploy reefs. This means screening locations with historical records of biogenic reefs in Danish waters and selecting sites that are suitable for restoring biogenic reefs.
  • Cultivating oysters and mussels on a large scale. This means collecting broodstock, the parent organisms that produce the larvae which, once they settle on a substrate, are known as spats and can be used as seeds. The seeds can then be deployed and develop into healthy adult mussels and oysters that can form a reef.
  • Deploying one or more reefs on the seabed at the selected site(s).

© Orsted

What challenges does the project seek to overcome?

A key challenge for the BioReef project is to produce mussel and oyster seeds on a large enough scale, and without disease. Horse mussel production has never been attempted on this scale before, nor has the establishment of biogenic reefs based on horse mussels in the North Sea.

Large-scale oyster production is more common, but the cultivation of European flat oysters for restoration can only succeed if they’re protected against the Bonamia parasite, which is partly responsible for the massive decline of the species in the North Sea.

DTU Aqua’s Section for Coastal Ecology has a large-scale hatchery and experimental nursery in northern Denmark that’ll be used for the BioReef project. The 750m2 hatchery opened in 2022. It contains facilities for biosecurity quarantine, broodstock conditioning, and larval production, along with settling tanks, a micro nursery, facilities for microalgae production, and a laboratory.

The hatchery is connected to the 450m2 experimental nursery where the initial grow-out will take place. DTU Aqua has additional grow-out facilities in the waters of the Limfjord, where the survival of young, individual organisms can be tested in real conditions.

DTU Aqua’s work for BioReef in the hatchery will lead to protocols that can be implemented elsewhere to produce seeds for other marine restoration projects.
Project timeline
2023: Collection of broodstock, screening of sites suitable for biogenic reef locations in Danish waters
2024: Site selection, development of hatchery protocols; grow-out testing
2025-2026: Further testing in the hatchery and on nearshore locations
2027: Deployment of reefs in selected location or locations.
The partnership between Ørsted and WWF partnership

The BioReef restoration project is an essential part of a five-year partnership between WWF and the renewable energy company Ørsted. The partnership was launched in October 2022 and the partners are exploring how to support a rapid transition to renewable energy, while addressing the global biodiversity crisis.

Learn more about the partnership at orsted.com/wwf  and panda.org/orsted.