Flat oyster production in hatcheries for restoration

For sustainable restoration of flat oyster (Ostrea edulis) beds in the North Sea, the genetic origin and diversity of the oysters used are crucial for the success and long-term resilience of these reefs. Flat oyster populations in the North Sea have strongly declined and no longer occur in large quantities, meaning that restoration projects depend on oysters sourced from elsewhere. To scale up restoration efforts from pilot projects to ecosystem-level impact, large quantities of oysters are required. Ideally, these oysters should originate from populations that are adapted to local environmental conditions. Hatchery-produced oysters are therefore increasingly used, as they provide a controlled and scalable source while avoiding impacts on remaining natural populations, limiting the risk of introducing invasive species, and allowing for monitoring of disease status.

 

This project focuses on the role of hatcheries in supporting large-scale flat oyster (Ostrea edulis) restoration in the Dutch North Sea. At the start of the project, there was a knowledge gap regarding the reliability of supply of hatchery-produced oysters, including their disease status, as well as their genetic diversity. To address this, the project combines a hatchery inventory with genetic analysis to assess whether hatcheries can deliver sufficient quantities of oysters with suitable characteristics for restoration. In doing so, it evaluates both the delivery capacity and reliability of hatcheries, and the genetic suitability of the produced oysters for long-term reef development.

 

The objectives of this project are to assess the reliability, frequency and volume of flat oyster supply from hatcheries, including their veterinary specifications such as infection with, or tolerance to, the parasite Bonamia ostreae. In addition, the project aims to evaluate the suitability of hatchery-produced oysters for large-scale Dutch North Sea restoration, by assessing whether the produced spat has sufficient genetic variation and by determining how many parental oysters contribute to each production cycle.

 

Program: Nature Regeneration North Sea
Image credit: Naomi Jansen, Natuur & Milieu

01-Jan-2025-01-Nov-2025

Project dates

Multiple hatcheries

Location

Wageningen Marine Research and Sas Consultancy

Project lead and partners

Ostrea edulis

Target species

Methods

The hatchery inventory was carried out using a combination of a European questionnaire, stakeholder workshops, and direct input from hatcheries. The questionnaire, distributed between February and October 2025, collected information from hatcheries and restoration projects across several European countries, including the United Kingdom, Ireland, France, the Netherlands, Italy, Denmark, and Germany. In addition, three workshops were organised with hatchery operators and restoration project representatives to discuss production capacity, scaling-up potential, and challenges in supply. Furthermore, detailed information was obtained directly from Dutch hatcheries during the period August to October 2025. The inventory focused on production capacity, supply reliability, frequency of delivery, and available product types such as larvae, single seed, and spat-on-substrate. It also considered constraints affecting production and supply, including disease regulations (particularly related to Bonamia ostreae), permitting requirements, technical limitations in hatchery processes, and market uncertainties influencing demand.

 

In parallel, a genetic analysis was conducted using flat oyster spat from five hatcheries located in the Netherlands, France, Sweden and Italy. Genomic DNA was extracted and analysed using SNP markers to assess genetic diversity and population structure. Measures such as heterozygosity and inbreeding coefficients were used, and relatedness between individuals was calculated to determine how many parental oysters contributed to each production cycle. The results were compared with genetic data from wild Dutch populations and previously used restoration stocks.

Results

The hatchery inventory shows that hatcheries and restoration projects across Europe are preparing for scaling up flat oyster production, both in terms of production capacity and expected demand. However, the results also indicate that supply is not yet fully reliable. A key issue identified is the interdependence between supply and demand: hatcheries are hesitant to significantly increase production without secure long-term demand, while restoration projects depend on a reliable supply before committing to large-scale deployment.

 

In addition, several technical and logistical challenges remain. Hatcheries experience variability in larval survival and settlement success, which can lead to uncertainty in production outcomes. Differences in national regulations, particularly those related to disease status and transport of oysters, also create barriers for cross-border supply.

 

Despite these challenges, developments in the sector are positive. Production reliability is gradually improving, and some hatcheries are already able to deliver large volumes of oysters on substrate, provided sufficient preparation time is available. Hatcheries are also increasing their production capacity, and most are willing to scale up further if long-term demand becomes more certain. Overall, the outlook for scaling up flat oyster production is cautiously positive, although the number of hatcheries capable of delivering at the required scale is still limited.

 

The genetic analysis shows that flat oysters produced by the two Dutch hatcheries are genetically similar to local wild Dutch populations. In contrast, oysters produced in hatcheries in France, Sweden and Italy, as well as oysters originating from Irish and Norwegian populations previously used in restoration, differ more strongly in their genetic composition. This indicates that the use of locally sourced hatchery oysters may better preserve local genetic characteristics.

 

In addition, the analysis revealed that in some hatcheries only a limited number of parental oysters contribute to reproduction during a production cycle. This can result in reduced genetic diversity and an increased risk of inbreeding in the produced spat. These findings highlight the importance of managing broodstock and reproduction processes carefully. The implementation of breeding programmes is therefore essential to ensure sufficient genetic diversity in hatchery-produced oysters used for restoration.

Tips & Tricks

Lessons learned

  • Reliable supply of flat oysters depends strongly on the alignment between hatcheries and restoration projects, particularly long-term demand.
  • Uncertainty in demand limits investment in scaling up hatchery production.
  • Genetic diversity in hatchery-produced oysters is not guaranteed and requires careful broodstock management.
  • Using locally adapted oyster populations can improve survival and restoration success.

 

Design improvements

  • Develop structured breeding programmes to ensure sufficient genetic diversity.
  • Optimise the full production chain: broodstock selection, larval rearing, settlement, and deployment.
  • Improve substrate choice, feeding regimes, and environmental conditions in hatcheries.
  • Better match hatchery production to the specific requirements of restoration sites.

 

Decommissioning

  • Long-term demand for oysters is partly linked to offshore developments (e.g. decommissioning).
  • Uncertainty in these developments can limit investment in hatchery capacity.

 

Permitting

  • Disease regulations (especially Bonamia ostreae) and transport rules remain key challenges.
  • Differences between countries can complicate cross-border supply.
  • Ongoing improvements in regulation may help scaling up restoration efforts.

Partners

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