North Sea ReViFES – Oyster Hatcheries and Settlement Techniques

The European flat oyster (Ostrea edulis) once formed extensive reef systems throughout the North Sea, creating complex habitats that supported marine biodiversity and contributed to ecosystem functioning. During the twentieth century, however, these reefs largely disappeared due to overfishing, seabed disturbance, and habitat degradation. As interest in oyster reef restoration grows, offshore wind farms have emerged as promising locations for recovery, as bottom-disturbing activities are generally excluded from these areas.

 

A major challenge for large-scale oyster reef restoration is the lack of a reliable supply chain capable of producing sufficient quantities of healthy oyster larvae and settled spat. While suitable restoration sites increasingly exist, practical bottlenecks remain throughout the process, from hatchery production and larval development to successful settlement and reef establishment.

This project investigated how these bottlenecks can be addressed by following the oyster restoration pathway from hatchery to reef. The research focused on improving larval production, understanding oyster larval behaviour, and identifying settlement substrates that can support successful reef development. Together, these studies provide insight into the critical steps required to scale up European flat oyster restoration in the North Sea.

The project contributes practical knowledge for hatcheries, restoration practitioners and offshore wind developers seeking to support oyster reef recovery. By improving our understanding of oyster production and settlement processes, the findings help inform future restoration projects and increase the likelihood of establishing self-sustaining oyster reefs in offshore environments.

The objective of this project was to identify and overcome key bottlenecks in the oyster restoration supply chain. By investigating hatchery diet optimisation, larval swimming behaviour and settlement substrate performance, the project aimed to develop evidence-based solutions that support large-scale restoration of European flat oyster reefs in the North Sea.

2020-2025

Project dates

North Sea (Borkum Reef Grounds in Germany/NL, Voordelta in NL, Noss Head in Scotland, Wadden Sea in NL)

Location

North Sea ReViFES consortium: NIOZ Royal Netherlands Institute for Sea Research, Wageningen University & Research (WUR), University of Groningen, Utrecht University, TU Delft, Van Oord and Waardenburg Ecology.

Project lead and partners

European flat oyster (Ostrea edulis)

Target species

Methods

This project examined key bottlenecks in the supply chain for European flat oyster (Ostrea edulis) reef restoration, from hatchery production to larval settlement. Three complementary studies were conducted, combining laboratory experiments, hatchery trials and field-based settlement tests. Together, the studies investigated how larval production can be improved, how oyster larvae behave during dispersal and settlement, and which substrates are most suitable for reef establishment.

European oyster: Hatchery diet optimisation
A literature review was conducted to identify factors limiting larval performance in European oyster hatcheries. Polyunsaturated fatty acids (PUFAs) were identified as a potential nutritional bottleneck. Eight microalgal species were evaluated, including four species already used in hatcheries and four novel species selected based on their nutritional profiles. Five experimental diets were tested under controlled laboratory conditions to assess larval growth. The most promising algal species, Ruttnera spectabilis, was subsequently tested in an operational oyster hatchery and compared with a standard mixed-species hatchery diet. Larval growth and settlement success were monitored over a three-week period.
European oyster: Larval swimming behaviour
A laboratory tracking system was developed to quantify oyster larval swimming behaviour. Individual larvae were placed in a chamber with parallel swimming lanes and recorded using a high-resolution camera system. Swimming speed, distance travelled and directional movement were measured over 90-minute observation periods. Experiments compared larvae of different ages and investigated whether larvae showed preferences for settlement cues by offering a choice between oyster shell, limestone and control conditions. The system was designed to improve understanding of larval movement and settlement behaviour at small spatial scales.
European oyster: Settlement substrate selection
Settlement experiments were conducted to determine which substrates are most suitable for oyster reef restoration. Twenty substrate types were tested across four separate trials in Ireland, the Netherlands, and France. The materials included conventional scour protection rocks, substrates commonly used in shellfish restoration projects, aquaculture spat collectors, and several novel bioinspired materials designed specifically for oyster restoration. Experiments were performed under both controlled and natural conditions using basket-based and tile-based test designs. Following each trial, oyster spat were counted and settlement densities were compared across substrate types to identify the most promising materials for restoration applications.

Resutls

The three studies produced practical insights into the key stages of European flat oyster restoration, from hatchery production to reef establishment. Together, the results demonstrate how hatchery performance can be improved, why active deployment remains necessary for restoration projects, and which substrate types are most suitable for promoting oyster settlement in offshore environments.

European oyster: Hatchery diet optimisation
The results show that hatchery performance can be improved through diet optimisation. The novel microalgal species Ruttnera spectabilis produced larval growth rates comparable to those achieved with a standard mixed-species hatchery diet, while also achieving higher settlement rates. This demonstrates that R. spectabilis can successfully replace existing hatchery diets without compromising larval performance.
The findings provide a practical opportunity to improve hatchery production of European flat oysters and increase the availability of source material for restoration projects. A key next step is the large-scale cultivation and implementation of R. spectabilis within commercial hatchery operations.
European oyster: Larval swimming behaviour
The swimming experiments showed that oyster larvae are unable to swim against dominant tidal currents and therefore cannot actively control their dispersal over large distances. As a result, natural larval movement alone is unlikely to deliver sufficient numbers of oysters to restoration sites in offshore wind farms.
At smaller spatial scales, however, larval behaviour remains important. Once oyster larvae are positioned close to suitable substrates, swimming and settlement behaviour can influence where they eventually attach. These findings highlight the importance of active deployment strategies combined with carefully selected settlement substrates.
European oyster: Settlement substrate selection
Settlement success differed considerably between substrate types. Bioinspired materials such as SeaCrete and BESE-reef paste consistently achieved the highest settlement densities and performed particularly well in hatchery and field trials. The results demonstrate that substrate selection is one of the most important factors influencing oyster settlement success.
At the same time, conventional materials such as granite remain highly relevant for large-scale restoration. Although settlement density is generally lower than on specialised bioinspired materials, granite scour protection is already widely used within offshore wind farms and can be deployed over much larger surface areas. Calcareous and shell-based materials also showed favourable settlement performance.
Implications for oyster reef restoration
A key outcome of this project is that successful oyster reef restoration requires improvements across the entire restoration chain rather than a single intervention. Hatchery optimisation can increase the availability of larvae, active deployment is required to overcome dispersal limitations, and suitable settlement substrates can significantly improve establishment success.
The results indicate that bioinspired materials are particularly useful for creating dense starter reefs, while conventional scour protection materials can provide restoration opportunities at larger scales. However, substrate alone is unlikely to restore oyster reefs. Because natural oyster recruitment remains limited in much of the North Sea, active introduction of oyster spat or broodstock remains essential for successful reef establishment.

Tips & Tricks

Hatchery

  1. What you feed larvae makes a real difference.
    It is worth testing several algae species before deciding on a hatchery diet. Small changes in feed composition can improve larval performance and settlement success.
  2. Always test a new feed in a working hatchery.
    Laboratory results are useful, but real hatchery conditions can influence performance in unexpected ways.

Deployment

  1. Build active deployment into your plan from day one.
    European oyster larvae cannot reach restoration sites on their own. If you want oysters in a specific location, you need to bring them there.
  2. Substrate choice matters.
    Once larvae are close to a restoration site, they do not settle randomly. Some materials are more attractive for settlement than others.

Substrate

  1. Choose substrates based on your restoration scale.
    Bioinspired materials such as SeaCrete and BESE-reef paste can achieve high settlement densities, while granite scour rock remains the most practical option for large-scale application in offshore wind farms.
  2. Rougher and more varied surfaces attract more oysters.
    Materials with more texture generally achieve better settlement than smooth, uniform surfaces.
  3. Do not rely on substrate alone to start a reef.
    With very limited natural oyster recruitment in the North Sea, suitable substrate alone is usually not enough. Oyster spat or live oysters often need to be introduced as well.

Let's talk

Would you like to know more about this project or get involved? Send an email to the project lead.

k.didderen@waardenburg.eco