Bonamia-Free Oysters for North Sea Restoration

Bonamia-free Oysters for North Sea Restoration

The Breeding Line / KOPON project aimed to improve the hatchery production of Bonamia-free European flat oysters for nature restoration in the Dutch North Sea. As natural recovery of flat oyster reefs is very limited and restoration requires millions of oysters, the project focused on developing a reliable hatchery-nursery method to produce Bonamia-free spat for large-scale outplacement.

This project addressed a key bottleneck in flat oyster restoration: unpredictable larvae and spat production in hatcheries. It combined hatchery practice, disease screening, algae nutrition research, bacterial monitoring, and field deployment trials to understand potential improvements in broodstock conditioning, larval survival and spat performance. The goal was to create a practical and scalable supply of disease-free oysters for restoration in the North Sea.

2020-2023

Project dates

Texel, Yerseke and Kamperland (hatchery/cultivation); Voordelta near Brouwersdam (deployment/field trial).

Location

The Rich North Sea, Stichting Zeeschelp, Roem van Yerseke, NIOZ Texel, Wageningen Marine Research (WMR) Yerseke, HZ University of Applied Sciences, Waardenburg Ecology

Project lead and partners

European flat oyster (Ostrea edulis)

Target species

Stand-alone structures

Methods

The project used a four-year applied research approach across three hatchery locations with different environmental and technical conditions. Comparable experiments were carried out at Zeeschelp, Roem van Yerseke and NIOZ to identify which factors most strongly influenced success in broodstock conditioning, larval rearing and spat production. These factors included broodstock origin, conditioning temperature, feeding regime, algae composition, water treatment, hygiene and bacterial pressure. Results were discussed during regular KOPON meetings every 6–8 weeks, and the work plan was adjusted annually based on new insights and experiences.

Monitoring included non-destructive Bonamia screening of broodstock, Bonamia testing of produced spat, tracking of larval growth and survival, algae quality analyses, bacterial sampling, and field monitoring of deployed spat in the Voordelta. An integrated monitoring and data compilation approach was used across locations so that results could be compared between hatcheries and between batches. WMR and NIOZ were responsible for the content of the monitoring plan, while implementation was carried out by the cultivating partners.

The project was developed within The Rich North Sea programme to address the shortage of suitable flat oyster seed for restoration. Because earlier hatchery production had been erratic and unreliable, The Rich North Sea brought together research institutes, hatchery operators and restoration partners in the KOPON working group to openly share knowledge, compare experiences and accelerate practical improvements in hatchery and nursery production.

Photo of oyster monitoring (source: the Rich North Sea)

Results

The project showed that reliable production of Bonamia-free flat oyster spat is possible in the Netherlands. Over four years, the work led to better control of broodstock conditioning, more insight into larval bottlenecks, practical biosecurity measures, and successful spat production suitable for restoration. Outplacement trials in the Voordelta showed survival and growth of deployed spat, especially for larger size classes.

Broodstock conditioning worked best with a gradual increase in temperature to about 22–24°C combined with mixed diets of diatoms and flagellates. Broodstock of Dutch origin could be conditioned successfully and produced larvae over a period of roughly 10–12 weeks. For larval growth and survival, the first week of culture generally performed well, while the second half of larval culture was the most critical phase due to bacterial pressure and nutritional limitations. Diatoms appeared to be an important part of the diet. For spat, larval quality at settlement was the key factor: when competent larvae were in good condition and hygiene was maintained, spat survival was good, and juveniles grew from a few millimetres to several centimetres over the following months.

A major practical result was that Bonamia-free spat production became reliable, and that suitable material for restoration can now be sourced in the Netherlands. The report also concludes that single spat production was generally more efficient than spat-on-shell production, while larger outplanted spat tended to survive better in the field than smaller spat.

KOPON Final Report (2019–2023)

Tips and tricks

Open collaboration and frequent exchange between hatcheries, researchers and restoration partners strongly improved progress. Working adaptively, reviewing results regularly and sharing failures as well as successes proved essential in moving from erratic culture outcomes to more stable spat production.
Important improvements included cleaner hatchery hygiene, closer attention to bacterial pressure, refinement of broodstock conditioning, comparison of algae diets, and the use of biosecurity procedures for Bonamia-free production. The project also showed that substrate choice matters: spat on shell works in sheltered conditions, but more exposed offshore conditions may require more robust substrates such as artificial reef structures or stone-based solutions.
Permitting and certification were important themes in the project. The report states that a certified or permitted hatchery-nursery procedure for offshore outplacement is still needed in the Netherlands. Although Bonamia-free spat was produced according to protocol, certification was not yet available. The report recommends that a formal procedure for certification and licensing should be developed and implemented as soon as possible.

The Flat oysters breeding line

Working towards reliable flat oyster production for restoration in the North Sea

Watch this video to learn more about the Breeding Line / KOPON project and how partners worked together to develop Bonamia-free flat oyster spat for nature restoration.