Larval Dispersal and micro-siting for restoration of European Flat Oyster (Ostrea edulis) on the Frisian Front

The European flat oyster (Ostrea edulis) was once a keystone species in the North Sea but is now functionally extinct offshore due to overharvesting and disease. Restoration efforts aim to reintroduce oysters at sites that combine suitable habitat conditions with hydrodynamic features that support larval retention, as natural recruitment sources are absent. The Frisian Front is a depositional zone recently partially closed to bottom trawling. It lies within the historic distribution range and appears to have suitable habitat conditions, according to existing habitat suitability models. Restoration efforts can only be effective if deployed oyster material remains in place. This study evaluates several sites on the Frisian Front, designated for oyster restoration in terms of larval retention, stability requirements of deployment material and local fine-scale habitat conditions.

Field work indicated that there were distinct differences between two sites, despite their relative proximity. One was relatively muddy, with a median grain size between 129 and 171 µm,  a silt content between 17.1 and 31.6% and dominated by heart urchins (Echinocardium cordatum). The other site consisted of much coarser sand (median size range 208-406 µm) and much lower silt percentages of between 0.3 and 11.5 %, as well as lower abundance of macrobenthos species and shell material. 

Metocean analyses indicated that both these sites were unsuitable for deployment of e.g. oyster spat on loose shell material as this would be dispersed quickly. A certain amount of weighting of deployment material (cages, gabions) is required.

Using a Lagrangian particle tracking approach coupled to a high-resolution hydrodynamic model, we simulated passive transport of virtual larvae released from both candidate sites during the main spawning period. Larvae were tracked for up to 20 days, reflecting the pelagic larval duration of O. edulis. Results indicate very low local retention and minimal connectivity between sites, with larvae consistently advected eastward. While both sites are located in areas of moderate habitat suitability, dispersal trajectories suggest that larvae may reach zones with higher suitability downstream, including fisheries exclusion areas and wind farm zones. These findings highlight that hydrodynamic retention and habitat quality must be jointly considered in offshore restoration planning. Achieving self-sustaining populations at the Frisian Front will likely require reinforcement strategies until upstream larval sources are established.

Although there is still much unknown about effective offshore restoration of reef-building species, combining research on different essential aspects will give the highest chance of success.