Samenvatting

Native European flat oyster (Ostrea edulis) reefs have strongly declined in the Dutch Delta, motivating restoration efforts that use engineered substrates to support settlement and early reef development. In Lake Veere, Project Asta tests the Mother Reef concept: biodegradable clay blocks that are conditioned and pre-seeded with oyster larvae prior to deployment. A key uncertainty for early-stage restoration is predation pressure by mobile predators such as the European green crab (Carcinus maenas). This study investigated how reef structure and oyster shell size influence crab predation under controlled conditions.

A controlled tank experiment (Experiment I, Shell size) was conducted in three runs between 13 October and 21 November 2025. Each run consisted of six tanks (four treatment tanks with structured Mother Reef substrates and two control tanks with loose oysters). Tanks were maintained for 96 hours at approximately 18 °C, and each tank contained one standardised male crab. In total, 715 juvenile oysters were monitored (360 small, 355 large).

Across all runs, 18.9% of oysters were consumed, with variation among runs (16.2%, 24.1%, and 16.3%). When pooled across runs, oysters on structured Mother Reef substrates experienced lower proportional predation (16.5%) than oysters in loose-oyster controls (23.7%), indicating that structure provided partial refuge. Small oysters showed higher proportional consumption than large oysters in both treatments, but Mann-Whitney U tests on tank-level proportions showed no statistically significant size effect (all tests: p ≥ 0.11 at α = 0.05).

Overall, the results suggest that Mother Reef structure can reduce short-term crab predation compared with loose oysters, while size-selective predation within the tested juvenile size range is weak or inconsistent. Recommendations include prioritising structural complexity and deployment strategy (layout, site selection, timing), ensuring spat reach a basic level of shell robustness, and continuing with follow-up work (including the planned Elevation × Size experiment and longer-duration studies better aligned with field conditions).