Crassostrea gigas has an irregularly shaped, deeply cupped shell that can exceed 150 mm in length. The left valve is cemented to the substrate, and once attached the animal cannot move. Originally from the northwest Pacific, it was introduced to Europe for aquaculture in the 1960s–70s and has since established feral populations from southern Norway to the Mediterranean. Wild spawning occurs when summer water temperatures exceed 18–20 °C, producing larvae that drift for 2–3 weeks before settling.
The irony of Pacific oyster fouling is that the same species valued as a product in one operation becomes a pest in another. Wild spat settling on mussel ropes, cage nets, and pontoons add rigid, sharp-edged mass that damages equipment and injures workers during handling. On mussel longlines, oyster fouling can account for 30–50 % of harvested weight, all of which must be stripped and discarded. In areas where feral populations have exploded — parts of the Wadden Sea, the Irish coast — oyster fouling now rivals blue mussel fouling in severity.
Manual scraping and mechanical tumbling are the primary removal methods. Lime treatment (applying hydrated lime to exposed shells) kills spat but requires careful handling. Timing matters: treating equipment in late summer, shortly after the settlement peak, catches spat while they are small and weakly attached. The cost calculator can help quantify fouling losses for shellfish operations. For a wider view of hard-fouling species in your region, explore the organisms database or see the anti-fouling methods comparison.
Control Methods
Manual scraping Lime treatment Mechanical cleaning