Introduction, impact and management of the Pacific oyster Crassostrea gigas in Dutch coastal waters Aad Smaal, Karin Troost & Jeroen Wijsman Institute for Marine Resources and Ecosystem Studies

INTRODUCTION C. gigas in the OOSTERSCHELDE Flat Oyster (Ostrea edulis) culture since 1870 Problems with overstocking (1890), shell disease (1930), severe winters (1963) and Bonamia (1980) Introduction Pacific Oyster (Crassostrea gigas) in 1964 to restore oyster culture Pacific oyster culture:1550 ha bottomplots meanwhile proliferation: by (unexpected) reproduction in warm summers and larval dispersal

WAD Oosterschelde and Wadden Sea are main areas for shellfish bottom culture OS musselplots oysterplots

OYSTER PROLIFERATION ON TIDAL FLATS: reconstruction on the basis of aerial photographs and ground truth 1980: 15 ha 1990: 210 ha 2002: 640 ha 2005: 800 ha

TOWARDS AN OYSTERSCHELDE ? OYSTER STOCK OOSTERSCHELDE 2000 - 2005 ha mln kg kg/m2 oysterbed total alive flesh (ww) TIDAL FLATS 800 200 25 15 1 GULLIES 700 ? ? ? ? ROCKY SHORES 100 ? ? ? ?

FURTHER PROLIFERATION 20 days

OYSTERS WADDEN SEA 2005 Biomass Surface N obs *10^6 kg Ha 2002: 0.1 n.d. 4 2003 0.15 n.d. 4 2004: 11 400 40 2005: 28 550 87

Colonization of the German Wadden Sea - Sylt Oyster culture: 1986 - Proliferation from Dutch Wadden Sea 1998 North Sea Texel Sylt Eastern Wadden Sea Western Wadden Sea The Netherlands Germany

PACIFIC OYSTER INVASION : characteristics epifauna, reef builder, resistant, filter feeder BEFORE AFTER

OTHER DOMINANT FILTER FEEDERS Cockle Native infauna, dynamic, resilient Mussel Native epifauna, forms bed structures, slow recovery, resistant

OTHER INVASIVE SHELLFISH SPECIES Slipper limpet 1930 Drilling mussel 1896 Soft clam 1250 ? Mainly infauna, No reef building Razor clam 1978

HYPOTHESIS Exotic species that are able to build resistant structures may change ecosystems more dramatically than resilient species As the oyster is typically a resistant species (eco-engineer) the introduction may induce a regime shift See: Smaal et al, 2005. IN: The Comparative Roles of Suspension-Feeders in Ecosystems, Dame RF Olenin S (Eds.) Kluwer Academic Publishers, Dordrecht.

Food role in the - Shellfish culture IMPACT CONSEQUENCES Food role in the - Shellfish culture Recruitment ecosystem - Nature conservation Space - Recreation

Carrying capacity problem: www.keyzones.com FOOD: competition Biomass Filtration capacity (5 – 25 l/h) Competition with other filter feeders: decrease of mussel growth Oyster 32% Cockle 24 % days Mussel 44 % Meat content of marketed mussels 15 20 25 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Carrying capacity problem: www.keyzones.com

RECRUITMENT: - gregarious settlement of oysters: positive feedback filtration of (mussel) larvae: competition PhD project Karin Troost 1000 2000 3000 4000 5000 6000 oysterbed control nr of larvae

Oysters settle on mussel and SPACE: Oysters settle on mussel and cockle beds Oysters settle in subtidal areas including mussel culture plots - Oysters take over rocky shore fauna on average > 60 % coverage Oysters form new habitat (reefs) on tidal flats

ECOSYSTEM ROLE OF OYSTER BEDS suspension feeding capacity: HIGH recruitment capacity: HIGH habitat type: epifauna or infauna: EPIFAUNA eco-engineering potential = substrate formation: POSITIVE FEEDBACKS = reef building: YES ability to form structures: HIGH RESISTANCE

REGIME SHIFT ? If oyster become dominant than changes in - species composition - food availability for waders - local geomorphology - habitat complexity - resilience - regime shift CONSEQUENSES ?

CONSEQUENSES for SHELLFISH CULTURE: - increased competition = production capacity change NATURE CONSERVATION: - less food for birds = carrying capacity change RECREATION: - health risk = multi-user conflict

WHAT TO DO? step 1 - Biomass reduction step 2 - Maintenance step 3 – Exploitation 2006: Experimental fishery 2 * 25 ha BACI oysters, sed, birds, mf 10 mln kg = 20.000 m3 100 shiploads

If you cant beat them, eat them