1. 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

2. 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

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

4. 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

5. TOWARDS AN OYSTERSCHELDE ?
OYSTER STOCK OOSTERSCHELDE
ha mln kg kg/m2 oysterbed
total alive flesh (ww)
TIDAL FLATS
GULLIES 700 ? ? ? ?
ROCKY SHORES 100 ? ? ? ?

6. FURTHER PROLIFERATION
20 days

7. OYSTERS WADDEN SEA 2005 Biomass Surface N obs *10^6 kg Ha
2002: n.d. 4
n.d. 4
2004:
2005:

8. 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

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

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

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

12. 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, IN: The Comparative Roles of Suspension-Feeders in Ecosystems, Dame RF Olenin S (Eds.) Kluwer Academic Publishers, Dordrecht.

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

14. 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:

15. 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

16. 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

17. 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

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

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

20. 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 = m3
100 shiploads

21. If you cant beat them, eat them