Large-scale Pacific oyster removal experiment in the Oosterschelde (The Netherlands)
Jeroen Wijsman1 & Aad Smaal2 1Wageningen Marine Research 2Aquaculture & Fisheries Group, Wageningen University & HZ University of Applied Sciences
2 Oosterschelde • 341 km2 • 104 km2 intertidal area • Nature conservation area • Shellfish culture
3 Shellfish culture, mainly bottom culture
• Mussels (25 Mkg yr-1) • Pacific oysters (3 Mkg yr-1) • Flat oysters (0.25 Mkg yr-1)
Flat oyster Pacific oyster (Ostrea edulis) (Crassostrea gigas)
4 Pacific oysters
• Traditional culture since 1870 flat oyster (Ostrea edulis) • Susceptible to Bonamia ostrea 1000 • Mass mortality after severe winter 1962/1963 800 • Pacific oysters introduced in 1964 • Plans to make Oosterschelde freshwater lake 600 • Oosterschelde too cold for reproduction 400
• 1971 fist observed in the wild (ha) Area 200
0 1970 1980 1990 2000 2010 Year
5 Development oyster reefs
5 km 1980:2005:1990: 15766212 ha ha
6 WARNING Take care of oyster shells with sharp edges. They can Impact pacific oysters cause deep cuttings
• Competition with mussels and cockles for food and space • Not a good food source for birds
• Growth on sluices and dykes Grazing pressure (Mm3 d-1) • Recreation
7 Large-scale removal of oysters (2006)
• 4 experimental plots (12.5 ha each) • 2 littoral and 2 sublittoral • Dumped at 2 locations
8 Location Zandkreek
9 Location Vondelingsplaat
10 Fishing with mussel dredges
• Dutch mussel farmers
11 Effort
• 1000 boat hours 70000
• 20 boat hours per ha 60000
) 50000 1 • Costs ca €300 000 -
40000 350 lossen (256 uur) 300 30000 vissen (679 uur)
250 Catch (kg hr 20000 200
150 10000 Inspanning (uren) Inspanning Effort (hrs) Effort 100 0 50 25/feb 4/mrt 11/mrt 18/mrt 25/mrt 1/apr 8/apr 0 Date Vondelingsplaat Zandkreek Zilverput Marollengat Zandkreek Vondelingsplaat Zilverput Marollegat
12 Monitoring
• Amount of oysters before and after • Sediment composition • Benthic fauna • Birds • Morphology (erosion/sedimentation)
13 Results
5 T0 4.5 • Total 12 500 tons removed T1 3 4 • 22 000 m , equivalent to 650 3.5 ) 20 ft sea containers 3 • 3 620 tons living oysters (~3% Mkg 2.5 2
of total stock) Total ( Totaal (miljoen kg) 1.5 1 0.5 0 Zilverput Marrolegat Vondelingsplaat Zandkreek
14 Zandkreek before after
15 Zandkreek before after
16 Vondelingsplaat before after
17 Vondelingsplaat after
18 Wading birds Dunlin
Oystercatcher Curlew
60
50
40 VondelingeOysterbed oesterbank 30 VondelingeFished weggevist oysterbed Vondelinge referentie aantal uren aantal Reference 20
10
0 Wulp Bontje Kanoet Rotgans Zaagbek Middelste Bergeend Scholekster Zilverplevier Kl Zilverreiger
19 Recovery • Shell remains in the bottom • High recruitment in 2007 • Recovery of oysterbeds 5 to 8 years
20 Repeated removal by oyster farmers
• Since 2006 fishery on wild 2005 2019 oysters in eastern part Oosterschelde • Repeated fishery for spat and substrate
21 Goods and services pacific oysters
• Provide habitat • hard-substrate organisms • Fish and birds • Reduce turbidity • Sedimentation • Wave protection • Nutrient cycling • ...
22 Conclusions
• Dense oyster reefs could be removed using mussel dredges • All shell remains should be removed to prevent recolonization • Oysters did not die at dumping locations, oysters should be grinded • Fishery should be repeated to prevent recolonization • Oyster reefs could have impact, but also provide ecosystem services
23 Thanks for your attention!
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