2. Contents
• Introduction/ Problem definition
• Objectives
• System Boundary
• Assumptions
• Methods
• Conceptual Model, Parameter Values & Stella
Model
• Results and Scenarios
• Limitations & Improvement
• Recommendations
2
3. Introduction
• The Wadden Sea is one of the largest tidal
systems of the world which is
internationally important as a stop-over
for migrating birds.
• The edible cockle, Cerastoderma edule is
one of the most abundant species of
molluscs found in the Wadden Sea.
• They are important for fisheries and also
as food source for migratory birds.
3
4. Problem definition
• Cockles were initially
harvested by manual
gathering method
with rakes.
4
• However, since the early 1960s the
exploitation was scaled up by the
introduction of mechanized suction-dredge
cockle fishing technology.
• This led to increased harvesting rate and a
change in sea sediment characteristics.
http://www.esfjc.co.uk/methods.htm
5. Objectives
• To determine how much cockle fishing can be
allowed to prevent the collapse of the cockle
stock.
• To determine the relationship between cockle
biomass and predation rate by birds.
• To determine how changes in sediment silt
content affect the carrying capacity of the
system for cockle biomass.
• To determine how cockle suction by
dredging boats affects sediment silt content.
5
6. System Boundary
• The model system
is the entire part of
Dutch Wadden sea.
• The area covered is
3000 km2.
• Time frame used is
25 years.
6
Case Study Area
Western Europe
Source: http://www.waddensea-secretariat.org/news/publications/maps.html
7. Assumptions
• Only migratory birds are preying on
cockles and rate of predation is constant
• Fishing is done by mechanical suction
dredging only
• Mortality of cockles is only due to fishing
and predation
• Constant rate of growth, predation, and
fishing
7
8. Methods
• Review of relevant literature
• Group discussions/ with the lecturers
• Formulation of conceptual model
• Development of a Stella model
• Parameterization and sensitivity analysis
of the model
• Validation through check of internal
consistency.
8
10. Parameter Values
10
Variable Value Unit Source
Cockle Biomass
(Nc)
168,700 tons
Smaal et al (2005) Does the Introduction of the Pacific
Oyster Crassostrea Gigas lead to Species
Shifts in the Wadden Sea
Cockles Growth
Rate
(rc)
0.754 Per year
Montserrat Ramon,2003 Population dynamics and
secondary production of the cockle Cerastoderma
edule(L.) in a back barrier tidal flat of the wadden Sea,
SCL MAR..67(4): 429-443
Carrying
Capacity (K)
1,687,000 Tons Estimated
FISHING (F)
Variable Value Unit Source
Cockles
harvesting rate
(rf)
80,000
Tons/
year
(in
1989)
Wolff (2005) The exploitation of living resources in the
Dutch Wadden Sea: a historical overview. Helgol Mar Res
59: 31–383
Number of
fishing boats
(Nf)
32
Boats
(in
1981)
Ens B.J et al (2004) The effects of shellfish fishery on the
ecosystems of the Dutch Wadden Sea and Oosterschelde
((EVA II)
11. 11
PREDATION (P)
Variable Value Unit Source
Population of
predatory birds
(Common Eider)
(B)
311,000 Birds
Laursen, K., Blew, J., Eskildsen, K., Günther,
K., Hälterlein, B., Kleefstra, R., Lüerßen, G.,
Potel, P., Schrader, S. 2010. Migratory
Waterbirds in the Wadden Sea 1987-2008.
Wadden Sea Ecosystem No.30. Common
Wadden Sea Secretariat, Joint Monitoring
Group of Migratory Birds in the Wadden Sea,
Wilhelmshaven, Germany.
Consumption
rate by predation
birds
(rb)
168.5 x 10-3
Tons/Bird/
year
Ens B.J et al (2004) The effects of shellfish
fishery on the ecosystems of the Dutch
Wadden Sea and Oosterschelde ((EVA II)p.
116 of 212
12. 12
SEDIMENT CHARACTERISTICS/ ACCUMULATION/LOSS
Variable Value Unit Source
Sediment mass 397,5x106 Tons Calculated
rate of silt
accumulation in
sediment (rs)
1.6 x106 Tons/year
Jonge, V.N. de. Essink, K., Boddeke, R. (1993) The
Dutch Wadden Sea: a changed ecosystem.
Hydrobiologia 265: 45 - 71.
Percentage of silt
in sediments
1.75 Percent
Beukema, J.J. & R. Dekker (2005) Decline of
recruitment success in cockles and other bivalves in
the Wadden Sea: possible role of climate change,
predation on post larvae and fisheries. Marine Ecology
Progress Series 287, 149–167.
Mass of Silt in
Sediment (Ns)
6,956,250 Tons Calculated
Mass of sediment
lost to dredging
(rd)
23.85 Tons/year Calculated
Rate of Silt loss to
dredging
1.5 percent
Newell R.C el.al (1998) The Impact of Dredging Works
in Coastal Waters: A Review of the sensitivity to
Disturbance and Subsequent Recovery of Biological
Resources on the Sea Bed. Oceanography and
Marine Biology: an Annual Review. 36: 127-78
Mass of silt lost to
dredging
0.36 Tons/year Calculated
Rate of Natural
erosion (re)
2 Tons/year Estimated
15. Results – Logistic Growth Curve
• Natural growth of cockles in the absence of
predation and fishing
15
16. Results – Effect of Fishing
16
• Fishing can be sustained by harvesting with between 12-40
boats at the constant rate of 2500 tons/boat/year. Using more
than 45 boats collapse the cockles biomass
• 1- logistic growth
• 2- 10 boats
• 3- 32 boats
• 4- 45 boats
• 5- 50 boats.
17. Results – Effect of Predation
17
• At the predation rate of 0.1685 tons/year, migratory birds have a
small impact on cockles biomass
• When the consumption rate by predatory birds is trippled then the
cockles biomass collapse
18. Results – Effect of Mechanical Dredging
18
• When the silt content in the sediment decreases below 0.5 % due to
natural erosion and dredging then the carrying capacity of the
system drops and the cockles biomass collapses.
19. Results – Effect of mechanical suction dredging on the silt
content in sediment
19
• When the rate of natural erosion is much smaller than the silt
accumulation rate , then the effect of sediment loss to dredging is
not seen even with an increase in the number of dredging boats..
• However, when the natural erosion rate is equal to the silt
accumulation rate, the effect of silt loss to dredging can be seen.
20. Limitations
• Other animals were not considered as
predators
• Only the population of the Common Eider
was used as bird population.
• Effect of temperature on cockles was not
considered as a factor that could affect
cockle biomass
• Data used for the model are gathered from
different years
20
21. Knowledge gaps for an
improved model
• Restriction of the system boundary to the
intertidal area on the Wadden Sea where
the cockles are fished.
• Working with more recent data.
• Obtaining the data from literature rather
than making calculation and estimate.
21
22. Recommendations
• The number of fishing boat should be
restricted to below 45 at the fishing rate of
2500 tons/year to prevent the collapse of the
cockles stock.
• The number of fishing boats should be
further reduced during seasons when there is
high silt loss to natural erosion.
• The amount of silt in the sea sediment should
not be below 0.5 % to maintain the carrying
capacity of the system.
22
23. References
• Beukema, J.J. & R. Dekker (2005) Decline of recruitment success in cockles and other bivalves in the
Wadden Sea: possible role of climate change, predation on post larvae and fisheries. Marine Ecology
Progress Series 287, 149–167.
• Ens B.J et al (2004) The effects of shellfish fishery on the ecosystems of the Dutch Wadden Sea and
Oosterschelde ((EVA II)p. 39 of 212
• Jonge, V.N. de. Essink, K., Boddeke, R. (1993) The Dutch Wadden Sea: a changed ecosystem.
Hydrobiologia 265: 45 - 71.
• Laursen, K., Blew, J., Eskildsen, K., Günther, K., Hälterlein, B., Kleefstra, R., Lüerßen, G., Potel, P.,
Schrader, S. 2010. Migratory Waterbirds in the Wadden Sea 1987-2008. Wadden Sea Ecosystem
No.30. Common Wadden Sea Secretariat, Joint Monitoring Group of Migratory Birds in the Wadden
Sea, Wilhelmshaven, Germany.
• Montserrat Ramon (2003) Population dynamics and secondary production of the cockle
Cerastoderma edule(L.) in a back barrier tidal flat of the wadden Sea, SCL MAR..67(4): 429-443
• Newell R.C et.al (1998) The Impact of Dredging Works in Coastal Waters: A Review of the sensitivity
to Disturbance and Subsequent Recovery of Biological Resources on the Sea Bed. Oceanography and
Marine Biology: an Annual Review. 36: 127-78
• Smaal et al (2005) Does the Introduction of the Pacific Oyster Crassostrea Gigas lead to Species Shifts
in the Wadden Sea. NATO Science Series IV: Earth and Environmental SeriesVolume 47:pp 277-289
• Van Dam A.A (2013) Environmental Modelling. Lecture Notes. Unesco-IHE
• Wolff (2005) The exploitation of living resources in the Dutch Wadden Sea: a historical overview.
Helgol Mar Res 59: 31–38
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