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Interactions between fishing, sediment and

cockles in the Wadden Sea

By: Adhitya Wirayasa (38806), Charity Chinkusu (35670),Olajumoke Kayode (31423)

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Contents• Introduction/ Problem definition• Objectives• System Boundary• Assumptions• Methods• Conceptual Model, Parameter Values & Stella

Model• Results and Scenarios• Limitations & Improvement• Recommendations

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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.

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Problem definition• Cockles were initially

harvested by manual gathering method with rakes.

• 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

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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.

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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.

Case Study Area

Western Europe

Source: http://www.waddensea-secretariat.org/news/publications/maps.html

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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

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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.

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Conceptual Model

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Parameter ValuesVariable Value Unit Source

Cockle Biomass(Nc)

168,700 tonsSmaal 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)

32Boats (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)

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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

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SEDIMENT CHARACTERISTICS/ ACCUMULATION/LOSSVariable Value Unit Source

Sediment mass 397,5x106 Tons Calculatedrate of silt accumulation in sediment (rs)

1.6 x106 Tons/yearJonge, 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 CalculatedRate of Natural erosion (re) 2 Tons/year Estimated

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Main EquationsCockles Biomass• dNc/dt = rcNc (1-Nc/K)Predation• P=B*rb

Fishing• F=Nf*rfSediment Silt• dS/dt=A-E-D

A=ra

E=re

D=Ns*rd*Nf

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Stella Model

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Results – Logistic Growth Curve

• Natural growth of cockles in the absence of predation and fishing

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Results – Effect of Fishing

• 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.

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Results – Effect of Predation

• 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

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Results – Effect of Mechanical Dredging

• 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.

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Results – Effect of mechanical suction dredging on the silt content in sediment

• 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.

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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

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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.

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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.

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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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THANK YOU