Analyses of decrease in cockle population in the Dutch Wadden Sea

Scientific Study 2013 10 Pages

Environmental Sciences


Analyses of decrease in cockle population in the Dutch Wadden Sea and its economic and environmental effects on the ecosystem


The Wadden Sea is a shallow inshore area of about 6000km2 extending from the Netherlands to Denmark (Jonge et al., 1993). This study focuses on the Dutch part of the Wadden Sea with an area of 3000km2 located between the North Sea and the Netherlands mainland. The Dutch Wadden Sea is a unique ecosystem that has provided provisional, regulating, and cultural and habitat/supporting ecosystem functions and services. Cockle (cerastoderma edule) shell fishing is one of the major provisional services that Dutch Wadden Sea provides. The filter feeders species help in the water purification of the system and this account to regulating service. Another important regulating service offered by this system is biological control by predation. Habitat for species is a supporting service also from the Dutch Wadden Sea (Beukema and Cadee 1996). Biodiversity is fundamental to many ecosystem services. It provides sustainability and resilience vital for livelihoods and coping strategies of many people, especially the rural poor. As a result of different biodiversities especially two distinct shore birds species (Oystercatchers and Eiders) present in the system, recreation and tourism has immensely contributed to income generation in the Dutch Wadden Sea, thereby serving as a cultural service. Additionally scientific and education information is as well a major cultural service provided by this system. Primary production mainly phytoplankton is moreover a supporting service from this system.

One of the most dominant shell fish in the Dutch Wadden Sea is cockle. Due to its provisional service importance, it is being harvested. Based on this case study, the main problem of the Dutch Wadden Sea is the decrease in cockle population. The three main causes of this decrease were loss of fine silts and organic matter in the cockle breeding sediments, failure in cockle bivalve recruitment (Beukema et al., 2001) and low survival of cockles. These causes were brought about by various drivers of change on the ecosystem. These drivers include both direct and indirect drivers. Cockles were initially harvested by manual methods, but since 1960s the exploitation was scaled up by the introduction of mechanized suction dredge cockle fishing technology, which sucks a layer 3-5 cm deep and 1 meter wide from sea bed (Swart and Jelte 2008). The technology was introduced to meet the high market demand for cockles. Consequent to the problem, primary and secondary stakeholders were deeply concerned about the consequences of this problem on the ecosystem. The main predators of cockles are oystercatchers and eiders (SMIT, 1983). The Dutch Wadden Sea is a good habitat for these birds because of their food (cockles) availability.

There are various impacts (effects) and responses resulting from this system problem. The impacts of the ecosystem problem include loss of biodiversity, loss of income from cockle fishing, loss of income from tourism due to high mortality in shorebirds. Since there is severe food shortage, diverse species in the Dutch Wadden Sea migrated to other ecosystem in search of food. Also during suction dredging as sediments are being extracted from the sea bed, there is also loss of filter feeders since not only cockles are being removed from the extracted sediment (Beukema et al., 2001). The response of banning mechanized fishing in the ecosystem on 23 November, 2004 by the Dutch Government immensely contributed to loss of income from cockle fishing. The tourist sector makes use of shorebirds abundance present in the Dutch Wadden Sea to generate income from tourists. But because of high mortality among the shorebirds, the sector loose serious income generation from this aspect. Another response was the alternative of embarking on gas exploitation from the system and investing a considerable amount of the money (EUR 800 million) from gas exploitation profits in the system nature, infrastructure and research (Anonymous 2004c).

The problem of the Dutch Wadden Sea gave rise to some externalities and trade-offs and also affected the ecosystem and its human well-being. The change (positive or negative) in shorebirds abundance in other [destination] ecosystem through migration could be seen as an externality. Habitat destruction and loss of income from tourism are also externalities. Mechanical suction dredging cockle fishing at the expense of tourism, supporting services and genetic resources are the major trade-offs.

The main objective of this study is to analyze the problem of decrease in cockle population in the Dutch Wadden Sea and its economic and environmental effects on the ecosystem. This could be achieved through these specific objectives; (1) analyze the environmental effects due to this reduction. (2) analyze the socio-economic effects caused by this decrease.


The method used for this case study includes:

- Forming a problem tree from the case study conceptual model
- Analyses of ecosystems functions and services by integrating the Millennium Ecosystem Assessment [MA] (MA, 2005) and The Economics of Ecosystems and Biodiversity (TEEB) (De Groot et al., 2010)
- Analyses of the drivers of change and the DPSIR (Drivers, Pressure, State, Impact and Response) of the Dutch Wadden Sea
- Stakeholder analyses of the ecosystem
- Analyses of the human well-being and livelihoods

a. The conceptual model of the study case

Firstly, a state representing cockle stock was placed. Then two rates representing the shorebirds (eider and oystercatcher) were linked to the cockle stock. Secondly, cockle fishing by mechanical suction dredging which represent outflow rate was also linked to the cockle biomass stock. Lastly, Habitat destruction due to silt extraction rate was also linked to the cockle stock.

The problem tree

The focal problem which is the decrease in cockle population was centered in the middle of the tree. The tree was constructed from the lowest level. High demand for cockles led to the overharvesting of cockles, then to meet this demand cockle fishing by suction dredging was introduced. Suction dredging cockle fishing with winter storms will finally lead to loss of silts and organic matter in the sediment that eventually causes decrease in cockle population. The middle branch was due to climate change (Beukema and Dekker 2005) mainly severe and mild winters that resulted into failure in bivalve recruitment which is the second main cause of decrease in cockle population in the study area. The third branch is characterized as natural mechanism. Cockle predation by shorebirds (consumption of adult cockles) and shore crabs and shrimps (consumption of early stage cockles) will lead to low survival of cockles (Beukema and Cadee 1996). The shore crabs and shrimps operate during the mild winter's temperature.

One of the three effects of the cockle population decrease in the Dutch Wadden Sea was elevated concentrations of algae. Second effect was high mortality among shorebirds leading to loss in biodiversity thereby attracting fewer tourists that resulted in loss of income. The third effect is the decrease in shell-fishing yield which also led to loss of income.

b. Analyses of ecosystems functions and services using Millennium Ecosystem Assessment and TEEB study

The Millennium Ecosystem Assessment and TEEB were integrated and used for the analyses of the study are. The ecosystem functions and services generated from these two categories were Provisional services (cockles, fish, gas exploitation, sand excavation, shells for ornamental resources and genetic resources), regulating services (water purification by filter feeders, moderation of extreme events, erosion control and biological control by predation), cultural services (recreation, tourism, aesthetic and spiritual aspects, scientific and education information) and Habitat/supporting services (habitat for species, soil formation, nutrient cycling and primary production).



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Title: Analyses of decrease in cockle population in the Dutch Wadden Sea