The Modeling of Sustainable Cockle-Fishery in the Wadden Sea, The Netherlands

Sustainability in cockle-fishery activities in the Wadden Sea is presently a
debate among fishery firms, government and conservationists in The Netherlands.
Before 80's the mechanized fishing with dredging exploitation of cockles
originally existed in relative harmony with nature conservation in the area.
This ended when the over-fishing continued during the series of mild winters in
1988,1989 and 1990 that resulted into three consecutive years without spatfall
of cockles (Wolff, 2001). As a result, a dramatic decrease of cockle stock
caused unsteady conditions between fishery and natural conservation. Since,
mechanized over fishing for cockles may harm nature into two main ways: It may
cause food shortages to birds feeding on large shellfish stock and damage
important habitats like cocklebeds (Scheiffarth, et al 2001). As an outcome,
the high mortality of shorebirds was reported because the species feeds on the
sizes of cockles that are also preferred by cockle-fisheries (Ens et al., 2003;
Verhulst et al, 2004; Kamermans, 2002). Then, the policy decision was made to
ensure survival of birds to satisfy at least 60% of their needs at minimum size
of cockle stock (Wolff, 2001) . On the other hand, there are studies in the
Dutch Wadden Sea that show the negative long-term impacts on spatfall of
cockles because of changes in the sediment characteristics from fine silt to
coarse grains done by suction dredging used for cockle fishery purposes (Dare,
et al 2004; Piersma, 2001). The main request of the Fisheries was that the
model should represent the profit, government, and conservationist interests.
In aim of this goal the group applied the Environmental System Analysis (ESA)
based on scientific published data in order to develop a model, which would run
several scenarios to figure out the requirements of the stakeholders in the
area.

OBJECTIVES OF THE MODEL

1. Seek the optimum number of boats for cockles
dredging which will not disturb the cockle stock and the feeding needs of
cockle-catching bird population.

2. Run the predator-prey model to find out the
relationship between cockles and seabirds population.

3. Test the feedback
between cockles recovery and silt content in the sediment. Modelling is
available per request.