The Nursery function of the Westernscheldt for fish and crustaceans

This report shows the results of the desk study on the impact of the deepening of the Westernschedt on the nursery function for fish and shrimp. In order to determine effects of the deepening hypotheses have been formulated based on the assumption that intertidal and shallow subtidal areas with maximum current velocities of 0.5 ms 1 are suitable as nursery areas. It is hypothesized that these areas will decrease due to the deepening activities. In order to test these hypotheses a theoretical framework on the functioning of nursery areas has been developed. Various studies have been done, but there is still a gap in the knowledge with respect to basic information on the fish and crustacean fauna of the Westernscheldt, especially for the pelagic nursery-type species. It will not be possible to study trends in the abundance of juvenile fish and shrimps as a consequence of the deepening activities for pelagic nursery-type species as well as for demersal nursery-type species due to this lack of knowledge. A list has been formulated describing possible nursery species of the Westernscheldt based on the geographical position and the knowledge of the (a)biotic environment of the Westernscheldt estuary. As well as a lack of basic information on the fish and crustacean fauna, no adequate description of the reference situation before the deepening activities were started is available and it will not be possible to determine and quantifythe full extent of the effects of the human activities on the nursery function of the Westernscheldt. It is expected that major changes due to the deepening will be the decrease in the intertidal and shallow subtidal areas. Despite the clear lack of information it is conclued, based on the above assumption, that the deepening of the Westernscheldt will have a negative effect of the functioning as a nursery, at least for the demersal type nurseries. The formulated hypotheses seem to be correct for the demersal-type nurseries. In order to quantify the impact of human activieties on the nursery function the gap in the basic knowledge needs to be filled. The required information can be gained by detailed field study, whereby the distribution of species in combination with the (a)biotic environment and carrying capacity of the Westernscheldt should be studied for the important nursery-type species. Using the gained knowledge in the ecotope approach it will be possible to model impact of human activities on the functioning of the Westernscheldt as a nursery.