Het mosselbestand op de PRODUS - vakken en de effecten van de visserij daarop : Effecten van mosselzaadvisserij op sublitorale natuurwaarden in de westelijke Waddenzee

The Dutch mussel farming industry is based on the use of young mussels fished from naturally occurring mussel beds as raw material. The Wadden Sea is a designated protected area, and therefore the uptake of this mussel spat needs a permit under the Nature Conservation Act. In order to assess whether the natural values of the area will be affected by the fishery, insight is needed into the effects of mussel fishery on the nature in the Wadden Sea, and more particularly for the conservation objectives set out in the framework of Natura 2000 for the Wadden Sea. Therefore the Ministry of Economic Affairs and the Producer Organisation of Dutch mussel survey initiated the research project ‘PRODUS’ in 2006. Part of PRODUS is a comparative study at 40 sites with paired, adjacent research plots of 4 ha each, one of which was open to fishing the other completely closed to fishing. The sites were positioned in newly developed mussel spat beds. These study sites could not all be established at the same time because there needed to be enough new spat in each area, which depends on natural spat fall, and this varies significantly from year to year. In 2006 the sampling started in 10 sites, then the number of sites increased each year from 2010 to eventually include 40 sites. Mussels are generally fished twice a year; in the autumn on the newly formed spat beds, particularly in beds that are relatively unstable and therefore have a greater chance of disappearing in winter; and a second time following the spring, in the remaining beds in the more stable areas. In the study, the effects of the autumn and the spring fishery were analyzed separately. Parallel to the research in open and closed plots was a Wadden Sea wide survey conducted in 2008 in which the benthic composition in the sublittoral areas of Wadden Sea was determined. In the period 2008 - 2010 a comparison was also made of the benthic composition and environmental characteristics of wild mussel beds and cultured mussel plots. Furthermore, in 2009 and 2010 some of the larger mussel beds were permanently closed to fishing, and a study was conducted to investigate what the consequences for these mussel beds were. The present report deals with the development of the mussel stock in each of the sites, in the mussel beds that were closed in 2009 and 2010 and their backgrounds in relation to fishing.