Shrimp fishery and natural disturbance affect longevity of the benthic invertebrate community in the Noordzee-kustzone Natura2000 area

The Noordzeekustzone is an important fishing ground for fishing vessels targeting brown shrimp (Crangon crangon). Shrimp trawling is by far the dominant fishing activity in this area. However, the effect of shrimp fishery on the benthic invertebrate community has never been clearly established. It is important to establish this effect (or lack of effect) because the Noordzeekustzone is a designated Natura 2000 area, with a policy target to improve the quality of the seafloor habitat (so-called H1110b,permanently submerged sand banks).In this paper, the longevity composition of the benthic community is studied in relation to environmental variables. First the longevity composition is estimated for seafloor habitats and the effect of depth,grainsize, tidal shear stress and trawling intensity on the longevity composition is estimated and used to derive quantitative relationships that can be used to determine the changes in the benthic community and the effect of natural and human pressures. The analysis is carried out using dredge sampling data,which effectively samples only the larger individuals (>0.5cm) in the upper 7cm of the sediment. The methodology developed in the FP7-project BENTHIS (Rijnsdorp et al, 2015) was used to assess the changes in the benthic community in the Noordzeekustzone and the importance of all those candidate factors with special attention to fishing effort. In this study we find clear evidence that intensive shrimp trawling is associated with a reduction in the longevity of the benthic invertebrate community. However, the direction and intensity of that impact is determined by the wind regime in the area, which we take as a proxy for the degree and/or frequency of natural disturbance of the seafloor. The entire Noordzeekustzone area is subject to strong natural disturbance, and it has often been suggested that trawling has no effects in such areas. Our analysis shows otherwise. Even within this highly dynamic area there is a clear gradient along the magnitude of natural disturbance, in the effect of shrimp fishing. At the lower end, we find that shrimp trawling truncates community longevity, while at the higher end shrimp trawling actually enhances longevity. The mechanism for this reversal remains to be studied. To our knowledge, this is the first study wherea clear effect of shrimp trawling on the benthic ecosystem has been found in empirical data.This study shows a statistically significant effect of an admitted economic activity on the seafloor in a Natura2000 area where protection of seafloor habitat is one of the key reasons for the protection. The admission of shrimp trawling in this area has been granted based on an appropriate assessment which concluded that there are no known significant effects of the activity on the seafloor habitat. The statistically significant results of this study indicate that there are effects and hence warrant furtherstudy to determine significance in the sense of the natura2000 framework.