Has trawling turned the Dutch seafloor into a high-production fish farm? (KB-14-012-049)

Around the globe, marine soft sediments on continental shelves are affected by bottom trawl fisheries.

In this study, we explore the effect of this widespread anthropogenic disturbance on the species

richness of a benthic ecosystem, along a gradient of bottom trawling intensities. We use data

from 80 annually sampled benthic stations in the Dutch part of the North Sea, over a period of

6 years. Trawl disturbance intensity at each sampled location was reconstructed from satellite

tracking of fishing vessels. Using a structural equation model, we studied how trawl disturbance

intensity relates to benthic species richness, and how the relationship is mediated by total benthic

biomass, primary productivity, water depth, and median sediment grain size. Our results show a

negative relationship between trawling intensity and species richness. Richness is also negatively

related to sediment grain size and primary productivity, and positively related to biomass. Further

analysis of our data shows that the negative effects of trawling on richness are limited to relatively

species-rich, deep areas with fine sediments. We find no effect of bottom trawling on species richness

in shallow areas with coarse bottoms. These condition-dependent effects of trawling suggest

that protection of benthic richness might best be achieved by reducing trawling intensity in a strategically

chosen fraction of space.