Dose-response relationship between shrimp trawl fishery and the macrobenthic fauna community in the coastal zone and Wadden Sea

The potential environmental effect of the brown shrimp fishery in the shallow shelf of the North Sea and Wadden Sea is debated because the fishery operates for a large part in designated Natura2000 sites. In this fishery a beam trawl is pulled along the seafloor, potentially affecting organisms living on and in the seabed. Because the effect by the trawl used in the shrimp fishery is unclear, has long been debated, and hardly studied to date, we carried out an experimental study to test short to medium term effects of the fishery on the benthic community in an area with a decadal long fishery. The study consisted of two complementing parts: one experiment focussed on the effect of a single fishing event in 15 different areas throughout the distribution area of shrimp fishery along the coast and in the Wadden Sea; in another experiment we measured the dose-response relationship between fishing intensity and the benthic community in one area in the Wadden Sea. Unfortunately, the first experiment was severely hampered by unplanned fishing by fishermen. Instead of the originally planned BACI design with 15 replicates, the first experiment was analysed by correlating actual fishing intensity and benthos community parameters. We found a negative relationship between fishing pressure and the change in number of species from pre- (T0) and post-fishing (T1, eight weeks later), but not in total density or Pielou's evenness. In the second experiment the change between T0 and T1, ten weeks later and T2, eight months later) in Pielou's evenness was negatively related to fishing pressure. The change in total density and the number of species were unrelated to fishing intensity. The pattern in evenness was primarily due to the positive relationship of the change of Ensis leei with fishing pressure. For eight of the ten most common individual macrofauna species, there was no relationship between their development and fishing pressure, while for two species the change in density showed a negative relationship with fishing pressure in the T0-T1 (Ascidia sp.) or T0-T2 comparison (Cerastoderma edule). We interpret the observed response in Ensis leei as their ability to relocate and quickly colonise space, that became vacant because the original inhabitants were disturbed, removed or died. Especially this last finding, that an apparently mobile opportunistic species is able to colonise vacant space is a fishery-related mechanism that adds to the understanding of the potential effect of shrimp fisheries.