A new apparatus for the direct measurement of the effects of otter trawling on benthic nutrient releases

An innovative Towed Trawl Simulator Sledge (TTSS) which has been tested in the field is described. The new instrument, which incorporates six water sampling bottles and video observation equipment, is designed to simulate the effects of the passage of otter trawl groundropes over seafloor sediments. The TTSS is towed from a surface vessel at speeds similar to those of commercial trawls and the water bottles, triggered simultaneously by electrical or acoustic signals, collect water samples in the plume of disturbed sediment behind the groundrope. The TTSS has been tested in the continental shelf fishing grounds of Heraklion Bay (Eastern Mediterranean) where quantitative measurements of the physical (sediment resuspension) and chemical (nutrient release) effects on surface sediments caused by the passage of otter trawl groundrope typical of that used in the Cretan fishery trawling were performed. The performance of the TTSS is also validated against an instrumented full-scale trawl. It is suggested that the TTSS should be used for the study and modelling of the effects of otter trawling and similar fishing processes on nutrient supply from seafloor sediments and the resulting ecosystem responses.