Anthropogenic disturbance on the NW Sicilian Canyons (western Mediterranean): trawling-induced impact and marine litters

Abstract

Industrial bottom trawl fishery and marine litter are concerning environmental threats exerting a strong impact on deep-sea ecosystems, menacing the maintenance and preservation of renewable natural resources. One of the main goals of the EU Eurofleets-2 ISLAND (ExplorIng SiciLian CAnyoN Dynamics) expedition was to explore the role of bottom trawling in altering the natural sedimentary dynamics along the submarine canyons of the NW Sicilian Margin (western Mediterranean) and to quantify the marine litters within them. An holistic approach was adopted across a depth range of 150-800 m, spanning from the analysis of the seafloor complexity from Multi Beam (MB) bathymetry, the quantification of sedimentation rates and organic carbon content in surface sediments collected with a multi corer, the analysis of litters and benthic habitats from ROV videos. Remote sensing data and samples were collected based on the distribution of the trawling effort averaged from the Vessel Monitoring System (VMS) data analzyed over a period of 6 years (2009-2015). Submarine canyons with null, low and high fishing effort were explored and compared. Results showed a potential impact of trawling on the large-scale seascape heterogeneity, smoothing the morphology of small sediment-starved gullies on the regions undergoing the maximum effort. ROV videos showed differences in the habitat complexity between adjacent regions undergoing different trawling effort, with impacted areas having a strongly reduced biodiversity, with some exceptions for scavengers. Multi core analysis revealed that submarine canyons undergo enhanced sedimentation rates due to resuspension processes generated by the trawlers crossing their axes either the surrounding regions. Analysis of ROV videos show a strong qualitative difference in the amount of litters between trawled and untrawled canyons. Whereas trawled canyons show absence of litters on their grounds, untrawled canyons host a high amount of litters, which was widespread either organized in several small islands, 1 to 3 m wide. Canyons most probably function as a sink hole for marine debris transported with bottom currents up to the visited depth of 800 m. Plastic is the greatest contributor to seabed litter (40%), followed by large items such as tires, oil drums and metal sheets (20%) and fishing material (longlines and nets - 15%). These compelling evidences suggest that NW Sicilian canyons are largely subject to the effect of threatening human activities, with an evident impact at different spatial and temporal scales