Methods for evaluating debris on the deep sea floor

The distribution and abundance of larger marine debris were investigated using four different methods (bottom trawling, pole trawling, videophotography and submersibles) during oceanographic cruises undertaken from November 1993 to July 1998. Different types of debris were enumerated, particularly pieces of plastic, plastic and glass bottles, metallic objects, glass, and various materials including fishing gear. The results were highly variable depending on the methodologies used as well as hydrodynamic, geomorphologic or anthropic factors. Concentrations reached more than 200 pieces of debris per hectare in some areas. In most stations sampled, plastic bags accounted for a very high percentage (more than 90%) of total debris. In the Mediterranean Sea, most debris was found in canyons along the continental slope. The adequacy of each method was considered in terms of qualitative and quantitative analysis. For larger areas, pole trawling was the most suitable method whereas narrow eroded areas such as canyons required submersibles.