A trawl survey of seafloor macrolitter on the South African continental shelf

Abstract

Demersal trawls provide an index of seafloor macrolitter abundance, but there are no published data from sub-Saharan Africa. We collected litter items from 235 trawls conducted to assess fish abundance off South Africa. Only 17% of trawls contained litter (3.4 items·km^-2, 2.1 kg·km^-2 but only 0.2 kg·km^-2 excluding four megalitter items). Plastic items predominated (88%), of which 77% floated once cleaned of epibionts. One LDPE bag manufactured three months before being caught carried pelagic goose barnacles Lepas anserifera, confirming that biofouling leads to rapid sinking of floating plastics. Fishery/shipping wastes comprised 22% of litter items (98% by mass; 73% excluding megalitter items); the remainder was general waste - mostly packaging or other single-use items - that could come from land- or ship-based sources. Litter was more abundant in deep water close to Cape Town. The annual demersal trawl survey is a useful way to monitor seafloor litter off South Africa.

Keywords: Fishing gear; Monitoring; Seafloor litter; Sedimentation; Single use plastics.