South African Environmental Observation Network Offshore Demersal Invertebrate Monitoring Part 1

Study Extent

Demersal research trawl gear are used to collect invertebrates as bycatch during fish surveys. The trawl gear used is German Otter trawl with otter trawl boards and a footrope rope-wrapped chain. The mesh size of the codend liner was 40 mm stretched mesh. See Atkinson et al 2011 doi: 10.2989/1814232X.2011.572378 and Axelsen and Johnsen 2015 doi:10.1111/fog.12079

Sampling

Samples were collected using a research demersal trawl net towed for 30 minutes per trawl, unless hard ground was encountered or net was damaged in which case shorter tow durations are noted. Sampling unit is nautical miles squared.

Quality Control

Data were validated by engaging with experts where necessary and using various field guides. Coordinates of the start position of the trawl are provided and due to the depths at which the trawls operate and the uncertainty of where the vessel is in relation to the trawl net on the seafloor, no further useful co-ordinate uncertainty can be provided.

Method steps

1. N/A

All marine invertebrates that are retained in the research trawl net from each trawl deployed are identified and recorded. These surveys take place in collaboration with the annual demersal research surveys conducted by the Fisheries Department, currently under the Department of Forestry, Fisheries and Environment (DFFE).

1. Psilaster
   rank: genus

Total of 316 species belonging to 10 marine Phyla

1. Haliporoides
   rank: genus

West and South coasts of South Africa, extending from the border with Namibia at the Orange River Mouth to Port Alfred