Use a square mesh instead of a diamond mesh codend on trawl nets
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
Background information and definitions
Trawling is a method of fishing that involves pulling a fishing net (trawl) through the water behind one or more boats. Trawl nets can catch a considerable number of unwanted organisms, including non-commercially targeted species and organisms under the legal-size limit. Nets are traditionally made of diamond-shaped mesh throughout, including at the codend. To reduce the amount of unwanted organisms caught, a codend made of square mesh can be used instead of diamond mesh, with the aim to allow smaller unwanted organisms to escape, while retaining the commercially targeted ones (Broadhurst et al. 2010).
Evidence related to the use of modified codend in combination with a mesh panel (“bycatch reduction device”) or a grid (“turtle excluder device”) are summarised under “Threat: Biological resource use – Fit one or more mesh escape panels/windows to trawl nets and use a square mesh instead of a diamond mesh codend” and “Fit one or more soft, semi-rigid, or rigid grids or frames to trawl nets and use a codend of different mesh geometry or size”. Evidence related to other codend modifications are summarised under “Threat: Biological resource use – Use a larger mesh codend”.
Broadhurst M.K., Millar R.B. & Brand C.P. (2010) Diamond-vs. square-mesh codend selectivity in southeastern Australian estuarine squid trawls. Fisheries Research, 102, 276–285.
Supporting evidence from individual studies
A replicated, paired, controlled study in 2007 in two areas of the English Channel, southwest England, UK (Wade et al. 2009) found that in one of two areas a trawl net fitted with a square mesh codend caught less non-commercial unwanted invertebrates (discard) compared to a standard trawl fitted with a diamond mesh codend. In one of two areas examined, using a square mesh codend instead of a standard diamond mesh codend reduced the biomass of invertebrate discard by 11% (square: 794 vs diamond: 889 kg). The square mesh codend caught similar amounts of commercially targeted species (megrim and anglerfish in that area) to the standard trawl and caught 26% more commercially important shellfish. In the other area, the square mesh codend caught similar biomass of invertebrate discard (1,746 kg) as the diamond mesh codend (1,842 kg), and similar amounts of commercially targeted species (Dover sole and plaice in that area). Two designs of trawl nets were towed simultaneously: a modified beam trawl with an 80 mm square mesh codend, and the industry standard beam trawl with an 80 mm diamond mesh codend. Twelve tows/area were conducted in July–August 2007. The catch was sorted into commercially important species (target and commercial unwanted catch) and discard species. Commercial organisms were counted, and discards were further sorted into benthic invertebrates and finfish and each were weighed.Study and other actions tested