Fit one or more mesh escape panels/windows to trawl nets and use a square mesh instead of a diamond mesh codend
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 cone-shaped fishing net (trawl) through the water behind one or more boats. The net is wide at the opening and narrows to a bag or ‘codend’, tied at the end with a drawstring, where organisms are trapped. Trawl nets can catch a considerable number of unwanted organisms, including non-commercially targeted species and organisms under the legal-size limit. Standard trawl nets are made from diamond-shaped mesh. To reduce the amount of unwanted organisms, a net can be modified by fitting one or more mesh “escape panels” in the outer mesh of the net before the codend, in combination with a codend made of square mesh instead of diamond mesh. These modifications are designed to allow smaller unwanted organisms to escape the main body of the net and the codend, while retaining the commercially targeted organisms.
Evidence related to the use of each modification (mesh panels and square-mesh codend) separately, 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 on trawl nets”.
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 fishing with a trawl net fitted with a square mesh codend and two large square mesh release panels (“bycatch reduction devices”) reduced the biomass of non-commercial unwanted invertebrate catch (discard) compared to a standard trawl. Across the two areas, the modified trawls caught 39–45% less invertebrate discard (349–730 kg) compared to the standard trawls (635–1,207 kg). However, they caught 22–82% fewer commercial shellfish (94–101 individuals), compared to standard trawls (120–570 individuals). The modified trawls also caught 22% less commercially important fish in one area, but those were worth more per kg than the fish caught in the standard trawls. Two designs of trawl nets were towed simultaneously: a modified beam trawl with an 80 mm square mesh codend and fitted with two large 200 mm square mesh release panels (one to release weed and one to release invertebrates), and the industry standard beam trawl with an 80 mm diamond mesh codend. Seven to nine tows/area were conducted in July–August 2007. The catch was sorted into commercially important species (target and non-target commercial 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