Fit one or more soft, semi-rigid, or rigid grids or frames to trawl nets and use square mesh instead of a diamond mesh at the 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 caught, one or more soft, semi-rigid or rigid grids or frames can be fitted to the inner side of the net before the codend, in combination with a codend made of square mesh instead of diamond mesh. The grid or frame is designed to prevent larger organisms, such as turtles, from entering the net/codend and allow smaller unwanted organisms to escape the codend, while retaining the commercially targeted organisms.
Evidence related to the use of grids in combination with other “bycatch reducing devices” are summarised under “Threat: Biological resource use – Fit one or more mesh escape panels/windows and one or more soft, rigid or semi-rigid grids or frames to trawl nets”. Evidence related to the use of each modification (grids and square-mesh codend) separately, are summarised under “Threat: Biological resource use – Fit one or more soft, semi-rigid, or rigid grids or frames 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 2012 in the Gulf of St Vincent, off the coast of South Australia (Gorman & Dixon 2015) found that trawl nets fitted with a rigid U-shaped grid (“bycatch reduction device”) and a square-oriented mesh codend resulted in lower catch rates of three dominant groups of unwanted invertebrate catch species, compared to unmodified nets. Compared to unmodified nets, the modified nets led to a 92% decrease in catch rate (kg/h) of sponges, 78–82% decrease in catch rate of crabs and other crustaceans, and a 61% decrease in catch rate of molluscs (excluding commercially valuable species of octopus, squid and cuttlefish; raw data not provided). A 15% decrease in catch rates of the commercially targeted western king prawn Penaeus latisulcatus was recorded due to reduced catch of less valuable smaller-sized prawns. In May 2012, unwanted catch of invertebrates in modified and unmodified nets were compared (see paper for details). Nets were deployed by four vessels during 29 paired hauls for 30 min (one modified; one unmodified; side-by-side simultaneously). All invertebrates were identified, sorted as commercial prawn catch or unwanted catch, and weighed.Study and other actions tested