Fit one or more soft, semi-rigid, or rigid grids or frames to trawl nets
Overall effectiveness category Likely to be beneficial
Number of studies: 2
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. To reduce the amount of unwanted organisms, one or more soft, semi-rigid or rigid grids or frames can be fitted to the inner side of the net before the codend. This grid or frame is designed to prevent larger organisms, such as turtles or sharks, from entering the net/codend, while retaining the commercially targeted organisms (Brčić et al. 2015; Sala et al. 2011).
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” and in combination with a modified codend under “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”.
Brčić J., Herrmann B., De Carlo F. & Sala A. (2015) Selective characteristics of a shark-excluding grid device in a Mediterranean trawl. Fisheries Research, 172, 352–360.
Sala A., Lucchetti A. & Affronte M. (2011) Effects of Turtle Excluder Devices on bycatch and discard reduction in the demersal fisheries of Mediterranean Sea. Aquatic Living Resources, 24, 183–192.
Supporting evidence from individual studies
A replicated, paired, controlled study in 2001 in areas of seabed in the Gulf of Carpentaria, northern Australia (Brewer et al. 2006) found that the effects of fitting a grid (“turtle excluder device”) to trawl nets, on large sponges and small unwanted catch (invertebrates and fish combined) varied with the device orientation. Nets fitted with a device oriented either ‘downward’ or ‘upward’ caught 82–96% fewer large sponges, compared to unmodified nets, but only the ‘downward’ devices reduced the weight of small unwanted catch (by 8%; data not provided for the ‘upward’ device). Compared to unmodified nets, nets fitted with a ‘downward’ device reduced the catch of commercially targeted prawns by 6%, while those with an ‘upward’ device caught similar amounts. The use of a “turtle excluder device” has been compulsory since 2000 in the Australian prawn fishery (as well as the use of a “bycatch reduction device”). Commercial vessels towed twin Florida Flyer prawn trawl nets from each side of the vessel in August–November 2001. Nets with one of 23 grid designs (rigid or semi-rigid frame with ≤120 mm bar spacing and an opening of ≥700 mm) grouped as either ‘upward’ (9 devices) or ‘downward’ (14 devices) oriented (267 nets examined for small unwanted catch, 392 for sponges) and an unmodified net (339 for sponges, 703 for small unwanted catch) were randomly assigned to either side of the vessel. Total weights of small unwanted catch (<300 mm), commercially targeted prawns, and counts of sponges (>300 mm) were recorded.Study and other actions tested
A replicated, paired, controlled study in 2014 in a sandy area in Spencer Gulf, Southern Australia (Kennelly & Broadhurst 2014) found that, when fitted to trawl nets, two grids reduced the number and biomass of unwanted giant cuttlefish Sepia apama and blue swimmer crabs Portunus armatus caught, compared to conventional nets without grids. Compared to conventional nets, nets fitted with a small grid resulted in a 50% decrease in the number and a 60% decrease in the biomass of giant cuttlefish caught, as well as a 40% decrease in the number and a 48% decrease in the biomass of blue swimmer crab caught. Nets fitted with a large grid resulted in 34% decrease in the number and a 37% decrease in the biomass of giant cuttlefish caught, as well as a 34% decrease in the number and a 50% decrease in the biomass of blue swimmer crab caught. There were no differences in cuttlefish abundance and biomass between the grid sizes. Catch of commercially targeted western king prawns Melicertus latisulcatus was reduced by 8% when using a small grid compared to a large grid and the conventional net (which had identical catches). Two grids were tested: a small grid (1.4 m long, 45° angle) and a large grid (1.98 m long, 30° angle) (see paper for full details). For 30 min at night, a trawler towed two identical nets (one on each side) fitted with a 41 mm mesh codend during simultaneous, paired deployments: one net fitted with a grid and one unmodified conventional net (eight deployments using small grids, seven using large grids). For each deployment, the weight and numbers of cuttlefish and crabs were recorded, as well as the weight of other unwanted catch. Weight and size of prawns were also recorded.Study and other actions tested