Fit mesh escape panels/windows and a size-sorting grid (rigid or flexible) to a trawl net
Overall effectiveness category Awaiting assessment
Number of studies: 6
Background information and definitions
Trawling is a method of fishing that involves pulling a fishing net (trawl) through the water behind one or more vessels. Trawls may be fitted with devices to allow unwanted catch to escape (often termed “bycatch reduction devices”). A variety of unwanted catch reduction devices have been developed aiming to enable specific sizes or groups of unwanted fish to escape the trawl net. These include rigid or flexible size-sorting or excluder grids, and also large square-, or diamond-mesh panels, fitted into the codend of a trawl net (Sinclair & Valdimarsson 2003). Positioning of both types of devices as well as opening size, mesh size or bar spacing and orientation can affect the type and sizes at which fish can escape.
Evidence for the individual effects of mesh escape panels/windows and size-sorting escape grids in trawl nets is summarized under ‘Fishing gear modification - Fit mesh escape panels/windows to a trawl net’, ‘Fit a size-sorting escape grid (rigid or flexible) to a fish trawl net’ and ‘Fit a size-sorting escape grid (rigid or flexible) to a prawn/shrimp trawl net’. Similar interventions of various combinations of unwanted catch reduction devices, including mesh panels/windows and escape grids, are summarized under ‘Fishing gear modification’.
Valdemarsen J.W., Suuronen P. 2003. Modifying Fishing Gear to Achieve Ecosystem Objectives. In Responsible Fisheries in the Marine Ecosystem. Food and Agriculture Organisation of the United States & CABI Publishing, 426 pp.
Supporting evidence from individual studies
A replicated, paired, controlled study in 1995–1996 of two fished areas of seabed in the Gulf of Carpentaria, Australia (Brewer et al. 1998) found that prawn trawl nets fitted with a square mesh escape panel in combination with a rigid size-sorting grid caught fewer unwanted fish, sharks and stingrays (Elasmobranchii) compared to an unmodified conventional trawl net. In the first of two trials, catch weights of unwanted fish in nets with a square mesh window and a rigid grid (Nordmøre) were reduced by 29–39% relative to a conventional net. In the second trial (commercial conditions), a square mesh window and grid (Super Shooter) caught fewer sharks (3) and stingrays (0) compared to a conventional trawl net (sharks: 12, stingrays: 2). Shark and stingray data were not tested statistically. In addition, target prawn Penaeidae catches were reduced by 17–34% in the square mesh window/Nordmøre system and average catch weight decreased from 41 kg in conventional nets to 38 kg in the square mesh window/Super Shooter system. Catch data were collected from deployments of two modifications of a standard prawn trawl net with a diamond mesh codend, each with a square mesh escape window grid fitted behind a rigid size-sorting grid (Nordmøre or Super Shooter) (see original paper for gear specifications). The modified nets were towed in paired deployments with other modified net designs and/or standard unmodified nets in Australia’s Northern Prawn Fishery area; during experimental trials in October 1995 (73 paired tows, 2h duration) and in commercial trials in October 1996 (24 paired tows, 2–3 h duration).Study and other actions tested
A replicated, paired, controlled study in 2000 in an area of soft seabed in the Indian Ocean, off Western Australia (Broadhurst et al. 2002) found that the effect of prawn trawl nets fitted with square mesh escape panels and rigid grids on non-target fish catch compared to a standard net, varied with the position of the escape panel. Compared to a standard net, trawl nets with the square mesh panel located in the rear section of the codend further away from the grid, reduced the non-target catches of two of seven non-commercial, and two of three commercial fish species, by between 50–76% in number and 47–73% by weight (see original paper for species individual data). Catches of the five other species were similar between the nets. No differences in non-target catches of the 10 fish species were found between the net with a grid and panel located forward of the codend (nearer the grid) and the standard net (see original paper for species individual data). In addition, the total catch weights of the target prawns Penaeidae were reduced in both grid/panel nets, by 12–14%. In August 2000, two sets of 10 paired trawl deployments (40 min each) were done in Shark Bay using one of two designs of modified trawl and a standard trawl net (47 mm diamond mesh, no grid or panel) simultaneously. The modified trawl nets were standard nets fitted with a Nordmøre rigid grid (100 mm bar spacing) located in front of the codend, and a square mesh escape panel (47, 94 and 155 mm mesh sections) at either the rear or front section of the codend (see original paper for gear specifications). All catch was sorted, counted, and weighed.Study and other actions tested
A replicated study in 2003 of an area of seabed in the Atlantic Ocean off Portugal (Fonseca et al. 2005) found that bottom trawl nets fitted with a square mesh escape window in addition to a rigid size-sorting escape grid, enabled the escape of high proportions of undersized commercially targeted and non-target fish species, and the main means of escape (window or grid) varied between bottom and mid-water dwelling species. Data were not statistically tested. The proportion (by number) of individuals of commercially targeted fish below their respective minimum landing sizes that escaped was 62–79% (grid) and 8–15% (square mesh window) for two bottom dwelling species; and 0–14% (grid) and 60–100% (window) for two pelagic commercial species. For non-target species, the percentage (by weight) of escaped individuals of one bottom dwelling species was 48% (grid) and 1% (window), and for two pelagic species the grid excluded 13–17% and the window 17–72%. In September 2003, a total of 26 trawl net deployments were done by research vessel off the north west coast of Portugal at 40–150 m depth. Trawl nets were fitted with either a Nordmøre grid (plastic, 1.5 × 1 m, 30 mm bar spacing) on its own (17 hauls), or a Nordmøre grid and a square mesh window (1.8 m long, 50 mm mesh size) inserted just behind the top section of the grid (9 hauls). A ‘flapper’ net guided catch to the bottom of the grid, the upper 40 cm of which had no bars to allow retained catch to pass into the codend, while catch that passed through the grid (excluded) was retained by an inner net. A cover attached over the square mesh window collected fish escaping through the meshes of the window (see original paper for gear specifications). Fish collected in the codend, inner net and cover were sampled.Study and other actions tested
A replicated, paired, controlled study in 2005 of an area of seabed in the Indian Ocean, off Mozambique (Fennessy & Isaksen 2007) found that a prawn trawl net with a square mesh escape window and a size-sorting escape grid reduced the overall discarded catch (fish and invertebrates) compared to a conventional trawl net. Average catch rates of discards (90% fish, 10% invertebrates) were lower in the net with a square mesh panel and size-sorting grid (30 kg/h) compared to a conventional trawl net without a panel or grid (56 kg/h). In addition, average catch rates were also reduced by a square mesh panel (panel: 37 kg/h, conventional: 50 kg/h), or grid (grid: 36 kg/h, conventional: 52 kg/h) alone, compared to a conventional trawl without either. Catch rates of retained fish were similar between nets (panel/grid: 4 kg/h, conventional: 5 kg/h), and the catch of targeted prawn (mostly Fenneropenaeus indicus) was lower in a panel/grid net (panel/grid: 6 kg/h, conventional: 8 kg/h). Data was collected in February 2005, from a total of 23 trawl deployments (6–21 m) using a twin-rigged trawler towing a test net and a conventional diamond mesh trawl net side by side. Test nets were the conventional design fitted with either: a square mesh escape panel (143 mm mesh size) and a rigid grid (‘Nordmøre’, 100 mm bar spacing) (eight deployments); a square mesh panel alone (11 deployments); or a grid alone (four deployments). See original paper for gear specifications.Study and other actions tested
A replicated, paired, controlled study in 2012–2013 in a fished area of seabed in the Atlantic Ocean, off Suriname (Willems et al. 2016) found that a shrimp trawl net fitted with a square mesh escape panel in combination with a size-sorting escape grid reduced the overall catch of rays and individuals of three of five ray species, compared to a standard commercial trawl net, but larger rays had higher escape rates than smaller rays. Overall ray catch rate was reduced by 36% in nets with a panel and grid compared to without. By species, between 32–77% fewer individuals of sharpsnout stingray Dasyatis geijskesi (panel/grid: 38, without: 161 ind), longnose stingray Dasyatis guttata (panel/grid: 440, without: 741 ind) and smooth butterfly ray Gymnura micrura (panel/grid: 572, without: 858 ind) were caught, and catches were similar between trawl types for cownose ray Rhinoptera bonasus (panel/grid: 8, without: 11 ind) and smalleyed round stingray Urotrygon microphthalmum (panel/grid: 171, without: 181 ind). Rays caught in the panel/grid net were on average 21% smaller than rays caught in the standard net (panel/grid: 26 cm, without: 32 cm), significantly smaller for sharpsnout (38%) and longnose stingrays (23%), and catch rate of all species combined declined with increasing size in the panel/grid net (data reported graphically). Trials were done on Atlantic seabob shrimp Xiphopenaeus kroyeri fishing grounds during eight commercial trips from February 2012 to April 2013. A total of 65 simultaneous deployments of a standard diamond mesh trawl net (45 mm mesh size codend) fitted with a square mesh panel (150 mm mesh size) and a grid (aluminium ’Super Shooter’ turtle excluder device, 10 cm bar spacing), and a standard trawl net were completed (2.5–3.5 knots, 1 h). All rays caught were identified, counted and wing width recorded. See original paper for gear specifications.Study and other actions tested
A replicated, paired, controlled study in 2010–2013 of a seabed area in the English Channel, UK (Vogel et al. 2017) found that bottom trawl nets fitted with square mesh escape panels or cylinders in combination with a sorting grid(s) of different designs, increased the size-selectivity of whiting Merlangius merlangus compared to a standard trawl net. Overall, both nets tested improved the escape of whiting under 30 cm in length (minimum landing size 27 cm) compared to a standard net. A square mesh panel with two consecutive flexible grids allowed whiting of all lengths to escape, while a square mesh cylinder with one rigid grid allowed significant escape of whiting up to 31 cm length. Data were reported as statistical model results and catch probability curves. Trials were done in June 2010 and November 2013 by commercial trawlers fishing parallel to each other: one rigged with a modified net and the other a standard net. In the first trial, a net fitted with a square mesh panel and two flexible grids of different designs was tested (18 paired deployments). In the second trial, a net fitted with a square mesh cylinder around the entire section circumference and an aluminium grid (30 mm spaced vertical bars) was tested (21 paired deployments) (see original paper for gear specifications). Fish length and weight in catches were recorded. Random sub-sampling was done when catches were large.Study and other actions tested