Fit a size-sorting escape grid (rigid or flexible) to trawl nets and use a square mesh instead of a diamond mesh codend
Overall effectiveness category Awaiting assessment
Number of studies: 3
Background information and definitions
Escape grids are frames of metal, plastics or mesh inserted in or near the codend of trawl nets to try and prevent unwanted species or sizes of catch from entering the codend. They are size selection mechanisms, the sizes at which individuals are sorted dependent on the type of panels and the mesh size and shape. Behaviour of individual fish species may also influence how effective a grid is at allowing escape; some fish will swim upwards in a net whilst others might swim down or to the sides. The effectiveness of escape panels is therefore likely to be dependent on the behaviour, shape, and size of the unwanted fish. Furthermore, constructing the codend using a square shaped mesh instead of a diamond mesh helps to maintain the net’s structure under strain, allowing a greater chance for fish to escape through the gaps of the mesh. Combining an escape grid with a square mesh codend may further increase the chance of fish escape for a greater range of species or sizes.
The effects of using just a size-sorting escape grid or just a square mesh codend in trawl nets are summarized under ‘Fishing gear modification - Fit a size-sorting escape grid (rigid or flexible) to a fish trawl net’, ‘Fit a size-sorting escape grid (rigid or flexible) to a prawn/shrimp trawl net’ and ‘Use a square mesh instead of a diamond mesh codend in a trawl net’, respectively. Similar interventions of various combinations of unwanted catch reduction devices, including escape grids and square mesh codends are summarized under ‘Fishing gear modification’.
Supporting evidence from individual studies
A replicated, paired, controlled study in 2006 of an area of seabed in the North Sea, UK (Catchpole et al. 2006) found that prawn trawl nets fitted with a rigid size-sorting escape grid and a square mesh instead of a diamond mesh codend, reduced the catch of all sizes of unwanted whiting Merlangius merlangus and the overall amounts of plaice Pleuronectes platessa, small cod Gadus morhua and small haddock Melanogrammus aeglefinus, compared to a standard trawl (no grid, diamond mesh codend). Average catch numbers of whiting of all length groups (11–41 cm) were reduced with a grid and square mesh codend (with: 0–23 fish/tow, without: 5–254 fish/tow). Average catch numbers of plaice, haddock and cod were lower for up to half of the length groups in nets with a grid and square mesh codend (plaice, with: 4–12 fish/tow, without: 1–22 fish/tow; haddock, with: 0 fish/tow, without: 4–52 fish/tow; cod, with: 0–2 fish/tow, without: 4–45 fish/tow). These included the smaller size groups of haddock and cod (see paper for data by length group). In addition, catches of marketable sizes of the target species Norway lobster Nephrops norvegicus were reduced in trawls with a grid and square mesh codend (with: 0.8–0.9, without: 1.2–2.0 baskets/trawl) but discards were also lower (with: 0.5, without: 2.4 baskets/trawl). In March 2006, a total of 10 paired deployments were made by a twin-rig vessel in the Farn Deeps Nephrops fishing ground off the coast of England. The vessel towed two trawl nets simultaneously: one 80 mm Nephrops trawl fitted with a metal grid (Swedish grid) and a 70 mm square mesh codend; and one standard 80 mm Nephrops trawl net with a 85 mm diamond mesh codend.Study and other actions tested
A replicated, paired, controlled study in 2002–2006 of two seabed areas in the Skagerrak and Kattegat off Sweden/Denmark (Valentinsson & Ulmestrand 2008) found that prawn trawl nets fitted with size-sorting escape grids (two types) and a square mesh instead of a diamond mesh codend, reduced the discarded catches of four of five fish species compared to a standard trawl net with no grid and a diamond mesh codend. Average discarded catch rates of cod Gadus morhua, whiting Merlangius merlangus, haddock Melanogrammus aeglefinus and plaice Pleuronectes platessa were lower in trawl nets with a grid and square mesh codend for two of two grids (all species, with: 0.4–18.8 kg/tow, without: 2.5–67.4 kg/tow), and hake Merluccius merluccius was lower for the flatfish grid (with: 3.3 kg/tow, without: 9.3 kg/tow), and similar for the Nordmøre grid (with: 0.0 kg/tow, without: 0.3 kg/tow). In addition, the weight of undersized and marketable target catch of Nephrops norvegicus was lower for one of two grids (flatfish grid) and similar for the other. Data were collected from two trials on commercial Nephrops fishing grounds in Skagerrak/Kattegat in November 2002 and June 2006. A total of 17 paired deployments were made by two twin-rig vessels towing two nets simultaneously: one net fitted with either a 35 mm rigid grid (Nordmøre) or a ‘flatfish’ grid (horizontal bars) and a 70 mm square mesh codend, and one net with either a 70 or 90 mm diamond mesh codend (see paper for gear specifications). All catches were sorted and weighed.Study and other actions tested
A replicated, randomized, paired, controlled study in 2002 of a coastal seabed site in the Coral Sea, Australia (Courtney et al. 2014) found that prawn trawl nets fitted with a size-sorting escape grid (a turtle excluder device) and a square mesh codend reduced the catches of the most frequently caught unwanted fish species, but not of less frequently caught species compared to a standard diamond mesh codend, and overall fish catch rates were similar to a square mesh codend alone. For seven of eight unwanted fish species caught in 26–85% of tows, average catch rates were lower in nets with a grid and square mesh codend (with: 0–93 g/ha, standard: 6–134 g/ha) and were similar for one species (with: 7 g/ha, standard: 5 g/ha). For a further 18 unwanted fish species caught in 6–31% of tows, there were no differences in average catch rates between grid/square mesh codend nets and standard nets (with: 0–43 g/ha, without: 1–27 g/ha). In addition, the catch rates of most unwanted fish (24 of 26 species) were similar to a square mesh codend without a grid (see paper for individual data). Catch rates of the target eastern king prawns Melicertus plebejus were similar between all net types (grid/square mesh codend: 264, square mesh codend: 267, standard: 274 g/ha). In July 2002, data were collected from 65 paired trawl deployments done over 10 nights off the coast of Queensland. Three different codends were tested against a standard diamond mesh codend: a standard diamond mesh with a metal, top-opening grid (Wick’s turtle excluder device), the grid in combination with a square mesh codend, and a square mesh codend alone (see paper for specifications). Each codend design was randomly assigned to the two trawl nets every 12 tows. Each tow was 2 nm long, at 2.2 knots.Study and other actions tested
Referenced paperCourtney A.J., Campbell M.J., Tonks M.L., Roy D.P., Gaddes S.W., Haddy J.A., Kyne P.M., Mayer D.G. & Chilcott K.E. (2014) Effects of bycatch reduction devices in Queensland's (Australia) deepwater eastern king prawn (Melicertus plebejus) trawl fishery. Fisheries Research, 157, 113-123.