Decrease the circumference or diameter of the codend of a trawl net
Overall effectiveness category Awaiting assessment
Number of studies: 13
Background information and definitions
Trawling is a method of fishing that involves pulling a fishing net (trawl/seine) through the water behind one or more vessels. In many fisheries, particularly edible crustacean fisheries (e.g. prawns), trawls often retain unwanted small non-target or non-commercial fish species. Behavioural differences between species can be exploited to reduce the amount of unwanted catch retained in a trawl net. In trawl nets with a smaller circumference, the lateral openings of the netting mesh are larger than in larger circumference trawls, which allows more fish to pass through the netting (Armstrong et al. 1990). This may be due to different hydrodynamic pressures in the codend, which may stimulate a behavioural response in fish and contribute to their escape, which may not occur in crustaceans and other groups of species (Broadhurst & Kennelly 1996).
Related interventions describing other modifications to the overall design of different types or parts of trawl gear are summarized under ‘Fishing gear modification - Modify the design or configuration of trawl gear (mixed measures)’, ‘Change the size of the main body of a trawl net’, ‘Modify the design or configuration of trawl doors’, ‘Modify a bottom trawl to raise parts of the gear off the seabed during fishing’ and ‘Modify design or arrangement of tickler chains/chain mats in a bottom trawl’.
Armstrong D.W., Ferro R.S.T., MacLennan D.N. & Reeves S.A. (1990) Gear selectivity and the conservation of fish. Journal of fish Biology, 37, 261–262.
Broadhurst M.K. & Kennelly S.J. (1996) Effects of the circumference of codends and a new design of square-mesh panel in reducing unwanted by-catch in the New South Wales oceanic prawn-trawl fishery, Australia. Fisheries Research, 27, 203–214.
Supporting evidence from individual studies
A replicated, controlled study in 1986–1988 on bottom fishing grounds in the northern and central North Sea, UK (Reeves et al. 1992) found that fishing nets (trawls and seines) with a narrower codend diameter improved the size-selectivity of haddock Melanogrammus aeglefinus, cod Gadus morhua and whiting Merlangius merlangus compared to standard and wide diameter codends. Across three codend mesh sizes and lengths of the extension piece, the length at which fish had a 50% chance of escape was greater for narrow (2.2 m) diameter codends for haddock (18–32 cm), cod (20–36 cm) and whiting (21–39 cm), compared to standard (3.2 m) codend diameters (haddock: 14–28 cm, whiting: 18–35 cm, cod: 16–32 cm) and wide (4 m) codend diameters (haddock: 11–25, whiting: 15–28, cod: 12–28 cm). Data were collected during three surveys on two commercial vessels in 1986–1988: two deploying a seine net and one a bottom trawl. Deployments were made with codends of three nominal diameters: narrow, 2.2 m (seine: 107, trawl: 36 hauls), standard, 3.2 m (seine: 105, trawl: 35 hauls) and wide, 4 m (seine: 116, trawl: 36 hauls); three mesh sizes (nominal 80, 90 and 100 mm) and three extension piece lengths (nominal 0.1, 9.1 and 13.7 m). Covers over each codend retained the fish escaping through the meshes for sampling.Study and other actions tested
A replicated, controlled study in 1991 of an area of seabed in the North Sea off Scotland, UK (Galbraith et al. 1994) found that bottom trawl nets of smaller codend diameter improved the size selectivity of haddock Melanogrammus aeglefinus, whiting Merlangius merlangus and cod Gadus morhua compared to standard and wider diameters. Across three codend mesh sizes, the length at which fish had a 50% of escape increased with decreasing codend diameter for haddock (small: 30–40 cm, standard: 25–35 cm, wide: 20–30 cm), whiting (small: 36–46 cm, standard: 29–40 cm, wide: 23–33 cm) and cod (small: 35–43 cm, standard: 29–37 cm, wide: 23–32 cm). Data were collected in August 1991 from 40 trawl deployments using nine test codends by two commercial Scottish demersal pair trawlers. Combinations of three nominal codend diameters (2.2 m, 3.2 m and 4.2 m) and three nominal codend mesh sizes (90 mm, 100 mm and 110 mm) were tested with at least four hauls/combination. Codend covers of smaller mesh size retained escaping fish catch.Study and other actions tested
A replicated, paired, controlled study in 1995 of an area of seabed in the Tasman Sea off New South Wales, Australia (Broadhurst & Kennelly 1996) found that decreasing the codend circumference in prawn trawl nets reduced the discarded catch of commercial and non-commercial small fish in three of five cases compared to a larger standard circumference codend. In codends of standard diamond mesh, average catch numbers of discarded fish were lower in small circumference codends compared to larger for two of two species: stout whiting Sillago robusta (small: 600 fish/haul, large: 1,800 fish/haul) and long-spined flathead Platycephalus longispinis (small: 500 fish/haul, large: 1,800 fish/haul). In diamond mesh codends fitted with square mesh panels, average number of discarded fish was lower with the smaller circumference for one of three species, stout whiting (small: 300 fish/haul, large: 800 fish/haul), and similar for long-spined flathead (small: 400 fish/haul, large: 700 fish/haul) and red spot whiting (small: 70 fish/haul, large: 60 fish/haul). In addition, commercial target king prawn Penaeus plebejus average catch weights were similar between codend circumferences (both 5–7 kg/haul). Data were collected in March 1995 on commercial prawn-trawl grounds using a commercial trawler fishing with a triple rigged net. Four codends were tested in pairs using the outer two nets (number of deployments not reported): 100 mesh and 200 mesh (standard) circumference, each set with without square mesh panels (see original paper for gear specifications). After each tow codend catches were sorted separately and the numbers and weights of commercial and non-commercial retained and discarded species recorded.Study and other actions tested
A replicated, randomized, paired, controlled study in 2002 in an area of seabed in an estuary leading to the Tasman Sea off New South Wales, Australia (Broadhurst et al. 2004) found that prawn trawl codends of smaller circumference (two types) reduced the unwanted catch of six fish species in only three of 12 cases compared to a larger circumference. In diamond mesh codends, average number of unwanted fish catch was lower with a smaller circumference than a larger for three of six species (small: 18–25 fish/haul, large: 45–75 fish/haul) and similar for three (small: 2–6 fish/haul, large: 2–8 fish/haul). In square mesh codends, there were no differences in average catch numbers of the six species between a smaller circumference of tapered design and a larger, non-tapered, codend (small, tapered: 0–9 fish/haul, large, non-tapered: 0–8 fish/haul). See original paper for data by individual species. In addition, the square mesh codends caught lower average numbers of unwanted fish for four of the six species. Data were collected on a commercial prawn Penaeidae trawler using a twin-net configuration on commercial grounds in Lake Woolooweyah in the Clarence Estuary in March 2002. Combinations of four experimental trawl nets codends (see original paper for gear specifications), with size-sorting escape grids (Nørdmore type, 20 mm bar spacing) were tested in pairs, one either side of the vessel: two 40 mm diamond mesh codends of either 100 or 200 meshes (standard) circumference, and two square mesh codends (one tapered from 82 to 54 meshes, and one 110 mesh non-tapered). All catch in each codend was sorted and counted by species.Study and other actions tested
A replicated, paired, controlled study in 2002 of an area of fished seabed in the northern North Sea off Norway (O'Neill et al. 2008) found that decreasing the codend circumference of a bottom fish trawl improved the size-selectivity of haddock Melanogrammus aeglefinus, and for saithe Pollachius virens in one of two cases, compared to a standard larger circumference. For haddock, the length at which fish had a 50% chance of escape was greater for both small (60 meshes) and intermediate (80 meshes) circumferences compared to the larger standard 100 meshes (small: 37–45 cm, intermediate: 33–45 cm, standard: 27–40 cm). For saithe, the length at which fish had a 50% escape likelihood was higher (41–65 cm) in the small 60 mesh circumference codend compared to the standard 100 mesh codend (40–77 cm), but the 80 mesh codend did not differ from either (40–49 cm). Data were collected from 22 paired deployments on a twin-rig trawler in August–September 2002. Diamond mesh (120 mm nominal size) codends of three circumferences (60 meshes, 80 meshes and standard 100 meshes) were tested each deployed alongside a small mesh (50 mm) trawl to measure size-selectivity. After each haul haddock and saithe number and length from each codend were recorded, and randomly sub-sampled when catches were high.Study and other actions tested
A replicated, paired, controlled study in 2007 of an area of seabed in an estuary leading to the Tasman Sea off New South Wales, Australia (Broadhurst & Millar 2009) found that using square mesh codends of smaller circumferences in prawn trawls resulted in lower catches of one of four non-target fish compared to larger circumferences. Average catch numbers of unwanted pink-breasted siphonfish Siphamia roseigaster decreased with decreasing codend circumference (small: 8 fish/haul, intermediate: 20 fish/haul, large: 25 fish/haul). However, for three other unwanted fish species (silver biddy Gerres subfasciatus, southern herring Herklotsichthys castelnaui and Ramsey’s perchlet Ambassis marianus – see original paper for species individual data) there were no statistical differences in average catch rates between codend circumferences (small: 7–25 fish/haul, intermediate: 4–18 fish/haul, large: 5–9 fish/haul). Average catch weights of all commercial target prawns Penaeidae were similar across codend designs (all 5 kg/haul). Experimental fishing was done in October and November 2007 on commercial prawn trawl grounds in Lake Wooloweyah on the Clarence River estuary using a local twin-rigged trawler. Three square mesh codends (27 mm nominal mesh) of varying circumference (standard 90 meshes, 150 meshes and 200 meshes) were deployed simultaneously with a small mesh (9 mm) codend, 18 deployments for each paired comparison. After each tow, codend catches were separated by species and numbers or weights recorded.Study and other actions tested
A replicated, paired, controlled study in 2005 of an area of seabed in the Tasman Sea, Australia (Graham et al. 2009) found that a smaller codend circumference in fish trawl nets did not reduce the discarded catch of five of five non-target fish species, or total discarded catch (fish and invertebrates). For five of five fish species (see original paper for species individual data), average catch numbers discarded were similar between small and large codend circumferences (small: 2–284 fish/haul, large: 54–502 fish/haul). The numbers (small: 753 fish/haul, large: 1,224–1,257 fish/haul) and weight (small: 104 kg/haul, large: 112–128 kg/haul) of total discarded catch (all fish and invertebrates) were also similar between codend circumferences. In addition, the number (small: 305 fish/haul, large: 685–1,468 fish/haul), but not weight (small: 94 kg/haul, large: 156–158 kg/haul) of commercial retained catch (fish and invertebrates) was lower in the smaller codend compared to the larger. Between March and November 2005, gears trials were done in a south-eastern Australian trawl fishery targeting school whiting Sillago flindersi. Three test codends, one 100 mesh and two 200 mesh circumferences, were tested in pairs during alternate deployments (16 paired deployments each) with a small mesh (40 mm) codend of 450 mesh circumference (see original paper for full specifications). Catches were counted and weighed by species and the lengths of the most abundant fish measured. Commercial species were divided into retained and discarded categories.Study and other actions tested
A replicated, controlled study in 2005 in an area of mud seabed in the Adriatic Sea off Italy (Sala & Lucchetti 2010, same experimental set-up as Sala & Lucchetti 2011) found that using a diamond mesh bottom trawl codend of standard circumference had reduced size-selectivity for one of three non-target fish species, and did not increase escape, compared to a larger circumference diamond mesh codend, however a change in mesh configuration (to square) did improve fish size-selectivity, irrespective of codend circumference. For diamond mesh codends, the predicted length at which fish had a 50% chance of escape was similar for a standard mesh circumference compared to a larger circumference for blue whiting Micromesistius poutassou (standard: 10.6 cm, large: 10.6 cm) and poor cod Trisopterus minutus (standard: 6.3 cm, large: 6.3 cm), but lower for European hake Merluccius merluccius (standard: 8.7 cm, large: 10.1 cm). However, escape of all three species was not affected by diamond mesh codend circumference (data reported as statistical results). A codend of square mesh with similar or larger circumference as the diamond mesh codends, resulted in greater 50% escape lengths for all three species compared to the diamond mesh codends (blue whiting: 13.6 cm, poor cod: 9.2 cm, hake: 12.6 cm). Gear trials were done on a research vessel in May and September 2005 in the Western Pomo pit area (a Norway lobster Nephrops norvegicus fishing ground, 210 m depth). Two diamond mesh codends (standard 280 and larger 326 mesh circumference) and one square mesh codend (70 meshes) were tested during 19, 13 and 20 deployments, respectively. All codends had a nominal 40 mm mesh size (see original paper for full gear specifications). Covers over each codend collected catch escaping from the meshes. All catch was weighed by species, and fish lengths recorded.Study and other actions tested
A replicated, controlled study in 2005 in two areas of seabed in the Adriatic Sea off Italy (Sala & Lucchetti 2011, same experimental set-up as Sala & Lucchetti 2010) found that diamond mesh bottom trawls with a smaller circumference codend had improved size-selectivity for European hake Merluccius merluccius and red mullet Mullus barbatus compared to a larger codend circumference, across two codend mesh sizes. For 56 mm mesh codends, the length at which fish had a 50% chance of escape was greater with a small (240 mesh) circumference codend than a larger (280 mesh) circumference for hake (small: 16.3 cm, large: 12.0 cm) and mullet (small: 12.8 cm, large: 10 cm). Similarly, for a 48 mm codend mesh size, the 50% escape length was greater with a small (280 mesh) circumference compared to a large (326 mesh) circumference (hake, small: 11.5 cm, large: 10.4 cm; mullet, small: 10.7 cm, large: 7.5 cm). In addition, an increase in mesh size alone resulted in an increase in size selectivity for both species. Gear trials (68 trawl deployments of 1 h duration) were carried out by research vessel in May and September 2005 on two fishing grounds (one 15–20 m depth, one 180–200 m depth) in the Central Adriatic. Four codends were tested combining small (240 and 280 meshes) or large (280 and 326 meshes) circumferences (or codend rigging ratio) with large (56 mm nominal) or small (48 mm nominal) mesh sizes (see original paper for full gear specifications). Small mesh (20 mm) covers over each codend collected catch escaping through the meshes. All catch was weighed by species, and fish lengths recorded.Study and other actions tested
A replicated, controlled study in 2009 on bottom fishing grounds in the Baltic Sea off Bornholm, Denmark (Wienbeck et al. 2011) found that reducing the circumference of a diamond mesh trawl codend improved the size-selectivity of Atlantic cod Gadus morhua compared to a larger circumference, and the effect was greatest in combination with mesh orientation turned by 90°. In two of two cases (standard and turned diamond mesh), the estimated length at which cod had a 50% chance of escape was greatest with a smaller circumference codend compared to a larger codend, and was higher in codends that also had meshes turned by 90° (standard, small: 39 cm; standard, large: 34 cm; turned, small: 42 cm, turned, large: 39 cm). Gear trials were done on a research vessel in October 2009 using four codend designs: two with small circumferences (44 and 46 meshes) and two with larger circumferences (91 and 92 meshes), each with or without the netting direction turned by 90° (seven deployments each). Each codend had an average mesh size of 114 mm and was fished one at a time from the same trawl body (see original paper for full gear specifications). A smaller mesh (80 mm) cover attached over each codend collected fish escaping through the meshes. Fish in the codend and cover catches were weighed by species, and cod lengths recorded.Study and other actions tested
A replicated study in 2008–2010 on fishing grounds in the Western Baltic Sea, off Germany (Herrmann et al. 2015) found that bottom trawls with a smaller codend circumference had improved size-selectivity for Atlantic cod Gadus morhua and similar size-selectivity for European plaice Pleuronectes platessa and European flounder Platichthys flesus, compared to larger circumference codends. Irrespective of twine number (single or double) and mesh orientation (standard or turned diamond), the predicted length at which cod had a 50% chance of escape was greater for smaller (44 mesh) circumference codends compared to larger (88 mesh) circumference codends (small: 48 cm, large: 42 cm). There were no differences in the 50% escape lengths between codend mesh circumferences for plaice (small: 25 cm, large: 25 cm) and flounder (small: 24 cm, large: 24 cm), and these were not dependent on netting direction (twine number not tested). Gear trials were done in September 2008 (32 deployments) and March 2010 (18 deployments) by a research vessel in the Arkona Basin. Five diamond mesh codends were tested, constructed using ultra strong polyethylene twine (“Dyneema”): three of 44 meshes circumference and two of 88 meshes circumference, and with either single or double twine, and standard diamond mesh or mesh turned by 90°. A smaller mesh (80 mm) cover attached over each codend collected fish escaping through the meshes. Fish in the codends and covers were weighed by species, and lengths recorded.Study and other actions tested
A review in 2016 of 40 experimental fishing trials in the northeast Atlantic Ocean (Fryer et al. 2016) found that overall, decreasing the circumference of trawl codends resulted in an increase in the size-selectivity of haddock Melanogrammus aeglefinus. The length at which haddock had a 50% chance of escape from the codend was greater by 1.3 cm for every reduction of 10 meshes in circumference around the codend. In addition, the 50% escape length increased by 3.4 cm for every 10 mm increase in codend mesh size and by 1.4 cm for every 1 mm decrease in twine thickness. The study was a meta-analysis of the effects of various changes to codend characteristics, including the number of meshes around the circumference, on the selectivity of haddock in the northeast Atlantic. Data were from 40 trials, covering the years 1991–2009, taken from published studies and other data collected by a Scottish fisheries research organisation (Marine Scotland Science).Study and other actions tested
A replicated, controlled study in 2014–2015 of three fished areas of seabed in the North Sea, Skagerrak and Baltic Sea, Northern Europe (Mortensen et al. 2017) found that choice of modification to bottom trawl gears made by fishers as part of an unrestricted trial, including decreasing codend circumference, increasing mesh size, adding square mesh escape panels and using a coverless trawl, reduced the total discarded catch (fish and invertebrate species) in one of two areas, but not overall, compared to using the regulatory gears. In two of three areas, the average total discarded catch of seven commercial target species (six fish and one invertebrate – see paper for species individual data) was lower for modified trawl gears (Skagerrak: 18 kg/tow, Baltic Sea: 256 kg/tow) compared to the regulated trawls (Skagerrak: 25 kg/tow, Baltic Sea: 328 kg/tow). In the other area (North Sea) average total discard was greater for modified (18 kg/tow) than regulated (13 kg/tow) trawl gears. For all three areas combined, there was no statistical difference in average total discard between modified and regulated trawl gears (modified: 52 kg/tow, regulated: 65 kg/tow). Gear trials were undertaken by twelve Danish bottom trawlers (three to six vessels/area) between December 2015 and July 2015. The fishers were challenged to reduce overall discarding of seven commercial species by modifying or developing new gears and/or changing fishing practice (see original paper for details of all modifications/gears used). Vessels switched between modified and conventional regulatory gears between fishing trips. Data were collected from 421 fishing trips and 2,642 haul deployments.Study and other actions tested