Rotate the orientation of diamond mesh in a trawl net
Overall effectiveness category Awaiting assessment
Number of studies: 6
Background information and definitions
Trawl fisheries are a common type of fishing practice around the world. The codend of a trawl net is designed to retain fish and often leads to large numbers of unwanted fish being caught. This can be reduced by modifying the codend design to increase the size selectivity of the gear. One way this might be done is to change the orientation of the meshes of the net. Standard trawl codends are constructed from diamond shaped meshes. During deployment the diamond meshes close under tension and the size of the opening through which fish can escape is drastically reduced. Rotating or turning the orientation of diamond mesh involves the knots of the meshes being tied in such a way that the mesh is turned through 90 degrees to the water flow (often referred to as T90 mesh). This results in the meshes being held more open under tension and maintains a larger gap through which fish can escape. This may in turn help to reduce the catches of unwanted smaller fish.
Note: Rotated, or T90, diamond mesh is not to be confused with nets made of square mesh, which consist of diamond mesh turned through 45 degrees to the water flow to create a square shape ensuring the meshes remain almost fully open (see ‘Fishing gear modification – ‘Use a square mesh instead of a diamond mesh codend in a trawl net’.).
Evidence for a similar intervention applied to trawl nets only is summarized under ‘Fishing gear modification – ‘Use a square mesh instead of a diamond mesh codend in a trawl net’. For interventions describing the effects of different mesh types in fishing gear more generally, but including trawl nets, see ‘Fishing gear modification - Use a larger mesh size’.
Supporting evidence from individual studies
A review in 2010 of 10 studies of various trawl gear modifications in bottom fishing grounds (one of which assessed rotating diamond mesh) in the Kattegat and Skagerrak, northern Europe (Madsen & Valentinsson 2010) reported that rotating the diamond mesh in a prawn trawl net codend by 90° improved the size selectivity of cod Gadus morhua compared to a standard diamond mesh codend of the same mesh size. Data were not statistically tested. The length at which cod had a 50% chance of escaping was 32.2 cm in a turned diamond mesh codend and 23.7 cm in a standard diamond mesh codend (both 99 mm mesh size). The review summarised data from 10 studies between 2005–2010 on the effects on cod catch and size selectivity of various modifications to trawl nets targeting Norway lobster Nephrops norvegicus. One of the 10 studies identified provided size selectivity data for diamond mesh codends with and without rotated mesh from 16 trawl deployments for each codend type (original study written in Danish).Study and other actions tested
A replicated, controlled study in 2009 of bottom fishing grounds in the Baltic Sea, Denmark (Wienbeck et al. 2011) found that turning the orientation of diamond mesh netting in the codend of a trawl net by 90° improved size selectivity for cod Gadus morhua compared to the standard mesh orientation. In two of two trials, the length at which cod had a 50% chance of escaping was greater in turned diamond mesh trawl nets (39–42 cm) than standard trawl nets (34–39 cm). Data were collected by research vessel surveys on fishing grounds south of the island of Bornholm in October 2009. Two trawl codends of netting made with the diamond meshes turned 90° relative to the standard (one with 46 and one with 91 meshes circumference) were tested against two standard diamond mesh nets (44 and 92 meshes circumference). Eight deployments of the 90°/46 mesh circumference and seven deployments of each of the 90°/91 mesh circumference and both the standard trawl nets were made.Study and other actions tested
A replicated, controlled study in 2011 of an area of seabed in the western Baltic Sea, Denmark (Herrmann et al. 2013) found that rotating the diamond mesh of trawl net codends by 90° increased the size selectivity for cod Gadus morhua, but not plaice Pleuronectes platessa, compared to standard diamond mesh, and was influenced by twine number. The length at which cod had a 50% chance of escape was higher with turned diamond mesh than standard diamond mesh at any given twine thickness (2–8 mm), and for both of the turned mesh orientations decreased with increasing twine number (turned, single: 39–42 cm, standard: 31–42 cm; turned, double: 29–41 cm, standard: 21–41 cm). For plaice, 50% escape likelihood was lower with turned meshes at all twine thicknesses and number (turned, single: 20–24 cm, standard: 24–25 cm; turned, double: 17–24 cm, standard: 24–25 cm). Data were collected from 43 experimental trawl deployments (90–180 min, 32–49 m depth) in the Arkona Basin in March–April 2011. Twelve codends were tested: six with diamond mesh turned by 90°, and six with standard mesh and either single or double twine at one of four twine thicknesses (3, 4, 6 or 8 mm)(see original paper for specifications). Each of the 12 codends was fished alternately, one at a time, from the same trawl. Covers attached over each codend collected fish escaping through the meshes. The lengths of cod and plaice in the codends and covers were measured to the nearest cm.Study and other actions tested
A replicated, controlled study of an area of seabed in a coastal bay in the Aegean Sea, off Turkey (Tokaç et al. 2014) found that rotating the direction of diamond mesh in a trawl codend by 90° increased the size selectivity of red mullet Mullus barbatus and common pandora Pagellus erythrinus, but not annular sea bream Diplodus annularis, compared to the standard diamond mesh direction. The length at which red mullet and common pandora had a 50% chance of escape was larger in codends with turned diamond mesh of three mesh sizes compared to standard diamond mesh (mullet, turned: 12–18 cm, standard: 9–15 cm; pandora, turned: 10–13 cm, standard: 9–11 cm), and there was no difference for annular sea bream at all mesh sizes (9–12 cm). Trials were done by research vessel in Izmir Bay during several periods between December to May. Three bottom trawl codends with diamond mesh turned by 90° were tested against two codends of standard diamond mesh orientation. A total of 61 valid deployments of 30 min were made: 13–17 hauls of turned diamond mesh codends of each of 40, 44 and 50 mm mesh size; and 10–11 standard diamond mesh codends, each of 40 mm and 50 mm mesh size. Al codends were attached to the same trawl net. Codend catches were sorted by species and fish length recorded. The year the study took place was not reported.Study and other actions tested
A replicated, controlled study in 2008–2010 on fishing grounds in the western Baltic Sea, Denmark (Herrmann et al. 2015) found that turning the mesh orientation in diamond mesh trawl codends by 90 degrees did not improve the size selectivity for cod Gadus morhua, plaice Pleuronectes platessa and flounder Platichthys flesus, compared to standard diamond mesh orientation, but was influenced by twine type and codend circumference. Data were reported as statistical model results. For all three species, there was no effect of codend mesh orientation (diamond turned by 90° or standard diamond) on size selectivity between all codends tested. However, size selectivity was improved for all three species by twine type (higher in codends made from the flexible compared to the standard polyethylene twine) and by reducing the codend circumference for cod, but not for plaice and flounder. Five codends made from a flexible, strong twine (Dyneema) of a similar mesh size (110 mm) and twine thickness (2.5 mm), but different mesh orientation (turned 90° or standard), codend circumference (44 or 48 meshes), and number of twines (single or double), were tested during two experimental fishing trials in the Arkona Sea in September 2008 and March 2010. Two further turned mesh codends made from standard 5 mm single twine netting were also tested. Selectivity data for cod >33 cm only was collected from 36 deployments during the first trial, and for cod (>33 cm), plaice and flounder from up to 24 deployments in the second trial.Study and other actions tested
A replicated, paired, controlled study in 2006 of two fished seabed sites in the southern North Sea off Belgium and England, UK (Bayse et al. 2016) found that turning the diamond shaped mesh in the codends of beam trawls by 90° increased the size selectivity of two round-bodied fish species, but not of three flatfish species, compared to standard beam trawl codends. The lengths at which fish had a 50% chance of escaping were greater in turned diamond mesh codends for two round-bodied fish: whiting Merlangius merlangus (turned: 26 cm, standard: 12 cm) and pouting Trisopterus luscus (turned: 19 cm, standard: 11 cm); and for flatfish, they were similar for dab Limanda limanda (turned: 14 cm, standard: 14 cm) and plaice Pleuronectes platessa (turned: 13 cm, standard: 14 cm) and lower for sole Solea solea (turned: 19 cm, standard: 20 cm). By size class, all lengths of whiting and pouting larger than 10 cm had higher size selectivity in turned diamond mesh codends, while dab, plaice and sole larger than 16, 15 and 19 cm, respectively, had lower selectivity (data reported as selection curves). Trials were done by research vessel in January 2006 on fishing grounds along the Belgian coast and in the outer Thames Estuary off England. Data was collected from 15 deployments of two 4 m beam trawls towed side by side, each with a different codend: one with the netting orientation turned by 90°, and the other a traditional diamond mesh orientation (both 80 mm mesh size; see original paper for specifications). Covers attached over each codend collected escaped fish. Lengths of fish captured in the codends and covers were measured (sub-sampled when numbers were very high).Study and other actions tested