Use a different twine type in a trawl net
Overall effectiveness category Awaiting assessment
Number of studies: 5
Background information and definitions
Although many commercial fishers must abide to set rules around the size of mesh they can use in their trawls, altering the thickness of mesh twine can impact the effectiveness of the mesh size. A thin twine allows the mesh to be more flexible which might help more fish smaller than the mesh opening to escape when forcing themselves against the trawl (Lowry & Robertson 1996) but the thinner twine might be weaker against heavy catch loads. A thick twine creates a more rigid, strong mesh (Lowry & Robertson 1996). It is believed that this thicker twine could present a greater visual barrier to fish which might discourage them from escape attempts.
Evidence for similar interventions is summarized under ‘Fishing gear modification - Modify the design or configuration of trawl gear (mixed measures)’. See also, ‘Use a larger mesh size’.
Lowry N. & Robertson J.H.B. (1996) The effect of twine thickness on cod-end selectivity of trawls for haddock in the North Sea. Fisheries Research, 26, 353–363.
Supporting evidence from individual studies
A replicated, controlled study in 1993 in a seabed area in the North Sea, UK (Lowry & Robertson 1996) found that using a thinner twine thickness in a fish trawl codend improved the size-selectivity of haddock Melanogrammus aeglefinus compared to thicker twine. The length at which haddock had a 50% chance of escaping was lower with thinner twine thickness (thin: 25 cm, thick: 24 cm). The authors noted the difference would be expected to be larger if the two codends had been of the same mesh size. Data were collected in the Moray Firth off Fraserburgh in June 1993 from 13 experimental trawl deployments by a commercial trawler in depths of 91–100 m (2 h duration, 2.5 kn speed). Separate deployments of two codends with different twine thicknesses were tested, 3.5 mm (7 hauls) and 5.2 mm (6 hauls). Measured mesh size was 95 mm for the thin twine and 100 mm for the thick twine. A small mesh cover attached over each codend collected fish escaping through the meshes. Codend and cover catches were recorded, and haddock lengths measured.Study and other actions tested
A replicated, controlled study in 2004 of one shallow inshore and one deeper offshore seabed area, in the central Adriatic Sea, off Italy (Sala et al. 2007) found that using a thinner diameter of twine in the codend of a trawl net improved the size-selectivity of five of five fish species. Across both areas, the length at which fish had a 50% chance of escaping was highest using thin twine compared to thick twine for whiting Merlangius merlangus (thin: 11 cm, thick: 8 cm), European hake Merluccius merluccius (thin: 9 cm, thick: 8 cm), red mullet Mullus barbatus (thin: 9 cm, thick: 7 cm), common pandora Pagellus erythrinus (thin: 9 cm, thick: 7 cm) and poor cod Trisopterus minutus capelanus (thin: 10 cm, thick: 7 cm). Data were collected by research vessel from trawl deployments on two different fishing grounds. In the offshore area (35 nm from the coast, 70 m depth), 34 trawl deployments were done between April 2004–May 2004. In the inshore area (5 nm off the coast near Ancona, 20 m depth), 20 deployments were made in September 2004. Two codends of different twine thicknesses were tested: 2.4 mm diameter (thin) and 2.9 mm diameter (thick – industry standard) and alternated daily on the same trawl net body. Small mesh covers attached over each codend sampled the fish escaping through the meshes.Study and other actions tested
A replicated study in 2011 of a fished area of seabed in the western Baltic Sea, off Denmark/Germany (Herrmann et al. 2013) found that size-selectivity of Atlantic cod Gadus morhua and plaice Pleuronectes platessa increased with decreasing twine thickness of a trawl net codend, and was also influenced by twine number (single or double) and mesh orientation (0° or turned by 90°). The estimated length at which cod had a 50% chance of escaping increased with decreasing thickness of single twine mesh (thinnest: 42 cm, thickest: 31–39 cm), irrespective of mesh orientation (however the difference between mesh orientations was greater at thicker twine thicknesses – see paper for data). The same increase in size-selectivity with decreasing twine thickness was found for plaice (thinnest: 24–25 cm, thickest: 24–25 cm). In addition, for a given twine thickness, turning mesh orientation by 90° increased selectivity of cod but decreased selectivity of plaice; and changing from single to double twine reduced selectivity of cod (see paper for data). Data were collected in March–April 2011 from 43 alternate deployments of 12 codends with different combinations of twine thickness (3 mm to 8 mm), twine number (single or double) and mesh orientation (standard diamond or turned by 90°). Haul duration was 90–180 minutes, at depths of 32–49 m. A small mesh cover attached over each codend collected fish escaping through the meshes. Cod and plaice in the codends and covers were sampled, and their lengths measured.Study and other actions tested
A replicated, controlled study in 2008 in an area of seabed in the western Baltic Sea off Denmark/Germany (Herrmann et al. 2015) found that trawl codends made of a flexible thin-twined netting improved the size-selectivity of cod Gadus morhua, plaice Pleuronectes platessa and flounder Platichthys flesus, compared to a conventional polyethylene twine, and for the flexible netting, there were no effects of changes to the number of twines (single or double) and netting orientation (0° or 90°), but cod selectivity decreased with increased codend circumference. For all three species, the length at which fish had a 50% chance of escaping was greater in codends made of 2.5 mm flexible thin twine compared to standard 5 mm single twine polyethylene codends (data reported graphically). For different designs of the flexible thin twine, there was no effect of twine number (cod tested only) or mesh orientation, but reducing the number of meshes in the codend circumference increased the size selection of cod but did not affect the size selection of plaice and flounder (data presented graphically – see paper). Data were collected on two surveys in September 2008 and March 2010. A total of 70 trawl deployments were carried out using different codend types, alternately fitted to the same trawl net. Five codends were constructed from a thin but ultrastrong twine (“Dyneema”) and differed in number of twines, netting orientation, and mesh circumference. Two were standard polyethylene codends (thicker and less flexible twine). Small mesh covers attached over each codend collected fish escaping through the meshes.Study and other actions tested
A replicated, controlled study in 2003 of bottom fishing grounds in the North Sea, UK (O'Neill et al. 2016) found that changing the thickness of twine (three diameters) in a trawl net codend did not improve the size-selectivity of unwanted haddock Melanogrammus aeglefinus or plaice Pleuronectes platessa. The lengths at which fish had a 50% chance of escaping capture were similar for all three twine thicknesses for both haddock (thin: 35–36 cm, medium: 35–36 cm, thick: 31–35 cm) and plaice (thin: 28.6–29.4 cm, medium: 28.6–30 cm, thick: 28.4–29.8 cm). Data were collected from 30 trawl deployments on a commercial fishing vessel east of Scotland in October 2003. Three twine sizes were tested (thin: 4.1 mm, medium: 4.6 mm, thick/conventional: 5.1 mm) during separate hauls and using the covered codend method to collect fish escaping through the meshes. Haul duration was 40–211 min. Haddock and plaice lengths were measured and if catches were large, subsampled.Study and other actions tested