Study

The influence of twine thickness, twine number and netting orientation on codend selectivity

  • Published source details Herrmann B., Wienbeck H., Moderhak W., Stepputtis D. & Krag L.A. (2013) The influence of twine thickness, twine number and netting orientation on codend selectivity. Fisheries Research, 145, 22-36.

Actions

This study is summarised as evidence for the following.

Action Category

Rotate the orientation of diamond mesh in a trawl net

Action Link
Marine Fish Conservation

Use a different twine type in a trawl net

Action Link
Marine Fish Conservation
  1. Rotate the orientation of diamond mesh in a trawl net

    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.

    (Summarised by: Chris Barrett)

  2. Use a different twine type in a trawl net

    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.

    (Summarised by: Chris Barrett)

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