Size selection in codends made of thin-twined Dyneema netting compared to standard codends: a case study with cod, plaice and flounder

Published source details Herrmann B., Wienbeck H., Stepputtis D., Krag L.A., Feekings J. & Moderhak W. (2015) Size selection in codends made of thin-twined Dyneema netting compared to standard codends: a case study with cod, plaice and flounder. Fisheries Research, 167, 82-91.
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This study is summarised as evidence for the following.

	Action	Category
	Use a different twine type in a trawl net Action Link
	Rotate the orientation of diamond mesh in a trawl net Action Link
	Decrease the circumference or diameter of the codend of a trawl net Action Link

Use a different twine type in a trawl net

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.

(Summarised by: Rosslyn McIntyre)
Rotate the orientation of diamond mesh in a trawl net

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.

(Summarised by: Rosslyn McIntyre)
Decrease the circumference or diameter of the codend of a trawl net

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.

(Summarised by: Rosslyn McIntyre)