Effect of netting direction and number of meshes around on size selection in the codend for Baltic cod (Gadus morhua)

Published source details Wienbeck H., Herrmann B., Moderhak W. & Stepputtis D. (2011) Effect of netting direction and number of meshes around on size selection in the codend for Baltic cod (Gadus morhua). Fisheries Research, 109, 80-88.
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This study is summarised as evidence for the following.

	Action	Category
	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

Rotate the orientation of diamond mesh in a trawl net

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.

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

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.

(Summarised by: Rosslyn McIntyre)