Use of selective devices in trawls to support recovery of the Kattegat cod stock: a review of experiments and experience

Published source details Madsen N. & Valentinsson D. (2010) Use of selective devices in trawls to support recovery of the Kattegat cod stock: a review of experiments and experience. ICES Journal of Marine Science, 67, 2042-2050.
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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
	Fit mesh escape panels/windows to a trawl net Action Link

Rotate the orientation of diamond mesh in a trawl net

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).

(Summarised by: Chris Barrett)
Fit mesh escape panels/windows to a trawl net

A review in 2005–2010 of ten trawl gear studies in bottom fishing grounds in the Kattegat and the Skagerrak, Northern Europe (Madsen & Valentinsson 2010) reported that the effectiveness of selective devices in prawn trawl nets in reducing the amount of unwanted small Atlantic cod Gadus morhua varied with the device used, and that square mesh escape windows had limited or no significant effect on unwanted cod catches but did improve the overall size selectivity of nets compared to conventional diamond mesh codends. Square mesh escape windows reduced the number of undersized (<40 cm) cod caught in one of three studies (by 59%) and no effect was reported for the other two. Increasing the mesh size in the window was reported to have no significant effect (one study) but the location of the window did (two studies). Overall, trawl nets with square mesh escape panels/windows had higher selection lengths (the length at which the fish have a 50% chance of escape/capture) than diamond mesh codends without (with: 27–30 cm, without: 15–26 cm – not statistically tested). The review summarised the effects of different codend selective devices on catches of cod from studies of trawl nets used to target Norway lobster Nephrops norvegicus in the Kattegat and Skagerrak. The selective devices were mesh escape windows, increased codend mesh size, square mesh codends, turned mesh codends and a sorting box (see paper for specifications and data).

(Summarised by: Chris Barrett)