Species-selective Nephrops trawling: Swedish grid experiments

Published source details Valentinsson D. & Ulmestrand M. (2008) Species-selective Nephrops trawling: Swedish grid experiments. Fisheries Research, 90, 109-117.
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This study is summarised as evidence for the following.

	Action	Category
	Fit a size-sorting escape grid (rigid or flexible) to trawl nets and use a square mesh instead of a diamond mesh codend Action Link
	Use a different design or configuration of size-sorting escape grid/system in trawl fishing gear (bottom and mid-water) Action Link

Fit a size-sorting escape grid (rigid or flexible) to trawl nets and use a square mesh instead of a diamond mesh codend

A replicated, paired, controlled study in 2002–2006 of two seabed areas in the Skagerrak and Kattegat off Sweden/Denmark (Valentinsson & Ulmestrand 2008) found that prawn trawl nets fitted with size-sorting escape grids (two types) and a square mesh instead of a diamond mesh codend, reduced the discarded catches of four of five fish species compared to a standard trawl net with no grid and a diamond mesh codend. Average discarded catch rates of cod Gadus morhua, whiting Merlangius merlangus, haddock Melanogrammus aeglefinus and plaice Pleuronectes platessa were lower in trawl nets with a grid and square mesh codend for two of two grids (all species, with: 0.4–18.8 kg/tow, without: 2.5–67.4 kg/tow), and hake Merluccius merluccius was lower for the flatfish grid (with: 3.3 kg/tow, without: 9.3 kg/tow), and similar for the Nordmøre grid (with: 0.0 kg/tow, without: 0.3 kg/tow). In addition, the weight of undersized and marketable target catch of Nephrops norvegicus was lower for one of two grids (flatfish grid) and similar for the other. Data were collected from two trials on commercial Nephrops fishing grounds in Skagerrak/Kattegat in November 2002 and June 2006. A total of 17 paired deployments were made by two twin-rig vessels towing two nets simultaneously: one net fitted with either a 35 mm rigid grid (Nordmøre) or a ‘flatfish’ grid (horizontal bars) and a 70 mm square mesh codend, and one net with either a 70 or 90 mm diamond mesh codend (see paper for gear specifications). All catches were sorted and weighed.

(Summarised by: Rosslyn McIntyre)
Use a different design or configuration of size-sorting escape grid/system in trawl fishing gear (bottom and mid-water)

A replicated, paired, controlled study in 2005 on bottom fishing grounds in the Skagerrak and Kattegat, Sweden (Valentinsson & Ulmestrand 2008) found that changing the design of size-sorting escape grids (two types) did not typically reduce the overall catches of discarded fish of five commercial species compared to a standard grid design. For one of two grid designs (flexible), average catch weights of discarded whiting Merlangius merlangus and haddock Melanogrammus aeglefinus were reduced compared to the standard (rigid) grid (flexible: 0.1–0.6 kg, standard: 0.6–1.6 kg), but cod Gadus morhua and hake Merluccius merluccius discards were similar between grids (flexible: 1.7–3.1 kg, standard: 1.6–3.8 kg) and plaice Pleuronectes platessa discards were higher in the flexible grid design (flexible: 2.3 kg, standard: 1.7 kg). For the other grid design (15 cm gap), no differences were found in average discarded weights of whiting, haddock and hake compared to the standard grid (gap: 0.0–0.2 kg, standard: 0.0–0.1 kg), but cod and plaice discards were higher (gap: 1.8–3.0 kg, standard: 1.0–1.3 kg). In addition, fewer discards of the target Norwegian lobster Nephrops norvegicus were caught with the flexible grid compared to the standard (23.3 vs 30.2 kg) and similar amounts with the gap grid (both 0.6 kg). Data were collected in June and November/December 2005, from 24 deployments of a twin trawl towing experimental nets and standard nets simultaneously at five locations in the Skagerrak and Kattegat. Experimental nets were fitted with one of two grid types: a flexible sorting grid (plastic, 35 mm bar spacing, ten hauls), and a rigid grid with a 15 cm open gap at the bottom (35 mm bar spacing, 14 hauls). The standard grids were aluminium (35 mm bar spacing). All nets had 70 mm square mesh codends.

(Summarised by: Rosslyn McIntyre)