Improving gear selectivity of whiting (Merlangius merlangus) on board French demersal trawlers in the English Channel and North Sea

Published source details Vogel C., Kopp D., Morandeau F., Morfin M. & Méhault S. (2017) Improving gear selectivity of whiting (Merlangius merlangus) on board French demersal trawlers in the English Channel and North Sea. Fisheries Research, 193, 207-216.
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This study is summarised as evidence for the following.

	Action	Category
	Fit mesh escape panels/windows and a size-sorting grid (rigid or flexible) to a trawl net Action Link
	Fit a size-sorting escape grid (rigid or flexible) to a fish trawl net Action Link
	Fit mesh escape panels/windows to a trawl net Action Link
	Use a larger mesh size Action Link

Fit mesh escape panels/windows and a size-sorting grid (rigid or flexible) to a trawl net

A replicated, paired, controlled study in 2010–2013 of a seabed area in the English Channel, UK (Vogel et al. 2017) found that bottom trawl nets fitted with square mesh escape panels or cylinders in combination with a sorting grid(s) of different designs, increased the size-selectivity of whiting Merlangius merlangus compared to a standard trawl net. Overall, both nets tested improved the escape of whiting under 30 cm in length (minimum landing size 27 cm) compared to a standard net. A square mesh panel with two consecutive flexible grids allowed whiting of all lengths to escape, while a square mesh cylinder with one rigid grid allowed significant escape of whiting up to 31 cm length. Data were reported as statistical model results and catch probability curves. Trials were done in June 2010 and November 2013 by commercial trawlers fishing parallel to each other: one rigged with a modified net and the other a standard net. In the first trial, a net fitted with a square mesh panel and two flexible grids of different designs was tested (18 paired deployments). In the second trial, a net fitted with a square mesh cylinder around the entire section circumference and an aluminium grid (30 mm spaced vertical bars) was tested (21 paired deployments) (see original paper for gear specifications). Fish length and weight in catches were recorded. Random sub-sampling was done when catches were large.

(Summarised by: Natasha Taylor)
Fit a size-sorting escape grid (rigid or flexible) to a fish trawl net

A replicated, paired, controlled study in 2009 of a seabed area in the southern North Sea, France (Vogel et al. 2017) found that a fish trawl net modified with a flexible size-sorting escape grid caught less whiting Merlangius merlangus overall but did not reduce the amount of undersized whiting compared to a standard trawl net without a grid, in a mixed-species bottom trawl fishery. Catch numbers of whiting across all lengths were generally lower with a grid, and the most caught of any given length was 7,000 fish with a grid and 10,000 fish without (data not statistically tested). However, average whiting length was not statistically different between nets (grid: 27.3 cm, no grid: 27.5 cm). In February 2009, trials were done on two 20–24 m commercial trawlers fishing parallel to each other: one rigged with the test net and one with the standard net (see paper for specifications). A total of 13 paired deployments were completed. Nets were identical apart from the test net had a 1.25 x 0.75 m flexible grid, and vertical bars with 20-mm spacing, and the standard net had no grid but a mandatory 80 mm square mesh panel. Both commercial and non-commercial fish catches were sampled. Total length/fish and weight/species were recorded. Random sub-sampling was done when catches were large.

(Summarised by: Natasha Taylor)
Fit mesh escape panels/windows to a trawl net

A replicated, paired, controlled study in 2013 of seabed areas in the English Channel and southern North Sea, UK (Vogel et al. 2017) found that fish trawl nets fitted with escape sections of square mesh (square mesh cylinders) caught less whiting Merlangius merlangus overall, but did not increase the average size of whiting caught, compared with standard diamond mesh trawls without a square mesh cylinder. In two of two comparisons, the numbers of whiting caught were lower in nets with square mesh cylinders (maximum 1,500–3,000 whiting) than standard nets (maximum 4,200–5,000 whiting) (data presented as selectivity distributions), but the average length of the whiting caught was not statistically different (square mesh cylinder: 24.7–25.6 cm, standard: 23.3–25.0 cm). Trials were done in April and November 2013 by commercial trawlers fishing parallel to each other: one rigged with a modified net and the other a standard net. In the first trial on 20–24 m vessels, 15 paired deployments were done with a standard (80 mm diamond mesh with mandatory 80 mm square mesh panel) net modified with an 80 mm section of square mesh around the circumference and a standard net with just the mandatory 80 mm square mesh panel. In the second trial, 13 deployments on 16–20 m vessels tested a net with a square mesh cylinder alone and a standard diamond mesh net without any square mesh (see paper for specifications). Both commercial and non-commercial fish catches were sampled. Total length per fish and weight per species were recorded. Random sub-sampling was done when catches were large.

(Summarised by: Natasha Taylor)
Use a larger mesh size

A replicated, paired, controlled study in 2010 of an area of seabed in the southern North Sea, France (Vogel et al. 2017) found that a trawl net made of larger mesh (‘large mesh trawl’) did not reduce the catches of unwanted small whiting Merlangius merlangus, compared to a trawl net of standard mesh size. Average whiting length was similar in catches between the large mesh trawl and a standard trawl (large: 27.0 cm, standard: 26.9 cm). Data were collected in January 2010 from 38 paired deployments by two 20–24 m commercial trawlers fishing parallel to each other: one rigged with a large mesh trawl net and the other a standard net (see paper for detailed specifications). Weights of commercial and non-commercial portions of the catch and total lengths of individual whiting were recorded. Random sub-sampling was done when catches were large.

(Summarised by: Natasha Taylor)