The effect of mesh configuration and codend circumference on selectivity in the Mediterranean trawl Nephrops fishery

Published source details Sala A. & Lucchetti A. (2010) The effect of mesh configuration and codend circumference on selectivity in the Mediterranean trawl Nephrops fishery. Fisheries Research, 103, 63-72.
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This study is summarised as evidence for the following.

	Action	Category
	Decrease the circumference or diameter of the codend of a trawl net Action Link
	Use a square mesh instead of a diamond mesh codend in a trawl net Action Link

Decrease the circumference or diameter of the codend of a trawl net

A replicated, controlled study in 2005 in an area of mud seabed in the Adriatic Sea off Italy (Sala & Lucchetti 2010, same experimental set-up as Sala & Lucchetti 2011) found that using a diamond mesh bottom trawl codend of standard circumference had reduced size-selectivity for one of three non-target fish species, and did not increase escape, compared to a larger circumference diamond mesh codend, however a change in mesh configuration (to square) did improve fish size-selectivity, irrespective of codend circumference. For diamond mesh codends, the predicted length at which fish had a 50% chance of escape was similar for a standard mesh circumference compared to a larger circumference for blue whiting Micromesistius poutassou (standard: 10.6 cm, large: 10.6 cm) and poor cod Trisopterus minutus (standard: 6.3 cm, large: 6.3 cm), but lower for European hake Merluccius merluccius (standard: 8.7 cm, large: 10.1 cm). However, escape of all three species was not affected by diamond mesh codend circumference (data reported as statistical results). A codend of square mesh with similar or larger circumference as the diamond mesh codends, resulted in greater 50% escape lengths for all three species compared to the diamond mesh codends (blue whiting: 13.6 cm, poor cod: 9.2 cm, hake: 12.6 cm). Gear trials were done on a research vessel in May and September 2005 in the Western Pomo pit area (a Norway lobster Nephrops norvegicus fishing ground, 210 m depth). Two diamond mesh codends (standard 280 and larger 326 mesh circumference) and one square mesh codend (70 meshes) were tested during 19, 13 and 20 deployments, respectively. All codends had a nominal 40 mm mesh size (see original paper for full gear specifications). Covers over each codend collected catch escaping from the meshes. All catch was weighed by species, and fish lengths recorded.

(Summarised by: Chris Barrett)
Use a square mesh instead of a diamond mesh codend in a trawl net

A replicated, controlled study in 2005 in one area of muddy-sandy seabed in the Adriatic Sea, Italy (Sala & Lucchetti 2010) found that a square mesh codend improved the size selectivity of a prawn trawl net for European hake Merluccius merluccius, blue whiting Micromesistius poutassou and poor cod Trisopterus minutus capelanus, compared to diamond mesh codends of standard and large circumferences. Across both surveys, the estimated lengths at which 50% of fish are predicted to escape were greater in hauls with the square mesh codend than the two diamond mesh codends for three commercially important species: hake (square: 12–16 cm; diamond: 8–11 cm), blue whiting (square: 14–18 cm; diamond: 11–15 cm), and poor cod (square: 10–13 cm; diamond: 6–10 cm). Catch comparison trials were done during two research vessel surveys in the Western Pomo pit (210 m depth; a Norway lobster Nephrops norvegicus fishing ground) in May and September 2005. Three codends were tested, all nominal 40 mm mesh size: a square mesh codend (70 meshes circumference); and two diamond mesh codends, one of conventional circumference (280 meshes) and one larger (326 meshes). Over the two surveys a total of 20 deployments were done with the square mesh codend, and 19 and 13 deployments with the standard and large diamond mesh codends, respectively. A cover attached over each codend sampled the catch escaping through the meshes.

(Summarised by: Chris Barrett)