Effect of Codend Circumference on the Size Selection of Square-Mesh Codends in Trawl Fisheries

Abstract

It is well established that increasing mesh number in the circumference of a diamond-mesh trawl codend can reduce size selection for round fish, whereas selection for flat fish species is unaffected. This effect has also been documented in Mediterranean trawl fisheries. In contrast, no information is available with regard to the effect of increasing mesh number in the circumference of square-mesh codends on the size selection of round fish and flat fish species. A field study was devised to bridge this gap and formulate proposals aimed at improving trawl fishery management. Size selection data were collected for a round fish species, red mullet (Mullus barbatus), and two flat fish species, Mediterranean scaldfish (Arnoglossus laterna) and solenette (Buglossidium luteum). Fishing trials were conducted in the Adriatic Sea (Central Mediterranean) using three square-mesh codends that differed only in mesh number around the circumference. Results demonstrated that increasing the number of meshes from 107 to 213 reduced the 50% retention length (L50) for red mullet by 2.5 cm but did not affect size selection for the two flat fish species. In some fisheries, regulatory provisions regarding the number of meshes in the circumference should therefore be carefully considered both for diamond- and square-mesh codends.

Towards more selective Mediterranean trawl fisheries: are juveniles and trash excluder devices effective tools for reducing undersized catches?

“…Tokac et al 2014, Sola and Maynou 2018. These include changes in the cod-end circumference (Sala et al 2016), the use of square-mesh panels inserted in front of the cod-end (e.g. Brčić et al 2015) and the adoption of sorting grids (Sardà et al 2005, claramente que las rejillas pueden contribuir a reducir sustancialmente las capturas no deseadas de DPS y HKE de talla ilegal en las pesquerías de arrastre de fondo del Mediterráneo.…”

Old Info for a New Fisheries Policy: Discard Ratios and Lengths at Discarding in EU Mediterranean Bottom Trawl Fisheries

“…The variety of factors (e.g., economic, legal, cultural, natural, biological, technical) affecting discarding render the issue quite complex for fisheries scientists and managers (Bellido et al, 2011;Santiago et al, 2015). Several solutions have been proposed and enforced to mitigate discards (e.g., SigurDardóttir et al, 2015;Rijnsdorp et al, 2016), however, it is widely recognized that they need to be adapted to the local features of each fishery (Hall and Mainprize, 2005;Johnsen and Eliasen, 2011;Rochet et al, 2014;Sala et al, 2016Sala et al, , 2017.…”

The influence of mesh size and shape on the size selection of European hake (<em>Merluccius merluccius</em>) in demersal trawl codends: An investigation based on fish morphology and simulation of mesh geometry

“…This situation for square mesh codends is also supported by the underwater in situ photos of square mesh codends presented in Robertson (1986). Further, an experimental study by Sala et al (2016) explained why L50 values tended to decrease with an increasing number of meshes in circumference for square mesh codends.…”

“…In the Mediterranean, studies have compared selectivity in diamond and square mesh codends (Ordines et al 2006, Lucchetti 2008, Özbilgin et al 2012. In addition to mesh size and type, other codend designs such as an SMP codends (Bahamon et al 2006, Tokaç et al 2010 and differences in codend circumference Lucchetti 2011 andSala et al 2016) have been investigated. These experiments have established selectivity estimates for specific mesh sizes, mesh types or codend constructions, and the results have been to some extent case-specific or are limited to a specific mesh size or gear design.…”