Abstract
In many trawl fisheries, codend size selectivity is supplemented by adding selection devices to the gear. In the Barents Sea gadoid fishery, combining diamond mesh codends with sorting grids is compulsory. However, the use of grids increases the costs and complexity of the gear, causing discontent among fishermen and prompting researchers to seek alternative solutions. Lastridge ropes are ropes attached to the selvedges of the codend. In this study, we tested the effect of shortening the lastridge ropes of two diamond mesh codends with different mesh sizes on the size selectivity of cod (Gadus morhua), haddock (Melanogramus aeglefinnus), and redfish (Sebastes spp.). Shortening the lastridge ropes by 15% increased the mesh opening during the fishing process, which significantly improved the size-selective properties of the codends. Further, the L50 values were always higher for the codends in the short lastridge configuration. Therefore, codends with shortened lastridge ropes may be a simpler alternative to sorting grids in this fishery, and they may be applicable to many other fisheries in which additional selection devices are used.
Highlights
Diamond mesh codends are the most widespread and simplest size-selection device used in demersal trawls, and in some fisheries, size selectivity relies solely on the selective properties of this type of codend (Cheng et al 2019)
The p value for the model with the lowest Akaike information criterion (AIC) value among the models considered was >0.05, which indicates that the difference between the experimental points and the model used in every case could be coincidental (Table 2)
The results demonstrated that both increasing mesh size from 128 to 137 mm and shortening the lastridge ropes for both codends, so that they were 15% shorter than the stretched codend netting, can change the selection properties of the codend for cod, haddock, and redfish (Figs. 3–4)
Summary
Diamond mesh codends are the most widespread and simplest size-selection device used in demersal trawls, and in some fisheries, size selectivity relies solely on the selective properties of this type of codend (Cheng et al 2019). As the catch in the codend builds up during towing and haul-back, tension increases and the longitudinal forces in the mesh bars close the codend meshes, affecting selectivity In many fisheries the approach adopted has been to insert additional devices into the gear, such as square mesh panels (Graham et al 2003; Herrmann et al 2015; Cuende et al 2020) or sorting grids (Sistiaga et al 2008; Brinkhof et al 2020), to supplement codend size
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