Size selection of cod by rigid grids—Is anything gained compared to diamond mesh codends only?

Abstract

Size selective grids were introduced into the Barents Sea demersal trawl shery in 1997. It was believed at the time that in comparison to standard diamond mesh codends, grids had better selective properties, i.e. narrower selection range. Few studies have directly compared the selective properties of the combined grid and codend with that of the codend only. In this study we describe two experiments carried out in 2002 and 2003, where we directly compared the properties of codend selectivity for cod (Gadus morhua ) with the combined selectivity of a Sort-V grid and a codend. In 2002 we used small-meshed covers, whereas the twin trawl method was used in 2003. In both experiments we compared the selectivity of a 135 mm codend only with that of a trawl tted with a 135 mm codend and a 55 mm grid, which is the mandatory con guration. In 2003 we also estimated the selectivity of a 155 mm diamond mesh codend, having an L 50 similar to that of the grid and 135 mm codend combination. The results presented no evidence that the grid and mesh combination had sharper size selection than codend meshes. The introduction of the mandatory use of grid in the shery in 1997 therefore only increased L 50 , and corresponded to a mesh size increase of around 20 mm. Mean selection length of the grid was inversely related to catch rates in the 2002 experiment when large catches were taken. Grid selection appeared less a ected by seasonal variations in degree of stomach fullness or condition than mesh selectivity. Other situations where grid selection might perform better than mesh selection are discussed.

Understanding the Size Selectivity of Redfish (Sebastes spp.) in North Atlantic Trawl Codends

“…5 trawl constructed from 155 mm mesh (the top front panel and the wings were built from 200 mm mesh). The selection system installed in the trawl was a Sort-V sorting grid (see Jørgensen et al, 2006) combined with a 135 mm codend. The codend was built with 8 mm braided polyethylene twine (called "Euroline premium"), was 70 meshes long and 70 meshes around.…”

“…Today, redfish is mainly harvested with diamond mesh codends or combined systems using sorting grids together with diamond mesh codends (NAFO, 2011;Gunnarsson et al, 1998;Jørgensen et al, 2006). The few available published redfish size selectivity studies mainly involve diamond mesh codends; the results from such studies indicate that mesh size can affect size selectivity of this species (Lisovsky, 2001;Lisovsky et al, 2006).…”

Improving release efficiency of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) in the Barents Sea demersal trawl fishery by stimulating escape behaviour

“…The compulsory size selectivity device for the trawlers targeting cod and haddock (Melanogrammus aeglefinus) in the Barents Sea consists of a rigid sorting grid with a minimum bar spacing of 55 mm and a diamond mesh cod end with a minimum mesh size of 130 mm. Fishermen are allowed to use three different grid systems: the Sort-X double grid system (Larsen and Isaksen 1993); the Sort-V single grid system (Jørgensen et al 2006;Herrmann et al 2013); and the Flexigrid double grid system . The sorting area of these grids is limited, and fishermen report that fish accumulate in front and behind the sorting grids at high catch rates (>10 t·h -1 ).…”

Size selectivity patterns in the North-east Arctic cod and haddock fishery with sorting grids of 55, 60, 70 and 80mm

“…During the last two decades, many studies in the North-east Arctic have focused on improving the escape (Larsen and Isaksen, 1993;Kvamme and Isaksen, 2004;Jørgensen et al, 2006;Grimaldo et al, 2007) and survival (Ingolfsson et al, 2002;Suuronen, 2005) of juveniles from fish trawl nets. The sorting grid, which was used on a voluntary basis in the North-east Arctic (north from 62 • N) gadoid fishery since 1993, became mandatory in 1997.…”