Selectivity of a 50-mm diamond mesh knotless polyethylene codend for commercially important fish species in the Aegean Sea

Abstract

This research investigated effects of changing the diamond mesh size on codend selectivity in Mediterranean fisheries. The selectivity of a typical 50-mm diamond knotless polyethylene (PE) codend used in the Turkish fishery in the Aegean Sea was measured for commercially important species, in particular hake (Merluccius merluccius), horse mackerel (Trachurus trachurus), anglerfish (Lophius piscatorius) and John Dory (Zeus faber). Fishing trials were carried out on the commercial trawler ÔHapulog˘luÕ between 9 and 12 December 2006 using a modified trawl net. Selectivity data were collected by the covered codend method and analysed by means of a logistic equation (Maximum Likelihood Method). The mean selectivity curve was estimated from individual hauls, taking betweenhaul variations into account. Mean mesh size of the codend was 49.4 mm as measured by digital calliper. Mean values for 50% retention length of hake and horse mackerel were estimated to be 11.4 and 15.6 cm total length; corresponding selection ranges were 4.1 and 5.5 cm respectively. The 50-mm diamond mesh codend showed adequate selectivity compared to the minimum landing size (MLS) for horse mackerel, while for hake it selected specimens in a size range far lower than the MLS. No selectivity values could be determined for anglerfish or John Dory. To design a more selective codend for the Turkish demersal trawl fishery, not only mesh size regulations but also other codend characteristics and netting material properties must be urgently considered.

Effect of mesh size and codend circumference on selectivity in the Mediterranean demersal trawl fisheries

“…An increase in mesh size from 48 mm to 56 mm could be made fruitless by an increase in codend circumference of only 13-17%. In agreement with Galbraith et al (1994) and Tosunoglu et al (2008) we have found that selectivity increases with larger mesh size but also decreases with wider codends. In the Mediterranean Sea, the effect of circumference might be not evident because the scarce catch size does not allow the meshes of the codend to stay open (Sala and Lucchetti, 2010).…”

Codend selectivity for jack mackerel and whitefin jack and unequal split parameter estimates observed in trouser trawl experiments

“…This suggests that there was an insufficient sieving effect of the mesh on jack mackerel with girths that were small enough to pass through the mesh in this study. Previous studies [33][34][35][36][37][38][39] have reported that the selection factor of diamond mesh codend for Trachurus fish varies from 2.0 to 3.43, and that of square mesh codend varies from 3.4 to 4.3, based on the measurement of the total length of the fish ( Table 4). The l 50 in fork length for the jack mackerel Trachurus japonicus estimated in this study corresponded to the l 50 in total length of 12.95 cm with the regression equation in Eq.…”

Evaluation of sorting grids for deepwater rose shrimp (Parapenaeus longirostris) in the Eastern Mediterranean demersal trawl fishery

“…No anglerfish specimens were caught in Lb of either grid, mostly because of the body shape of the species. Tosunoğlu et al. (2008b) reported that due to the body shape (head and body depressed) no individuals could escape and that all (from 9 to 57 cm total length) were retained in the 50 mm diamond mesh codend.…”