Approach and methodology used in fishing experiments with square mesh cod-end in Adriatic Sea

The present paper presents the approach and methodology employed to measure the selectivity and the effectiveness of the square meshes applied to the type of trawl most commonly used at Adriatic Sea. Seventeen fishing experiments were completed to this end by the vessels of research DALLAPORTA and ANDREA. The selectivity of square mesh and diamond mesh shape bottoms were achieved separately by the use of a double 20 mm mesh size cod-end, equipped with plastic hoops to avoid masking the bottom tested. The adopted hanging ratio of square meshes panel is 25%. To compare the effectiveness of the two systems, the hauls were completed alternatively with one then the other type of cod-end, the yields expressed as a whole and and by species being given in number and weight in kg/km2. The trawl geometry and efforts exerted on the trawl gear were constantly measured during each haul, using Scanmar equipment and tension sensors. The selectivity parameters were then calculated for each haul using software CC2000 while the average selectivity curves were obtained by the method of Fryer (1991). Compared with the observations obtained for diamond mesh bottoms of same dimensions, no significant difference was noted, neither between the vertical and horizontal openings, nor between the forces of resistance of the trawls. The specific composition is also identical but with weaker yields for the square mesh; for instance if one obtains a profit in weight of 64% for hake the total effectiveness is decreased by 20 %. In conclusion, the use of a square mesh bottom trawl involves a selectivity higher than with diamond meshes. This higher selectivity however involves an economic loss due to the youngest fraction of stock, as it is the case for the red mullet.