Selectivity of turned meshes codends for Deepwater Rose Shrimp (Parapenaeus longirostris), Horse Mackerel, (Trachurus trachurus) and European Hake (Merluccius merluccius) in the Aegean Sea

Abstract

More than fifty published on demersal trawl codend selectivity studies have been carried out in the Mediterranean waters of Turkey since the mid-1980s. In this study, in an attempt to enhance the size selectivity of three species in the Aegean Sea trawl fisheries, three 90 o turned meshes codens were investigated. Experiments were carried out in the Kuşadası Bay of the Eastern Mediterranean, between 19 January and 29 March 2015 with commercial stern trawler. The meshes of tested codends were turned 90 o , two of them have 44 mm mesh size with 300 and 150 meshes around codend circumferences (44T300 and 44T150). The other has 40 mm mesh size with 165 meshes around codend circumferences (40T165). Individual hauls and mean selectivity parameters were estimated by using the CC2000 and EC-Modeller softwares, respectively. The mean L50 values of 44T300, 44T150 and 40T165 were 17.5 ± 0.2 mm, 18.6 ± 0.1 mm and 17.1 ± 0.2 mm carapace length for shrimp, 16.4 ± 0.4 cm, 17.1 ± 0.4 cm and 14.8 ± 0.2 cm total length for horse mackerel, respectively. The pooled L50 values of, 12.8 ± 0.5 cm, 13.2 ± 0.6 cm and 12.1 ± 0.1 cm total length for hake. While there are no differences between L50 values of 40T165 and 44T300 for deepwater rose shrimp (p>0.05), there are significant differences for horse mackerel (p<0.05). The likelihood ratio test results showed that there were no significant differences among all codends for European hake. In conclusion, any effort such as using square mesh, turned mesh codends or halved the number of meshes in the codend to release immatures of some species, is probably to cause the loss of marketable sizes of other species in the Mediterranean. This economical loss of revenues is the essential concern of fishermen that prevents them using alternative codend designs, though such designs are technically applicable. Consequently, the losses of income caused by using T90 mesh codend and halving the number of meshes around codend circumference should also be investigated in future studies.

Estimating selectivity of experimental diamond (T0) and turned mesh (T90) codends in multi-species Mediterranean bottom trawl

“…age L50 value above the MCRS of 20 cm (STECF, 2015), obtained with the T90 configuration, whereas the higher values reported in the literature have been obtained with a 60 mm DM (19.50 cm, Aldebert & Carries, 1991;16.64 cm, Soldo, 2004;18.10 cm, Belcari et al, 2007). The size selection properties of the T90 mesh described herein are much greater than those found by Genç et al (2018), who tested a 40 mm and 44 mm mesh (average L50 values, 12 -13 cm). To our knowledge, no other published data on the selectivity of the T90 mesh are available for this species.…”

“…This configuration is obtained by turning a typical diamond netting by 90°; as a result, its meshes remain more open under the weight of the catch, enabling smaller specimens to escape (Madsen et al, 2012). Most of the work on the T90 mesh has been conducted by comparing it to a standard DM codend in Turkish waters (Tokaç et al, 2014;Deval et al, 2016;Ilkyaz et al, 2017;Genç et al, 2018;Kaykac et al, 2018). The T90 codend, like the SM codend, generally provides greater selectivity than the DM codend (Tokaç et al, 2014;Deval et al, 2016) even though its size selection properties vary according to the cross-sectional morphology of target species (Herrmann et al, 2013;Tokaç et al, 2014;Bayse et al, 2016).…”

Exploring the feasibility of technological transfers of two by-catch reduction devices in the crustacean bottom trawling of the central Mediterranean

“…Because adopting a larger mesh size in the codend implies a strong reduction in the valuable fraction of catch, different technical solutions are being tested to modify the mesh shape to reduce the unwanted by-catch from trawling in the Mediterranean Sea. Considering the selectivity of the T90 (i.e., a diamond mesh turned 90°) codend, this solution was tested in the Adriatic Sea (Petetta et al, 2020) and the Aegean Sea (Dereli and Aydin, 2016;Deval et al, 2016;Gençet al, 2018), but not in the Strait of Sicily. The T90 codend, compared to other meshes currently used by trawlers, should remain more open during fishing operations, allowing undersized fishes and shrimps to escape.…”