Relative Catch Performance of Two Gear Modifications Used to Reduce Bycatch of Undersized Fish and Shrimp in Mediterranean Bottom Trawl Fisheries

Abstract

The catch of large quantities of sublegal-sized fish and shrimp is a pervasive feature of bottom trawl fisheries, particularly in the Mediterranean demersal mixed fisheries where regulations traditionally allow small mesh sizes. To address these concerns, two bottom trawl net selectivity trials were carried out in 2019 and 2020 on fishing grounds worked by the trawl fleet of Spanish Mediterranean, under normal commercial operating conditions with volunteer trawlers of the local fleet. The traditional otter bottom trawl employed in the demersal mixed fishery was modified with a 50-mm T90 panel on the extension piece under two different configurations (front of the extension piece and back of the extension piece). A second modification consisted of inserting a selective grid in the extension piece of the standard bottom trawl net. The species investigated in the demersal mixed fishery were European Hake Merluccius merluccius, Red Mullet Mullus barbatus, Striped Red Mullet Mullus surmuletus, and the deepwater rose shrimp Parapenaeus longirostris. Important selectivity improvements were observed for European Hake and deepwater rose shrimp, particularly in the selective grid trial, where 95% and 100%, respectively, of undersize specimens escaped through the grid. The design with the T90 panel in the back of the extension piece allowed for a reduction of 35% of sublegal-sized individuals of European Hake, but no difference was gained in the sizes of both Mullus spp. retained in the cod end. The adoption of these gear modifications might contribute to reducing discards of sublegal-sized fractions of the fisheries target species.

Effect of Extension Piece Design on Catch Patterns in a Mediterranean Bottom Trawl Fishery

“…The authors observed a significant reduction, in the experimental net, of the catch of undersized individuals, but also of legal sized individuals. The same results were obtained by Maynou et al (2021). They tested two experimental nets that, besides reducing the extension piece mesh size (50 mm T90 vs. 53 mm diamond of standard net), 13.2 1.9 (Tokaçet al, 1998) maintained or increased the number of meshes around extension circumference (210 and 220 vs. 210 of standard net) and, in the 40 mm square mesh codend, reduced the number of meshes around circumference (106 and 104 vs. 130 of standard net).…”

Testing experimental devices in the extension piece to increase the selectivity of bottom trawl in the Nw Mediterranean

“…The results obtained by those experiments are contrasting. In the case of Sola and Maynou (2018a) and Maynou et al (2021), the T90 placed in the extension piece determined an increase in the size at first catch of European hake, well above the MCRS, as well as an increase in the catch rates of this species. For red mullet and striped red mullet, the legal mesh size of codend (40 mm square or 50 mm diamond) already allows catching individuals larger than the MCRS and the use of the T90 in the extension piece would mainly result in a commercial catch reduction.…”

“…Recently, some experiments have been carried out in the Mediterranean to evaluate whether the T90 configuration can lead to a significant increase in selectivity if applied to different sections of the trawl net, other than the codend. Sola and Maynou (2018a) and Maynou et al (2021) in the Catalan Sea, and Petetta et al (2022) in the Adriatic Sea experimented trawl nets with the T90 configuration placed in the extension piece, i.e. the section of the net just in front of the codend.…”

“…The 90°t urned mesh configuration (T90) is a simple rotation by 90°of a traditional diamond mesh netting; as a result, the meshes remain open during towing and under the weight of the catch, enabling smaller specimens to escape (Wienbeck et al, 2011;Madsen et al, 2012). The T90 in the extension piece of the net has been recently tested in the Catalan Sea (Sola and Maynou, 2018a;Maynou et al, 2021) with promising results (e.g., the increase of modal length of the catch of European hake and red mullets), and in the Adriatic Sea (Petetta et al, 2022).…”

Monitoring global fishing activity in proximity to seamounts using automatic identification systems

“…The Mediterranean Sea hosts a large trawl fishery that is characterised by small mesh sizes (40‐mm square mesh) to capture deep‐water shrimp species alongside demersal fish species (Sala et al, 2015). However, this also results in high levels of bycatch that can account for up to 65% of total catch including the exploitation of juvenile and undersized individuals (Maynou et al, 2021; Tsagarakis et al, 2014). Indiscriminate trawling practices have also resulted in substantial alterations in benthic community composition in this region with many VME indicator species now largely absent from these areas (De la Torriente Diez et al, 2022; Grinyó et al, 2020; Pierdomenico et al, 2018).…”