Stimulating release of undersized fish through a square mesh panel in the Basque otter trawl fishery

Abstract

17Discards of regulated species in the Basque mixed trawl fishery are a challenge. In 2006, a square mesh 18 panel (SMP) was introduced in the fishery to increase the release efficiency of undersized fish. However, 19 studies have shown that the selectivity in this fishery is based on codend selectivity and the release 20 through the SMP is inefficient due to low contact between fish and the SMP. In order to improve contact, 21we tested four different gear configurations that use different stimulators to lead fish to the panel: without 22 stimulation, with stimulation based on ropes, with stimulation based on ropes and floats, and with 23 stimulation based on LED lights. The experiment was carried out on three of the potential choke species 24 for the fishery: hake (Merluccius merluccius), horse mackerel (Trachurus trachurus), and blue whiting 25 (Micromesistius poutassou). The results showed that stimulators did not significantly improve the release 26 efficiency of hake and horse mackerel through the panel. For blue whiting, stimulation with floats had a 27 significant positive effect on release efficiency, whereas LED light-based stimulation had the opposite 28 effect. In general, the contribution of the SMP to the overall release efficiency of the selective system 29 (SMP+codend) was low. Underwater recordings confirmed that the stimulators generally were not able to 30 lead fish towards the SMP. 31 Keywords: Square mesh panel (SMP); Basque bottom otter trawl; Release efficiency; Contact probability 32 33 *Manuscript including abstract Click here to view linked References et al. (2014) analyzed available data from observer discard monitoring, catch landings, and/or 53 nominal fishing effort from 2011 to 2013 and found that the total discard of the Spanish fleet 54 operating in ICES VIIIabd was around 60-65% of the total catch. Thus, unwanted catches and 55 discards constitute a substantial waste that negatively affects the sustainable exploitation of 56 marine resources (Kelleher, 2005). This perception has motivated the establishment of the 57 Landing Obligation (LO) under the provisions of Article 15 of the 2013 reform (EU, 2013). Its 58 main objective is to eliminate discards of commercially exploited stocks. By 2019, all EU 59fisheries are obliged to land the catches of regulated species to be counted against the quota. 60In recent decades, several fishing regulations have been implemented specifically to 61 stimulate the recovery of hake (ECminimum codend mesh size for trawlers fishing the northern stock of European hake in the Bay 63 of Biscay was changed from 70 mm to 100 mm diamond mesh. In 2006 (EC, 2006), fishermen 64 were given the alternative of using a 70 mm diamond mesh codend combined with a square 65 mesh panel (SMP) (2 m long, 1 m wide, 100 mm mesh size) inserted in the upper panel of the 66 extension piece of the trawl instead of a 100 mm diamond mesh codend. Currently, the gear 67 composed of the SMP with a 70 mm diamond mesh codend is the one most used by the fleet. 68Several studies have ...

Optimizing size selectivity and catch patterns for hake (Merluccius merluccius) and blue whiting (Micromesistius poutassou) by combining square mesh panel and codend designs

“…Regarding SMP, the following designs were considered: (i) small SMP located at the top panel of the extension piece (SMP TS ), which is included in current regulation of the fishery; (ii) large SMP located at the top panel of the extension piece (SMP TL ); (iii) small SMP located at the bottom panel of the extension piece (SMP BS ); (iv) large SMP located at the bottom panel of the extension piece (SMP BL ) and (v) absence of SMP in the trawl. Earlier studies showed that the SMP TS performs unsatisfactorily for some species due to lack of contact between fish and the SMP TS [ 19 , 23 ]. We therefore considered different SMP designs meant to optimize the release efficiency of hake and blue whiting.…”

“…The second design considered was a larger SMP placed at the upper panel, potentially increasing the chance for contact with the panel. Underwater observations in earlier studies showed that hake prefer swimming close to the lower panel [ 19 , 23 , 47 ], whereas blue whiting has an erratic behavior in the SMP area, swimming quickly either towards the SMP or the codend [ 23 ]. Thus, the third and fourth designs consisted respectively of the small SMP in (i) and a larger-size panel placed at the bottom panel of the trawl.…”

“…Thus, the third and fourth designs consisted respectively of the small SMP in (i) and a larger-size panel placed at the bottom panel of the trawl. Considering that hake individuals, besides entering the trawl close to the lower panel, do not actively swim inside it [ 23 ], we tested a SMP BL that was bigger than SMP TL in case (iv). The aim was to potentially offer more chances to hake individuals to attempt escape.…”

“…From experimental designs (i) and (ii), selectivity data for the SMP BS , SMP BL and CD D were obtained, whereas selectivity data for the CD S was obtained from design (iii). The selectivity data of SMP TS and SMP TL for hake and blue whiting was obtained from the sea trials conducted by Cuende et al [ 23 ] and Cuende et al [ 24 ], respectively. These two studies were carried out in 2017 and 2018 respectively, during same fishing period and similar fishing ground and depth.…”

“…These two studies were carried out in 2017 and 2018 respectively, during same fishing period and similar fishing ground and depth. Using information from the experimental sea trials in Cuende et al [ 23 , 24 ], the selectivity of all ten gear combinations was subsequently modeled.…”