Mediterranean demersal fisheries are characterized by a critical combination of high fishing effort and low size at first capture of commercial species (Lleonart and Maynou 2003, Vasilakopoulos et al. 2014, Colloca et al. 2017). This is mainly due to the progressive growth in engine power of trawlers, the rapid technological advances in fish finding and fishing technology, and the traditional use of fine mesh size in the cod-end (Fiorentini et al. 2004). Due to its low selectivity, Mediterranean bottom trawling produces a high amount of discards and, although several technical solutions have been developed for discard reduction (e.g. Tokac et al. 2014, Sala et al. 2015, Brčić et al. 2017), the management measures adopted have been mostly limited to increasing the mesh size in the cod-end (e.g. Tsagarakis et al. 2017).

In 2006, a specific regulation for the management of Mediterranean fisheries was approved by the European Union (Reg. UE 1967/2006), emphasizing the importance of the Mediterranean basin for the Common Fishery Policy (CFP). In this regulation, a set of measures were adopted to improve the sustainable exploitation of Mediterranean stocks. In particular, minimum conservation reference sizes (MCRS) were defined for the most important species, in addition to a minimum legal mesh size in the trawl net cod-end of 40-mm square mesh or 50-mm diamond mesh.

Over the last decade, the reduction of discarding of the non-commercial fraction in fisheries has become an increasingly important objective from the socioeconomic, ecological, and moral point of view (Bellido et al. 2011, Santiago et al. 2015, Maynou et al. 2018). The CFP has defined specific management measures to reduce discarding of catches of regulated species, including the landing obligation (Article 15 of EU Reg. 1380/2013) of all Mediterranean species subject to MCRS (Veiga et al. 2016). These species include the deep-water rose shrimp (DPS), Parapenaeus longirostris (Lucas, 1846), and the European hake (HKE), Merluccius merluccius (Linnaeus, 1758).

The DPS is one of the main target species of trawl fishery in several Mediterranean sectors and represents about 40% of total trawl landing in the Strait of Sicily (central Mediterranean) (Milisenda et al. 2017). In this area, DPS is caught by trawl vessels fishing on the outer continental shelf and upper slope in geographical sub-areas (GSAs) 12 (northern Tunisia), 13 (Gulf of Hammamet), 14 (Gulf of Gabes), 15 (Malta) and 16 (Southern Sicily). The available scientific information indicates that fishing fleets of Italy, Malta and Tunisia exploit a single shared DPS stock (Fiorentino et al. 2013) with a total catch of about 7540 t in 2016. This catch is mostly due to the Italian (70%) and Tunisian (30%) vessels, the Maltese yield being negligible. The main commercial species associated with DPS trawling is the European hake (Merluccius merluccius; HKE), amounting to about 10% of the total trawl landing.

According to Milisenda et al. (2017), the annual discards due to DPS trawl fisheries in the Strait of Sicily accounted for about 36% of the total catch. This fraction also included undersized specimens of species under MCRS according to Reg. UE 1967/2006, such as DPS and HKE, which are consequently subject to the European discard ban.

Reducing discards in multi-species trawling (Stergiou et al. 2009, Sala et al. 2015) is a main challenge in Mediterranean fisheries. In fact, the legal mesh size adopted is suitable for catching crustaceans, cephalopods and medium-size fish such as red mullets, but it is too small to avoid the catch of juveniles of large-sized fish such as hake and monkfish (Caddy 1990). For the latter species, the adoption of a bigger cod-end mesh size would be required to increase their size of first capture, as observed in numerous experiments carried out in Mediterranean waters testing different combinations of mesh geometry and mesh size in the trawl cod-end (Kaykaç et al. 2009, Dereli et al. 2016 and references herein).

However, a significant mesh size increase would easily break up the current catch profile of Mediterranean bottom trawlers, with a significant loss of small-sized marketable species and individuals of other commercial species (e.g. Ragonese et al. 2001). Alternative selection tools and technical solutions to the classical mesh size increase in the cod-end have proved to be effective in improving trawl selectivity without a major loss of commercial fish and shellfish (e.g. Tokac et al. 2014, Brčić et al. 2017, Sola and Maynou 2018). These include changes in the cod-end circumference (Sala et al. 2016), the use of square-mesh panels inserted in front of the cod-end (e.g. Brčić et al. 2015) and the adoption of sorting grids (Sardà et al. 2005, Massutí et al. 2009, Özvarol 2016). In particular, the studies carried out on sorting grids have revealed clear advantages associated with their use in mixed trawl fisheries, such as improved selectivity and by-catch reduction (e.g. Massutí et al. 2009, Aydın and Tosunoğlu 2011), a stable selection pattern during the tow (Suuronen and Sardà 2007), and a higher survival rate of specimens escaping from the grid bars compared with a conventional cod-end mesh (e.g. Brčić et. al. 2015, Lucchetti et al. 2016). The main disadvantages are related to handling on board trawl nets equipped with grids, the potential occlusion of the grids by debris, and their sporadic reduced sorting capacity when a high number of fish arrive simultaneously (Suuronen and Sardà 2007). Although all these studies point to the positive impact of sorting grids on the size at first capture of commercial fish and crustaceans (e.g. Massutí et al. 2009, Aydın and Tosunoğlu 2011), their adoption in Mediterranean trawl fisheries is still very low. Lack of sorting grid implementation could be linked to several causes, including fishers’ resistence to changing fishing traditions and customs, and, consequently their unwillngness to modify their fishing gear substantially. Other possible causes are linked to the lack of a suitable technology transfer from researchers to fishing enterprises (e.g. Morrissey and Almonacid 2005), as well as the overall weakness of fishing management in the Mediterranean region (Colloca et al. 2017).

In this study we tested the efficiency of sorting grids in improving the selectivity of trawlers targeting DPS in the Strait of Sicily. Three different types of sorting grids were designed in collaboration with some trawl vessel captains of the Mazara del Vallo fleet and tested during ad hoc designed fishing trials. The objective of the study was to quantify the effects of the sorting grids on the catch of undersized specimens of DPS and HKE and to elucidate both the technical constraints and the possible trade-offs in terms of fishery sustainability.

The absolute number of specimens caught between test and control was significantly different only for DPS (test=5691; control=9970), while for HKE no significant difference was found (test=1566; control=1569) (Fig. 3). The length frequency distributions of DPS and HKE showed a similar shape between test and control. However, the proportion of undersized specimens was lower in test hauls, with clear differences between the two species. In the case of DPS, mainly G1-SM40 led to a reduction in the catch of shrimps of less than 20 mm carapace length (CL) (–6%), while all three grids produced less catch of HKE <20 cm TL (G1-SM40 –20%; G2-ST20 –6%; G3ST25 –7%) compared with the control (Fig. 3).

The parameters and R² of the fitted mean probability functions of the final GAMMs are presented in Table 2. The GAMM for JTED G1-SM40 showed that the estimated P_St of DPS increased from 0 to 0.3 for shrimps between 10 and 18 mm CL, corresponding to a decrease in catch probability of 100% to 60% compared with the control. P_St appeared constant at about 0.3 for 18-25 mm CL DPS, increasing progressively up to 0.5 (no differences from the control) for 30 mm CL shrimps (Fig. 4). In the case of HKE, the estimated P_St ranged between 0.2 and 0.5 for 15-20 cm TL, although with a higher uncertainty due to the low catch. P_St increased up to 0.6 for 20-24 cm TL, but over this size there was no clear significant catch effect of the grid (Fig. 5). The estimated P_St values using G2-ST20 for DPS were quite similar to those obtained with G1-SM40. In fact, the P_St increased from 0 to 0.3 for shrimps between 9 and 18 mm CL (decreasing in catch probability between 100% and 60% compared with the control), remained quite costant at about 0.28 between 18 and 23 mm CL, and after that, size increased progressively up to 0.5 (no differences from the control) at 27 mm CL (Fig. 4). In the case of HKE, P_St ranged between 0.4 and 0.5 for 10-21 cm TL (decreasing catch probability by up to 20% compared with the control), increased to 0.6 (increasing the catch probability by up to 20% compared with the control) for 15-26 cm TL, and after that size there was high uncertainty due to the low catch (Fig. 5). With G3-ST25 the estimated DPS P_St values ranged between 0.1 and 0.2 for 10-20 mm CL (decreasing catch probability by up to 69% compared with the control), and after this size the P_St increased slowly to 0.6 for 20-30 mm CL. The estimated P_St HKE ranged between 0.3 and 0.5 for 10-20 cm TL (decreasing catch probability by up to 35% compared with the control), and over this size there was no clear significant catch effect of the grid (Fig. 5).

The P_us values with G1-SM40 were 0.37 and 0.19 for DPS and 0.25 and 0.51 for HKE during the diurnal and nocturnal hauls, respectiPvely (Table 3). For G2-ST20 the P_us values during the diurnal and nocturnal hauls were 0.38 and 0.16 for DPS and 0.40 and 0.47 for HKE, respectively. Similarly, with G3-ST25 the P_us values during the diurnal and nocturnal hauls were 0.33 and 0.18 for DPS and 0.48 and 0.44 for HKE, respectively. These values were similar between diurnal (0.37, 0.38 and 0.33) and nocturnal hauls (0.19, 0.16 and 0.18) for DPS, clearly showing an efficient reduction of undersized shrimp catch in the cod-end of ~45% in daytime and ~73% at nighttime, compared with the control. The P_us for HKE were different (0.25, 0.40 and 0.48) during the diurnal hauls, showing a reduction of undersized hake of 0.38% to 72%, while were close to 0.5 during the nocturnal hauls (0.51, 0.47 and 0.46) showing no differences in the retention of undersized HKE between test and control at night (Table 3).

The results of our catch comparison experiment showed that the use of JTEDs mounted on traditional trawl nets of Mediterranean trawlers can be beneficial in reducing the by-catch of undersized individuals (i.e. individuals below the MCRS) of two important commercial species, the deep-water rose shrimp and the European hake. The three different types of JTEDs that were tested produced significant and sometimes contrasting differences in the catch of small individuals of the two species. The JTED consisting of a stretched square mesh size of 40 mm (G1-SM40) produced a high sorting ability for both species in terms of reduction of juvenile catch (DPS –60%, P̅_St=0.26; HKE –44%, P̅_St=0.37) with a minor loss of the marketable fraction for DPS (DPS –25%, P̅_St=0.48; HKE –39%; P̅_St=0.40).

The results clearly demostrated also the good efficiency in reduction of undersized specimens using G2-ST20 (DPS –59%, P̅_St=0.27; HKE –34%, P̅_St=0.43) and G3-ST25 (DPS –73%, P̅_St=0.18; HKE –38%, P̅_St=0.40). However, the latter showed quite a high loss of the marketable DPS fraction (–36%).

The observed differences between day and night in catch proportion of undersized DPS and HKE in trawl nets equipped with and without JTED might be associated with the juveniles’ daily feeding migrations towards the sea surface at night (e.g. Savenkoff et al. 2006, Carpentieri et al. 2008), as well as to the different behaviour of the two species (e.g. swimming capability). In particular, the reduced occurrence of DPS during nocturnal hauls is associated with a reduced catch proportion of undersized specimens: from 45% less during the day to 73% less at night than the control. The scarce occurrence of HKE juveniles close to the bottom at night (Orsi Relini et al. 1997) could affect the similar estimated probability (P_us~0.5; both gears catching equal numbers of specimens) of retaining a fish among the nocturnal hauls with and without JTEDs. Vice-versa, during the diurnal hauls, when HKE juveniles aggregate near the bottom, the JTEDs’ efficiency is clearly recognizable for the tested grids (G1-SM40 –62%, P_us~0.25; G2-ST20, –39%, P_us~0.4; G3-ST25 –50%, P_us~0.33). Even if our results need to be confirmed across seasons, oceanographic conditions, benthic communities and substrata typology, the adoption of the JTED G1-SM40 could significantly reduce the trawl catches of undersized DPS and HKE, suggesting its potential usefulness to minimize the overall discard franction. Consequently, seeking compliance with the landing obligation, the JTED G1-SM40 should be tested in commercial trawlers operating in different conditions to verify its overall sorting ability also for other species and to evaluate its potential to improve the status of Mediterranean overexploited stocks. It is worth noting that this selection device is low-cost and, according to our experience, easy to mount on commercial trawl nets.

Our findings are consistent with previous results obtained in other areas of the Mediterranean Sea, emphasizing the sorting ability of grids mounted on trawl nets. In fact, in the eastern Mediterranean demersal trawl fishery, Aydın and Tosunoğlu (2011) evaluated the use of sorting grids of 10- and 15-mm bar spacing, demonstrating a substantial improvement for DPS selectivity. In the same area, Aydın et al. (2011) analysed the efficiency of sorting grids with 20-mm bar spacing, demonstrating a substantial separation between fish and shrimp. However, a loss of 23% and 25% of legal-sized DPS in the cod-end was recorded using 44-mm diamond and 40-mm square mesh, respectively. In the Balearic Islands (western Mediterranean), a flexible sorting grid with 15- and 20-mm bar spacing improved trawl selectivity by increasing the L₅₀ for DPS and HKE and decreasing the discarded fraction (Massutí et al. 2009). However, although some studies have explored the survival rate of discarded species in Mediterranean trawl fisheries (e.g. Revill 2012, Tsagarakis et al. 2018, Özvarol and Bolat 2017), there is little knowledge on the mortality rates of DPS and HKE that escape through sorting grids. This aspect should be closely investigated through experimental surveys in the near future to obtain an overall assessment of the effects produced by the adoption of the grids on the stocks of the two species (Vitale et al. 2018).

The overexploitation of most demersal stocks in the Mediterranean (e.g. Colloca et al. 2013) increases the need to find technological solutions to improve gear selectivity, reduce discards, minimize the impacts on habitats and increase the survival rate of individuals discarded by fishers or escaping through the cod-end. In our study the engagement of local fishers was extremely useful to design appropriate sorting grids for the traditional trawl nets used by Sicilian trawlers to target DPS. Due to the increasing importance of trawling for deep-water shrimp in the Mediterranean Sea (Pinello et al. 2018), the identification of ad-hoc technical tools to reduce the catch of juveniles is important for the sustainability of the fishery. In order to achieve this objective in the shrimp trawl fishery, a spatial-based approach including the closure of the nurseries of main commercial species to bottom trawling was considered a promising management measure (Maiorano et al. 2009, Russo et al. 2014, Colloca et al. 2015). The GFCM recently adopted the closure of three stable nurseries of DPS and HKE in the northern sector of the Strait of Sicily (REC.CM-GFCM/40/2016). A next step in the management of the fishery could be to integrate this "drastic” measure with the adoption of new technological solutions able to consistently reduce the impact on juveniles/undersized specimens. In this context, JTED G1-SM40 could be a useful selective device for achieving the management goals of the EU (i.e. art. 14 of EU Reg. 1380/2013) and the GFCM. Although more quantitative data should be gathered to generalize the results obtained in this study, the adoption of ad hoc–designed sorting grids such as G1-SM40 may be important for improving the exploitation pattern of Mediterranean trawlers and have a positive impact on important species such as DPS and HKE.