香京julia种子在线播放

Unwanted Catches (UCs) are common and critical problems in fisheries worldwide, as they can alter marine biodiversity and at the same time could worsen overexploitation of commercial species (Kelleher, 2005). UCs constitute the portion of the total catch that is returned to the sea, and typically include undersized individuals of commercial species and non-marketable species or with low market value (Alverson et al., 1994; Kelleher, 2005; Bellido et al., 2011). Recent assessment indicate as more than 40 percent of global catch is discarded outboard (Davies et al., 2009). The quantity of the UCs mainly depends on the characteristics of fishing grounds (e.g., geographic area, type of habitat, abundance of either juveniles of commercial species or non-commercial species etc.) and on the fishing gears used (Catchpole et al., 2005; Kelleher, 2005). In the Mediterranean Sea, demersal trawling is responsible for about 50 percent of the total discard produced (STECF, 2006), highlighting the low level of selectivity of this sector (FAO, 2016), while a lower discard rate characterizes other gear belonging to industrial pelagic fishing, such as the pelagic trawl or purse seine. As regards artisanal fisheries, few data have been collected to date on the discarded fraction, but the first results obtained show that this sector has a higher level of selectivity (FAO, 2019). Other factors influencing the UCs production include technical characteristics of the vessels and gear (e.g., mesh size) (Graham et al., 2008), fishing strategies (Lleonart and Maynou, 2003), fishing season and environmental conditions (Milisenda et al., 2017b). Nonetheless, “the decision of discarding part of the catch is a choice that fishermen ultimately take, and is mainly driven by economic reasons or fishing regulations” (Batsleer et al., 2013; Condie et al., 2014).

Under the current trawl net selectivity patterns used in southern European fisheries, undersized individuals of some important commercial species, such as European hake (Merluccius merluccius) and Atlantic horse mackerel (Trachurus trachurus), predominate in the discard (Milisenda et al., 2017b; Damalas et al., 2018; Despoti et al., 2020), especially during the recruitment period (FAO, 2016). The combined provisions of art. 14 on “Avoidance and minimization of unwanted catches” and art. 15 on “Landing Obligation – LO” under the reformed Common Fisheries Policy – CFP (Reg. EC n. 1380/2013) has the objective of reducing the excessive practice of discarding unwanted catch at sea. The new Common Fisheries Policy promotes the technical measures aimed at reducing and discouraging the capture of specimens below the Minimum Conservation Reference Size (MCRS, Reg. EC No 1967/2006), demanding from fishermen also to land all the catches of regulated species. Given the specific characteristics of each Mediterranean sub-basin and the objective of moving toward regional-based fisheries management in the reformed CFP, “local management plans are needed as solutions to set up the most appropriate measures at regional level” (Vitale et al., 2016). In the southern European waters, both Mediterranean and Atlantic, there are many measures, both regulatory and technical, aimed to minimize unwanted catch including, for instance the discard ban, gear selectivity improvement, fisheries restricted areas, and “monitoring and control measures” (Karp et al., 2019; Common Fisheries Policy – CFP).

Even the Marine Spatial Planning (MSP) must take into account the reduction of discarding and its spatial dynamics, i.e., where and when the natural stocks and fleets interact (Maiorano et al., 2009; Bellido et al., 2019; D’Andrea et al., 2020). One possible approach to reduce discards is to avoid areas and seasons where UCs is more likely (O’Keefe et al., 2013; Vilela and Bellido, 2015; Paradinas et al., 2016), and for this reason data on the spatial distribution of undersized organisms is an important source of information to reach lower discard production at the regional level (Vitale et al., 2016). Highlighting the distribution and persistence of areas where high abundance of organisms under the MCRS is present can support the designation of no-take zones or Fishery Restricted Areas (FRAs) and their inclusion in conservation networks (Colloca et al., 2015). The GFCM has already started working in this direction, decreeing the formation of different FRAs in the Mediterranean Sea. Three of these are located in the Strait of Sicily as part of the multiannual management plan for hake and deep-water rose shrimp fisheries (“East of Adventure Bank,” “West of Gela basin,” and “East of Malta bank”) and another one in the Adriatic Sea (“Jabuka/Pomo Pit”) as an important essential fish habitat for hake and Norway lobster (Nephrops norvegicus). Besides areas closed to trawling according to the EC Reg. 1967/2006 (e.g., 3 miles or 1.5 miles from the coast/50 m isobath), two FRAs (Fishing Restricted Areas) are present in geographical subarea 9 of Mediterranean Sea (Ligurian Sea and Northern Tyrrhenian Sea): (1) Off the coasts of Argentario promontory (50 km², from 160 to 220 m depth); (2) in the Gulf of Gaeta (125 km² from 100 to 200 m depth). Both FRAs are located in areas with high concentration of juveniles (nursery areas) of European hake and deep-water rose shrimp. According to the Italian Ministry (MIPAAF) Decree (22nd January 2009), fishing activities with towed gears are forbidden in the two FRAs from January 1st to June 30th. In May 2020, in the West Med Multiannual Plan (WM MAP) Spain established two FRAs located in the geographical subareas 2 and 5. In the Portuguese Atlantic area, two Fishing Restricted Areas are established. These two FRAs are limited temporal closures, from the first day of December until the last day of February for hake and from the beginning of May to the end of August for Norway lobster. The main objective of both FRAs is to protect juveniles and their spawning stock of these two species from trawling or any other fishing activities using towed gears. Moreover, in the Mediterranean Sea there is an increasing interest in spatial based measures as management tools for achieving several objectives related to fisheries sustainability, such as species conservation, reduction of trawl impact on demersal community, restoration of essential fish habitat and preservation of vulnerable marine ecosystem (Fiorentino et al., 2003; Maiorano et al., 2009; De Juan and Demestre, 2012; Russo et al., 2014b, 2019; Pipitone et al., 2014; Colloca et al., 2015). Of course, the rationale of these measures is inspired by the fact that fishing effort pattern, at the scales of both individual vessels and fleets, is highly heterogeneous in space and time (Russo et al., 2013, 2019).

The aim of this paper is to identify critical areas for the production of UCs (and therefore, implicitly and intuitively, for the occurrence of discards) in four important fishing areas of the southern European waters, including both Mediterranean Sea and Atlantic Ocean off the Portuguese coast. These areas are detected by matching hotspots of high densities of individuals below MCRS, with high levels of trawling effort. The output of this study could provide a base for supporting the application of spatial based fishery management in order to minimize UCs and definitively improve the sustainability of demersal fisheries.

The present study provides a large scale assessment of the distribution and temporal persistence of areas with high density of individuals below MCRS of important commercial demersal species, based on a time series of bottom trawl surveys data. The study shows also the overlap of hot spots of undersized individuals with the areas that are persistently used by commercial trawlers. The most important result of the analysis is the first evaluation of the importance of different areas in the Mediterranean Sea and the Atlantic waters off the Continental Portuguese coast as potential source of discards for species subjected to the landing obligation in bottom trawling, due to MCRS in the Mediterranean or TAC for the Atlantic area. These results provide a relevant knowledge-base for the development of a spatial planning approach to fisheries management in order to mitigate negative effects of discarding.

Although our data comes from scientific surveys conducted in a given period of the year, previous studies have shown that the aggregation areas of juveniles of the species included in the study remain substantially unchanged in the different periods of the year (Abella et al., 2008; Colloca et al., 2009; Fortibuoni et al., 2010; Garofalo et al., 2011). The data on the spatial and temporal distribution of the trawl fleet was obtained starting from daily data, therefore the hotspots of the highly-impacted fishing areas obtained (fishing effort Hot Spots) represent a good approximation of the average annual distribution, and the overlap zones between the PI indices and EPI a good indicator of reality. Moreover, we are aware on the possible error that could come from the temporal mismatch of the survey and the VMS/AIS data. However, to minimize this error we have considered the persistence over time of hotspots distribution of the fishing fleet. The persistent areas, calculated using the VME/AIS 2012–2016 time series (5 years) should well represent the areas “persistently” most impacted by trawling in the various areas. From a biological point of view, the fishing fleet is distributed according to the distribution of the fishery resource, which in turn is distributed according to environmental characteristics that hardly change drastically over the time span used in this study, therefore also the distribution of the fishing fleet could be considered stable in the considered time. Furthermore, we are aware that the species considered in the present study do not cover all the species regulated by the MCRS or the TAC, but since they represent in some cases up to 90 percent of the under-sized fraction discarded (Despoti et al., 2020), our results may be considered representative of a generalizable trend.

We found a quite high stability of the density hot spots of undersized juveniles as already observed for the nursery areas of these species in the North Mediterranean Sea (Colloca et al., 2015). This can be due to the combined effect of connectivity between spawning and recruitment areas, and the role of habitats for juvenile growth and survival, and effects of fishing pressure (Lortie et al., 2004; Guisan and Thuiller, 2005; Ferrier and Guisan, 2006; Algar et al., 2009; Wisz et al., 2013). Based on the frequency of occurrence (Table 1), European hake, is the species showing the widest distribution of individuals below MCRS in all the four case study areas, thus making it one of the mostly discarded species by the Mediterranean and Portuguese trawl fisheries (Borges et al., 2001; FAO, 2016; Milisenda et al., 2017b). As showed in Table 2, this species is the only species whose areas of persistence (IP) fall within the EPI areas in all four areas considered. European hake recruits and juveniles are abundant on the continental shelf, and in particular at the shelf break (Colloca et al., 2003, 2015). “The shelf break is a high-energy zone as a result of the interplay of tides, storm waves, breaking internal waves and bottom turbulence produced by fronts separating shelf and slope waters (Herman, 1972; Vanney and Stanley, 1983). The high production of plankton at the shelf break makes it an important feeding ground for fish (Williams et al., 2001) and other organisms” (e.g., cephalopods, crustaceans, macro-epibenthos, etc.) (Colloca et al., 2003). Results also show persistence areas with high abundance of deep-water rose shrimp, below the MCRS on the continental shelf, in proximity of the shelf break.

The main persistence areas of small-sized horse mackerel were found in shallower waters (∼100 m) than those of the other two species. As observed in previous studies the deep shelf habitats seem play a key role for the recruitment of horse mackerel (Milisenda et al., 2017a; Rumolo et al., 2017). This pattern of distribution in areas around 100 m depth makes juveniles of horse mackerel the ones with the highest spatial extension values of overlap between IP and EPI, compared to other species (Table 2). In fact, as shown in Figure 7, the areas of temporal persistence of the commercial fishing effort are mainly distributed along the 100-m bathymetry, acting mainly on the IP areas of the T. trachurus (Areas 1, 2, and 3), and of P. longirostris (Areas 2 and 3).

According to Colloca et al. (2015), the three main oceanographic mechanisms proposed by Bakun (1996) to explain the main spatial patterns of fishery resources, i.e., (1) “nutrient enrichment” (e.g., upwelling, water mixing), (2) “concentration” (convergence, frontal formation, and water column stability) and (3) “retention within (or drift toward) appropriate habitats,” enable the establishment and co-occurrence of nursery areas of different species. An exhaustive literature exists on the spatial distribution of juveniles of main commercial species in the Mediterranean Sea (Fiorentino et al., 2003; Maynou et al., 2003; Colloca et al., 2009, 2015; Fortibuoni et al., 2010; Garofalo et al., 2010, 2011; Giannoulaki et al., 2013; Druon et al., 2015; Ligas et al., 2015). However, these studies often differed in the criteria used to identify juveniles (e.g., young of the year, first cohort of the 0 age group immature specimens) and their main purposes were not always linked to the management of local fisheries. In our study we propose to use MCRS of target commercial species to explicitly identify stable concentration areas of undersized individuals in a clear management framework aimed at minimizing the discards of trawl fisheries also considering the interspecific spatial overlap.

Our results show the presence of four important areas, one in each case study, where individuals of important commercial species below the MCRS permanently coexist. These areas are located in front of Tarragona (Catalan Sea), in the eastern part of Adventure bank (Strait of Sicily), around Elba Island (Ligurian and northern Tyrrhenian Seas) and off Lisbon (Continental Portuguese coast), mainly between 100 and 200 m depth. The eastern part of Adventure Bank is an important area where nurseries of several commercial species are located (Fiorentino et al., 2003; Fortibuoni et al., 2010; Garofalo et al., 2011; Colloca et al., 2015; Di Lorenzo et al., 2018). Here, the surface current Atlantic Ionian Stream, generates some semi-permanent mesoscale features (e.g., upwelling, eddies, and fronts) (Robinson et al., 1999; Lermusiaux and Robinson, 2001). In particular, the eastern area of Adventure Bank is characterized by large cyclonic vortex, which produces an upwelling at its center, that in turn increases primary and secondary production (Agostini and Andrew, 2002). This area is exploited by the trawlers of Sciacca and Mazara del Vallo, being the most important fishing fleet operating in the area. Our results showed the importance of the bottoms between 100 and 200 m depth as those where undersized European hake, deep-water rose shrimp and horse mackerel aggregate. These results are in line with what was found by Gristina et al. (2013) in the Strait of Sicily. Most juveniles concentrate in the circalittoral and in particular on the Coastal Terrigenous Mud biocenosis. It is worth noting that three fisheries restricted areas (FRA) for the protection of part of the nurseries of European hake and deep-water rose shrimp have been established in 2016 (FAO, 2016) and are in force since July 2019 in the northern part of the Strait of Sicily.

In the Catalan Sea, important stable aggregations of undersized specimens were found in our study south of Tarragona and off the Ebro delta. These areas are coincident with the main nurseries of hake reported by Maynou et al. (2003) and are likely related to two factors that enhance local productivity, especially in the deeper continental shelf, close to the shelf break. First, the Ebro River outflow in combination with the local upwelling generated by the south flowing coastal current along the shelf break (Liguro-Provençal-Catalan or Northern current, Millot, 1999) increases biological productivity in the area off the Ebro delta, especially along its northern margin. In contrast, the highly stratified summer water column, characterized by limited vertical movement, high depletion of surface nutrients, and with a primary production restricted to the deep chlorophyll maximum (DCM, usually located between 60 and 80 m depth), at the deepest levels of the photic zone (Estrada et al., 1993; Maynou et al., 2020). The presence of the shelf-slope front combined with the seasonal cycle is the second factor contributing to enhancing the productivity in the southern part of Catalonia, particularly at the DCM level (Estrada and Salat, 1989). Despite the concentration of biological productivity in the southern part of Catalonia, the fishing activity is based on 20 harbors of similar importance, spaced regularly along the coast. Harbors in the northernmost part, near the French border, exploit the highly productive fishing grounds near the Gulf of Lions.

The largest IPI area was found in the Tyrrhenian seas. “The circulation in the Tyrrhenian Sea is organized in a series of cyclonic (anti-clockwise) and anticyclonic (clockwise) gyres determined by the wind” (Artale et al., 1994; Gasparini et al., 2005). These vertical movements and water turbulences enriches the upper layer with nutrients, giving the Tyrrhenian Sea a relatively high concentration of nutrients within the Mediterranean basin (Nair et al., 1992; Nezlin et al., 2004). These favorable trophic conditions allow the presence in these areas of the largest hake nursery in the western Mediterranean basin (Orsi Relini et al., 2002; Abella et al., 2008; Colloca et al., 2009; Ligas et al., 2015; Cantafaro et al., 2017). In the northern part of Elba Island, the wide shelf is characterized by soft bottoms proper to trawling, favoring the development of the fleet of Viareggio, which is the largest one in the Ligurian Sea. South of Elba Island, the shelf is slightly narrower and fishing activity is concentrated on the continental slope. Important trawling fleets operate in this area (Porto Santo Stefano, Porto Ercole, and Castiglione della Pescaia), targeting European hake, red mullet (Mullus barbatus), horned octopus (Eledone cirrhosa), deep-water pink shrimp, Norway lobster and giant red shrimp (Aristaeomorpha foliacea).

The west coast of Portugal is an important persistent nursery area of the species studied in this work, mainly in the northern and central part of the coast, near the outflow of major rivers. Three factors influence overall productivity in this area: this coast is characterized by an upwelling phenomenon during the summer months providing nutrients (Peliz and Fiúza, 1999; Edwards et al., 2005) and increasing overall productivity (Loureiro et al., 2005); in autumn, there is an increase of pigment concentration in the northern part of the western coast (Peliz and Fiúza, 1999). One of the most important factors influencing nursery areas seems to be the water outflows from the two most important rivers along Portugal’s coastline (Tejo and Douro), which contribute with nutrients for the high productivity areas reported near the river mouths (Peliz and Fiúza, 1999). According to Peliz and Fiúza (1999), the persistence of a strong pigment front near the shelf break zone establishes the limits between the rich coastal area and the oligotrophic oceanic waters. Relvas et al. (2007) also indicates that the establishment of convergence zones in the shelf break is significant for the retention of larvae, a factor that appears to limit the distribution of the nursery areas. Part of the nursery area for M. merluccius, located in the south part of Portugal, is already protected by a fishing regulated area created by the Regulation Council 850/98. This area excludes trawl activities from December to the end of February. The southern part of Portugal is explored by the southern fleet, located mainly in Portimão and Olhão fishing harbors and dominated by crustacean trawlers. Several other important fishing harbors, from Sesimbra to Matosinhos, explore the central and north fishing grounds operating in the persistence areas, however, the trawl fleet has a larger mobility being able to spread their activity at national level.

The safeguard of the areas with a high presence of young organisms is a priority for the management of Southern European fisheries resources because juvenile fish are very vulnerable to a fine-mesh trawl fishery (Caddy, 1993), particularly when they assemble in nursery areas, and a decrease in fishing mortality of immature fish is fundamental for more sustainable fisheries (Beverton and Holt, 1957). The effects of nurseries protection were assessed in a simple way by estimating the fraction of recruits in the stock that benefit directly by the closure (Colloca et al., 2013). Modeling the spatial distribution of specimens below MCRS could offer fine scale management possibilities to mitigate discarding practices (Jennings et al., 2012). Our results provide a significant contribution for developing functional marine spatial plans (MSP), identifying the principal vulnerable areas, where individuals of important commercial species below the MCRS permanently coexist (Bastardie et al., 2017).

Moreover, one common reason for failure of the closure areas strategy management is the shift of fishing effort from the closure into the neighboring area still open to fishing (Agardy et al., 2011; O’Keefe et al., 2013), leading an increase of by-catch in these surrounding areas, undermining or completely nullifying any bycatch reduction achieved by the removal of effort within the closure (Hiddink et al., 2006; Powers and Abeare, 2009). Therefore, fisheries managers should not only consider the by-catch reduction within a closure area, but also the potential by-catch production outside the closure area (Wilen et al., 2002; Halpern et al., 2004; Powers and Abeare, 2009). In this direction, the combined approach used in this study, combining fisheries independent (Scientific surveys) and fisheries dependent data (distribution of commercial fishing activity), can represent a winning solution. In fact, from the scientific surveys we can obtain the distribution of organisms under MCRS in the space, we can highlight the areas most affected by fishing, and identify the areas where fishing pressure should not be concentrated (e.g., by means of spatial closure). Furthermore, we are aware that the trawl net used in scientific surveys has a smaller cod-end mesh size compared to the commercial trawl nets, therefore it is capable of capturing also the smallest fraction of the population. For this reason, we used survey data to identify the “potential” discard areas (areas with high density of small-sized individuals) and not the undersized organisms that will be caught by commercial fishing. In addition, the results of this work may contribute to an augmented compliance with the landing obligation implemented by the new CFP, and a reduction in the fishing mortality of juveniles that is currently weakening the productivity of Mediterranean demersal stocks (Colloca et al., 2013), by highlighting the “potential” discard areas in the four studied regions.

In conclusion, the strength of our results is to make available for the existing (e.g., WestMed Multi Annual Plan of the European Union) and future management plans the spatial distribution of areas where the probability to catch organisms under the legal minimum size is significantly high. In the current scenario of the new CFP, the creation of management measures to make trawl fisheries more selective is mandatory, with knowledge about the key areas where high density of individuals under MRCS are usually present critical. In this way, instead of applying strict measures covering the whole fishing grounds, an adaptive spatial management based on temporal or permanent area closure can be enforced to reduce the catch of undersized specimens and protect the habitats that are important for recruitment processes. Therefore, considering that uncertainty pervades fishery management, mainly in a multispecies fishery where it seems unfeasible to optimize the yield for all stocks at the same time, specific actions like the Hot Spots conservation of individuals below MCRS could represent an action toward sustainable fisheries.

The datasets presented in this article are not readily available because some of the original raw data are protected by confidentiality. Requests to access the datasets should be directed to GM, giacomo.milisenda@szn.

GM created the original outline for the manuscript. GM, GG, TR, and LD’A conducted the data analysis. FF, FC, FM, AL, CM, LB, JG, KE, and SV provided input into drafting the manuscript. All authors participated in the editing and final preparation of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.