Minimum Conservation Reference Size Research Articles

The challenge of harvesting common sole (Solea solea) in highly selective trawl fisheries

Common sole (Solea solea) is an economically important species in several European trawl fisheries, yet little is known about the size selective properties of codends used in bottom trawl fisheries targeting sole. This study presents results from a sea trial conducted in the inner Danish waters where common sole is fished in a seasonal trawl fishery using a 90 mm diamond mesh codend with a mandatory large mesh escape panel. To improve understanding of the selectivity in this gear, we in addition tested a plain 90 mm diamond mesh codend without an escape panel. This combination of codend mesh size and large mesh escape panel is part of an ambitious management plan aimed at eliminating bycatch of cod (Gadus morhua) in trawl fisheries in the inner Danish waters. In the fishery for common sole, we found a severe mismatch between gear regulations and minimum conservation reference size of the target species. The outcome is a highly inefficient fishery in which only 22 % (CI: 18–27 %) by weight of the marketable sole is retained in the 90 mm diamond mesh codend. Further, we estimated that 25 % (CI: 16–35 %) of the sole entering the codend would contact the mandatory escape panel and escape, resulting in a total loss of 83 % (CI: 79–87 %) of marketable sole through the mandatory gear. The inefficiency in this fishery demonstrates the need for other means than gear specifications to regulate this type of fishery.

Single-species quotas drive discards by multi-species trawlers in the Celtic Seas ecoregion when their relative abundance fluctuates

Abstract Our study investigated discarding patterns by bottom otter trawlers over 17 years prior to the full implementation of the Landing Obligation. A generalized additive modelling approach was applied to at-sea sampling data to examine the most important drivers of the discards per unit effort (DPUE) of cod, whiting, hake, and haddock in the Celtic Seas ecoregion from 2002 to 2018. Many operational, environmental, regulatory, and spatio-temporal factors were tested. Fish above and below the minimum conservation reference size (MCRS) were modelled separately to compare drivers of discarding of these categories of unwanted catch. In the full time period analysis, the three most important factors in determining DPUE were geographic location, an interaction between geographic location and year, and year interacting with month. Further analysis of a subset of the time series indicated that quota availability significantly impacted the DPUE for <MCRS haddock, <MCRS whiting, and >MCRS hake, while the quota allocated for one species often significantly influenced the DPUE of other species and size classes. Quota interacting with abundance had a significant impact for all >MCRS species and <MCRS hake. This indicates that changes in relative abundance compound discarding under quota constraints. Collectively, these results empirically demonstrate that single-species quotas majorly contribute to discards in a mixed-fisheries context.

Length matters: Effects of fishing gear and fishing behavior on the catch efficiency of demersal seines

Raiding seals pose a big problem to gillnet fishers in areas with high seal abundances. As moving to active gears could be a potential solution for that problem, one gear of particular interest in areas with relatively flat sea bed structure is the so-called “MiniSeine” – a demersal seine that is reduced in size so it can be operated from small vessels. Besides its ability to catch most of the species targeted by gillnet fishers, it offers various advantages compared to other active gears. To reduce the gear in size to fit on a small vessel, the present study assessed how seine rope length (4 coils vs 8 coils), seine rope diameter (18 mm vs. 22 mm) and seine net configurations of different sizes and shapes (three different designs) affect the catch efficiency and the ratio of fish below minimum conservation reference size (MCRS). In general, shorter seine ropes (4 coils) resulted in significantly lower catches than longer ones (8 coils), except for the smallest seine configuration. Larger seine net configurations and longer seine ropes caught generally less fish below MCRS, both up to around 10 %. The seine rope diameter did not affect catches significantly. As potential “adjusting screw” to counteract lower catches due to shorter seine ropes, the effect of different layout patterns of the seine ropes on the catch efficiency was assessed. Laying out the second seine rope perpendicular to the first one can increase the fishing area to more than three times while CPUE showed a tendency to be increased compared to the standard layout pattern. Besides its importance for a successful development of the MiniSeine, these findings are of equal interest for the large-scale demersal seine fishery.

Sorting grid usage to reduce the catch of undersized narrow‐clawed crayfish (Pontastacus leptodactylus Eschscholtz, 1823) in fyke net fisheries

AbstractSustainability of natural stocks of crayfish is critical because of their economic and ecological importance. Crayfish (Pontastacus leptodactylus), one of the most important species in Turkish inland fisheries, has been facing serious threats recently. One threat the species faces is high landed bycatch of undersized crayfish because of selective fishing gear, such as fyke nets. We estimated size‐selectivity of sorting grids with various bar spacings for crayfish to reduce bycatch. Six bar spacings (15.0, 17.5, 20.0, 22.5, 25.0, and 30.0 mm) were tested alongside commercial crayfish fyke nets. Mean selectivity (L50) (±95% CI) was 6.90 (6.63–6.99) cm for 15.0 mm bar spacing, 7.52 (7.16–8.02) cm for 17.5 mm, 8.52 (8.32–8.71) cm for 20.0 mm, 9.70 (9.28–9.90) cm for 22.5, 10.75 (10.34–11.17) cm for 25.0 mm, 11.80 (11.31–12.24) cm for 30.0 mm, and 8.79 (8.05–9.29) cm for commercial fyke nets. The proportion of undersized specimens caught be reduced from 32.27% for commercial fyke nets to as low as 2.46% by using sorting grids. The codend of 22.5 mm bar spacing was more selective, with significantly lower discard rates, and more effective for individuals above the minimum conservation reference size than commercial fyke nets.

Evaluating the Sustainability of an Eastern Mediterranean Gillnet Fishery Based on the Catches of Undersized Individuals and the Reproductive Period of Targeted Species

The catch composition of a coastal gillnet fishery in the Eastern Mediterranean Sea was analyzed through a two-year experimental fishing survey. Seven fish species occurred regularly in the hauls. Surmullet, Mullus surmuletus, which is the most valuable demersal fish in Greek waters and the intended target of the gillnets in small-scale fisheries, was the most abundant and systematically caught species. Almost all surmullets were larger than their minimum conservation reference size. However, three commercially exploited species (Diplodus annularis, Pagellus acarne, and P. erythrinus) were caught systematically as undersized individuals. In addition, these three species were caught mostly as immature individuals. Moreover, the operational season of the surveyed métier overlapped completely with the reproductive period of five commercially exploited species (D. annularis, M. barbatus, M. surmuletus, Sphyraena sphyraena, and Trachurus trachurus). Improvements and the establishment of additional technical measurements should be considered for the small-scale gillnet fisheries in the studied area to attenuate their detrimental effects and achieve a better compromise between sustainable exploitation of the local multi-species fish resources and the need for an economically sustainable practice.

One mesh does not fit all: A dual compartment codend provides flexible selectivity opportunities to manage mixed fisheries

Fisheries management is moving from a single-species to an ecosystem-based approach to better balance the technical, biological, and economic aspects of the many mixed-species fisheries in the world. Most mixed fisheries are conducted with trawl gears, which have a hereditary challenge of providing a species and size selectivity that meets the development in management objectives of reducing unwanted catch. When fishing on sympatric species, a codend that allows separation of species into different compartments during towing can provide separated catch fractions with contrasting selectivity opportunities. In a case study, we quantified the processes of vertical separation and length-based selectivity for a dual compartment codend having an upper, large mesh compartment intended for cod (Gadus morhua) and a lower, small mesh compartment intended for Nephrops (Nephrops norvegicus). This allowed identification of which processes to improve to reduce unwanted catch. The gear concept delivered a complex selectivity profile while enabling high flexibility to adjust selectivity, also at sea, and provides a tool for the industry to ensure better compliance with increasing management ambitions. An unusual selectivity curve for cod resulted from the contrast in mesh size between the two compartments. Cod and Nephrops smaller than and just above the minimum conservation reference size (MCRS) were efficiently released through the 120 mm square meshes in the upper and the 60 mm square meshes in the lower compartment, respectively. Cod (47%) had a low probability of escaping when caught in the lower compartment, and Nephrops (9%) were lost when caught in the upper compartment. A compartment in the dual compartment codend may easily be changed at sea to adjust the selectivity according to the mix of species encountered, available quota portfolio, and management regime.

Effects of the implementation of T90 extension and 52 mm square mesh codend on the bottom trawl hake fishery of the north western Mediterranean

The Spanish bottom trawl fleet operating in the Mediterranean currently uses 40 mm square mesh codends. Its selectivity is still too low to overcome the overall problem of high proportions of immature specimens in the catches. Even for some target species such as the European hake (Merluccius merluccius), there are high proportions of individuals smaller than the minimum conservation reference size (MCRS). The aim of the present work is to assess the selectivity of three different configurations of the traditional net (TRA) used in the bottom trawl hake fishery off the northwest Iberian Peninsula, each including the following modifications: i) an extension piece made of 90° turned diamond mesh (T90), ii) a 52 mm square meshed codend (52S), and iii) both modifications combined (EXP). The experimental fishing survey applied two methodologies: alternate hauls to compare the different net configurations; and the covered codend method to calculate the selectivity parameters of 52 mm square meshed codend for the main commercial species. Catch comparisons showed no discard reduction using the T90 extension piece for any of the species analysed, nor any selectivity improvement. Conversely, the 52 mm square meshed codend showed a clear discard reduction for M. merluccius and a generalized improvement of selectivity for most commercial species. This improvement raised the 50% retention length (L50) for M. merluccius to 22.2 cm, well above its MCRS (20 cm), allowing to escape 90% of the undersized individuals. However, the implementation of the 52 mm square meshed codend would involve important economic losses for main target species, like M. merluccius and Mullus barbatus, representing up to 32 and 28% of the incomes, respectively. Considering all analysed species, economic losses using the 52 mm square meshed codend would represent 27% of the incomes obtained using the current 40 mm square meshed codend in force. Despite it, transition analyses showed that the yield per recruit of the main target species would recover after two years, and even increase up to 30% and 17% for M. merluccius and M. barbatus, respectively, after the fourth year of the implementation of the 52 mm square meshed codend.

Recreational harvesting of the wedge clam (Donax trunculus) in southern Portugal: Characterization of the activity based on harvesters’ perception and local ecological knowledge

This study was the first attempt to characterize the recreational harvesting of the wedge clam (Donax trunculus). For this purpose, 403 face-to-face interviews were performed in the summers of 2018 and 2019 in southern Portugal. The majority of the harvesters frequented the selected beaches during holidays and collected wedge clams as a hobby for an appetizer. The respondents reported an inter-annual variation in their catches of wedge clam with a decreasing abundance in 2018 followed by a recovery in 2019, but with more recreational harvesters collecting undersized individuals. Although most harvesters declared to consume D. trunculus, only a few bought wedge clams being the clams’ size a secondary factor in their valorization. The majority of the recreational harvesters did not know the species' minimum conservation reference size nor the legally allowed maximum daily catch. It was estimated that recreational harvesters can collect 13 tonnes of wedge clams during the summer season along the southeastern coast of Algarve. Even though manual harvesting is not an efficient fishing technique, the lack of selectivity of harvesters can eventually compromise the resilience of D. trunculus populations. Despite most respondents affirmed to get informed about marine biotoxins, most surveys were done during periods of interdiction for shellfish harvesting. The present results revealed that the population is still unaware of the health risks of consuming shellfish contaminated with biotoxins. Educational activities and awareness campaigns are required to further inform recreational harvesters about the regulations in force for this activity and the sanitary threats of marine biotoxins.

Reproductive Biology of Male European Hake (Merluccius merluccius) in Central Mediterranean Sea: An Overview from Macroscopic to Molecular Investigation.

The improvement of scientific knowledge about overexploited fishery resources allow us to provide scientific advice for their management and stock protection. By using a multidisciplinary approach, the aim of the study was to characterize, for the first time in the Central Mediterranean Sea (GSA 17), the reproductive biology of males of M. merluccius, currently highly exploited. A multi-year sampling from January 2017 to December 2019 was performed to exhaustively evaluate the sex ratio of the stock, while the 2018 annual sampling was selected to investigate the reproductive pattern of males. Individuals in spawning conditions were found every month, proving that M. merluccius is an asynchronous species, reproducing all year round, with a seasonal reproductive peak in spring/summer, as indicated by GSI. Five gonadal development stages were defined to fully describe the reproductive cycle of males. The macroscopic and histological L50, respectively 18.6 cm and 15.4 cm, were both below the Minimum Conservation Reference Size (MCRS). According to the mRNA levels, fsh and lh played a significant role during spermiation, whereas the gnrhr2a was involved at the beginning of sexual maturity. In the testis, fshr and lhr reached maximum expression levels before the spermiation. The hormonal stimuli of 11-ketotestosterone and its receptor were significantly higher when the specimen was in reproductive activity.

Assessment of size at first maturity for Ruditapes philippinarum from Arcachon Bay (French Atlantic coast): New insights for fishery management

Regarding fisheries, management strategies aim at regulating access to the resource and at enforcing resource conservation in order to achieve sustainable exploitation. In this context, Minimum Conservation Reference Size (MCRS) is a technical measure based on scientific knowledge of the species characteristics (sexual maturity, individual growth and natural mortality). Reproductive capacity is generally apprehended by using the size at first maturity SL50, the size at which 50% of the individuals are mature. The SL50 was unknown for the Arcachon Bay Manila clam population (Ruditapes philippinarum). This study provides a first estimate based on the histological analysis of a large sample of individuals (1420 clams ranging from 10 to 41 mm) from four distinct bay inner sites and collected when the majority of the individuals are mature (from June to August). Our methodological choice was made to allow comparison with other European R. philippinarum deposits but also to be recognized by the EU; an interest of practices harmonization was besides revealed for bivalves. Following a logistic regression analysis, the SL50 was estimated to be 26.7 mm for the totality of the samples considered, with moderate spatial variability. If only sexed individuals were taken into account in the estimation, the SL50 was 24.5 mm for females and 21.6 mm for males. All sites and hypsometric levels combined, it is estimated that SL50 is reached at an average age of 1.6 years, 32 mm at 2.3 years and 35 mm at 2.9 years with high variability at the scale of site-hypsometric level pairs. Considering the SL50 and the spawning activity usually observed intra-basin, at least half of the clam population would have spawned at least once before their potential catch if the MCRS was reduced from 35 mm (current regulations) to 32 mm (regulations under study). This information constitutes essential knowledge to be included in Joint Recommendations when considering a MCRS revision in a European context that recognizes a more important place dedicated to regionalization.

Changes in catch efficiency and selectivity in the beam trawl fishery for sole when mechanical stimulation is replaced by electrical stimulation

Beam trawl fisheries for sole are characterised by large amounts of unwanted bycatch (i.e. discards) consisting of fish below the Minimum Conservation Reference Size, unwanted fish due to low commercial value or lack of quota, and benthic invertebrates. In order to reduce the quantity of discards, a substantial part of the Dutch beam trawl fleet was allowed to replace the conventional tickler chain beam trawl (BT) with the pulse trawl (PT) on an experimental basis. The PT used electrical stimulation to immobilise and capture fish. Here we study whether pulse trawling reduced the amount of discards by comparing catch rates of landings and discards of BT and PT in the period 2009–2018 for a wide range of species. The PT caught (kg.km-2) significantly more marketable sized sole (Solea solea, 48 %), turbot (Psetta maxima, 8 %), brill (Scophthalmus rhombus, 28 %) and whiting (Merlangius merlangus, 95 %), and significantly less marketable sized plaice (Pleuronectes platessa, −16 %), cod (Gadus morhua, −32 %) and gurnards (−12 %). No significant difference was found for dab (Limanda limanda), gadoids, or rays and sharks. Among discards, the PT caught more undersized sole (27 %) and whiting (42 %) but less undersized plaice (−21 %), dab (−19 %) and grey gurnard (Eutrigla gurnardus, −31 %). The observed differences in species selectivity are discussed in relation to the response of fish to bottom trawl gear and the effects of pulse stimulation. For the benthic invertebrates Ophiuroidae, bivalves and crabs the PT caught fewer individuals (between −38 % and −57 %). No significant difference was observed in sea urchins and sea stars. Overall, this study shows that the transition from the BT to PT resulted in a 36 % decrease (95 % prediction interval: 31–42 %) in discards (kg.hour-1).

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

In the Mediterranean, experiments and technical measures aimed at improving the selectivity of bottom trawl nets mostly concern modifications to the codend. Grids have been tested in various areas, but have not been adopted for management purposes so far. The present study aims to evaluate whether the adoption of diamond meshes turned by 90° (T90 configuration) and a selection grid with 2 cm bar spacing, both placed in the extension piece of a commercial bottom trawl net, can contribute in reducing the capture of specimens under the Minimum Conservation Reference Size (MCRS) for the main commercial species. The results indicated that the T90 configuration has no effect compared to the commercial net commonly used by fishers. The grid showed a reduction of the catches of European hake individuals between 11 and 19 cm Total Length (TL). However, the use of this device reduced the catch of many species, such as red mullet, deep-water rose shrimp and broad tail short fin squid. This reduction was observed also for specimens above MCRS. In conclusion, the T90 applied to the extension piece does not provide any improvement in selectivity, contrary to what has been observed when the T90 is placed at the codend. In contrast, the grid could be a possible way to increase selectivity. However, the use of grid should be further investigated to identify settings that can avoid the loss of a substantial fraction of commercial catch.

Characterization of discards along a wide bathymetric range from a trawl fishery in the NW Mediterranean

Discards represent a loss of natural resources and negatively affect the sustainability of fisheries. Information on discards, such as diversity and size of the species discarded is essential to better manage trawl fisheries. Thus, this study aims to gain knowledge on discards from the Catalan bottom trawl fisheries in the NW Mediterranean Sea to offer information needed for fisheries management. Discard ratios, species composition of discards and lengths of the discards were analyzed from data collected on board commercial trawlers, from November 2018 to December 2020 and analyzed by depth, zone, and season. Discard ratio varied among depths from 30.5% in the shallowest depth (20 – 70 m) to 14.3% in the deepest (400–700 m), with depth being the main factor determining the species composition of discards. This study also focused on the discards of six main commercial species, i.e. European hake (Merluccius merluccius), red mullet (Mullus barbatus), black-bellied angler (Lophius budegassa), poor cod (Trisopterus capelanus), deep-water pink shrimp (Parapenaeus longirostris) and Norway lobster (Nephrops norvegicus). The length at which 50% of individuals were discarded (L50d) was lower than the Length at First Maturity (LFM) and the Minimum Conservation Reference Size (MCRS) for the six species. These findings provide relevant information to comply with the Marine Strategy Framework Directive’s (EU Directive 2008/56/EC) ecosystem approach to fisheries and to evaluate the implementation of measures for fisheries’ best management practices, such as gear selectivity and spatial planning.

Effect of codend mesh sizes on the size selectivity and exploitation pattern of cocktail shrimp (Trachypenaeus curvirostris) in shrimp trawl fishery of the South China Sea

This study aims at improving fishery management by testing and comparing the size selectivity and exploitation pattern of diamond-mesh codends, with mesh sizes ranging from 25 to 54 mm, for cocktail shrimp (Trachypenaeus curvirostris) in the South China Sea (SCS). Beginning from 25 mm, the minimum mesh size regulated by the shrimp trawl fishery industry, we investigated how the mesh sizes of the codends would affect the size selectivity and exploitation pattern in reference to the minimum conservation reference size (MCRS, 7.0 cm total length) of cocktail shrimp in the SCS. According to our results, the selective properties of the codend mesh sized at 25 mm in line with the regulation failed to protect the undersized individuals of cocktail shrimp because of its 50% retention length of 5.85 cm (confidence interval, CI: 5.16–6.18 cm), far less than the MCRS of cocktail shrimp. As the retention probability of a shrimp with a MCRS length was as high as 97.45 % (CI: 86.86–100.00%), more than 40% of the undersized shrimp was retained. It was proved that increasing the codend mesh size did not significantly improve the size selectivity and exploitation pattern which might be improved by other gear modifications.

Length-girth relationships of 24 marine fishes in the northern Aegean Sea (eastern Mediterranean Sea)

The knowledge of morphological relationships and particularly of those concerning fish body girth (G) with total length (TL) is necessary in gear selectivity and specifically the technical measures to avoid capture of undersized individuals. This study concerns 24 marine species exploited by the small-scale coastal fleet in the Aegean Sea (eastern Mediterranean Sea), for 6 of which, the TL-G relationships are mentioned for the first time in the Mediterranean Sea and the adjacent seas. Samples were collected seasonally, from April 2016 to February 2017. The coefficients of the linear regression of body girth in three body positions, (Geye, posterior to the eye; Ghead at the posterior end of the operculum; Gmax at the maximum body depth), with the total length were estimated for each species and for the groups formatted when Geye, Ghead and Gmax were plotted against total length for all the species combined. Statistically significant differences among the three groups were detected (ANCOVA, P<0.001). Comparison of the total length-body girth relationships for 18 species previously studied in different geographic areas of the Mediterranean and the adjacent seas, showed differences mainly with the results from Portuguese waters for certain species populations. Based on the resulted equations, the maximum girth (G max) corresponding to the Minimum Conservation Reference Size (MCRS) and to the total length at maturity (L m) were calculated for each species. Identified mesh sizes respective to G max values were quite larger than the minimum legal mesh size for gillnets and the inner sheet of trammel nets, indicating that the relevant current fisheries regulations cannot meet the requirements for sustainable exploitation of fish resources.

Chamelea gallina reproductive biology and Minimum Conservation Reference Size: implications for fishery management in the Adriatic Sea

BackgroundThe striped venus clam Chamelea gallina is an economically important species in Adriatic Sea fisheries. The use of hydraulic dredging for its catch has a long history in Italy and its management faced several stages of development in the last 40 years. A great effort has been made in the past two decades to move from poorly or weakly managed fisheries to a well-structured co-management system to improve the sustainability of this fishery. However, a prerequisite for appropriate resource management is a sound knowledge of the biology and reproductive strategy of the species.ResultsWe investigated three major biological features– the gametogenic cycle, size at sexual maturity and partial fecundity – by microscopic, histological and video analysis techniques. We demonstrated that its breeding season is driven by rises in seawater temperature and chlorophyll-a concentration and that its spawning period lasted from March to September. Size at sexual maturity was reached very early in the life cycle. As regards partial fecundity – the number of mature oocytes potentially released by females with ripe gonads in a single release event – varied in relation to size. Nevertheless, the reduction on the Minimum Conservation Reference Size (MCRS) from 25 to 22 mm (Delegated Regulation (EU) 2020/2237) lead to a 40% reduction in the number of emitted eggs.ConclusionsWe suggest that the ability of Adriatic clam stocks to withstand the strong fishing pressure of the past 40 years and the present one is due to their high reproductive potential and multiple spawning events combined with the effect of management measures (closed areas/seasons, quota, MCRS) and technical constraints on the gear and the sieve on board. Moreover, since the reduced MCRS for Venus shells is still larger than the size at maturity, it will probably not be detrimental to the reproductive capacity of the stock.

Comparing size selectivity and exploitation pattern of diamond-mesh codends for Southern velvet shrimp (Metapenaeopsis palmensis) in shrimp trawl fishery of the South China Sea.

In this study, size selectivity and exploitation pattern of six diamond-mesh codends with different mesh sizes, ranging from 25 to 54 mm, for Southern velvet shrimp (Metapenaeopsis palmensis) were tested and compared in a shrimp trawl fishery of the South China Sea (SCS). We used a codend with a mesh size of 25 mm (D25) as a starting point, which is the minimum mesh size (MMS) currently regulated in the studied area. Four different fishing population scenarios were applied to quantify and compare how mesh sizes of codends used would impact the size selectivity and exploitation pattern for the target shrimp species. The results demonstrated that the D25 codend was not proper for protecting juvenile shrimp at the studied area. By applying this legal codend, L50 (50% retention length) of the target shrimp species was below its minimum conservation reference size (MCRS, 7.0 cm total length), the retention probability of shrimp with a length of MCRS was above 95% CI [91–99] and more than 43% of undersized shrimp was retained. To mitigate the bycatch issue of undersized shrimp, increasing the mesh size in the diamond mesh codend is a simple and effective option. However, the loss of catch efficiency for marketable shrimp is a major concern while increasing the mesh size. A good compromise between releasing undersized shrimp and maintaining the legal individuals is manifested by using the codend with 35 mm mesh size (D35). Our study will be beneficial for the management of shrimp trawl fisheries in the SCS.

An overview of bottom trawl selectivity in the Mediterranean Sea

In the Mediterranean Sea, where bottom trawling for demersal species is the most important fishery in terms of landings, around 75% of the assessed fish stocks are overfished. Its status as one of the world’s most heavily exploited seas and the one subject to the highest trawling pressure has become a global concern. An extensive review of bottom trawl selectivity studies was performed to assess the sustainability of this fishery in the Mediterranean; the selectivity parameters were collected from 93 peer-reviewed publications of 10 countries, totalling 742 records and 65 species. The review highlighted that i) the catch of bottom trawls commonly employed in the Mediterranean, even complying with current regulations on codend meshes, still includes immature individuals for 64-68% of the species investigated, and individuals under the minimum conservation reference size (MCRS) for 78% of the species investigated, and that ii) the MCRS set for 59% of the species analyzed is well below their length at first maturity, and is therefore ecologically inadequate. Although square-mesh codends are slightly more selective, the models developed herein demonstrate that improving size and species selectivity would require considerably larger meshes, which may significantly reduce profitability. The urgent need to reduce the biological impacts of bottom trawling in the Mediterranean should be addressed by promoting the adoption of more ecologically sustainable fishing gears through the introduction of more selective meshes or gear modifications.

Estimating overall size-selection pattern in the bottom trawl fishery for four economically important fish species in the Mediterranean Sea

The management of multispecies fisheries, such as the Mediterranean bottom trawl fishery, is always a challenge. However, information on gear selectivity and discards has been studied separately so far. In this paper, the overall size-selection pattern by the trawl codend in the sea and by the fisher onboard the vessel is investigated for four commercially important fish species, Mullus barbatus, Mullus surmuletus, Pagellus erythrinus and Lophius budegassa, using different codends. For each species, the selection model used offered the possibility to simultaneously describe the escape, discard, and landing probability. The results, useful for fisheries management, showed that the codend made of 40 mm diamond meshes was always detrimental for the stocks. The 40 mm square meshes codend compared to that of 50 mm diamond meshes was more appropriate for the sustainability of both Mullus species, providing also a lawful catch along with greater compliance to the rules fisher behaviour, negligible discards and the lowest possible economic losses for the fisher. None of the codends was effective for P. erythrinus in achieving the minimum conservation reference size (MCRS) of the species. All codends were harmful to L. budegassa as the majority of juveniles were retained in the codend, resulting in negligible escapees, a high discard probability, and landings of a size much lower than the length at first maturity of the species. Further studies are needed to be conducted in the future for other species, since the trawl fishery in the Mediterranean is a multi-species fishery.

Reburial potential and survivability of the striped venus clam (Chamelea gallina) in hydraulic dredge fisheries

The striped venus clam (Chamelea gallina) is the main edible bivalve living in Italian waters. According to Regulation (EU) 2020/2237, undersized specimens (total length of the shell, < 22 mm) must be returned to the sea. C. gallina specimens of different size classes that had undergone hydraulic dredging and mechanized sorting were analysed for reburial ability in a laboratory tank and for survivability in the laboratory (135 clams, 21 days) and at sea (320 clams, 15 days). In the tank experiments, the reburial times (T50 and T90) and the upper (+) and lower (−) confidence intervals (CIs) of the whole sample were about 4 h (CI+ 4.4, CI− 3.6) and 8 h (CI+ 8.2, CI− 7.7), respectively, and were significantly shorter for the medium-sized clams (22–24.9 mm) than for the smallest (< 21.9 mm) and the largest (> 25 mm) specimens. For the field survivability experiments, clams under and above the minimum conservation reference size were placed in separate metal cages. Survival rates were 94.8% and 96.2% respectively in the laboratory and at sea, without significant differences between the two experiments or among size classes. These findings conclusively demonstrate that C. gallina specimens returned to the sea have a very high survival probability and that they can contribute to mitigate the overexploitation of natural populations.