Mediterranean Bottom Trawl Research Articles

Every animal matters! Evaluating the selectivity of a Mediterranean bottom trawl fishery from a species community perspective.

Bottom trawl fisheries often catch several species simultaneously. However, most studies addressing the catch performance and selectivity of a specific trawl focus on a few commercially important or most vulnerable species requiring management measures. By contrast, the present study considers the multispecies nature of Mediterranean bottom trawl fisheries through a holistic approach that accounts for the full species community in the catches. Specifically, we evaluated and compared the catch performance of the two codends allowed for this fishery, made of 40 mm square (SM40) and 50 mm diamond (DM50) meshes. Results showed that 50 and 80% of the catch in weight and count numbers, respectively, consisted of species without commercial value, demonstrating that large proportions of the catch are not considered when using the existing approach to evaluate the ecological impact of the fishing activity. Significant differences in catch profiles between the two codends were observed, especially for two commercial flatfish species, Arnoglossus laterna and Citharus linguatula, with larger contributions in the SM40. Further, the SM40 codend had a significantly higher retention, compared to DM50 codend, for specific sizes of Merluccius merluccius and Mullus barbatus. The outcomes of the study can be useful for the Mediterranean bottom trawl fisheries management.

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

The catch composition of bottom trawls is commonly refined and improved through changes in codend design. Measures like reducing the number of meshes in codend circumference or turning diamond netting by 90 degrees are well known to improve the size selectivity of fish species with rounded cross-sectional shape. Based on this we speculated whether the same measures, if applied in other parts of a bottom trawl, would provide similar benefits as in the codend. Therefore, experiments were carried out by deploying these changes to the trawl extension piece in a Mediterranean bottom trawl fishery. However, for European hake and monkfish, results showed no indication of improved selectivity or catch pattern compared to the standard extension piece in the trawl. Contrary, for red mullet, one of the most important species in this fishery, reducing the number of meshes in the circumference of the extension piece jeopardized the size selection obtained in the trawl with a standard extension piece. The lesson learnt from this study was that the design changes that work for the codend do not necessarily work for other parts of the trawl. In fact, they can even have negative effects.

Effect of ground gear modification on bycatch of rays in mediterranean bottom trawl fishery

Bycatch of rays and skates in towed fishing gears represents one of the major threats to these relatively slow-growing marine species. The objective of this study was to modify ground gear in a bottom trawl fishery to increase the escape of these species during towing without associated loss of target catch. Sea trials were carried out with a research vessel in Mersin Bay, North-eastern Mediterranean. Experimental ground gear was modified by cutting the rigging twine between the fishing line and the footrope in the central part of the ground gear. Capture of three unwanted bycatch species were estimated. The probability of capture of guitarfish (Rhinobatos sp.) and common stingray (Dasyatis pastinaca) was significantly reduced to 8% and 20% for guitarfish and stingray, respectively compared to standard ground gear. The results for spiny butterfly ray (Gymnura altavela) were inconclusive due to the wide confidence intervals. Further, the catch comparison results for five out of six target species investigated did not show significant reduction in catch efficiency when using experimental gear compared to the standard trawl. Only for common sole (Solea solea) the modified trawl had significantly lower catch efficiency than the standard trawl. We believe that this technical measure for reducing unwanted bycatch in bottom trawls has a potential to be adopted by the fishery due to being an efficient, low-cost measure which does not create additional challenges during handling of the gear.

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

AbstractThe 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.

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.

Discards reduction of non-commercial benthic species from a simple net modification

The high discards rates of non-commercial benthic species is one of the main problems of the Mediterranean bottom trawl fishery developed on the continental shelf. The present work analyzes data from 22 bottom trawls, carried out in parallel by two commercial trawlers between 70 and 90 m depth on maërl beds off eastern Menorca (Balearic Islands). One vessel used an experimental trawl gear (EXP) equipped with a lighter and smaller groundrope in the mouth of the net, leaving a gap without net between the main groundrope and the secondary one. The other vessel used a trawl gear traditionally used by the fleet operating in the area (TRA), without that modification. Catches, discards and commercial yields between both gears were compared. Discarded biomass of non-commercial benthic species with EXP was reduced by half when compared to TRA, particularly for rhodoliths (54%) and ascidians (51%). The reduction of discarded rhodoliths is especially relevant in the Balearic Islands where sensitive habitats, such as maërl beds, are an important part of the benthic communities. The modification of the net, at the same time, does not imply a reduction of yields of commercial captures and can easily be implemented in any bottom trawl net. It is concluded that this could be an appropriate and plausible technical measure to improve the ecological efficiency and the sustainability of the Mediterranean bottom trawl fishery.

Estimating selectivity of experimental diamond (T0) and turned mesh (T90) codends in multi-species Mediterranean bottom trawl

The effect of shifting from a diamond mesh codend (T0) to a 90° turned mesh codend (T90) on the size selectivity of seven commercially important species in the Mediterranean bottom trawl fishery was evaluated. During sea trials conducted in the north-western Adriatic, two experimental codends made of 54 mm nominal mesh size netting that differed only in mesh configuration were alternately mounted on the same trawl. Overall, the size selectivity was significantly improved for all the species analysed, shifting from T0 to T90. The difference in the average expected L50values between T90 and T0 was particularly marked in European hake (Merluccius merluccius, 21.26 vs 11.26 cm total length), common squid (Loligo vulgaris, 12.06 vs 7.88 cm mantle length) and mantis shrimp (Squilla mantis, 20.78 vs 13.35 mm carapace length). Both codends had an excessive size selectivity especially for red mullet (Mullus barbatus), Mediterranean horse mackerel (Trachurus mediterraneus) and whiting (Merlangius merlangus), thus involving a commercial loss. These findings demonstrate the efficiency of the T90 codend tested at excluding undersized specimens, especially of hake, whose average L50was above the minimum conservation reference size of 20 cm. The adoption of this practical and inexpensive solution could help improve the management of the demersal resources targeted by the Mediterranean bottom trawl fishery.

Effect of a lateral square-mesh panel on the catch pattern and catch efficiency in a Mediterranean bottom trawl fishery

The current legal codends used in Mediterranean bottom trawl fisheries are at high risk of retaining undersized individuals of several commercial fish species. This entails that codends alone are unable to provide the desired exploitation pattern. A simple technological measure that potentially can provide higher release efficiency of undersized fish are Square-Mesh Panels (SMPs). SMPs are often applied in the upper section of the trawl belly, just ahead the codend. However, recent studies in the Mediterranean have demonstrated that SMPs mounted in this position provided limited release efficiency, because very few fish came into contact with their meshes. In attempt to improve SMPs efficiency in the Mediterranean bottom trawl fisheries, we applied them on the lateral sides of the last tapered section of the trawl belly, just ahead of the codend, and fitted two guiding panels in the trawl belly to enhance fish-SMP contact. We compared the catch performance of a standard commercial trawl with a 52 mm diamond-mesh codend and of a similar trawl fitted with lateral SMPs (70 mm mesh size) in the belly using a twin trawl. The study focused on red mullet (Mullus barbatus), a commercially important species, but data for gurnard (Chelidonichthys lucerna and Chelidonichthys cuculus) were also obtained and analysed. In contrast to previous research on SMPs mounted in the top panel of the trawl, in this study SMPs induced a significant effect on catch performance for red mullet, demonstrating that their lateral position involved greater fish-SMP contact. However, since the test trawl lost a significant amount of legal-sized red mullet compared with the standard trawl, the effect was not wholly positive, possibly due to an excessively large mesh size. Therefore, future studies should be encouraged to test lateral SMPs with smaller mesh sizes.

Combining selection models and population structures to inform fisheries management: a case study on hake in the Mediterranean bottom trawl fishery

The reduction of juvenile catch and discards are important targets in fisheries policy. This work examines how selection models can predict the size structure (LFD) of discards and landings considering the effects of area, time period, gear and fisher behaviour. Additional exploitation indicators related to the gear used, fisher's selection pattern, and discards were also estimated. The approach is demonstrated in a study concerning hake in the Mediterranean trawl fishery, focusing on high (HRA) and low (LRA) recruitment areas in the Saronikos Gulf (Eastern Mediterranean) during two periods (June, September) using two codends (40 mm square-40S, 50 mm diamond-50D mesh). The predicted discards LFDs revealed generally higher percentages in the HRA in June when using the 50D. The predicted landings LFDs showed higher percentages in the LRA in September for both codends, but undersized hake were always included. LFDs and exploitation indicators indicated that both codends were inappropriate for sustainable fishing of hake in the HRA, where 50D performed worse than the 40S. Fishing with both codends in the LRA in September revealed the lowest discards rates and minimum sizes of landings close to MCRS (minimum conservation reference size). These results can provide information to fisheries management aiming to protect juveniles and reduce discards through spatio-temporal fishing closures.

Towards more selective Mediterranean trawl fisheries: are juveniles and trash excluder devices effective tools for reducing undersized catches?

Mediterranean trawl fisheries are characterized by a critical combination of high fishing effort and low size at first capture for most commercial species. In this study we tested the use of sorting grids installed on bottom trawl nets to minimize the catch of undersized deep-water rose shrimp (Parapenaeus longirostris, DPS) and European hake (Merluccius merluccius, HKE). The catch of a traditional trawl net was compared with that obtained with the same net equipped with ad hoc designed juveniles and trash excluder devices (JTEDs) during an experimental survey. Three different JTEDs were tested: the first (G1-SM40) was built with a net of 40-mm square mesh; the second (G2-ST20) and third (G3-ST25) with vertical steel bars spaced 20 and 25 mm, respectively. The probability of retaining DPS and HKE by length class in trawl nets with and without JTED was analysed using generalized additive mixed models. With G1-SM40, the reduction of undersized individuals in the cod-end was about 60% and 44% for DPS and HKE, respectively. With G2-ST20, a 34% catch decrease of HKE individuals smaller than 20 cm total length was observed. A loss of marketable fractions of DPS was recorded with G1-SM40 (–25%) and G2-ST20 (30%). Finally, JTD G3-ST25 was efficient at reducing the catch of undersized specimens of DPS and HKE, but showed a higher loss of marketable fractions than the other JTEDs. Although further fishing trials would be required to maximize the sorting efficiency of the grids, the results obtained clearly indicated that grids can substantially reduce unwanted catches of undersized DPS and HKE in Mediterranean bottom trawl fisheries.

Assessment of the relative catch performance of hake, red mullet and striped red mullet in a modified trawl extension with T90 netting

We studied the relative catch performance of a modified trawl fitted with an extension piece using a 90º turned mesh (T90) in comparison with a standard trawl net used in NW Mediterranean bottom trawl fisheries employing a diamond mesh net. The comparison was made by means of paired experimental hauls using the same fishing vessel with alternate deployments of the standard net (control) and the experimental net. We used the catch comparison approach for three target species of the fishery: European hake, red mullet and striped red mullet. Our results show that the experimental net significantly reduces the catches of small-size hake and red mullet (though there was no discernible difference for striped red mullet), reducing unwanted catches of regulated species under the Landings Obligation. The overall catch rates of hake, pooled over all sizes, also increased by an estimated 50%, while the catch rates of red mullet and striped red mullet were significantly lower. However, considering all commercial species, the experimental net produced losses of commercial catch and income estimated at 17% and 18%, respectively, which may pose a barrier to the adoption of this relatively simple, inexpensive solution.

Predictive models for codend size selectivity for four commercially important species in the Mediterranean bottom trawl fishery in spring and summer: Effects of codend type and catch size.

Models to predict codend size selectivity for four major commercial species—European hake (Merluccius merluccius), Norway lobster (Nephrops norvegicus), deep-water rose shrimp (Parapenaeus longirostris), and Atlantic horse mackerel (Trachurus trachurus)–in Mediterranean bottom trawl fisheries were established based on data collected during fishing trials using the two legal codends: a 40 mm square-mesh codend and a 50 mm diamond-mesh codend. The models were applied to predict the extent to which size selection depend on codend type, also accounting for the potential effect of codend catch size and fishing season. The size selectivity of the two codends was evaluated and compared in identical simulated controlled conditions. Mesh type significantly affected the size selection of Norway lobster alone, with a slightly better performance of the 40 mm square-mesh codend. A high risk of retention of undersized individuals was predicted for both codends for all species except Norway lobster.

The “discard problem” in Mediterranean fisheries, in the face of the European Union landing obligation: the case of bottom trawl fishery and implications for management

Since the first introduction of the landing obligation (a.k.a. Discard ban) in 2015, the EU Mediterranean fisheries are facing some unforeseen challenges. The demersal bottom trawl fisheries, being the most significant contributors to the so-called 'discard problem', are confronted with the greatest challenges. Data from the Italian and the Greek fleet, spanning over two decades (1995–2015), were analysed with the intention of revealing the diversity and heterogeneity of the discard problem, especially for regulated species. Species composition of discards, as well as discarding rates, were shown to be irregular, fluctuating among areas, depth strata, seasons and years. Although fish dominated the discarded gross catch in weight, benthic invertebrates (other than commercial cephalopods and crustaceans) were the taxa discarded almost exclusively. The established minimum conservation reference size was largely ignored by fishers. From a management point of view, the present investigation suggests that the recently established Discard Management Plans lack scientific evidence (given the high intrinsic variability of the parameters and confusion regarding the rules) and provide exemptions from the landing obligation that will in practice allow the average Mediterranean bottom trawl vessel to continue business as usual. Moreover, detecting if these rules are actually respected is an almost impossible task for the Mediterranean control and enforcement authorities. Incentivizing the adoption of fishing technologies and practices that reduce pre-harvest mortality and post-harvest discards, while avoiding damage to sensitive marine species and habitats, seems the only way to move forward, rather than dealing with the problem after it has occurred.

Otolith chemistry of Champsodon nudivittis (Ogilby, 1895) and Nemipterus randalli (Russell, 1986) in Iskenderun Bay, Turkey

The study objective was to investigate the chemical composition of otoliths of two Lessepsian fish migrants, namely, Champsodon nudivittis and Nemipterus randalli, which thrive in the Iskenderun Bay, Turkey. The study specifically investigated the age structure and explored differences in chemical otolith composition in relation to age. Samples were collected using a traditional Mediterranean bottom trawl (mesh size 44 mm) at depths of 45 to 90 m. A total of 78 Champsodon nudivittis (size range, 6.0 to 14.0 cm) and 60 Nemipterus randalli (size range, 6.1 to 17 cm) were captured in May 2012. Age readings were carried out (sectioning technique). Additionally, the concentrations of Na, K, Li, and Ca were determined using flame photospectrometry. The results revealed that the concentrations of Na (5.70 mg/g) and K (4.45 mg/g) in otoliths of Nemipterus randalli were predominant elements after Ca (128.71 mg/g). The concentration of Li in otoliths was also statistically different in the two species. This study contributes to the knowledge of the otolith chemistry in the two Lessepsian fish species now living in the same (but new) geographical region.

Can a square-mesh panel inserted in front of the cod end improve size and species selectivity in Mediterranean trawl fisheries?

An experiment was conducted to assess the selectivity in a typical Mediterranean bottom trawl, equipped with a square-mesh panel inserted in front of the cod end, for Atlantic horse mackerel (Trachurus trachurus), European hake (Merluccius merluccius), red mullet (Mullus barbatus), poor cod (Trisopterus minutus), broadtail shortfin squid (Illex coindetii), and deep-water rose shrimp (Parapaeneus longirostris). The release efficiency of undersized individuals through the panel was low. The differences in selectivity between the gear with and without the panel were very small. The low release efficiency of the square-mesh panel was caused by the lack of fish contact with the panel as they drifted towards the cod end, since the average contact probability was estimated not to exceed 9% for any of the species investigated. A low probability of contact with the selection device was thus found to be the reason for the low efficiency of the square-mesh panel.

Preliminary assessment of discards survival in a multi-species Mediterranean bottom trawl fishery

Preliminary assessment of discards survival in a multi-species Mediterranean bottom trawl fishery

Old Info for a New Fisheries Policy: Discard Ratios and Lengths at Discarding in EU Mediterranean Bottom Trawl Fisheries

Discarding is considered globally among the most important issues for fisheries management. The recent reform of the Common Fisheries Policy establishes a landing obligation for the species which are subject to catch limits and, in the Mediterranean, for species which are subject to Minimum Conservation Reference Size (MCRS) as defined in Annex III to Regulation (EC) No 1967/2006. Additionally, several other initiatives aim to reduce unwanted catches of target and bycatch species, including species of conservation concern. This raises the need to study discarding patterns of (mainly) these species. In this work we collated a considerable amount of historical published information on discard ratios and lengths at discarding for species caught in EU Mediterranean bottom trawl fisheries. The main aim was to summarize the available historical records and make them more accessible for scientific and managerial needs, as well as to try identifying patterns in discarding. We show discard ratios and lengths at which 50% of the individuals were discarded (L50) for 15 species (nine bony fishes, three crustacean decapods and three elasmobranchs). Discard ratios were usually low for target species such as hake, red mullets and highly commercial shrimps and exemptions from the landing obligation under the de minimis rules could be sought in several cases. Discard ratios were usually higher for commercial bycatch species. Discarding is affected by a combination of factors and for a given species, especially for non-target ones, discards are likely to fluctuate within a fishery, across seasons, years and regions. For most species considered, L50s were lower than the MCRS (when in place) and length at first maturity. L50s of target species, such as hake, were very small due to the existence of market demands for small sized individuals. However, for species of low demand, like horse mackerels, a higher retention size was observed, often exceeding MCRS. Lengths at discarding are affected by legal provisions, market demands but also by biological, population and ecological traits. Understanding the factors that affect discarding constitutes the starting point for designing mitigation measures and management plans to reduce discards and improve the sustainability of the stocks.

Can a square-mesh panel inserted in front of the codend improve the exploitation pattern in Mediterranean bottom trawl fisheries?

A 50mm square-mesh panel was inserted in front of the codend in a Mediterranean bottom trawl as an attempt to improve the exploitation pattern for Trachurus trachurus (Atlantic horse mackerel), Merluccius merluccius (European hake), Mullus barbatus (red mullet), Trisopterus minutus capelanus (poor cod), Illex coindetii (broadtail shortfin squid), and Parapenaeus longirostris (deep-water rose shrimp). Escapement through the square-mesh panel was low, never estimated to be above 10% of the total escapement, implying that a square-mesh panel inserted in front of the codend only had minor effect on the exploitation pattern in the fishery.

An interaction index to predict turtle bycatch in a Mediterranean bottom trawl fishery

Incidental catch is the major threat to the survival of loggerhead turtles (Caretta caretta) in the Mediterranean and the main reason for their decline. More than 100,000 turtles are estimated to be caught annually due to fishing practices; 10–50% die. Bottom trawls are among the fishing gears exerting the worst impact on sea turtle populations, especially in the coastal waters of Tunisia and Libya, northern Adriatic Sea, the Mediterranean coastal areas of Turkey and Egypt, where the continental shelf is large and turtles in the demersal stage are commonly found also in winter. Seven adult-sized loggerhead turtles captured incidentally by bottom trawls in the central-northern Adriatic Sea and treated in rescue centres were tagged with satellite transmitters before release. In this paper Italian bottom trawl track data obtained from vessel monitoring systems (VMSs) were analysed by VMSbase R suite to identify the areas of maximum fishing effort in the Adriatic. These data were combined with satellite information on turtle distribution to provide an interaction index enabling prediction of potential trawl–turtle interaction hotspots and periods. The present pilot study can be considered as a risk-analysis approach directed at identifying the areas and times of possible trawling–turtle interactions in a Mediterranean trawl fishery. By identifying the areas and seasons at highest risk of turtle bycatch, the index here developed has the potential to provide key information to design and implement mitigation strategies. Vessel monitoring system is actually in force in several countries, therefore the approach here studied might have a worldwide application.

Influence of sea-bottom temperature and depth on swept area estimation from trawl surveys

The methodological approach to data collection and analysis in Mediterranean bottom trawl surveys has changed considerably since their beginnings in the mid-1980s. The introduction of new technologies in surveys greatly improved knowledge on trawl underwater geometry, but also raised a question on the reliability of the data obtained. The most commonly used trawl monitoring systems in the Mediterranean use the constant speed of sound when calculating horizontal distance from one sensor to another. However, the speed of the sound in seawater is not constant because it depends on water temperature, pressure, and salinity. This is known to affect the performance of the horizontally transmitting sonars, and it is reasonable to assume that unless it is properly compensated for, the readings from trawl monitoring systems can be incorrect, resulting in biased swept area estimates, and hence potentially producing bias in abundance estimates. In the knowledge that speed of sound depends on temperature, salinity and pressure, the Del Grosso (1974) equation was used to calculate the compensation coefficient for a series of theoretical depth and temperature data. A simple model is proposed in the current paper for the manual correction of the horizontal net opening and swept area estimates. Temperature and depth data obtained from the MEDATLAS project are used to show seasonal and spatial variation in the near-bottom speed of sound, and the results demonstrate that, unless compensated for properly, the error in the horizontal net spread ranges from –1.5% to 2.9%.