Bottom Trawl Research Articles

Selectivity-based management for reversing overexploitation of demersal fisheries in North-western Mediterranean Sea

Bottom trawling in the Mediterranean Sea has compromised the availability of demersal fishery resources, primarily due to decades of overexploitation beyond sustainability thresholds. Low selective bottom trawl nets have exacerbated this situation by catching large numbers of under-sized or immature individuals of targeted and not-targeted commercial species that are subsequently discarded. EU legislation requires EU Mediterranean countries to recover demersal fish stocks to sustainable levels by January 2025. This is a challenging requirement under current management scenarios that mainly focus on reducing fishing hours, compromising the activity of the fleets. While social networks among fishing fleets and fisherman associations are essential for facilitating the transition to sustainable exploitation, improving the selectivity of bottom trawl gears is proposed as a viable option beyond reducing fishing hours. This study presents the benefits of improving bottom trawlers net selectivity in multi-species NW Mediterranean coastal and deep-sea fisheries to reduce the catch of non-commercial sizes and discards. Data from selectivity experiments and literature review, fish market landings and a regional monitoring program in the FAO geographical sub-area 6 were used. Results indicate that in the short term, enhanced selectivity leads to an increase in the proportion of fish that are closer to their minimum conservation reference sizes or sizes at maturity, while minimising the impact on commercial catches. Sustainability may be achieved, though beyond 2025, with measures complementing selectivity. These include the adoption of semi-pelagic otter doors to mitigate the impact of trawlers on the seabed and the expansion of no-take marine reserves to ensure habitat recovery and spillover effects.

Seasonal and Spatial Distribution Characteristics of Sepia esculenta in the East China Sea Region: Transfer of the Central Distribution from 29° N to 28° N

The golden cuttlefish (Sepia esculenta) is an important cephalopod species with a lifespan of approximately one year. The species plays a crucial role in marine ecological support services and is commercially valuable in fisheries. In the seas around China, this species has emerged as the main target for cuttlefish fisheries, replacing Sepiella maindroni since the 1990s. Variations in oceanographic conditions associated with global warming could significantly impact the temporal-spatial distribution of the species. In this study, we performed bottom trawling surveys with four cruises during 2018–2019 in the East China Sea region to determine the current resource status and seasonal-spatial variations in S. esculenta. We found that the average individual weight (AIW) values were 4.87 and 519.00 g/ind at stations located at 30.50° N, 124.00° E and 30.50° N, 124.50° E, respectively, with the aggregation of larvae and parent groups in spring. The species was not distributed north of 32.00° N in summer. The catch per unit effort by weight (CPUEw) value decreased in the order of 2772.50→2575.20→503.29→124.36 g/h, corresponding to latitudes of 34.50° N→34.00° N→33.50° N→32.50° N 121.50° E in autumn. The most suitable fishing areas were the south of the East China Sea region in spring; the south of the East China Sea region extending to the center and outer parts of the East China Sea region in summer; the south of the Yellow Sea close to the Haizhou Bay fishing ground and the forbidden fishing line region of the Lusi and Dasha fishing grounds in autumn; and the south and center of the East China Sea region in winter. The most suitable sea bottom temperature (SBT) values from spring to winter were 14.76–20.53 °C, 19.54–22.98 °C, 11.79–17.64 °C, and 16.94–20.36 °C, respectively. The most suitable sea bottom salinity (SBS) values were 31.53–34.80‰ in spring, 32.95–34.68‰ in summer, 31.51–34.77‰ in autumn, and 33.82–34.51‰ in winter. We concluded the following: (1) the southern and northern areas of the East China Sea region are spawning and nursery grounds, respectively, in spring; (2) the central distribution is located at a latitude of 28.00° N in autumn and winter; and (3) the southern area of the Yangtze River to the north is a spawning ground in spring, and the areas located at 29.00–34.50° N, 124.00–124.50° E, and 28.00–30.50° N, 125.50–126.50° E are nursery grounds. The results of this study provide useful guidance for appropriate fisheries management, thereby avoiding a collapse in the S. esculenta population, which has been experienced in other species in this area.

Ideas and perspectives: Human impacts alter the marine fossil record

Abstract. The youngest fossil record is a crucial source of data documenting the recent history of marine ecosystems and their long-term alteration by humans. However, human activities that reshape communities and habitats also alter sedimentary and biological processes that control the formation of the sedimentary archives recording those impacts. These diverse physical, geochemical, and biological disturbances include changes in sediment fluxes due to the alteration of alluvial and coastal landscapes, seabed disturbance by bottom trawling and ship traffic, ocean acidification and deoxygenation, removal of native species, and introduction of invasive ecosystem engineers. These novel processes modify sedimentation rates, the depth and intensity of sediment mixing, the pore-water saturation state, and the preservation potential of skeletal remains – the parameters controlling the completeness and spatiotemporal resolution of the fossil record. We argue that humans have become a major force transforming the nature of the marine fossil record in ways that can both impede and improve our ability to reconstruct past ecological and climate dynamics. A better understanding of the feedback between human impacts on ecosystem processes and their preservation in the marine fossil record offers new research opportunities and novel tools for interpreting geohistorical archives of the ongoing anthropogenic transformation of the coastal ocean.

Characterizing behavioral responses of Pacific cod to bottom trawl vessels and gear using archival tag accelerometer data

Fish behavior during the capture process affects the overall efficiency of survey vessels and gear, influencing selectivity and catchability, which in turn influences stock assessments. The effects of behavior on capture are typically investigated through experiments that use video or acoustic technologies to observe behavioral responses of fish to vessels and gear directly. While these approaches provide valuable information about fish behavior, their use is limited to observing the movement of fish inside a small observed area. In this proof-of-concept study, we demonstrate a novel application of bio-logging Pop-up Satellite Archival Tags (PSATs) equipped with accelerometer sensors to observe behavioral responses of free-swimming demersal fish to approaching bottom trawl fishing vessels and gear. By combining PSAT and fishing vessel geolocation data from the Vessel Monitoring System, we characterized diving and herding responses of three individual Pacific cod (Gadus macrocephalus) at-liberty for 21–106 days to trawling fishing vessels. We found that fish descended as trawlers approached, presumably in response to noise generated by the fishing vessel and gear, which supports the previously untested hypothesis that Pacific cod dive in response to approaching fishing vessels and gear, similar to closely related gadids. Additionally, one 88 cm fish swam ahead of a trawl towed at 1.25 m·s−1 (i.e., 1.42 body lengths per second) for 55.6 minutes, covering a distance of 4.17 km before capture, which challenges previous assertions that Pacific cod are unlikely to outswim bottom trawl gear towed at typical towing speeds. This study demonstrates that opportunistically collected archival tag data can be used to improve knowledge of behavioral interactions between fish and fishing gear.

Rapid climate change increases diversity and homogenizes composition of coastal fish at high latitudes.

Rapid warming at high latitudes triggers poleward shifts of species' distributions that impact marine biodiversity. In the open sea, the documented redistributions of fish lead to a borealization of Arctic fauna. A climate-driven borealization and increased species diversity at high latitudes are also expected in coastal fish communities, but they have not been previously documented on a large, biogeographic scale. Here, we investigate the impact of temperature change over the last 25 years on fish communities along the coast of Norway. The study area, spanning different ecoclimatic zones between 62° and 71° N, harbors over 200 species of boreal and Arctic fish. Several of these fish species are harvested by coastal and indigenous communities, influencing settlement geography and livelihood. The long-term data on coastal water temperatures and fish species were obtained from monitoring stations and scientific surveys. Water temperature measured at three fixed sampling stations distributed along the coast show increased temperatures during the study period. The fish species distribution and abundance data were obtained from the annually standardized scientific bottom trawl survey program. Fish species richness, which was highest in the south, increased with warming first in the south and then, gradually, further north, eventually affecting biodiversity in the whole study area. Fish community composition showed a distinct latitudinal pattern early in the study, with Arctic fish species confined to the north and boreal species dominating the south. The poleward shifts eventually eroded this zoogeographic pattern, resulting in more boreal fish species in the north and an increased homogenization of species composition along the Norwegian coast. The climate-driven reorganization of fish communities affects coastal ecosystems that are exposed to fisheries, aquaculture, and other rapidly expanding human activities, stressing the urgent need for a climate adaptation of integrated coastal management.

Nanopore environmental DNA sequencing of catch water for estimating species composition in demersal bottom trawl fisheries

AbstractBycatch and discards, representing unwanted catches, undermine sustainable fisheries and hinder the conservation of vulnerable and endangered species. To effectively monitor bycatch and enhance the effectiveness of management measures while promoting sustainable fishing practices, reliable data is essential. Here, we explore the use of Nanopore metabarcoding to analyze the catch composition in demersal bottom fisheries. We collected eDNA samples directly from an onboard catch holding tank (catch water) for 10 fishing hauls from a fishing vessel operating in the Skagerrak (North‐East Atlantic). The approach involved sequencing a combination of long (~2 kb) and short (~170–313 bp) mitochondrial amplicons and was validated by analyzing a fishery‐related mock community sample and fishing haul replicates. Overall, the detection rate accuracy was 95% for landed species, and replicates obtained from the same fishing haul showed consistent results, validating the robustness of this approach. The detection rate accuracy for all caught species observed on board (including the non‐landed fraction) was 81%. Undetected species were always limited to species in low abundance, but may also be attributed to problems with identifying closely related species due to the impact of sequencing errors and limited diagnostic variation in the genetic regions used. In the future, such biases may be reduced by using additional markers to increase species discrimination power and applying newly available technological advantages in flow cell chemistry to improve sequencing accuracy. In conclusion, this study demonstrates the effectiveness of Nanopore eDNA sequencing of catch water for estimating species composition in demersal bottom trawl fisheries, including catches of non‐commercial and threatened and vulnerable species, without disrupting fishing activities. Incorporating eDNA analysis of catch water may therefore help facilitate effective monitoring, leading to better‐informed fisheries management, biodiversity conservation efforts, and the implementation of relevant legislation such as the EU landing obligation.

Guardians of the seabed: Nature inclusive design of scour protection in offshore wind farms enhances benthic diversity

In the past, a large part of the seabed of the southern North Sea was covered by hard substrates, including oyster beds, coarse peat banks, and glacial erratics. Human activities, particularly bottom trawl fisheries, led to the disappearance of most of these hard substrates, resulting in the loss of its associated diverse benthic life as well. However, the introduction of human-made structures such as offshore wind farms in the North Sea, offers a chance to provide habitat of similar functionality as the former hard substrates. The offshore wind farm infrastructure generally contains layers of rock material deployed at the base of the wind turbine foundations and cable crossings, so-called scour protection, aiming to prevent seabed erosion. The scour protection offers a unique habitat for rock-dwelling benthic organisms in an otherwise mostly soft-bottom environment. By designing the scour protection to be more nature inclusive, the biodiversity of benthic life can be increased. In this study we examined the effect of substrate material and grading of the scour protection on the epibenthic biodiversity in situ. This was done by deploying research cages containing crates (n = 15) with different types of substrates (concrete, granite, and marble) on the scour protection within an offshore wind farm in the Dutch North Sea. The study revealed a significant (p < 0.05) positive relation between available substrate surface (pebble size) and taxonomic richness. Furthermore, a biological trait assessment of living habits (Tube dwelling, Burrowing, Free living, Crevice dwelling, Epi/endobiotic, and Attached) revealed variations in habit modes across substrate types, with marble and concrete samples showing greatest divergence. Marble samples contained a higher prevalence of tube dwelling organisms, whereas concrete samples contained a relatively higher prevalence of free living, epi/endobiotic and crevice dwelling organisms. The findings support the value of nature inclusive scour protection designs, emphasizing that both taxonomic and functional diversity can be enhanced by increasing the available surface area of the scour protection and incorporating a variety of substrate types. By adopting these nature inclusive design components, the coexistence of renewable energy production and a diverse marine benthic community can be further optimized.

Do cold water corals provide an essential habitat for Helicolenus dactylopterus (Delaroche, 1809) in the Northwest Africa?

Cold water corals (CWC) provide habitats for many organisms including demersal fish. Bottom trawl observations have indicated a co-occurrence of the fish Helicolenus dactylopterus with CWC reefs, but a detailed understanding of this relation is lacking. To better understand the nature of this relation we have analyzed 85 video-lines from ROV dives conducted at 25-1700m depth off Morocco, Mauritania, and Senegal in 2020 and 2021. We annotated abundance, size, and behavior of the 552 specimens observed (32% juveniles and 68% adults), of these 82% occurred in CWC habitats at 400-600m depth. Both juveniles and adults were observed standing on the seafloor. Our observations are discussed considering available knowledge on feeding ecology and life cycle of H. dactylopterus. Our findings show that CWC provides an essential habitat for this species at least during parts of its lifecycle, however, more behavioral studies are needed for an in-depth understanding of this association.

Situation of Offenses Observed in Maritime Fishing and Proposal for Measures to Improve the Surveillance System: Case of the National Fisheries Surveillance and Police Center (CNSP) in Guinea

Research work on the theme “Situation of offenses observed in maritime fishing and proposal for measures to improve the surveillance system: case of the National Fisheries Surveillance and Police Center (CNSP) in Guinea » Conakry took place in the period from September 14 to December 14, 2022 inclusive. The objective of this work was to take stock of the offenses recorded in fishing with a view to proposing solutions for improving the surveillance system. To achieve this objective, the following methodology was adopted: consultation of executives and exploitation of archives; survey of surveillance agents; identification and descriptions of the surveillance system; identification of the offenses and fleet involved in the Guinean EEZ in recent years; determination and classification of the causes of offenses; evaluation of the surveillance system; proposal for improved measures of the surveillance system. Monitoring of this approach revealed that: the offenses committed in the Guinean EEZ are related to the fishing gear used; administrative documents and fishing operation zones. According to records, for the period from 2006 to 2021, there were 308 boardings. The rate of infringements in IP is 59%, 25% in PAM and 16% in PSI. The flags which have committed more IP infringements are Chinese 86 and Korean 60; in PSI the Chinese 17 and the Senegalese 16 and in PAM the Guineans 33, Senegalese 23 and the Leonese 18. The stakeholder survey revealed that the number of vessels monitored by VMS in 2021 was 79 in PI and 49 in PSI. The main documents inspected at sea are generally: registration certificates, crew, rescue equipment, gross tonnage, coastal state fishing authorization, maritime radio license. The fleets involved in the infringements in the Guinean EEZ from 2006 to 2022 numbered 135 including 80 demersal trawlers, 7 cephalopod boats, 21 shrimp boats, 22 pelagic trawlers and 5 cargo ships. The improvement of the surveillance system in Guinea may be more efficient, incombining the AIS and VMS system in fisheries surveillance, by creating a marking system for fishing gear, by purchasing very powerful launches and maintaining the confidentiality of patrol operations.

Edge computing based real-time Nephrops (Nephrops norvegicus) catch estimation in demersal trawls using object detection models

In demersal trawl fisheries, the unavailability of the catch information until the end of the catching process is a drawback, leading to seabed impacts, bycatches and reducing the economic performance of the fisheries. The emergence of in-trawl cameras to observe catches in real-time can provide such information. This data needs to be processed in real-time to determine the catch compositions and rates, eventually improving sustainability and economic performance of the fisheries. In this study, a real-time underwater video processing system counting the Nephrops individuals entering the trawl has been developed using object detection and tracking methods on an edge device (NVIDIA Jetson AGX Orin). Seven state-of-the-art YOLO models were tested to discover the appropriate training settings and YOLO model. To achieve real-time processing and accurate counting simultaneously, four frame skipping ideas were evaluated. It has been shown that adaptive frame skipping approach, together with YOLOv8s model, can increase the processing speed up to 97.47 FPS while achieving correct count rate and F-score of 82.57% and 0.86, respectively. In conclusion, this system can improve the sustainability of the Nephrops directed trawl fishery by providing catch information in real-time.

Predicting the distribution of megabenthic communities on deep-water seamounts with cobalt-rich crusts in the Magellan Seamount Chain in the northwestern Pacific ocean

Deep-water seamount ecosystems are sensitive to human activities and have a slow recovery rate after being disturbed. Bottom trawling and potential deep-sea mineral extraction could severely damage benthic communities on seamounts and seriously impact deep-sea ecosystems. Inadequate knowledge of the distribution of megabenthos on seamounts or their community structure hinders deep-sea conservation and management. In this study, based on a multidisciplinary dataset generated from recordings taken by human-occupied vehicle (HOV) and remotely operated vehicles (ROVs) along transects and environmental variables, a range of megabenthic morphotaxa were observed on two adjacent deep-water seamounts and predicted using species distribution models (SDMs). Accordingly, based on the predicted distribution of each morphotaxon, five distinct communities were identified through cluster analysis. The results of SDMs showed that environmental variables varyingly impacted the distribution of different morphotaxa, among which the average velocity and eastness direction of near-bottom currents, bathymetric position index (BPI), and backscatter intensity exerted the most significant influence on megabenthic distribution patterns. The distribution of five distinct communities showed a similarity of community composition on the two deep-water seamounts, suggesting a potential connectivity between the two seamounts. The distribution of communities revealed the spatial characteristics of vulnerability of deep-water seamounts at the community level, which could provide a direct basis for marine spatial planning of deep-sea ecosystems.

Biogeochemical dynamics in a marine storm demonstrates differences between natural and anthropogenic impacts

This study explores the impact of a wind storm on sediment resuspension and marine biogeochemical dynamics. Additionally, the storm took place during an expedition researching bottom trawling, enabling the direct comparison of certain natural and fisheries-related disturbances. The storm was initiated by a decline in atmospheric pressure and a 2 h period of gale force winds, which was followed by over 40 h of elevated bottom currents. Storm induced turbidity, potentially a cumulative post-fishing impact, was remarkably higher compared to what was observed in a recent trawling event. Storm-induced mixing and movement of water masses led to decreased silicate and increased phosphate concentrations in the water column, accompanied by lower salinity and higher fluorescence. The erosion depth of the seabed averaged around 0.3 cm during the peak turbidity period. Trawl-induced erosion in the area has been measured at over twice that depth, and has been linked to intermittent reductions in near-bed oxygen levels. In contrast, storm-induced turbidity coincided with increased oxygen due to wave mixing, suggesting inherent differences in how trawling and storms can oxidize reduced substances. These findings suggest that storms have a greater regional impact, whereas the local impacts of bottom trawling on biogeochemistry can be more significant.

The impact of ocean warming on selected commercial fisheries in New Zealand

AbstractWarming oceans affect the growth, reproduction and location of fish species. Using a bioeconomic framework, we estimate the relationship between sea surface temperature (SST) and the catch of de facto open‐access commercial fisheries of flatfish, trevally and jack mackerel in New Zealand. Assuming either a logarithmic or quadratic relationship between the SST and the carrying capacity of the fish stock, we consider three fishing methods (bottom trawl, set net and midwater trawl) and two measures of effort (count and duration). We show that ocean warming results in an increase in catch for all species if we assume a logarithmic relationship, with the highest marginal product of SST () for jack mackerel caught using midwater trawl. The highest marginal revenue product (computed by the MPSST, current price and fished area) is found for flatfish caught by set net. However, when assuming a quadratic relationship, there is a threshold above which catch starts to decrease, with the maximum catch of species ranging between 10 and 12.2°C. These results also vary spatially, with higher values of MPSST observed in areas closer to the South Pole. Our findings are relevant for any review of fisheries management systems in response to ocean warming.

Spatio-temporal trajectory data modeling for fishing gear classification

International Organizations urge the protection of our oceans and their ecosystems due to their immeasurable importance to humankind. Since illegal fishing activities, commonly known as IUU fishing, cause irreparable damage to these ecosystems, concerned organisms are pushing to detect and combat IUU fishing practices. The automatic identification system allows to locate the position and trajectory of fishing vessels. In this study we address the task of detecting vessels’ fishing gears based on the trajectory behavior defined by GPS position data, a useful task to prevent the proliferation of IUU fishing practices. We present a new database including trajectories that span 7 different fishing gears and analyze these as in a time sequence analysis problem. We leverage from feature extraction techniques from the online signature verification domain to model vessel trajectories, and extract relevant information in the form of both local and global feature sets. We show how, based on these sets of features, the kinematics of vessels according to different fishing gears can be effectively classified using common supervised learning algorithms with accuracies up to 90%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$90\\%$$\\end{document}. Furthermore, motivated by the concerns raised by several organizations on the adverse impact of bottom trawling on marine biodiversity, we present a binary classification experiment in which we were able to distinguish this kind of fishing gear with an accuracy of 99%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$99\\%$$\\end{document}. We also illustrate in an ablation study the relevance of factors such as data availability and the sampling period to perform fishing gear classification. Compared to existing works, we highlight these factors, especially the importance of using sampling periods in the order of minutes instead of hours.

On possibility of using Danish seine to assess the stocks of commercial fish species

In modern conditions of less number of research expeditions, more information is collected by fishing vessels, where Danish seine is used as a fishing gear. Biostatistical data obtained by bottom trawl and Danish seine were compared in the joint survey in the Ussuri Bay (Japan Sea) and showed good comparability. Availability of the Danish seine using for assessment of fish stocks in this area is concluded.

Distribution of pacific cod Gadus macrocephalus (Gadidae) and functional structure of the habitat in the northern Japan Sea and some issues on regulation of the fishery

Seasonal and bathymetric distribution of the main age groups of pacific cod in the northern Japan Sea are considered on the data of bottom trawl surveys in 1980–2023. The main wintering and spawning accumulations, where the fish concentrate from December to January and then spawn in February-March, are formed on the external shelf and the upper continental slope along the western coast approximately between Peter the Great Bay and 47о N and along the eastern coast between Cape Krillon and 47о 30′N. The juveniles have feeding migrations from April to May, while older age groups complete the migrations a month later. They migrate in three main directions: from the areas at the mainland coast – southwestward and northeastward and from the areas at western Sakhalin — southward. The juveniles feed on shallows in Peter the Great Bay, northern Tatar Strait, and on southwestern shelf of Sakhalin between the Nevelsky Bay and Cape Krillon, whereas the feeding grounds for older groups occupy two vast areas including Peter the Great Bay with the adjacent southeastern shelf of Primorye and the shelf of northern Primorye in the Tatar Strait (to the north from 47о N) with the whole shelf of Sakhalin. In the northern Tatar Strait (northward from 48о N), the cod belonged to the Primorye and Sakhalin stocks can feed together. The feeding season lasts from June to September. The older cod feed usually deeper that the juveniles. Back migrations of cod for wintering occur from October to November. Because of a joint feeding area where the mainland and Sakhalin stocks are able to mix, the bottom trawl surveys at Primorye coast cannot be used for correct assessment of the total available catch (TAC) in this fishing district. To improve quality of the cod fishery forecasts, the methods of cohort or production analyses should be used, with the control by bottom trawl surveys data, as alternative information about the state of cod stocks, taking into account the abovementioned patterns of seasonal distribution and migrations.

Spreading a demersal trawl without otter boards: A proof of concept using flexible foils

This manuscript documents an engineering proof of concept. We demonstrate that a deep-water demersal trawl targeting northern shrimp (Pandalus borealis) can be spread without otter boards using nothing more than flexible foils (i.e., kites) mounted near the wings of the trawl. A combination of numerical simulation, physical modeling in a flume tank, and full-scale sea trials were used to optimize the horizontal spreading of the demersal trawl. Spacing between the foils and the angle of attack to the direction of tow were found to be important parameters. To our knowledge this is the first successful attempt to spread a demersal trawl full-scale at-sea using nothing but flexible foils.

Lower fishing effort would benefit fish stocks and fisheries profitability in the Gulf of Lion in the Mediterranean Sea

The increasing pressure on marine resources due to the high demand for fish and fishery products from an increasing population can lead to the overexploitation of fish stocks worldwide. In addition, climate change has already impacted fish populations pushing them towards states never seen before with a risk of irreversible changes. Evaluating the socioeconomic consequences of climate and anthropogenic impacts is challenging due to the large variety of interactions between social, ecological, and economic systems that make future effects difficult to predict. In this study, we develop climate-socioeconomic scenarios for the Gulf of Lion’s most important commercial fisheries using the MEFISTO bioeconomic model. Here, the biomass, catches and profit changes for the main commercial species (anchovy, sardine, hake and red mullet) and three fleets (demersal trawlers, gillnets, purse seiners) are modeled. We project the potential future storylines of fisheries under four different climate-driven and socio-political scenarios considering changes along three components: (i) climate change affecting fish stocks’ productivity through physiological processes, (ii) economic changes impacting fish price and fishing costs and (iii) technological management (fishing effort). Under alternative assumptions of productivity, market and management changes, our results indicate that fishing mortality reductions would mitigate the impacts of climate change and economic volatility over the GoL fisheries. An improvement in the overfished fisheries' management seems to be the main driver of change by 2050, over the direct impact of climate change. The market volatility (fluctuations in fish and fuel prices) will also be key for the future viability of Mediterranean fisheries.

Understanding size selectivity of trawls using structural models: Methodology and a case study on fish sorting grids

Fish behaviour affects the performance of selection devices in fishing gears. Traditionally, fish behaviour in relation to selection devices is assessed by direct observation. However, this approach has limitations, and the observations are not explicitly incorporated in the selectivity models. Further, underwater observations and quantification of fish behaviour can be challenging. In this study we outline and use an indirect method to explicitly incorporate and quantify fish behaviour in trawl selectivity analysis. We use a set of structural models, which are based on modelling the actual processes believed to determine the size selection of the device, to discern which behaviours are most likely to explain the selectivity process. By bootstrapping we assess how confident we can be in the choice of a specific structural model and on discerning the associated behavioural aspects. We collected size selectivity data in the Barents Sea demersal trawl fishery targeting gadoids, where the use of a sorting grid is compulsory. Using our modelling approach, we obtained deeper understanding of which behavioural processes most likely affect size selectivity in the sorting grids tested. Our approach can be applied to other fishing gears to understand and quantify fish behaviour in relation to size selectivity.

Combined stomach contents and stable isotope analysis reveal feeding habits of Japanese jack mackerel (Trachurus japonicus) in the East China Sea

Japanese jack mackerel (Trachurus japonicus) is an economically important pelagic fish in the East China Sea. Knowledge is still limited regarding the feeding habits of this species in this area. Based on bottom trawl surveys conducted in the southern coastal area of Zhejiang during August in 2019 and 2020, we analyzed the feeding ecology of T. japonicus using stomach contents analysis and the stable isotope approach. The results indicated that the δ13C value for T. japonicus ranged from −17.72‰ to −14.30‰, and the δ15N value ranged from 10.89‰ to 13.03‰. Two-way analysis of variance indicated significant annual and ontogenetic variations in the δ13C value, thereby suggesting changes in the final nutrient sources for T. japonicus in different growth stages and years. The trophic level of T. japonicus was estimated as from 3.35 to 4.46. In general, the trophic level of T. japonicus increased with growth, although the ontogenetic variation was not significant. The main food groups identified for T. japonicus based on stomach contents analysis were fish, shrimp, crabs, cephalopods, and small crustaceans. The results obtained by stable isotope analysis showed that the nutrient contribution rate of small crustaceans to the diet of T. japonicus was highest in both years, followed by shrimp. The feeding habits of T. japonicus had obvious ontogenetic variations, and the average contribution of fish species increased as the size of the predator increased. The results obtained in this study provide detailed feeding information about T. japonicus in the East China Sea.