Effective monitoring of cetacean occurrence is essential, as their populations face multiple anthropogenic threats, including shipping strikes, noise pollution, fisheries bycatch and habitat degradation. Passive acoustic monitoring (PAM) is a reliable non-invasive method for detecting cetaceans underwater, yet its wider adoption has been constrained by the availability and cost of equipment. Recent advances in cost-effective, user-friendly devices offer new opportunities for expanding acoustic data collection. In this study, low-cost acoustic recorders and underwater cameras were deployed across a range of fixed and mobile platforms in the Western Mediterranean, including fishing gear (drifting pelagic longlines and trammel nets), anchored Fish Aggregating Devices (aFADs) and boat-based observational surveys. These deployments demonstrate the practical potential for opportunistic cetacean presence data collection through collaboration with non-academic stakeholders, including the fishing sector and non-profit organisations. We report visually confirmed acoustic encounters of the sperm whale (Physeter macrocephalus) and four dolphin species (family Delphinidae), demonstrating that affordable PAM recorders can be used to capture cetacean vocalisations such as echolocation clicks and whistles. Despite certain limitations, these instruments can enhance the accessibility of PAM to a wide range of users, facilitating cetacean presence data collection and providing insights into interactions with human pressures, particularly fisheries. Moreover, low-cost devices demonstrate strong potential for integration into citizen science initiatives, offering a scalable solution to reduce operational costs and allow underwater soundscape data collection across large spatial scales. As PAM gains increasing recognition within ocean observing frameworks, our study underscores the value of low-cost PAM technologies in supporting efforts to document cetacean occurrence while fostering more inclusive approaches to marine biodiversity monitoring.

Knowledge of the spatiotemporal behavior and interactions with fishing gear of bycatch species is essential to improve conservation and fisheries management strategies. We analyze fine-scale data from onboard observers, covering 25 fishing fleets from 2002 through 2018, to assess temporal trends and spatial variation in sea turtle bycatch in longline and purse seine tuna fisheries in the Atlantic and Indian oceans. For the spatial analysis, we used the latest Regional Management Units for Sea Turtles (RMUs). In 117,381 fishing sets (51,431 in longlines and 65,950 in purse seine), a total of 26,050 sea turtles were incidentally caught. Bycatch was significantly higher in the Atlantic than in the Indian Ocean. Some bycatch trends were observed in longline fisheries but should be interpreted with caution due to low observer coverage and uneven sampling. In purse seine fisheries, where observer coverage was higher, an increasing trend over time was found in the bycatch rate of the three most caught species during the study period. This could be related to an increase in population size as suggested in other regional-scale analyses. We identified RMUs that should receive further attention from the perspective of increased capture rates over time. Our large-scale analysis confirms that surface longline sets capture significantly more turtles than deep longline sets. Purse seine sets on fish aggregating objects (FOB) captured more turtles than sets made on free schools of fish. These results are of relevance for the management and protection of these threatened species and the development of management measures.

The Mediterranean Sea is a significant habitat for seabirds, including endemic and endangered species. However, French pelagic longline fisheries overlap with foraging zones of seabirds, posing risks of bycatch. Insufficient monitoring and reporting hinder efforts to understand and mitigate these interactions. To identify effective fisheries bycatch mitigation measures, documenting species-fishing gear interactions and foraging strategies for each species are needed. Seabird underwater interaction processes were investigated in situ on pelagic longline gears. Cameras affixed on branchlines 1–2 m above the hook were deployed between May 2022 and June 2024 to enable a precise description of interactions of various species with baited hooks, including the Yelkouan Shearwater (Puffinus yelkouan) and the Atlantic Puffin (Fratercula arctica). Foraging strategies are documented for the two pursuit-diving seabirds. The Yelkouan Shearwater foraged in association with conspecifics and competitive events were observed. Videos revealed cryptic events of successful and unsuccessful attempts in consuming the bait during the gear soak at depths to ca 10 m. Underwater interactions occurred during the gear soak on unattended gear, while the vessel was away from the site. The Yelkouan Shearwater and the Atlantic Puffin may have developed strategies to feed on pelagic longline bait. These observations suggest that bycatch mitigation approaches are needed during multiple phases of fishing operations, and not just during setting and hauling as is the prevailing paradigm. Technical seabird bycatch management measures tailored to regional fishing practices are proposed and discussed.

Knowledge of post-larval swordfish (Xiphias gladius) ecology in the world’s oceans is incomplete as they are rarely found in ichthyoplankton samplings or commercial catches and individuals are difficult to observe in the marine ecosystem. Analyses of stomach contents of apex predators can provide otherwise unobtainable ecological insights. Two well-preserved bills of post-larval swordfish were identified among the partially digested stomach contents of an adult male dolphinfish (Coryphaena hippurus) caught in the northern Tyrrhenian Sea. The lower jaw to fork lengths of the two predated swordfish were estimated at respectively, 18 and 22 cm, corresponding to 81- and 99-day-old swordfish hatched in the 2022 spawning season. This analysis and recent information on the reproductive dynamics of swordfish in the same area reported in the literature shed light not only on the early stages of swordfish growth but also on dolphinfish ecology in the Tyrrhenian Sea. The two species share the same habitat and are both caught with commercial pelagic surface longlines. Environmental shifts associated with climate change could unpredictably modify the reproduction dynamics, growth, recruitment and distribution of both species. This knowledge is important for the sustainable management of regional fisheries.

Fisheries can profoundly impact co-occurring species exposed to incidental capture. Spatiotemporal fisheries management holds substantial potential to balance socioeconomic benefits with ecological costs to threatened bycatch species. This study estimated the effect of the spatial and temporal distribution of effort by a western Pacific Ocean tuna purse seine fishery on catch rates of target and at-risk species by fitting spatially explicit generalized additive multilevel regression models within a Bayesian inference framework to observer data. Mean field prediction surfaces defined catch rate hotspots for tunas, silky sharks, rays, and whale sharks, informing the design of candidate area-based management strategies. Due to limited sample sizes, odontocete and marine turtle catch rate geospatial patterns were summarized using simple 2D hexagonal binning. Effort could be focused in two areas within core fishing grounds to reduce overlap with hotspots for silky sharks, rays, and whale sharks without affecting target catch. Effort could be shifted outside of core fishing areas to zones with higher target tuna catch rates to reduce overlap with hotspots for at-risk species. Sparse and small marine turtle and whale shark hotspots occurred across the fishing grounds. Results did not identify opportunities for temporally dynamic spatial management to balance target and at-risk catch rates. Research on the economic and operational viability of alternative spatial management strategies is a priority. A small subset of sets had disproportionately large odontocete captures. Real-time fleet communication, move-on rules, and avoiding sets on dolphin schools might reduce odontocete catch rates. Managing set association type and mesh size present additional opportunities to balance catch rates of at-risk and target species. Employing output controls that effectively constrain the fishery would alter the spatial management strategy to focus fishing within zones with the lowest ratio of at-risk bycatch to target tuna catch. Findings inform the design of alternative spatial management strategies to avoid catch rate hotspots of at-risk species without compromising the catch of principal market species.

Pelagic stingray (Pteroplatytrygon violacea) is the only species of stingray (Dasyatidae) that utilizes both pelagic and demersal habitats. It is the main bycatch species in pelagic longline fisheries targeting bluefin tuna (Thunnus thynnus) in the Gulf of Lions. In the Mediterranean Sea, their stock structure, behavioural ecology and movements are unknown. For the first time in the Mediterranean, 17 individuals (39–60 cm disc width) were tagged with pop-up satellite archival transmitting tags using a novel method of tag attachment to investigate horizontal and vertical movements. The tags were attached for between two and 60 days. Between the months of July and October, pelagic stingray occupied a temperature range of 12.5–26.6 °C, and a depth range extending from the surface to 480 m. Monthly trends in catch-per-unit-effort (CPUE) of pelagic stingray peaked in August and decreased by late autumn. Pelagic stingray may aggregate on the continental shelf during summer and move southwards in early autumn, and this movement pattern is considered in relation to the reproductive cycle and overwintering. At-vessel mortality was low, but there was varying evidence of post-release mortality, indicating the need for further work. Future work and options for bycatch mitigation are also discussed.

The blue shark (Prionace glauca), a highly migratory and wide-ranging shark, has been classified as Critically Endangered in the Mediterranean Sea by the International Union for Conservation of Nature since 2016 due to overfishing. Surprisingly, basic information on its biology and ecology, essential for its conservation, remains unknown, and its regional stock has not been assessed. A multi-year large-scale telemetry tagging programme was implemented to fill these gaps. A total of 39 blue sharks were equipped with satellite tags in three areas of the Western Mediterranean, and their movements and home ranges were examined by sex and size class. Their distributions and habitat preferences across seasons were identified using satellite-based environmental data. Tagged sharks were capable of long-range movement between distant regions, but they did not move neither towards the Eastern Mediterranean basins nor the North Atlantic. We identified the Gulf of Lions as the mating, parturition and nursery grounds. We demonstrated that sharks can use the general geostrophic current system for large movements. Our results support the current stock boundary in the Mediterranean based on international conventional tagging programmes and genetic studies, but the absence of connectivity between the two Mediterranean basins and the North Atlantic basin suggests that they have distinct stocks. Our findings provide critical input for developing effective conservation and management plans for this cosmopolitan species. For the first time in the Mediterranean, this study provides insights into the movement patterns of blue sharks and their habitat preferences.

Here we report findings regarding the reproductive biology of swordfish Xiphias gladius in Corsican waters in the north-western Mediterranean Sea. We sampled 558 swordfish (72-216 cm; lower jaw to fork length, LJFL) caught from June to September in 2019 and 2020 off Corsica's western and eastern coasts. We assessed swordfish reproductive activity using macroscopic gonad characteristics, trends of gonadal indexes for both sexes, and histological investigation of oocyte developmental stages. Size at first maturity (L50) was estimated at 139.2 & PLUSMN; 2.7 (95% CI) cm LJFL for females using histological techniques and 117 & PLUSMN; 1.7 cm (LJFL) for males using macroscopic criteria, corresponding to 3-4 and 2-3 year old-fish, respectively. Spawning occurred mainly from June to July, with a peak in July, characterized by a high gonadal index and histological and macroscopic ev-idence of reproductive activity. X. gladius spawning period is of relatively short duration, closely linked to SST, during which the females lay successive batches of eggs at a non-evaluated frequency. Batch fecundity (BF) and relative fecundity (RF) were estimated from five gravid females (135-209 cm LJFL) with unovulated, hydrated oocytes. BF was exponentially related to fish length with a mean BF of 1.94 & PLUSMN; 1.33 (mean & PLUSMN; SD) million hy-drated oocytes in females. RF ranged from 22.29 to 37.24 (29.05 & PLUSMN; 5.97; mean & PLUSMN; SD) hydrated oocytes per gram of body weight. Our results improve knowledge regarding the sexual maturity, reproductive period, and spawning areas of swordfish in Corsican waters. These data are essential for the spatiotemporal management of Mediterranean swordfish stocks. We also provide an overview of the available information related to the reproductive biology and dynamics of Mediterranean swordfish and demonstrate the ubiquitous presence of swordfish spawning grounds throughout the Mediterranean Sea.

Substantial global population declines in pelagic sharks have led to the introduction of management and conservation measures, including gear restrictions and no-retention policies, to curb declines and encourage stock recovery. As the rate of discarding sharks increases, there is a growing need to understand prognostic factors that influence their post-release survival (PRS) outcomes.PRS was measured with survival pop-up satellite archival tags attached to shortfin mako (Isurus oxyrinchus) and silky sharks (Carcharhinus falciformis) released or discarded from pelagic tuna longline fishing vessels operating in the Western and Central Pacific Fisheries Commission Convention Area. In total, 117 tags were deployed on 60 mako and 57 silky sharks captured as bycatch during commercial pelagic longline fishing trips in New Zealand (n = 35), Fiji (n = 58), New Caledonia (n = 10) and the Republic of the Marshall Islands (n = 14).Mako engaged in long-distance movements between New Zealand, Australia, Fiji and New Caledonia, while silky sharks tagged in the Marshall Islands showed evidence of seasonal movements eastward.PRS was determined for 110 sharks (57 mako, 53 silky sharks). Most tagged sharks of both species were uninjured (89%) at capture and most sharks (88%) survived post-release until tag loss or the programmed pop-up date (60 days). However, when considering a complete fishing interaction (haulback, handling, release), PRS estimates were markedly reduced to 48.6% and 52.3% for mako and silky sharks, respectively. For both species, survivorship was greater in large (>150 cm fork length) uninjured sharks and sharks released with low shark length to trailing branchline ratios.While these findings suggest that retention bans offer sharks an increased chance of survival, continued efforts should be made to improve handling and release practices, reduce trailing gear and minimize pelagic shark bycatch.

The blue shark Prionace glauca is a top predator with one of the widest geographical distributions of any shark species. It is classified as Critically Endangered in the Mediterranean Sea, and Near Threatened globally. Previous genetic studies did not reject the null hypothesis of a single global population. The blue shark was proposed as a possible archetype of the "grey zone of population differentiation," coined to designate cases where population structure may be too recent or too faint to be detected using a limited set of markers. Here, blue shark samples collected throughout its global range were sequenced using a specific RAD method (DArTseq), which recovered 37,655 genome-wide single nucleotide polymorphisms (SNPs). Two main groups emerged, with Mediterranean Sea and northern Atlantic samples (Northern population) differentiated significantly from the Indo-west Pacific samples (Southern population). Significant pairwise FST values indicated further genetic differentiation within the Atlantic Ocean, and between the Atlantic Ocean and the Mediterranean Sea. Reconstruction of recent demographic history suggested divergence between Northern and Southern populations occurred about 500 generations ago and revealed a drastic reduction in effective population size from a large ancestral population. Our results illustrate the power of genome scans to detect population structure and reconstruct demographic history in highly migratory marine species. Given that the management plans of the blue shark (targeted or bycatch) fisheries currently assume panmictic regional stocks, we strongly recommend that the results presented here be considered in future stock assessments and conservation strategies.

For many years, tremendous effort has been dedicated to developing new industrial tuna fisheries, while their adverse impacts on threatened marine species have received relatively little attention. In tuna fisheries, bycatch is the major anthropogenic threat to marine megafauna in general, particularly sharks. Research on the development of gear technology for bycatch reduction and potential mitigation measures helped tuna Regional Fisheries Management Organizations adopt bycatch reduction management measures. After reviewing past research on the development of mitigation measures for pelagic longline and tropical purse seine fisheries based on pelagic species' behaviours, we describe promising new approaches integrating recent technological breakthroughs. New innovations include autonomous underwater vehicles carrying cameras along with miniaturized sensors, aerial drones, computer simulation of fishing gear geometry, environmental DNA assays, computer visualizations and deep learning. The successful application of such tools and methods promises to improve our understanding of factors that influence capture, escape and stress of caught species. Moreover, results emerging from recent ethological research explaining the power of social connection and learning in the “fish world” such as social learning from congeners, habituation to deterrents, and how past fishery interactions affect responses to fishing gear should be taken into account when developing technical mitigation measures.

REPLYING TO A. V. Harry & J. M. Braccini Nature https://doi.org/10.1038/s41586-021-03463-w (2021) Our global analysis1 estimated the overlap and fishing exposure risk (FEI) using the space use of satellite-tracked sharks and longline fishing effort monitored by the automatic identification system (AIS). In the accompanying Comment, Harry and Braccini2 draw attention to two localized shark–longline vessel overlap hotspots in Australian waters, stating that 47 fishing vessels were misclassified as longline and purse seine vessels in the Global Fishing Watch (GFW)3 2012–2016 AIS fishing effort data product that we used. This, they propose2, results in misidentifications that highlight fishing exposure hotspots that are subject to an unexpected level of sensitivity in the analysis and they suggest that misidentifications could broadly affect the calculations of fishing exposure and the central conclusions of our study1. We acknowledged in our previously published paper1 that gear reclassifications were likely to occur for a small percentage of the more than 70,000 vessels studied, however, here we demonstrate that even using much larger numbers of vessel reclassifications than those proposed by Harry and Braccini2, the central results and conclusions of our paper1 do not change.

REPLYING TO H. Murua et al. Nature https://doi.org/10.1038/s41586-021-03396-4 (2021) Our previously published paper1 provided global fine-scale spatiotemporal estimates (1° × 1°; monthly) of overlap and fishing exposure risk (FEI) between satellite-tracked shark space use and automatic identification system (AIS) longline fishing effort. We did not assess shark mortality directly, but in addition to replying to the Comment by Murua et al.2, we confirm—using regression analysis of spatially matched data—that fishing-induced pelagic shark mortality (catch per unit effort (CPUE)) is greater where FEI is higher. We focused on assessing shark horizontal spatiotemporal overlap and exposure risk with fisheries because spatial overlap is a major driver of fishing capture susceptibility and previous shark ecological risk assessments (ERAs) assumed a homogenous shark density within species-range distributions3,4,5 or used coarse-scale modelled occurrence data, rather than more ecologically realistic risk estimates in heterogeneous habitats that were selected by sharks over time. Furthermore, our shark spatial exposure risk implicitly accounts for other susceptibility factors with equal or similar probabilities to those commonly used in shark ERAs3,5.

The Mediterranean Large Elasmobranchs Monitoring (MEDLEM) database contains over 3000 records (more than 4000 individuals) of large elasmobranch species from 20 different countries around the Mediterranean and Black seas, observed from 1666 to 2017. The main species included in the archive are the devil fish (1 813 individuals), the basking shark (939 individuals), the blue shark (585 individuals) and the great white shark (337 individuals).In the last decades other species such as the shortfin mako (166 individuals), the spiny butterfly ray (138) and the thresher shark (174 individuals) were reported with an increasing frequency. This was possibly due to an increased public awareness on the conservation status of sharks, and a consequent development of new monitoring programmes. MEDLEM does not have a homogeneous reporting coverage throughout the Mediterranean and Black seas and it should be considered as a database of observed species presence. Scientific monitoring efforts in the south-eastern Mediterranean and Black seas are generally lower than in the northern sectors and the absence in our database of some species does not imply their actual absence in these regions. Some considerations are made on the frequency and spatial distribution of records, size structure of the observed individuals for selected species, general area coverage and species involved as by-catch by fishing gear.

During the last decade, particular attention has been paid worldwide to the problem of by catch and discards in fisheries. Collaborative research between fishermen and scientists is important to fisheries management. Partnerships with commercial longline fishermen were developed to enable them to participate in two research projects in order to integrate their information, experience and expertise. These programmes, financed by the fishing industry and regional councils were designed to describe the activity of the fisheries, to assess the scale of fishery effects on the various taxa, to study ecology and explore spatial population genetic structure in the western part of the Mediterranean Sea of the blue shark (Prionace glauca) and stingray (Pteroplatytrygon violacea) and finally to propose mitigation measures to reduce impacts on elasmobranches, sea birds and sea turtles. Communication, education, post-implementation monitoring and long-standing collaboration are the key factors to success. This presentation shows the progress realized to date.

Fisheries can profoundly affect bycatch species with ‘slow’ life history traits. Managing bait type offers one tool to control species selectivity. Different species and sizes of marine predators have different prey, and hence bait, preferences. This preference is a function of a bait’s chemical, visual, acoustic and textural characteristics and size, and for seabirds the effect on hook sink rate is also important. We conducted a global meta-analysis of existing estimates of the relative risk of capture on different pelagic longline baits. We applied a Bayesian random effects meta-analytic regression modelling approach to estimate overall expected bait-specific catch rates. For blue shark and marine turtles, there were 34% (95% HDI: 4–59%) and 60% (95% HDI: 44–76%) significantly lower relative risks of capture on forage fish bait than squid bait, respectively. Overall estimates of bait-specific relative risk were not significantly different for seven other assessed taxa. The lack of a significant overall estimate of relative capture risk for pelagic shark species combined but significant effect for blue sharks suggests there is species-specific variability in bait-specific catch risk within this group. A qualitative literature review suggests that tunas and istiophorid billfishes may have higher catch rates on squid than fish bait, which conflicts with reducing marine turtle and blue shark catch rates. The findings from this synthesis of quantitative and qualitative evidence support identifying economically viable bycatch management measures with acceptable tradeoffs when multispecies conflicts are unavoidable, and highlight research priorities for global pelagic longline fisheries.