Effective marine protected area (MPA) networks require accurate knowledge of habitat distribution, yet in remote oceanic archipelagos, the logistical and financial costs of comprehensive surveys render this difficult to obtain. Species distribution models (SDMs) offer a practical alternative, however their application to MPA placement evaluation in data-deficient oceanic archipelagos remains limited. The Azores archipelago has an MPA network currently under consideration for expansion, yet site suitability remains unknown and knowledge of coastal reef-fish habitat distribution is incomplete, with extensive surveys being impractical. To address this, we used underwater visual census data collected between 2010 and 2023 across the archipelago, along with seabed and oceanographic variables, to construct SDMs predicting the distributions of six coastal reef-fish species using a random forest approach. These predictions identified areas of high biomass or probability of presence as proxies for high-suitability habitat, and the proportion covered by the present MPA network was assessed. Results suggest Azorean MPAs are generally placed in areas of predicted high-suitability habitat, although due to their small size, no-take reserves protect a minute proportion of this. MPAs with more lenient regulations cover a larger area of predicted high-suitability habitat, although as these permit certain extractive activities, the protection provided is questionable. Given the large proportion of shelf areas assigned legal protection that we suggest to be appropriately placed, we argue MPA policy should prioritise tighter restrictions and stronger enforcement over spatial expansion. We conclude that SDMs are a valuable tool for evaluating reef-fish habitat protection in remote, data-deficient oceanic archipelagos.

Abstract Background Biologging and telemetry have transformed our understanding of marine megafauna movement ecology. Yet, methodological constraints continue to limit data quality and deployment duration. Devices recording whale shark (Rhincodon typus) behaviours and movements have been used for decades, but they remain challenging to deploy and vary in success. Recently, spring-loaded clamp-based systems have emerged as one of the most widely used approaches to attach electronic tags to the fins of this globally endangered species. Currently, however, no consensus guidelines exist as to how to optimise this approach, potentially leading to continued underperforming deployments limiting analysis potential. Here, we synthesise experiences with clamp-based tagging worldwide through a targeted survey of whale shark researchers. We explore performance and challenges with a view to propose current best practices in the field. Results Whale shark researcher responses to the survey highlighted clamp-based systems as a practical and more widely applicable approach than drill-based methods, which are often used to secure tags to other large sharks. They also noted that clamps have greater retention potential and are suitable for a wider range of tags compared to dart-based methods, but are still constrained by design, placement, and deployment conditions. Researchers used a variety of materials and designs to build their own clamps, often facilitated by direct collaboration with each other or key manufacturers. Clamps produced highly variable outcomes, ranging from successful long-term satellite transmissions over 200 days and short-term biologging for 48 h at 20 Hz, to premature detachment and cases of fin damage. For long-term clamps, changes in position on the fin allowed for more stable satellite transmissions over time. Some clamp designs achieved data quantity and quality close to that of drilled deployments, demonstrating their potential to rival traditional methods while offering a less invasive approach. Results emphasised the ongoing need for technological refinement and rigorous evaluation of clamp performance and associated impacts. Conclusions Based on collective insights, we present a unified approach to clamp design and positioning, and identify key priorities for advancing this attachment technology, such as aiming for positions b-2 and c-2 on the fin and ensuring the clamp bridge distance (always between 30 and 50 mm) and tension are matched to shark size. Optimising clamp systems could substantially improve our ability to generate high-quality, long-duration movement data while minimising tagging impacts on the animal where possible. This could enhance ecological and conservation research outcomes for endangered whale sharks, with broader implications for tagging other large-bodied marine megafauna.

Entanglement in marine debris is a widespread threat affecting animals worldwide. However, most incidents remain underreported due to the challenges of systematically monitoring their occurrence. This study investigates the entanglement of marine vertebrates in plastic based on 17 yr (2008-2024) of opportunistic observations between the Azores Archipelago and mainland Portugal. The data was obtained from (1) sea users (tourist operators, scientists, fishery observers) and (2) a stranding network. Only events supported by clear visual evidence were included in the analysis. We report a total of 41 entanglements in either abandoned, lost, or otherwise discarded fishing gear (ALDFG; 56%) or in single-use plastics (SUPs; 44%) in 10 different vertebrate species (blue shark, swordfish, loggerhead and green sea turtles, Cory's shearwater, bottlenose dolphin, minke, Bryde, and sei whales, and Sowerby's beaked whale). Most individuals (78%) presented visible impacts from the entangling debris, typically external lesions (91%) or mortality (9%). Specific injuries in live animals included constrictions (46%), skin damage such as wounds or lacerations (39%), limb loss (11%), and body deformations (4%). Although opportunistic data may not fully capture the extent and severity of entanglements, we found that floating plastic exerts additional pressure on open ocean ecosystems. This is often overlooked due to a limited network of stakeholders supporting data sharing, which is crucial for understanding this ongoing threat. Overall, this study offers the first baseline evidence on plastic litter entanglements among several marine vertebrate species in the open NE Atlantic, supporting consistent monitoring for the EU Marine Strategy Framework Directive.

Seamounts are key biodiversity hotspots for marine ecosystems but highly vulnerable to human impacts, highlighting the urgent need for effective conservation measures. Their remoteness and deep habitats, however, challenge the implementation of the long-term monitoring programs necessary to quantify the protection effect for reliable conservation benchmarks. This study evaluates the potential of passive acoustic monitoring (PAM) combined with ecoacoustic indices as a practical and cost-effective tool for ecological assessment of seamounts. We use this approach to analyze the presence and diversity of fish sounds across four mid-north Atlantic seamounts of contrasting summit depths. Results showed a greater presence and diversity of fish sounds in shallower seamounts, likely reflecting higher habitat complexity and species richness. Acoustic activity also exhibited clear diel patterns, with the occurrence of several sound types showing marked differences between day and night, suggesting temporal variation in fish behaviour and habitat use. Variations in fish sound diversity was ascertained and cross-validated by three ecoacoustic indices: Acoustic Complexity Index (ACI), Entropy (H), and Sound Pressure Level (SPL), and supported by in-situ evidence from underwater visual census at the summits of shallow seamounts. These findings support PAM as a viable and valuable tool for assessing and monitoring fish biodiversity in seamounts, providing essential data at relatively low cost in support of management policies for these fragile ecosystems.

Fine-scale studies of juvenile shark ecology remain limited because existing tags are often too large or invasive to be deployed on sharks <2 m length. Consequently, high-resolution behavioral and physiological data from early life stages are scarce, despite their ecological relevance for understanding juvenile shark ecology and vulnerability–information needed to inform conservation status and management of shark populations. Here, we provide a proof-of-concept for a reproducible, minimally invasive fin-clamp attachment to address this knowledge gap. We assess tag stability, the quality of collected data, and the impact on post-tagging recovery and added hydrodynamic drag. We tested two iterations of fin-clamps (prototype and miniaturized) on eight juvenile smooth hammerhead sharks (0.97 to 1.45 m total length) using a tri-axial acceleration, depth, temperature, and video logger, achieving retention from 16 h to 19 days. The miniaturized fin-clamp (reduced size, weight, and improved release mechanism) enabled easier attachments in under 10 s and resulted in more consistent retention times with a more stable tag position. Corrected accelerometry-derived tailbeat frequency closely matched video-based estimates, validating the quality of acceleration data across both iterations. Asymptotic models of tailbeat dynamics indicated post-release recovery within 1.6–2.7 h. We applied the “jiggle method” to shark accelerometry data to estimate swimming speed, which was then used to parameterize computational fluid dynamics simulations. Results revealed a non-linear decrease in tag-induced penalties relative to shark size: high drag penalties for sharks ≤1 m total length but markedly lower drag penalties for individuals ≥1.2 m, providing empirical guidance on practical size thresholds for future tagging. The fin-clamp is a reproducible, minimally invasive system for high-resolution biologging of 1.2–2 m sharks, collecting unprecedented combined behavioral and video data for sharks of this life stage. By quantifying tag-induced hydrodynamic penalties and recovery periods, this study establishes empirical and ethical guidelines for its responsible application. The fin-clamp thus provides a tool to address current knowledge gaps constraining conservation and management of coastal shark populations, including identification of critical juvenile habitats, assessment of anthropogenic stressors, and characterization of movement patterns relevant to species recovery plans, with broader applicability to welfare-conscious biologging of other elasmobranch species.

Shark populations are declining globally, yet fundamental knowledge about their ecology and critical habitats across life history remains scarce, often preventing more effective conservation measures. To address these knowledge gaps for the vulnerable smooth hammerhead shark (Sphyrna zygaena), we conducted triple-element (carbon, nitrogen, and sulphur) stable isotope analysis of vertebral growth layers from individuals from the Atlantic Ocean. This approach, including the first analysis of δ34S in elasmobranch vertebrae, produced individual lifetime records of assimilated feeding ecology and habitat use from birth to their maximum recorded age. We found consistent ontogenetic shifts with early juveniles characterized by coastal-associated isotope signatures followed by a transition around 2.5 years towards a progressively more oceanic-associated habitat and diet until adulthood. Isotopic variation was greatest in juveniles and females, reflecting pronounced interindividual variability that may arise from the use of isotopically distinct nurseries, niche partitioning, or both. Sulphur isotope data revealed many of the clearest patterns, underscoring the value of δ34S in elasmobranch vertebrae as a robust ontogenetic environmental tracer. Taken together, our results demonstrate, how time-resolved stable isotope records can inform life stage-specific ecological dynamics, highlighting the importance of integrating ontogenetic patterns into management efforts of highly mobile marine species.

Nursery habitats are important to many fish populations, as they provide optimal conditions for the survival and growth of juveniles. Protection of these habitats can be critical to the conservation of endangered fish species. We investigate prospective essential habitats for early life stages of smooth hammerhead Sphyrna zygaena and tope Galeorhinus galeus, 2 highly migratory and endangered shark species. The study focusses on an oceanic island in the Azores archipelago, North Atlantic, where a nursery area for 2 to 4 yr old smooth hammerhead has been recently described. Baited remote underwater video systems and scientific longline fishing were used to sample across a depth range of 30-135 m on the island shelf. Young-of-the-year sharks were identified based on body size measurements. A primary smooth hammerhead nursery area was found to exist within the larger nursery site. Smooth hammerhead individuals of different sizes were segregated spatially, suggesting an ontogenetic shift in habitat use. By contrast, tope were not confined to the nursery area but seemed to use the entire island shelf as an essential habitat. Juvenile tope had a balanced sex ratio, while mature individuals were predominantly females. This study provides a first description of habitat use of natal smooth hammerhead and juvenile tope around an oceanic island in the mid-Atlantic. The results highlight that Faial Island is an essential habitat for critical life history stages of both species, suggesting that the Azores Archipelago might have a more important role for the North Atlantic populations of these sharks than previously thought.

Studying shark movement ecology is vital for understanding their ecological roles and supporting sustainable management and conservation strategies. However, such information remains scarce for deep-sea sharks. We used biotelemetry to investigate the spatial behaviour and movements of two endangered deep-sea predators, the kitefin (Dalatias licha) and bluntnose sixgill (Hexanchus griseus) sharks, in the Azores, northern Mid-Atlantic Ridge. We tagged a total of 21 kitefin sharks with acoustic transmitters (some including depth sensors) and seven sixgill sharks with different tag configurations: three were fitted exclusively with acoustic transmitters, two were double-tagged with both acoustic transmitters and pop-up satellite archival tags (PSATs), and two were tagged exclusively with PSATs deployed via a speargun-equipped submersible. Both species exhibited diel vertical migrations and unexpected high site fidelity (up to 4 years), using habitats that inter-connect seamounts, slopes, and island shelves. Sixgill sharks exhibited more extensive and diverse individual home ranges and vertical diel activity patterns than kitefin sharks. Sexual segregation was evident in kitefin sharks, with males and females displaying distinct differences in depth distribution and habitat use, supporting earlier hypotheses based on fisheries data. These behavioural patterns suggest that sixgill sharks function as wide-ranging, deep-sea opportunistic foragers and predators, traversing interconnected habitats in search of prey - including kitefin sharks, which occupy a more slope-associated mesopredator niche. Our novel findings support ecological theory suggesting that deep-sea sharks exhibit far more contrasting spatial ecologies than previously thought, driven by their life histories. These differences may have implications for their high vulnerability to fisheries and climate change-induced habitat degradation.

Wearable technology is now a primary tool for studying animal movement ecology, enabling key insights into species’ migrations, ecophysiology, and interactions across diverse taxa. However, using sensors to identify specific movement behaviors remains challenging without direct visual observations of the tagged animal. This difficultly arises because sensors only measure movement from a single point on the body, providing limited information about the underlying whole-body kinematics that contribute to behaviors. Although magnetometers are commonly limited to orientation data, we show they can be used to identify and describe the motions of spatially-isolated body appendages via an adhered magnet. Variations in magnetic field strength can then be linked to peripheral body movements to directly measure key behaviors. To illustrate the broad potential of this approach, we conducted four experiments with taxonomically diverse animals, providing measurements of ventilation rates in flounder, scallop valve angles, shark foraging, and squid propulsion. For each species, changes in magnetic field strength were correlated with appendage position to identify and characterize important behaviors that are difficult to measure with traditional tagging approaches. This novel method revealed that scallops modulated valve opening angles on a circadian rhythm. Similarly, flounder operculum beat rate occurred at 0.5 Hz, with most beats reaching only a few degrees in magnitude. When applied to a shark, magnetometry quantified jaw angle and chewing events while foraging. Finally, for a mobile epi- and mesopelagic squid, magnetometry revealed three prominent and coordinated fin and jet propulsion movements during high acceleration swimming. This method leverages comparatively small magnets to enable new measurements on fragile and diminutive structures. Magnetometry expands the scope for exploring new ecological and biomechanical questions in a previously understudied size class of marine species.

Species distribution models (SDMs) are an important tool for marine conservation and management, yet guidance on leveraging diverse data to build robust models is limited. We evaluated whether an integrated SDM (iSDM) framework outperforms traditional data pooling or ensemble approaches when synthesizing multiple data types. We trained traditional SDMs and iSDMs using three data types for the blue shark (Prionace glauca) in the North Atlantic: fishery-dependent marker tags, observer records, and fishery-independent electronic tags. We compared pooled and ensembled SDMs, built with boosted regression trees, to an iSDM explicitly designed to address data-specific biases while leveraging each dataset’s strengths. While all approaches produced robust models, performance varied among data types, with fishery-dependent data consistently yielding more accurate than fishery-independent data. Differences in performance stemmed from models’ abilities to capture spatiotemporal dynamics in training data. iSDMs accounting for seasonal variability yielded the most accurate estimates but were computationally intensive, emphasizing the need to align model purpose with integration methods. Our findings reveal key trade-offs in data integration methods, particularly in balancing predictive accuracy and feasibility. As diverse data sources grow, leveraging robust approaches will be vital for improving conservation and management strategies and understanding dynamic species distributions in a changing ocean.

Plastic additives, particularly phthalic acid esters and related compounds, are pervasive pollutants in marine ecosystems; however, little is known about their accumulation in large filter-feeding megafauna. Mobulid rays, such as the critically endangered sicklefin devil ray (Mobula tarapacana), may be particularly vulnerable to chemical exposure due to their continuous filter-feeding behaviour and occurrence in plastic-polluted oceanic regions. This study aimed to assess the presence of plastic-derived chemical pollutants, specifically plasticizers, in biopsies of M. tarapacana from the Azores Archipelago, North Atlantic. An ammonium formate-modified QuEChERS extraction procedure coupled with gas chromatogrpahy-mass spectometry analysis was developed, validated, and applied to fin biopsies collected from 25 individuals. The method demonstrated excellent linearity, recovery rates between 79 and 125 %, and low matrix effects. Four plasticizers: diisobutyl phthalate, dibutyl phthalate, di(2-ethylhexyl) adipate, and di(2-ethylhexyl) phthalate (DEHP) were detected above quantification limits in 8 of the 25 analysed samples. Among them, DEHP showed the highest concentrations (185-511 ng/g wet weight). These findings provide the first empirical evidence of plasticizer accumulation in M. tarapacana, highlighting that megafauna from the open ocean are not exempt from plastic-associated chemical contamination.

Animal movements and the associated energy costs dictate an individual’s scope for activity and habitat use. Yet in situ measurements of movement often fail to quantify whole-body movement and their physiological costs. These challenges lead to data gaps connecting how movement behaviours and energy output interact to constrain species’ biogeography. Here we combined swim tunnel respirometry and multi-positional field biologging data to estimate the energy output of squid (Loligo forbesii), an ecologically key marine invertebrate. Laboratory respirometry experiments revealed a strong correlation between body mass and metabolic rate during fin-contributed swimming, enabling energy cost estimates in the wild. Free-ranging squid enacted dynamic and diverse fin and jet swimming that varied on short time scales. Animals largely selected (66%) low-amplitude fin-contributed movements where fin waves propagated metachronally. Higher amplitude fin and jet movements were rare, accounting for 4% of time budgets. Application of the bioenergetic model on naturally exhibited behaviours estimated that animals consumed 3117 ± 532 mg O2 per day to fuel the predominant metachronal fin movements, an expenditure energetically comparable to that of similar-niche fishes. These unique data reveal substantial behavioural flexibility and indicate squid prefer low-cost movement behaviours that may enable squids’ high growth rates and successful competition with fishes.

The expansion of the world 's merchant fleet poses a great threat to the ocean 's biodiversity. Collisions between ships and marine megafauna can have population-level consequences for vulnerable species. The Endangered whale shark ( Rhincodon typus ) shares a circumglobal distribution with this expanding fleet and tracking of movement pathways has shown that large vessel collisions pose a major threat to the species. However, it is not yet known whether they are also at risk within aggregation sites, where up to 400 individuals can gather to feed on seasonal bursts of planktonic productivity. These "constellation " sites are of significant ecological, socioeconomic and cultural value. Here, through expert elicitation, we gathered information from most known constellation sites for this species across the world ( >50 constellations and >13,000 individual whale sharks). We defined the spatial boundaries of these sites and their overlap with shipping traffic. Sites were then ranked based on relative levels of potential collision danger posed to whale sharks in the area. Our results showed that researchers and resource managers may underestimate the threat posed by large ship collisions due to a lack of direct evidence, such as injuries or witness accounts, which are available for other, sub-lethal threat categories. We found that constellations in the Arabian Sea and adjacent waters, the Gulf of Mexico, the Gulf of California, and Southeast and East Asia, had the greatest level of collision threat. We also identified 39 sites where peaks in shipping activity coincided with peak seasonal occurrences of whale sharks, sometimes across several months. Simulated collision mitigation options estimated potentially minimal impact to industry, as most whale shark core habitat areas were small. Given the threat posed by vessel collisions, a coordinated, multi-national approach to mitigation is needed within priority whale shark habitats to ensure collision protection for the species.

The blue shark, Prionace glauca (Carcharhinidae), is the most abundant and wide ranging of all oceanic sharks, ranging from oligotrophic tropical systems to the highly productive slopes of temperate continental shelves (Druon et al., 2022; Vandeperre et al., 2014). It is also the most heavily exploited shark globally, with an estimated 10 million animals fished per year (Clarke et al., 2006; Pacoureau et al., 2021). Approximately three-quarters of tracked blue shark movements overlapped with the areas exploited by longline fisheries in the North Atlantic Ocean (Queiroz et al., 2019). To further darken this scenario, overlap with fishers is expected to be exacerbated in the future as oceans continue to warm, causing the expansion of the oxygen minimum zones and the compression (shoaling) of their vertical habitat in regions that are critical for this species (Le Croizier et al., 2022; Vedor et al., 2021). Blue sharks are ocean wanderers, with individual movements that can extend for thousands of kilometers and seem to be largely governed by the dynamic oceanographic structures and environmental gradients that they encounter (e.g., Humphries et al., 2010; Queiroz et al., 2012). Previous studies have found that blue sharks have a complex population structure, whereby young females tend to avoid aggressive courtship behavior from adult males, juveniles avoid areas with larger individuals, presumably to reduce the chances of cannibalism (Vandeperre et al., 2014), and adult females seek warmer water to possibly promote fertilization and embryo development (Hazin et al., 1994). Yet, the specific habitat preferences and putative migrations of individual blue sharks over interannual time scales remain unpredictable at the individual level and at local and regional scales. Other highly mobile sharks use both pelagic and neritic habitats, either seasonally or ontogenetically, eventually returning to specific locations over time (Chapman et al., 2015). According to Mayr (1963), any form of residency or fidelity to a specific place, regardless of the reason, can be treated as a form of philopatry. More recently, Chapman et al. (2015) proposed that residency and site fidelity are subsets of philopatric behavior, where residency broadly describes an individual exhibiting largely uninterrupted occupancy of a limited area for a specified period of time, and site fidelity can describe the return of an individual to a location where it previously resided after having left it for some defined period of time. The effective management of mobile species requires a considerable amount of information on individual movements in relation to the spatiotemporal distribution of threats, especially for species that display some form of site-faithful/seasonally residence (Chapman et al., 2015). While there exists enough behavioral, genetic, and fisheries data to conclude that many sharks may be at least moderately philopatric for nurseries, mating areas, feeding areas, or other localities (Hueter et al., 2005), it remains to be clarified whether strictly oceanic sharks such as the blue shark, display any sort of site fidelity, in particular long-term philopatry toward feeding areas, including provisioned remote shark dive locations. Testing this hypothesis requires long-term tracking or identification of individual sharks. However, this represents a difficult logistic challenge because, at present, there is a mismatch between the longevity of electronic tags and the multiannual time scale of blue shark movements and because blue sharks lack unique individual patterns present in other sharks such as the white shark (Carcharodon carcharias) or whale sharks (Rhincodon typus) that allow them to be resighted interannually at specific areas (Hewitt et al., 2018; McCoy et al., 2018). Here, we document, for the first time, the interannual resighting of two blue sharks at a provisioned remote shark dive site. “Scarface,” as it was nicknamed, is a large adult male blue shark, measuring 299 cm total length, sighted first in 2019, that we resighted across seven summers (2017–2023) at a popular shark diving spot in the Azores archipelago (mid North Atlantic). The individual was identified based on the unique scaring patterns on the left side of the head, which caused a permanent deformation of both mouth and left eye (Figure 1). There was also some less defined scaring over the gill slits, dorsal ridge, and rostrum that were most pronounced in 2017. These scarring patterns suggest that the injuries occurred not too long before the summer of 2017 and that they possibly resulted from interaction with fishing gear and handling before release. Each year, the individual was sighted within a very specific 9 km2 area located on the north slope of the channel between Faial and Pico islands. This area, called “Pedra do Sousa” is also a traditional fishing ground for the veined squid (Loligo forbesii) and a range of benthic fishes, with the bottom depth ranging between 150 and 400 m. Shark A23, a 220 cm (total length) male, was tagged during a shark dive, on 29 August 2022, at the Condor seamount MPA, a popular shark dive site, approximately 20 nautical miles west of “Pedra do Sousa”, used as an alternative shark diving site. Local shark dive operators from Faial and Pico islands routinely attract blue sharks with chum to these sites between July and October. Between shark tagging expeditions and shark diving trips, we sighted “Scarface” on 29 August 2017, 23 August 2018, 30 July 2018, 23 August 2019, 30 August 2020, and 19 August 2023. Local dive operators also reported the sighting of “Scarface” in the same area and across different years, always within a maximum of 1 week of our observation in that year. Although these sightings did not produce photographic records, the estimated size and reported scaring patterns suggest it was the same shark. Shark A23 was resighted at “Pedra do Sousa,” on the 28 August 2023 (Figure 1). This record of consecutive re-sightings challenges the notion that blue sharks are strictly oceanic wanderers, with little to no site fidelity, despite evidence that juvenile blue sharks exhibit some degree of residency at a regional scale (Vandeperre et al., 2014). Our observations suggest that these animals may reside in the area or, alternatively, that they are philopatric, and return to the same location regularly. The residency hypothesis is unlikely given the well documented highly migratory behavior of blue sharks, including individuals of both sexes previously tagged in the Azores (Vandeperre et al., 2014), and can cross ocean basins and show distinct pelagic habitat preferences depending on their life stage. Large males typically visit the Azores region when the water is warmest (from July to September) and return to lower latitudes in autumn (Vandeperre et al., 2014), given their lower tolerance to cooler temperatures compared with juveniles and females. This emigration also coincides with the end of the shark diving season in the Azores. If “Scarface” and A23 were locally resident, possibly conditioned by the regular provisioning during the summer, more frequent sightings would be expected during the 4-month season of shark diving. A more credible hypothesis is that they are philopatric for predictable feeding areas, returning to the Azores, specifically to the consistently provisioned areas, after putative long-range migrations as previously reported for blue sharks tagged in the Azores (Vandeperre et al., 2014). In fact, Vandeperre et al. (2014) reported regional philopatry in four blue sharks that returned to the Azores region after being tracked for more than 1 year. However, the spatial resolution associated with satellite tracking does not allow for the spatial resolution at the scale of local philopatry reported for these two sharks. The philopatry of these sharks to a very specific location, where food is regularly provided, implies that seasonal variation and patchy distribution of forage in part determines migratory patterns and that individual sharks somehow learn when and where forage is available and how to navigate to those spots. There are behavioral, genetic, and fisheries data to conclude that some shark species are philopatric, especially adult females that return to their natal areas to pup, while numerous other sharks may be at least moderately philopatric to nurseries, mating and feeding areas, or other ecological relevant localities (Chapman et al., 2015). Yet, philopatry has rarely been reported for oceanic sharks (e.g., Howey-Jordan et al., 2013; Madigan et al., 2015). Thus, the regular supply of food by diving companies to attract sharks can be an experimental test of the presence and strength of philopatry in blue sharks and other species. As mature males, the repeated occurrence of “Scarface” and one interannual resighting of A23 in this area is unlikely to be related to reproduction as males and females do not co-occur in this region during the summer (Aires-da-Silva et al., 2008; Vandeperre et al., 2014). It is possible that the regional philopatry may be related both to foraging and search for prey associated with the numerous seamounts and island slopes in the region, known to constitute hotspots for other species of oceanic megafauna (Afonso et al., 2020; Morato et al., 2010), as well as foraging in a specific location where food is seasonally provisioned. This could explain why these two sharks did not remain for more extended periods in the provisioning area, as bait/chum is an opportunistic meal that will not sustain a large blue shark. Although feeding associated with shark diving in the Azores is prohibited, this rule is not strictly observed (Fontes, personal observation). Shark provisioning is known to be associated with philopatry for other species. For example, great hammerhead sharks (Sphyrna mokarran) can display philopatric behavior toward shark provisioning locations where they are able to acquire the daily energetic requirements from provisioned food alone, consuming up to 4.75 kg of provisioned food per dive, over 5 months (Heim et al., 2021). Although the abundance of seamount-associated prey may drive regional philopatry of blue sharks, the presence of these two sharks across multiple years in a restricted location is more likely to be related to the consistent provisioning at these locations. Elasmobranchs are capable of using different orientation strategies and have spatial memory systems to navigate during long- and short-distance migrations (Heinrich et al., 2020; Keller et al., 2021). Specifically, Klimley et al. (2002) suggested that blue sharks could sense the earth's magnetic field to navigate across tens of kilometers on straight lines. It could be the case that “Scarface,” A23, and other blue sharks use the region's islands and seamount magnetic signature to navigate to their destination. Although Vandeperre et al. (2014) found evidence of regional site fidelity of satellite-tagged blue sharks in the Azores, this is the first evidence of philopatric behavior over the long term at a local scale. Satellite telemetry and genetic analysis can be used to infer migration corridors, population structure, and philopatry, which are essential to managing and protecting marine migratory species (Relano & Pauly, 2022). Yet, multiannual telemetry studies are limited by battery duration and long-term tag retention issues, and the spatial resolution of most satellite tags and genetic tools are not ideal to investigate philopatry at local scales. Our results highlight the value of citizen science and the collaboration between science and the diving industry to provide valuable information to support management and conservation decisions (Hoschke et al., 2023). Although our observations are limited to two individuals, these results add to previous studies that found some degree of regional site fidelity in blue sharks (Queiroz et al., 2012; Vandeperre et al., 2014) and, most importantly, provide the first evidence of long-term philopatry at a dive site, adding to our current understanding of the spatial ecology of this pelagic oceanic shark. Further research using longer term satellite telemetry, including the development of tags that can effectively last and report for multiple years, is needed to better understand this important aspect of the behavioral ecology of threatened pelagic sharks and the implications for management and conservation of the most intensively fished shark globally. The authors declare no conflicts of interest. Data (Fontes, 2023) are available in Figshare.com at https://doi.org/10.6084/m9.figshare.22041563.v1.

Tracking large-scale movements of fishes in the ocean's midwaters, below the euphotic zone and above the seafloor, is extremely challenging. Archival satellite telemetry devices rely on light, sea surface temperature, or bottom depth data to estimate location. Consequently, geolocation of fishes inhabiting the twilight (mesopelagic: 200-1000 m) and midnight (bathypelagic: 1000-4000 m) zones has been restricted to hypothesized movement routes, thereby precluding a baseline ecological understanding against which to assess potential anthropogenic impacts. We assessed the viability of comparing depth-temperature profiles measured by animal-borne satellite tags against those from 3D ocean-resolving models and incorporated known locations from acoustic telemetry to enable a quantitative framework for deep-sea geolocation. Testing of alternative, data-driven likelihood scenarios on a deep-water shark species assemblage with marked variation in modal depth distributions confirmed that the methodological frontier of geolocation can be advanced into the twilight and midnight zones. We identify key limitations in deep-water geolocation, and ways to overcome them, identifying a viable path for robust location estimates that can help address the knowledge gap on fish movement ecology in the deep sea. Our findings suggest that leveraging state-of-theart geolocation approaches, in combination with novel technologies, raises new opportunities for studying enigmatic deep-ocean ecosystems.

Abstract Global climate‐driven ocean warming has decreased dissolved oxygen (DO) levels (ocean deoxygenation) leading to expansions of hypoxic zones, which will affect the movements, behaviour, physiology and distributions of marine animals. However, the precise responses of animals to low DO remains poorly understood because movements and activity levels are seldom recorded alongside instantaneous DO in situ. We describe a new animal‐attached (dissolved oxygen measuring, DOME) archival tag with an optical oxygen sensor for recording DO, in addition to sensors for temperature and depth, a triaxial accelerometer for fine‐scale movements and activity, and a GPS for tag recovery. All sensors were integrated on a single electronic board. Calibration tests demonstrated small mean difference between DOME tag and factory‐calibrated DO sensors (mean relative error of 5%). No temporal drift occurred over a test period three times longer than the maximum deployment time. Deployments on four blue sharks (Prionace glauca) in the central North Atlantic Ocean showed regular vertical oscillations from the surface to a maximum of 404 m. Profiles from diving sharks recorded DO concentrations ranging from 217 to 272 μmol L−1, temperatures between 13°C and 23°C, and identified an oxygen maximum at ~45 m depth, all of which were consistent with ship‐based measurements. Interestingly, the percentage of time sharks spent burst swimming was greater in the top 85 m compared to deeper depths, potentially because of higher prey availability in the surface layer. The DOME tag described blue shark fine‐scale movements and activity levels in relation to accurately measured in situ DO and temperature, with the potential to offer new insights of animal performance in low oxygen environments. Development of a tag with physico‐chemical and movement sensors on a single electronic board is a first step towards satellite relay of these data over broader spatiotemporal scales (months over thousands of kilometres) to determine direct and indirect responses of marine animals to heatwave and deoxygenation events.

The world's largest extant fish, the whale shark (Rhincodon typus), is a docile giant that occurs in tropical and subtropical regions globally. It prefers to feed on plankton, which it filters through its wide mouth. Adult whale sharks have been reported to occur in large numbers during warm summers off the Azores, an isolated archipelago on the mid-North Atlantic ridge (PLoS ONE 2014; doi.org/10.1371/journal.pone.0102060). As this oceanic region is oligotrophic, the whale shark's favorite food is not abundant. Instead, we found that they prey on snipefish (Macroramphosus sp) that have been corralled into "bait balls" at the surface by large schools of bluefin (Thunnus thynnus) and tropical tunas, including bigeye (Thunnus obesus), skipjack (Katsuwonus pelamis), and yellowfin (Thunnus albacares). A feeding frenzy often ensues. With the baitfish corralled, the whale sharks then rely on powerful suction to fill their massive mouths with prey. This shark–tuna feeding association has rarely been observed elsewhere, yet in these islands it is the norm when both whale sharks and tunas are present. We have tagged whale sharks with high-resolution biologgers (with accelerometers, cameras, and gauges to measure location, pressure, and temperature) to help elucidate the ecological importance of this unique associative behavior.

Many predator species make regular excursions from near-surface waters to the twilight (200 to 1,000 m) and midnight (1,000 to 3,000 m) zones of the deep pelagic ocean. While the occurrence of significant vertical movements into the deep ocean has evolved independently across taxonomic groups, the functional role(s) and ecological significance of these movements remain poorly understood. Here, we integrate results from satellite tagging efforts with model predictions of deep prey layers in the North Atlantic Ocean to determine whether prey distributions are correlated with vertical habitat use across 12 species of predators. Using 3D movement data for 344 individuals who traversed nearly 1.5 million km of pelagic ocean in > 42,000 d, we found that nearly every tagged predator frequented the twilight zone and many made regular trips to the midnight zone. Using a predictive model, we found clear alignment of predator depth use with the expected location of deep pelagic prey for at least half of the predator species. We compared high-resolution predator data with shipboard acoustics and selected representative matches that highlight the opportunities and challenges in the analysis and synthesis of these data. While not all observed behavior was consistent with estimated prey availability at depth, our results suggest that deep pelagic biomass likely has high ecological value for a suite of commercially important predators in the open ocean. Careful consideration of the disruption to ecosystem services provided by pelagic food webs is needed before the potential costs and benefits of proceeding with extractive activities in the deep ocean can be evaluated.

Tuna Regional Fishery Management Organizations (tRFMOs) are increasingly interested in spatiotemporal management as a tool to reduce interaction rates with vulnerable species. We use blue shark ( Prionace glauca ) as a case study to demonstrate the critical first steps in the implementation process, highlighting how predictions of global habitat for vulnerable life stages can be transformed into a publicly -accessible spatial bycatch mitigation tool. By providing examples of possible management goals and an associated threshold to identify essential habitats, we show how these key areas can represent a relatively low percentage of oceanic area on a monthly basis (16-24% between 50 degrees S and 60 degrees N), yet can have relatively high potential protection efficiency (similar to 42%) for vulnerable stages if fishing effort is redistributed elsewhere. While spatiotemporal management has demonstrable potential for blue sharks to effectively mitigate fishing mortality on sensitive life stages, we identify inherent challenges and sequential steps that require careful consideration by tRFMOs as work proceeds. We also discuss how our single-species framework could be easily extended to a multispecies approach by assigning relative conservation risk before layering habitat model predictions in an integrated analysis. Such broader application of our approach could address the goals of tRFMOs related to reducing the ecosystem effects of fishing and pave the way for efficient fisheries co-management using an ecosystem-based approach.