The meagre (Argyrosomus regius) is a large teleost inhabiting the coastal waters of the East Atlantic Ocean and the Mediterranean Sea. Along the Iberian Peninsula, it is frequently targeted by both commercial and recreational fisheries. Despite its relevance, information on its population structure and movement ecology has been scarce until recently. While these studies have provided a foundation, information on the main migratory corridors along the coast is still missing. The current study aimed to identify primary migratory routes, aggregations, and residency sites along the Southwest coast of Portugal. Forty-nine adult meagres were captured and tagged with acoustic transmitters between 2018 and 2023. Their movement was monitored from 122 acoustic receivers, grouped into 25 arrays, which were deployed throughout the Gulf of Cadiz and along the Southwest coast of Portugal. A spatial network approach was applied to analyse these data. The locations near the tagging site and along the Southwest coast displayed high values of eigenvector and betweenness centralities, indicating their crucial role in facilitating the connectivity of adult meagre between the two regions. Furthermore, cooccurrences outside the spawning season highlight their role as potential feeding grounds. The number of detections and co-occurrences near the Guadiana Estuary provides further evidence that the location may serve as a spawning ground, although additional data are needed to confirm this. Our findings expand previous research, identifying key passages and aggregation sites for meagre along the coast. These results can inform management and conservation efforts, particularly in the context of marine spatial planning.

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.

Biotelemetry networks have transformed our capacity to study the movements and ecology of aquatic animals, yet their development has been geographically uneven over time. While North America and Australia (among others) already hosted mature, well-coordinated infrastructures in the early 2000s, Europe has long lacked a formal framework for collaboration. The European Tracking Network (ETN) emerged in 2017 as a grassroots response to this gap. Since then, ETN has grown tremendously and has become a sustainable network with a central data system and research infrastructure installed all across Europe, enabling collaborative research on a multitude of species and environments while maintaining a strong track record in capacity building. ETN operates through a decentralised model in which national and regional partners lead infrastructure deployment while contributing data to a shared data platform that adheres to FAIR principles and supports collaborative tools, interactive dashboards and a dedicated R-package. ETN has successfully enabled a growing body of transnational research on species movements, habitat connectivity and behavioural ecology, as well as meta-analyses that maximise data reuse and help reduce animal use. At the same time, ETN faces challenges common to large-scale telemetry initiatives, including the need for stable funding, potential equipment incompatibility, the risk of false-positive detections, a potential lack of community buy-in, and ongoing discussions around data ownership and proper attribution. By working proactively with the community and stakeholders, ETN could address most of these potential challenges through the development of open transmission protocols, by aligning data policies with community needs, and by enhancing stakeholder engagement. Looking ahead, ETN is positioning itself at the interface of digital twins, machine learning, multisensor integration and responsible stewardship. Together, these efforts are transforming ETN into a key component of a global aquatic animal observatory, supporting evidence-based management and conservation across European aquatic ecosystems and beyond.

Global blue carbon assessments are hindered by a lack of data from understudied seagrass regions, such as those of Western Africa. This study reports the first in situ records of organic carbon (OC) stocks and burial rates for seagrass beds at Arguin Island, Banc d'Arguin (Mauritania), Western Africa, measured in intertidal Zostera noltei and subtidal Cymodocea nodosa meadows. The major blue carbon sources in seagrass meadows since 1900 were assessed using sedimentary environmental DNA (eDNA) and chronostratigraphy. The OC stocks in the top 50 cm of the sediment cores were not significantly different between the beds of the two seagrass species and averaged 27.8 ± 7.14 Mg C ha−1, which is 5 times higher than that in adjacent unvegetated sediments. The OC sequestration rate for the past 100 years was 10.3 ± 1.4 g C m−2 year−1 in C. nodosa sediments and 12.3 ± 5.9 g C m−2 year−1 in Z. noltei sediments. Sedimentary eDNA analysis revealed that the major OC source within the C. nodosa and Z. noltei sediments has been the seagrass species itself, with low contributions from allochthonous eDNA reads. Carbon sources in Z. noltei meadows were more diverse than those in C. nodosa meadows. In bare sediment, diatoms were the major carbon source. The present study demonstrates the potential of sedimentary eDNA to reveal the major sources of organic matter in blue carbon ecosystems, improving our understanding of the provenance of sedimentary OC and thus carbon cycling processes. Additionally, it provides new OC stock and sequestration rate measurements from a region of the world that remains underrepresented in global blue carbon assessments.

Acoustic telemetry offers valuable opportunities to investigate individual variability in circadian-related and other behaviours and how environmental cues shape these patterns in wild fish populations. However, this potential has not yet been fully exploited. We conducted a meta-analysis on 44 datasets from 34 distinct marine and freshwater species and different types of data (acoustic detections, depth, acceleration and positioning). Our aim was to explore the potential of acoustic telemetry in identifying chronotypes as consistent among-individual differences in circadian-related behaviours. First, we applied hidden semi-Markov models to classify individual time series into active and rest states. Subsequently, we computed two classical circadian-related behavioural traits: awakening time (as the activity onset) and rest onset (as the activity offset). Subsequently, we identified distinct phenotypes by decomposing behavioural variation into within- and among-individual components based on repeatability scores. We found evidence of distinct chronotypes in 17 species, with average repeatability scores of 0.52 for awakening time and 0.43 for rest onset, revealing that chronotypes are common in aquatic species. Our findings highlight that both the data type, particularly acceleration sensors, and the number of detections are effective tools for exploring chronotypes. Our study proposes a novel approach to characterising daily activity patterns in aquatic species, predominantly in fishes, and provides guidelines for investigating chronotypes across diverse taxa. We emphasise the promise of biotelemetry and advanced statistical models for improving our understanding of the behaviour of aquatic species and highlight the value of synthesising across large data sets collected in networks of biotelemetryprojects.

The movement ecology of invasive species discloses critical information to estimate ecological impacts and inform management policies to control their spread. The Atlantic blue crab Callinectes sapidus Rathbun, 1896, has successfully invaded coastal and estuarine ecosystems in southern Europe, making it urgent to understand fundamental aspects of its ecology in the non-native range. Recent studies identified key prey species for the Atlantic blue crab and quantified niche overlap with other crab species; however, the areas where the impacts are likely to be greater remain unexplored. Thus, we aim at studying its spatial ecology and assess which areas of a tidal estuary (Guadiana, Portugal) are predominantly used by the invasive Atlantic blue crab through acoustic telemetry. A total of 24 specimens were collected in the estuary in 2022, tagged, and tracked for up to nine months using four acoustic receivers located at 1, 3, 11, and 20 km from the river mouth. Two predominant patterns were observed: 1) movements along the estuary synchronized with the tide, and 2) extended residency periods in the transition between the middle and upper estuary. These findings can now inform policy makers and be used to better design local management plans, namely in upper areas of the Guadiana estuary where the species tends to aggregate, making it easier to be targeted by artisanal fisheries, when compared to coastal areas.

Aggregations and social interactions play an important role in the movement ecology of many animals, including elasmobranchs. Several of these species have shown the capability of carrying out complex social behaviours, and the importance of sociality in this taxon is being realized. Although it is a growing field of study in the case of these organisms, these processes still need to be better understood, especially to support management and conservation policies. In this study, a long-term acoustic telemetry data set collected on Dasyatis pastinaca in a coastal marine protected area was analysed. A co-occurrence network analysis was done to investigate preferential associations among individuals, revealing non-random associations among them. The analysis revealed a few strong and consistent associations that were maintained across inter-migratory periods, as this species performs seasonal migrations to a nearby estuary, suggesting temporal stability of the observed associations. Moreover, individuals had similar average positions and a generally high overlap of space use in both periods, indicating some level of site fidelity to the fully protected area. Groups of up to 64% of tagged individuals were co-detected at a same receiver, particularly in the western side of the array. Despite our limited sample size, likely underestimating associations, these results show that in addition to their large-scale movement pattern, D. pastinaca is likely to also present active partner preference and spatial structure at a finer spatial scale. The nature of such results is relevant to support the protection of these species.

The widespread adoption of acoustic telemetry has transformed our understanding of marine species' behavior and movement ecology. However, accurately interpreting telemetry data, especially concerning tagging mortality, is essential for drawing valid conclusions. In this study, we scrutinized tagging mortality in 223 individuals across 14 species and evaluated the impact of tagging methodologies, including capture method and size effects. Results reveal high tagging survival rates overall, attributable to the resiliency of the studied species and the rigorous animal welfare protocols followed during tagging procedures. Our results highlight the importance of tailoring array designs to the specific mobility patterns of the studied species for accurate survival assessments. This research contributes to generalizing mortality assessments and clearing the path for more precise and reliable telemetry studies in the future.

Information on population structure and connectivity of targeted species is key for proper implementation of spatial conservation measures. We used a combination of genomics, biophysical modelling, and biotelemetry to infer the population structure and connectivity of Atlantic meagre, an important fisheries resource throughout its distribution. Genetic samples from previously identified Atlantic spawning locations (Gironde, Tejo, Guadalquivir, Banc d’Arguin) and two additional regions (Algarve and Senegal) were analysed using genome-wide SNP-genotyping and mitochondrial DNA analyses. Biophysical models were conducted to investigate larval dispersal and connectivity from the known Atlantic spawning locations. Additionally, thirteen fish were double-tagged with biotelemetry transmitters off the Algarve (Portugal) to assess movement patterns and connectivity of adult individuals. This multidisciplinary approach provided a robust overview of meagre population structure and connectivity in the Atlantic. Nuclear SNP-genotyping showed a clear differentiation between the European and African populations, with significant isolation of the few known Atlantic spawning sites. The limited level of connectivity between these subpopulations is potentially driven by adults, capable of wide-ranging movements and connecting sites 500 km apart, as evidenced by tagging studies, whilst larval dispersal inferred by modelling is much more limited (average of 52 km; 95% of connectivity events up to 174 km). Our results show sufficient evidence of population structure, particularly between Africa and Europe but also within Europe, for the meagre to be managed as separate stocks. Additionally, considering the low degree of larvae connectivity, the implementation of marine protected areas in key spawning sites could be crucial towards species sustainability.

Abstract Background The lack of compatibility between acoustic telemetry equipment from different manufacturers has been a major obstacle to consolidating large collaborative tracking networks. Undisclosed encrypted signal coding protocols limit the use of acoustic telemetry to study animal movements over large spatial scales, reduce competition between manufacturers, and stifle innovation. The European Tracking Network, in collaboration with several acoustic telemetry manufacturers, has worked to develop new transparent protocols for acoustic tracking. The results are energy-efficient transmission protocols accessible to all researchers and manufacturers. Today, the Open Protocols (OP) are already available to manufacturers and developers, and the first transmitters and receivers to implement them are already in the water. Results The main objective of this study was to confirm the compatibility between devices from different manufacturers using OP, characterise the acoustic range of each transmitter–receiver manufacturer combination, compare the detection efficiency to the standard protocols used at present (R64K and encrypted protocols), and assess its robustness against spurious detections. An international collaborative effort was made to conduct acoustic range tests in four main aquatic habitats: a river, a coastal lagoon, a coastal habitat, and the open sea. Receivers and transmitters from different manufacturers were deployed at increasing distances from each other using the same experimental design at each location. The decay of detection probability with distance was modelled for each transmitter–receiver manufacturer combination by applying logistic regression using a Bayesian approach. Furthermore, to thoroughly assess performance differences in an applied research context, we conducted a direct field comparison between groups of smolts tagged with OP and R64K tags, tracking their migration to the sea. Conclusions Our results confirm full compatibility between the tested devices, with negligible differences in the measured acoustic ranges between OP manufacturers and when compared to encrypted protocols. The OP was also robust against spurious detections, and the field comparison between OP and R64K showed equal performance. We hope these novel insights will encourage international research groups to promote OP-based studies to ensure compatibility and maximise the benefits of acoustic telemetry networks.

Elasmobranchs are a group of slow growing species whose populations are in decline mostly due to their susceptibility to overfishing. A common approach to protect marine species is to establish marine protected areas (MPAs). Data on the spatial ecology of species is key information for MPA implementation and management. However, this information is usually lacking, particularly for elasmobranchs. In this study, thornback rays were tagged with acoustic transmitters to track their movement patterns in a marine protected area in Portugal. Individuals were detected for up to 1323 days, and transient and resident behaviours were observed. Residents exhibited a seasonal pattern of presence, peaking during late winter and spring. Weekly occurrence range size reached its maximum from mid-summer to mid-autumn. Diel changes in movement, mainly as increases in activity, were detected during night and twilight. These findings highlight the seasonal and daily dynamics of thornback rays and how these can influence their protection in an MPA. The area where the LSMP is established appears to be dominated by males, indicating that most of the protection provided to this species is towards this sex. These results provide valuable insights for the conservation and adaptive management of this commercially relevant species.

Home range size is a fundamental trait that can affect the probability of fish being harvested and, at the same time, may be affected by fishing. The relationship between home range size and fishing will impact the effectiveness of fully protected areas (FPAs), as it will influence the number of fish moving into fished areas, affecting both spillover and edge effects. One hypothesis is that individuals within FPAs will present reduced home range size relative to individuals in fished areas. This pattern can be driven by demographic selection (e.g. fishing of individuals with large home ranges leaving the FPAs), improved habitat requiring less foraging movements, or behavioural changes associated with reduced fishing threats. To test the relationship between home range size and protection, we compiled 1143 individual-level home range sizes based on acoustic tracking, covering 17 species from 11 FPAs in 7 countries, with information on distance from FPA borders. A dichotomic analysis (in/out of FPAs) did not support a significant change in the home range size between FPAs and fished areas. However, continuous analysis across the FPA borders demonstrated reduced home range size within the FPAs. We did not find an effect of FPA age or size on this pattern. While we cannot pinpoint the underlying mechanism for the pattern revealed, we suggest behavioural changes as the main driver for reduced home range within FPAs. This mechanism will lead to more resident populations within FPAs, reducing fishing mortality within FPAs yet limiting spillover benefits to adjacent fisheries.

Understanding the spawning behavior of meagre (Argyrosomus regius) is crucial for fisheries management and conservation. Meagre forms large spawning aggregations in estuaries, yet details of its spawning grounds remain elusive. We tagged 41 individuals and monitored their movements throughout several spawning seasons. Detections inside estuaries were limited. Fish detected inside the Tejo estuary during the spawning season performed regular movements toward coastal areas, suggesting potential spawning outside estuaries. Our findings underscore the significance of understanding meagre's migratory patterns for effective fisheries management and conservation strategies, supporting the establishment of spatial and seasonal closures during the spawning season.

1. Elasmobranchs are slow-growing marine predators whose populations have declined due to their susceptibility to overfishing, especially affecting species of large size. Rostroraja alba is a large skate that is currently classified as critically endangered. 2. To address the effects of overfishing and promote the recovery of marine species and habitats, marine protected areas (MPAs) are a commonly implemented management strategy. A critical aspect of their success is using information on species' movement patterns; however, this is frequently not assessed for elasmobranchs. 3. In this study, acoustic telemetry was used to monitor 30 R. alba in the Professor Lu & iacute;s Saldanha Marine Park (Portugal) to assess their long-term movements in the context of a coastal MPA and to evaluate the contribution of this kind of spatial management to the conservation of this species. 4. Immature individuals were more frequently captured than adults, which postulates this area as a potential nursery. The average residency was moderate and relatively consistent throughout the year, as was area use. No marked size or sex-based differences in movement were noted. Individuals were more active and occurred in shallower waters at night and twilight, and occasional quick vertical movements into the water column, isolated or in sequence, were also noted. 5. In general, the LSMP seems to offer stable protection to immature R. alba, with no strong seasonal variation. These results provide relevant input for adaptive management measures of this and other similar MPAs and highlight the contribution of these efforts to the protection of nursery areas.

Based on acoustic detections from a network of five stations along the south coast of Portugal, 25 transits of tagged meagre were identified between 2019 and 2020, mostly in summer. Comparisons with hourly current measurements from a moored Acoustic Doppler current Profiler and an High Frequency Radar system indicate that meagre move westward with favourable alongshore flows at a confidence level > 90% (while eastward transits observations were too few to be significant). Several similar transits of different individuals occurred within two days suggesting a species response to environmental changes. The analysis of surface seawater temperature from ERA5 shows that meagre always swam westward towards colder water, despite their warm water affinities. As such, they reached food-rich upwelling areas when the water temperature was higher than usual (due to the westward advection of warm water), probably optimizing their feeding ability. The demonstrated alongshore meagre movements in response to temperature variations induced by the mesoscale coastal circulation is particularly relevant for the management of this high economic value species in southern Portugal.

Abstract Background The meagre, Argyrosomus regius, is a large coastal predatory fish inhabiting waters from the north-eastern Atlantic and Mediterranean Sea, where it is targeted by commercial and recreational fisheries. Previous genetic studies have found an unexpectedly high population differentiation not only between the Atlantic and the Mediterranean, but also along the Atlantic coast. However, the reasons underpinning this genetic barrier remained unclear. Likewise, even though the species is amongst the world’s largest marine teleosts, knowledge about its movement ecology and migratory behaviour remains notably scarce, and primarily reliant on fisheries-dependent data. Methods In this study, we used a combination of acoustic telemetry and pop-up satellite archival tags to investigate the movements of 22 adult meagre (70–143 cm total length) along the Southwestern coast of the Iberian Peninsula. Results Our results strongly suggest that the previously reported genetic differentiation is not maintained by limited adult dispersal/movement, as hypothesized. On the contrary, we documented some of the longest individual annual migrations ever recorded for a coastal teleost, up to > 2000 km, with frequent back-and-forth movements between the West and Southern Iberian coasts. Moreover, their detected regional movement patterns support the existence of a marked seasonal behavioural shift, with individuals being less active and moving to deeper waters during winter, and are consistent with spawning philopatry associated to their summer reproductive movements. Finally, we identified putative aggregation areas that may harbour important feeding/overwintering grounds. Conclusions These findings shed new light on the movement and behaviour patterns of meagre that may be of particular importance for the conservation and spatial management of this species throughout its range, and open the door to further research on functional connectivity.

Acoustic telemetry (AT) has emerged as a valuable tool for monitoring aquatic animals in both European inland and marine waters over the past two decades. The European Tracking Network (ETN) initiative has played a pivotal role in promoting collaboration among AT researchers in Europe and has led to a significant increase in the number of tagged and observed aquatic animals in transboundary European waters. While AT benefits decision-making and delivers essential data to management bodies, its potential for management decision-making mechanisms has yet to be fully harnessed. We reviewed existing research, studies, and organisational initiatives related to aquatic animal tracking and their utility in fisheries management in European waters. We found that AT has already contributed to many aspects of fisheries management, such as improved understanding of stock dynamics, identification of critical habitats, assessment of migration routes, and evaluation of the effectiveness of conservation measures. However, broader utilisation of tracking technologies is needed. By leveraging the full potential of AT, managers can make more informed decisions to protect, restore, and sustainably manage European waters and creatures that live therein.

The exploitation of marine resources has caused drastic declines of many large predatory fishes. Amongst these, sharks are of major conservation concern due to their high vulnerability to overfishing and their ecological role as top predators. The 2 protected and endangered shark species tope Galeorhinus galeus and smooth hammerhead Sphyrna zygaena use overlapping coastal areas around the globe as essential fish habitats, but data to assess their trophic ecology and niche partitioning are scarce. We provide the first comparative assessment of the trophic ecology, ontogenetic shifts, and niche partitioning of the co-occurring tope and juvenile smooth hammerhead around the Azores Islands, mid-north Atlantic, based on delta 13C, delta 15N, and delta 34S (CNS) stable isotope analysis of muscle tissue of the sharks and their putative prey species. Overall, isotopic niches of both species indicated a reliance on similar resources throughout the sampled sizes (tope: 35-190; smooth hammerhead 54-159 cm total length), with significant ontogenetic shifts. Topes displayed a gradual shift to higher trophic levels and a more generalist diet with increasing size (increasing delta 15N values and isotopic niche volumes, respectively), whereas smooth hammerhead diet shifted towards prey with lower delta 34S at a constant trophic level and a more specialized diet than tope of comparable body size (decreasing delta 34S and constant delta 15N and delta 13C values, respectively). Our results indicate contrasting ontogenetic shifts in delta 13C and delta 34S along with pronounced differences between niche overlap of life stages pointing to intra- and interspecific niche partitioning of habitat and prey.

Global blue carbon assessments are hindered by the lack of knowledge on important seagrass ecosystems in the world, such as the Banc d’Arguin (western Africa), a hypertrophic system receiving coastal upwelling waters and iron-rich Saharan dust. This study reports the first in situ records for western Africa seagrasses of organic carbon (OC) stocks and burial rates of intertidal Zostera noltei and subtidal Cymodocea nodosa meadows. A second goal was to assess the major blue carbon sources in seagrass meadows over the last centuries, using sedimentary eDNA analysis and chronostratigraphy along sediment core profiles. The top 50-cm sedimentary carbon stocks were not significantly different between species and averaged 27.8 ± 7.14 Mg ha-1, 5 times higher than in adjacent unvegetated sediments. The carbon sequestration rate for the past 100 years in C. nodosa sediments was 9.2 ± 1.5 g OC m-2 yr-1, and 12.3 ± 5.9 g OC m-2 yr-1 in Z. noltei. Sedimentary DNA analysis revealed that the major carbon source within C. nodosa and Z. noltei since the 14th century has been the species itself (92% and 74%, respectively) and thus that allochthonous contributions were low. Carbon sources within Z. noltei were more diverse than within C. nodosa, with contributions from C. nodosa (7%) and diatoms (7%). Other plant sources, including crop plants, were also important contributors from the 11th to the 15th centuries, indicating the existence of a milder climate with more vegetation. In bare sediment, diatoms were the major carbon source along the sediment profile (97%). We estimate that Banc d’Arguin holds a carbon stock of 3,800 ± 830 Gg OC, where intertidal and subtidal seagrass meadows sequester 3.8 and 4.3 Gg OC every year, respectively, rendering this ecosystem a global hotspot for blue carbon.