Sex is an important driver of variation in behaviour, ecology and physiology. Sex identification in the Critically Endangered European eel (Anguilla anguilla) currently requires fish sacrifice, or the use of morphological differences such as body length, which can be inaccurate in certain habitats and at intermediate body lengths. Non-lethal tools for accurate sex identification in this species are therefore required. Here, we test the efficacy of two minimally invasive methods of sex identification: (1) ultrasonography and (2) molecular markers taken from pectoral fin clips. Both methodologies were applied to riverine European silver eel and compared to histological sex identification. Ultrasonography accurately identified the gonads in female eel, with males identified by the absence of detectable gonadal structures. The level of expression of three molecular markers (dcn, LOC111853410, kera) previously used to identify sex in Japanese eel (Anguilla japonica) were evaluated. There was no significant differential expression of the three molecular markers between sexes, suggesting that these markers cannot be accurately used to identify sex in European eel. Our findings suggest that minimally invasive imaging using ultrasonography can be a reliable tool for identifying sex in early-stage adult migratory European eels, with particular use at intermediate sizes (400-500 mm) and animals in habitats where migration potential is limited and may regularly exceed typical growth sizes. This technique can be highly valuable for studies that address ecological, behavioural, conservation and management issues in this Critically Endangered species.

River connectivity is crucial for the European eel (Anguilla anguilla) to complete its complex life cycle, which is vital for upstream recruitment to the declining population of this critically endangered catadromous fish. Eel passes, or ladders, are frequently installed on riverine structures, such as dams and weirs, to mitigate barrier effects and restore connectivity for upstream migrating eel. Efforts to optimise the effectiveness of passes have previously focused on the ascent section, quantifying the effects of climbing substrate, longitudinal slope, lateral slope and flow rate. However, conditions at the pass crest also impact the rapidity and success of upstream movements. Using controlled experiments and custom-built eel passes with contrasting crest shapes (curved vs. sloped) and flow directions (ascending vs. descending), we quantified the effect of crest conditions on the attempt success, passage efficiency and speed of ascending juvenile eel. Only three of the four treatments (sloped ascending, curved descending and curved ascending) demonstrated passage efficiencies significantly greater than 50%. Transit speed at the crest was significantly quicker (~3.5 min) in passes with a curved crest shape and ascending flow compared to the control. Our findings indicate that simple modifications to the shape of the pass crest and the configuration of flow delivery can help minimise delay and enhance passage efficiency. This, in turn, will increase upstream migration success and contribute to conservation and management goals, such as the EU Eel Regulation and The Eels (England and Wales) Regulations 2009.

The concurrent pressures of climate change and chemical pollution, often studied in isolation, have been linked to increases in infectious disease that threaten biodiversity. Understanding their interconnected nature is vital, as the impacts of climate-mediated environmental changes can be exacerbated by chemical pollution and vice versa. Using data from 836 UK-stranded short-beaked common dolphins (Delphinus delphis) (n = 153 (analysed for polychlorinated biphenyl (PCB) blubber concentrations)) necropsied between 1990 and 2020, we show that PCB concentrations and sea surface temperatures (SSTs) are associated with an increased risk of infectious disease mortality. Specifically, a 1 mg/kg lipid increase in PCB concentration correlates with a 1.6% increase in disease mortality risk, while a 1 °C rise in SST corresponds to a 14% increase. Additionally, we derived a novel PCB threshold concentration (22 mg/kg lipid), defined as the level where PCB blubber concentrations are significantly associated with infectious disease mortality risk. International efforts to reduce carbon emissions have mostly failed, and despite regulatory efforts, PCBs remain a significant threat. We demonstrate the urgent need for conservation strategies that address both risk factors simultaneously to protect marine biodiversity.

It is widely recognized that the research techniques used to monitor and study wildlife have the potential to disturb and alter the behavior of the study species. However, other impacts on sensitive, non-target, species are rarely investigated. Here, we assessed the disturbance of waterbirds in relation to the boating activities associated with an acoustic telemetry study on European eel (Anguilla anguilla) in a UK water storage reservoir designated a Special Protection Area (SPA) for waterbirds. Counts of 11 species of waterbirds were surveyed pre-, during-, and post-activity. Boating activities had no impact on waterbird counts, but counts were correlated with cloud cover, air temperature, and wind speed; results may also reflect seasonal differences in bird numbers at this site. Our results indicate that the deployment of acoustic telemetry receivers and netting for studying fish, when applied with appropriate well-informed mitigation measures, can be undertaken without causing significant disturbance to waterbird species, including those that are threatened or sensitive to disturbance.

Top marine predators, such as odontocetes, pinnipeds, and seabirds, are known to forage around fishing boats as fishermen aggregate and/or discard their prey. Recently, incidents of humpback whales interacting with fishing boats have been reported. However, whether humpback whales utilise discard fish as a food source and how they forage around fishing boats is unknown. This study reports, for the first time, the foraging behaviour of a humpback whale around fishing boats. Three whales were tagged using a suction-cup tag containing a video camera, and a behavioural data logger in the coastal area of Tromso, Norway. Video data from one tagged whale showed that the whale remained in close vicinity of fishing boats for 43 min, and revealed the presence of large numbers of dead fish, fish-eating killer whales, fishing boats, and fishing gear. In waters with large numbers of dead fish, the whale raised its upper jaw, a motion associated with engulfing discard fish from fishing boats, and this feeding behaviour differed markedly from lunge-feeding observed in two other whales in the same area. This behaviour was defined as "pick-up feeding". No lunge feeding was seen on the data logger when the whale foraged around fishing boats. This study highlights a novel humpback whale foraging strategy: low energy gain from scattered prey but also low energy costs as high-energy lunge feeding is not required.

Coral reef ecosystems are highly threatened and can be extremely sensitive to the effects of climate change. Multiple shark species rely on coral reefs as important habitat and, as such, play a number of significant ecological roles in these ecosystems. How environmental stress impacts routine, site-attached reef shark behavior, remains relatively unexplored. Here, we combine 8 years of acoustic tracking data (2013-2020) from grey reef sharks resident to the remote coral reefs of the Chagos Archipelago in the Central Indian Ocean, with a satellite-based index of coral reef environmental stress exposure. We show that on average across the region, increased stress on the reefs significantly reduces grey reef shark residency, promoting more diffuse space use and increasing time away from shallow forereefs. Importantly, this impact has a lagged effect for up to 16 months. This may have important physiological and conservation consequences for reef sharks, as well as broader implications for reef ecosystem functioning. As climate change is predicted to increase environmental stress on coral reef ecosystems, understanding how site-attached predators respond to stress will be crucial for forecasting the functional significance of altering predator behavior and the potential impacts on conservation for both reef sharks and coral reefs themselves. Environmental stress on coral reefs is increasing. Authors show a stress-related reduction in residency of grey reef sharks on the remote reefs of the Chagos Archipelago, promoting more diffuse space use and increased time away from the shallow forereefs.

Eel Management Plans demand European silver eel (Anguilla anguilla) escapement to the sea of at least 40% of that expected historically in the absence of human impacts. Landlocked lentic waterbodies, such as drinking water reservoirs, host substantial numbers of eel, which could represent a significant contribution to catchment-based conservation targets. To optimize netting strategies for eel management policies, information on their aggregation behaviour is currently needed but lacking. We performed a fine-scale acoustic tracking study to monitor the movements of 86 European eel in a UK reservoir. Social network sampling and sensitivity analyses were used to assess whether eel aggregate at scales relevant for current capture techniques. European eel were found to aggregate at spatial and temporal scales of 50 m and 2 days, respectively, which complements current capture methodologies and recommendations. Aggregations were not driven by fixed resources, indicating that other factors, such as sociality, may drive aggregation behaviour. Results also show that current netting practices could be optimized by increasing netting lengths from 50 to 80 m. In addition to aiding conservation and management protocols, these results provide an ecological foundation for exploring the role of social behaviour in this Critically Endangered species.

A total of 7 RIS files. Three RIS files from three web searches (Scholar, SCOPUS, and Web of Science) conducted for the review "The drivers of anguillid eel movement in lentic water bodies: a systematic map" currently submitted for publication at Reviews ins Fish Biology and Fisheries. RIS file of the combined search data conducted in Zotero RIS of the results from backward citation chasing. RIS file of the results from forward citation chasing. RIS file of the combined results from onward and backward citation chasing.

Anguillid eels have fascinated humans for centuries, but our knowledge of these mysterious fish is still scant. There are 19 species or subspecies in the genus Anguilla, which are found globally, except in the eastern Pacific and southern Atlantic. Their common label 'freshwater eels' is a misnomer - all anguillids are facultatively catadromous, born in marine environments, developing in continental waters, with a proportion never entering freshwater at all. Anguillid eels have several life history traits that have allowed them to exploit a broad range of habitats. As such, anguillid eels play an important ecological role in both marine and freshwater environments as well as being commercially valuable. Because of this, anguillid eels are under threat from multiple stressors, such as barriers to migration, pollution, parasites, disease, climate change and unsustainable exploitation. Six species are listed as Threatened in the Red List of Threatened Species, and four are listed as Data Deficient. Strengthening conservation and management of these species is essential, and further research provides an exciting opportunity to develop a greater understanding of this mysterious clade of fish.

Environmental DNA (eDNA) methods are being increasingly used in proof-of-concept studies to detect shark species, many populations of which are experiencing severe declines. These methods are widely seen as the future of biodiversity monitoring, but they have yet to become established as routine monitoring techniques for elasmobranch species. Here, we developed species-specific quantitative PCR assays for the detection of grey reef shark (Carcharhinus amblyrhynchos) and silvertip shark (Carcharhinus albimarginatus). We assessed whether species-specific eDNA methods could infer the distribution of the two species around the atolls of the Chagos Archipelago, which, despite being surrounded by a large marine protected area, experience contrasting levels of illegal fishing leading to heterogeneity in shark population densities. We found that eDNA detections were significantly reduced and sporadic around the northern atolls, which are under high pressure from illegal fishing. By contrast eDNA detections of both species were ubiquitous and consistent around the highly protected atoll Diego Garcia. We postulate that current levels of illegal, unreported and unregulated (IUU) fishing is having a significant impact on the shark community in the northern atolls and suppressing local reef shark populations. In the northern atolls we also employed visual and acoustic telemetry techniques to reveal the distribution of reef sharks. We found that despite eDNA samples being taken directly after visual surveys, detection results did not correlate, suggesting a need for further optimisation of eDNA methods for detecting sharks. However, both species were detected by eDNA in sites where they were not observed, highlighting that the scale of the sampling environment must be considered when inferring eDNA results and showing that eDNA methods can be used to fill gaps in data from more established monitoring techniques. We conclude that eDNA methods should be used in combination with other techniques to provide a complete picture of shark distribution so that threatened species can be better protected.

Coral reefs are significantly threatened by multiple environmental stressors associated with climate change. While there is growing recognition of the importance of interacting stressors on coral reefs, so far this has been primarily limited to in situ studies. Satellite remote sensing has potential for investigating coral reef exposure to multiple environmental stressors at a global scale over multiple years; however, current satellite monitoring is primarily focused on thermal stress. Here we collate nine environmental variables (cloud cover, current, depth, salinity, wind, and four sea surface temperature‐based metrics) from readily available satellite datasets using the Google Earth Engine geospatial processing platform. Using ecological and health‐based thresholds obtained from the literature, we developed, using fuzzy logic (discontinuous functions), a Reef Environmental Stress Exposure Toolbox (RESET) for monitoring environmental stress exposure at multiple scales. Stress exposure scores for 3157 reefs were generated and mapped globally across 12 coral reef ecosystem regions. RESET was also applied to three case‐study reefs, previously well monitored for stress and disturbance using other methods. PCA analysis indicated that depth, current, sea surface temperature (SST) and SST anomaly accounted for the greatest contribution to the variance in environmental stress exposure in these three regions. Depth, degree heating weeks, and SST anomaly were identified as the potential drivers of inter‐ and intra‐region variation in environmental stress exposure. Individual variables were then integrated into a multi‐metric index of combined stress exposure which corroborated temporal and spatial differences due to known disturbance events. RESET provides a open access, easily interpretable set of tools and associated indices for monitoring environmental stress exposure on coral reefs, designed to inform conservation and management decisions. As such RESET has broad potential to assist in the monitoring of our increasingly imperilled coral ecosystems, in particular, those that are remote or inaccessible.

Natural history documentary films can be a powerful tool for wildlife conservation, providing an accessible means to increase public knowledge of the natural world. There has been an increasing focus in documentary films on the threats to biodiversity in recent years that has positively aided conservation efforts. However, potential ethical and welfare implications of natural history film making are often overlooked. Here, we consider the design and impact of the narratives used and the filming methods employed in natural history film making and their potential implications for conservation. Although these programmes are often lauded for their cinematography, filming techniques and practices should satisfy high ethical standards and should be evaluated to assess disturbance caused to wildlife and any associated negative behavioural and physiological impacts. This evaluation should include the direct impact of the filming, as well as considering the risk of viewers replicating human-wildlife encounters they see on film. Trends towards the use of highly dramatized storytelling, anthropomorphism and the inclusion of inaccurate information should also be addressed. Although some production companies have filming guidelines in place, this is not standard industry practice. Natural history films are an important means of educating and enthusing people about nature and its conservation; however, it is vital that films are made responsibly. To facilitate this discussion, we propose recommendations, including standardised industry-wide guidelines, codes of conduct and independent ethical reviews, for natural history film makers to mitigate and avoid negative impacts.

Coral reefs are critical ecosystems globally for marine fauna, biodiversity and through the services they provide to humanity. However, they are significantly threatened by anthropogenic stressors, such as climate change. By combining 9 environmental variables and ecological and health-based thresholds obtained from the available literature, we develop, using fuzzy logic (discontinuous functions), a Coral Reef Stress Exposure Index (CRSEI) for remotely monitoring coral reef exposure to environmental stressors. Our approach capitalises on the abundance of readily available satellite Earth Observation (EO) data available in the Google Earth Engine (GEE) cloud-based geospatial processing platform. CRSEI values from 3157 distinct reefs were generated and mapped across 12 important coral reef ecosystem regions. Quantitative analyses indicated that the index detected significant temporal differences in stress and was, therefore, able to capture historic change at a global scale. We also applied the CRSEI to three case-study reef ecosystems, previously well-monitored for stress and disturbance using other methods. PCA analysis indicated that depth, current, sea surface temperature (SST) and SST anomaly accounted for the greatest contribution to the variance in stress in these three regions. The CRSEI corroborated temporal and spatial differences in stress exposure from known disturbances within these reference regions, in addition to identifying the potential drivers of inter- and intra-region differences in stress, namely depth, degree heating weeks and SST anomaly. We discuss how the index can be further improved in future with site-specific thresholds for each stress variable, and the incorporation of additional variables not currently available in GEE. This index provides an open access tool, built around a free and powerful processing platform, that has broad potential to assist in the regular monitoring of our increasingly imperilled coral reef ecosystems, and, in particular, those that are remote or inaccessible.

Abstract A wide array of technologies are available for gaining insight into the movement of wild aquatic animals. Although acoustic telemetry can lack the fine‐scale spatial resolution of some satellite tracking technologies, the substantially longer battery life can yield important long‐term data on individual behavior and movement for low per‐unit cost. Typically, however, receiver arrays are designed to maximize spatial coverage at the cost of positional accuracy leading to potentially longer detection gaps as individuals move out of range between monitored locations. This is particularly true when these technologies are deployed to monitor species in hard‐to‐access locations. Here, we develop a novel approach to analyzing acoustic telemetry data, using the timing and duration of gaps between animal detections to infer different behaviors. Using the durations between detections at the same and different receiver locations (i.e., detection gaps), we classify behaviors into “restricted” or potential wider “out‐of‐range” movements synonymous with longer distance dispersal. We apply this method to investigate spatial and temporal segregation of inferred movement patterns in two sympatric species of reef shark within a large, remote, marine protected area (MPA). Response variables were generated using network analysis, and drivers of these movements were identified using generalized linear mixed models and multimodel inference. Species, diel period, and season were significant predictors of “out‐of‐range” movements. Silvertip sharks were overall more likely to undertake “out‐of‐range” movements, compared with gray reef sharks, indicating spatial segregation, and corroborating previous stable isotope work between these two species. High individual variability in “out‐of‐range” movements in both species was also identified. We present a novel gap analysis of telemetry data to help infer differential movement and space use patterns where acoustic coverage is imperfect and other tracking methods are impractical at scale. In remote locations, inference may be the best available tool and this approach shows that acoustic telemetry gap analysis can be used for comparative studies in fish ecology, or combined with other research techniques to better understand functional mechanisms driving behavior.

Abstract We are in the midst of a revolution in satellite technology, with the rapid development and advancement of small satellites (or SmallSats, i.e., satellites <180 kg). Here, we review the opportunities and challenges that such technology might afford in the field of conservation and ecology. SmallSat constellations may yield higher resolutions than those that are currently available to scientists and practitioners, increasing opportunities to improve environmental‐monitoring and animal‐tracking capabilities. They may cut access costs to end users, by reducing operational costs and bringing increased competition to the existing market. Their greater flexibility and affordability may moreover enable the development of bespoke constellations for specific conservation and ecological applications, and provide greater interoperability with ground‐based sensors, such as tracking devices and camera traps. In addition, SmallSats may serve as cost‐effective research and development platforms for new components and products. Combined, these benefits could significantly improve our ability to monitor threats to the environment as they unfold, while enhancing our understanding of animal ecology and ecosystem dynamics. However, significant hardware and software developments are required before such technology is able to produce, process and handle reliable and cost‐effective data, and the initial research and development costs still represent a major challenge. Further, we argue that much remains to be done to ensure these new data products become accessible, equitable and sustainable.

Climate change is predicted to have profound consequences for marine ecosystems. Due to the growing political and social drive to address its impacts, monitoring and mitigating climate change is a priority in marine policy in the UK. Cetaceans have been proposed as informative sentinel species for monitoring ocean health. Here, strandings data from four UK cetacean species were assessed for their use as a tool to aid policy makers monitoring climate change in marine environments. Data on stranded cetaceans were collected from 1990 to 2018 and differences in the proportions of stranded cold water adapted and warm water adapted species assessed using Generalised Linear Models (GLM), with 6-year periods and four regions of the UK included as explanatory variables. This modelling approach showed an increase in the proportion of stranded warm water adapted species over time across the UK and that differences in proportion of strandings between cold water and warm water adapted species can be detected between regions and 6-year periods, chosen as metrics to coordinate with reporting cycles for policy assessment needs. As such, these results show the potential for utilising strandings data to identify changing oceanic trends at the appropriate spatial and temporal scales for policy reporting in the UK. However, development of these analyses with a more detailed examination of these data at a finer resolution, incorporating other data sources, such as distribution trends and dietary stable isotope data, may be required before it is applicable as an indicator for trends in changes in climate.

Background There are now a wide array of field and laboratory techniques available for gaining insight into the movement and behaviour of sharks. Although acoustic telemetry may lack the fine-scale resolution of some satellite technologies, the low cost and longer battery life make it a powerful tool for investigating elasmobranch behaviour. Here, we develop a novel approach to analysing acoustic telemetry data, using detection gaps to infer movement patterns to and from monitored reef habitats, to investigate spatial and temporal segregation between two sympatric shark species in a large remote MPA. Methods A total of 102 grey reef sharks (Carcharhinus amblyrhynchos) and 76 silvertip sharks (Carcharhinus albimarginatus) were fitted with long-term acoustic transmitters and tracked inside a large acoustic array of reef-based receivers in the British Indian Ocean Territory MPA, between 2014 and 2018. From the resulting dataset (768,081 detections), movements between receivers and recursive loops to the same receiver were identified. Using the durations of inter-receiver movements (i.e. detection gaps), individual behaviours were classified into ‘restricted’ or potential wider ‘out of range’ movements. Drivers of these movements were identified using network analysis, GLMMs and multi-model inference starting from an a priori set of explanatory variables. Results In general, silvertip sharks were more likely to undertake ‘out of range’ movements than grey reef sharks. ‘Out of range’ movements were more common at night compared to during the day, and during the wet season compared to the dry season. In addition, the difference in ‘out of range’ movements between the two species increased at night. These results suggest spatial and temporal segregation of movements between the two species. Conclusions We present a novel analysis of detection gaps from acoustic telemetry data to infer differential movement patterns and describe how species organise in space and time. Furthermore, this approach shows that acoustic telemetry gap analysis can be used for comparative studies, or combined, with other research techniques to better understand the functional role of sharks in reef ecosystems, moving towards more informed strategies for the conservation and management of the marine environment.

Given the recent trend towards establishing very large marine protected areas (MPAs) and the high potential of these to contribute to global conservation targets, we review outcomes of the last decade of marine conservation research in the British Indian Ocean Territory (BIOT), one of the largest MPAs in the world. The BIOT MPA consists of the atolls of the Chagos Archipelago, interspersed with and surrounded by deep oceanic waters. Islands around the atoll rims serve as nesting grounds for sea birds. Extensive and diverse shallow and mesophotic reef habitats provide essential habitat and feeding grounds for all marine life, and the absence of local human impacts may improve recovery after coral bleaching events. Census data have shown recent increases in the abundance of sea turtles, high numbers of nesting seabirds and high fish abundance, at least some of which is linked to the lack of recent harvesting. For example, across the archipelago the annual number of green turtle clutches ( Chelonia mydas ) is ~ 20,500 and increasing and the number of seabirds is ~ 1 million. Animal tracking studies have shown that some taxa breed and/or forage consistently within the MPA (e.g. some reef fishes, elasmobranchs and seabirds), suggesting the MPA has the potential to provide long-term protection. In contrast, post-nesting green turtles travel up to 4000 km to distant foraging sites, so the protected beaches in the Chagos Archipelago provide a nesting sanctuary for individuals that forage across an ocean basin and several geopolitical borders. Surveys using divers and underwater video systems show high habitat diversity and abundant marine life on all trophic levels. For example, coral cover can be as high as 40–50%. Ecological studies are shedding light on how remote ecosystems function, connect to each other and respond to climate-driven stressors compared to other locations that are more locally impacted. However, important threats to this MPA have been identified, particularly global heating events, and Illegal, Unreported and Unregulated (IUU) fishing activity, which considerably impact both reef and pelagic fishes.

Global elasmobranch populations have declined dramatically over the past fifty years, and continued research into the drivers of their habitats and distributions is vital for improved conservation and management. How environmental factors influence elasmobranch behavior, habitat use, and movement patterns is still relatively poorly understood, in part because of the scale over which many of these animals roam and the remote nature of the marine ecosystems they inhabit. In the last decade there has been an explosion of satellite remote sensing (SRS) technologies that can cover these vast spatial scales for the marine environment. Consequentially, SRS presents an opportunity to analyze important environmental drivers in elasmobranch ecology and to aid management decisions for the conservation of declining populations. A systematic literature review was undertaken to synthesize the current use of SRS environmental data in elasmobranch research. In addition, to facilitate the use of SRS in this field moving forward, we have compiled a list of popular SRS data sources and sensors for common environmental variables in marine science. Our review of 71 papers (55 published in the last 10 years) identified ten SRS-derived environmental variables that have been used in elasmobranch studies, from a range of satellite sensors and data sources. Sea surface temperature and ocean productivity were the most frequently used variables. Articles primarily analyzed variables individually or in pairs, with few studies looking at a suite of interacting variables. Here, we present a summary of the current state of knowledge on the application of SRS, current gaps and limitations, and discuss some of the potential future directions in which we envisage this field developing. Threatened elasmobranch populations inhabit some of the world’s most remote marine ecosystems. With often global coverage, SRS presents an opportunity to analyze the important environmental drivers of elasmobranch ecology to aid management decisions for the conservation of declining and threatened populations.