David J. Curnick

Recreational fisheries in remote regions often operate under limited regulation and data availability, presenting challenges for sustainable management. Diego Garcia, located within the Chagos Marine Protected Area, hosts a unique recreational reef fishery embedded within a highly protected environment. We assessed the stock status of five key species— Lutjanus bohar , L. gibbus , L. monostigma , Caranx melampygus , and Variola louti —using the Length‐Based Spawning Potential Ratio (LB‐SPR) approach and length composition data collected through stereo‐video surveys. The average spawning potential ratio (SPR) across all species appears to be operating above 30%, a common threshold below which a stock can be considered overfished. The SPR of L. bohar and L. gibbus remained above 30% across all bootstrap iterations indicating confidence that these species are exploited at a sustainable level. L. monostigma , C. melampygus , and V. louti exhibited greater uncertainty in their estimated SPR, spanning all possible values with at least 50% of bootstrap iterations remaining above 30% SPR; for these species, caution should be applied until additional sampling can improve the resolution of these estimates. These findings suggest that the recreational fishery is operating within sustainable limits, likely due to access restrictions and diffuse fishing pressure across species. However, uncertainty in model outputs for data‐limited species underscores the need for continued biological sampling and catch monitoring.

Marine megafauna species are affected by a wide range of anthropogenic threats. To evaluate the risk of such threats, species’ vulnerability to each threat must first be determined. We build on the existing threats classification scheme and ranking system of the International Union for Conservation of Nature (IUCN) Red List of Threatened Species by assessing the vulnerability of 256 marine megafauna species to 23 at‐sea threats. The threats we considered included individual fishing gear types, climate‐change‐related subthreats not previously assessed, and threats associated with coastal impacts and maritime disturbances. Our ratings resulted in 70 species having high vulnerability ( v > 0.778 out of 1) to at least 1 threat, primarily drifting longlines, temperature extremes, or fixed gear. These 3 threats were also considered to have the most severe effects (i.e., steepest population declines). Overall, temperature extremes and plastics and other solid waste were rated as affecting the largest proportion of populations. Penguins, pinnipeds, and polar bears had the highest vulnerability to temperature extremes. Bony fishes had the highest vulnerability to drifting longlines and plastics and other solid waste; pelagic cetaceans to 4 maritime disturbance threats; elasmobranchs to 5 fishing threats; and flying birds to drifting longlines and 2 maritime disturbance threats. Sirenians and turtles had the highest vulnerability to at least one threat from all 4 categories. Despite not necessarily having severe effects for most taxonomic groups, temperature extremes were rated among the top threats for all taxa except bony fishes. The vulnerability scores we provide are an important first step in estimating the risk of threats to marine megafauna. Importantly, they help differentiate scope from severity, which is key to identifying threats that should be prioritized for mitigation.

Theory predicts that high population density leads to more strongly connected spatial and social networks, but how local density drives individuals’ positions within their networks is unclear. This gap reduces our ability to understand and predict density-dependent processes. Here we show that density drives greater network connectedness at the scale of individuals within wild animal populations. Across 36 datasets of spatial and social behaviour in >58,000 individual animals, spanning 30 species of fish, reptiles, birds, mammals and insects, 80% of systems exhibit strong positive relationships between local density and network centrality. However, >80% of relationships are nonlinear and 75% are shallower at higher values, indicating saturating trends that probably emerge as a result of demographic and behavioural processes that counteract density’s effects. These are stronger and less saturating in spatial compared with social networks, as individuals become disproportionately spatially connected rather than socially connected at higher densities. Consequently, ecological processes that depend on spatial connections are probably more density dependent than those involving social interactions. These findings suggest fundamental scaling rules governing animal social dynamics, which could help to predict network structures in novel systems.

Machine learning has the potential to revolutionize passive acoustic monitoring (PAM) for ecological assessments. However, high annotation and computing costs limit the field’s adoption. Generalizable pretrained networks can overcome these costs, but high-quality pretraining requires vast annotated libraries, limiting their current development to data-rich bird taxa. Here, we identify the optimum pretraining strategy for data-deficient domains, using tropical reefs as a representative case study. We assembled ReefSet, an annotated library of 57 000 reef sounds taken across 16 datasets, though still modest in scale compared to annotated bird libraries. We performed multiple pretraining experiments and found that pretraining on a library of bird audio 50 times the size of ReefSet provides notably superior generalizability on held-out reef datasets, with a mean area under the receiver operating characteristic curve (AUC-ROC) of 0.881 (±0.11), compared to pretraining on ReefSet itself or unrelated audio, with a mean AUC-ROC of 0.724 (±0.05) and 0.834 (±0.05), respectively. However, our key findings show that cross-domain mixing, where bird, reef and unrelated audio are combined during pretraining, provides superior transfer learning performance, with an AUC-ROC of 0.933 (±0.02). SurfPerch, our optimum pretrained network, provides a strong foundation for automated analysis of tropical reef and related PAM data with minimal annotation and computing costs. This article is part of the theme issue ‘Acoustic monitoring for tropical ecology and conservation’.

The evolution of marine fisheries has broadly influenced the ecology, distribution and abundance of coastal elasmobranchs. However, quantifying these interactions remains a challenge due to the scarcity of historical records. Fishers' local ecological knowledge (LEK) is an essential tool in fisheries research, providing an avenue to regain scarce information on rare or cryptic species and contextualise modern observations and trends. LEK was collected from 27 semi‐structured interviews with commercial, recreational and charter fishers to better understand the ecology and temporal population trends for the Critically Endangered angelshark (Squatina squatina) within Wales, United Kingdom. Questions were designed to identify fisher perceptions of socio‐economic, ecological and legislative factors that drove changes in fishing effort for vessels operating within the ‘Welsh Zone’ between 1968 and 2019. Understanding how fleet‐wide spatiotemporal changes to fishing effort and capacity within commercial, recreational and charter fisheries have influenced historical records and sightings of S. squatina is an important consideration for assessing its status in Wales and designing appropriate actions to safeguard the species. Our findings identify structural changes to fisheries in Wales and key driving factors that likely decreased fishers' likelihood of encountering S. squatina over a 51‐year period. Therefore, we suggest recent published estimates of population declines may be overstated. Furthermore, we highlight the importance of quantifying and incorporating changes in fishing effort at a regional scale into future modelling approaches for elasmobranchs and advocate for the increased consideration of LEK in the development of management strategies. Read the free Plain Language Summary for this article on the Journal blog.

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.

Many sharks, rays and skates are highly threatened and vulnerable to overexploitation, as such reliable monitoring of elasmobranchs is key to effective management and conservation. The mobile and elusive nature of these species makes monitoring challenging, particularly in temperate waters with low visibility. Environmental DNA (eDNA) methods present an opportunity to study these species in the absence of visual identification or invasive techniques. However, eDNA data alone can be difficult to interpret for species monitoring, particularly in a marine setting where its distribution can be influenced by water currents. In this study, we investigated the spatial and temporal distribution of elasmobranch species in two Special Areas for Conservation (SAC) off the coast of Wales. We took monthly eDNA samples for 1 year (starting September 2020 and March 2022 for the northern and southern SACs, respectively), and used metabarcoding to reveal the presence of elasmobranch species. We combined these data with hydrodynamic modelling and particle tracking methods to simulate the potential origins of the detected eDNA. We detected 11 elasmobranch species, including the critically endangered angelshark (Squatina squatina) and tope (Galeorhinus galeus). Most detections were in the spring and the fewest in the autumn. The particle tracking simulations predicted that eDNA was shed, on average, approximately 7 km and 15 km (in the northern and southern SACs, respectively) from the sampling stations at which it was detected. These results show that the two SACs represent important areas for elasmobranchs in the United Kingdom and demonstrate that eDNA methods combined with particle tracking simulations can represent a new frontier for monitoring marine species.

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.

Robust species-level methods for quantifying ecological differences have yet to be incorporated into conservation strategies. Here, we present a conservation prioritisation approach that integrates species trait data and extinction risk to quantify the contribution of individual species to overall functional diversity. The Functionally Irreplaceable with Risk of Extinction (FIRE) metric directs conservation action to species whose extinction is expected to result in significant losses of functional diversity. We applied our framework to sets of species at the global scale. First we assessed the world’s birds, highlighting congruent and divergent priorities identified by trait-based and phylogenetic approaches. Second, we applied FIRE to the world’s sharks, exploring the impact of imputed traits on prioritisation robustness. For birds and sharks, we show that prioritising by functional irreplaceability is an effective strategy to conserve exploited species. The FIRE metric provides a robust tool to facilitate the incorporation of functional diversity into conservation policy and practice, revealing species that may be overlooked by existing approaches.

The order Lamniformes contains charismatic species such as the white shark Carcharodon carcharias and extinct megatooth shark Otodus megalodon , and is of particular interest given their influence on marine ecosystems, and because some members exhibit regional endothermy. However, there remains significant debate surrounding the prevalence and evolutionary origin of regional endothermy in the order, and therefore the development of phenomena such as gigantism and filter-feeding in sharks generally. Here we show a basal lamniform shark, the smalltooth sand tiger shark Odontaspis ferox , has centralized skeletal red muscle and a thick compact-walled ventricle; anatomical features generally consistent with regionally endothermy. This result, together with the recent discovery of probable red muscle endothermy in filter feeding basking sharks Cetorhinus maximus , suggests that this thermophysiology is more prevalent in the Lamniformes than previously thought, which in turn has implications for understanding the evolution of regional endothermy, gigantism, and extinction risk of warm-bodied shark species both past and present.