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Understanding how reef-associated sharks utilise coastal reef habitats throughout their ontogeny is essential for their effective management. In this study, we assessed the distribution of sharks in the northern Caribbean Netherlands (Saba, Saba Bank and St Eustatius) and ontogenetic shifts in habitat and depth use of the 2 most observed species: Caribbean reef shark Carcharhinus perezi and nurse shark Ginglymostoma cirratum. We used stereo baited remote underwater video (stereo-BRUV) surveys at 376 sites. A total of 126 shark sightings (6 species) were recorded, with C. perezi (n = 72) and G. cirratum (n = 42) most frequently observed. The probability of recording at least 1 shark per deployment ranged from 0.19 to 0.37 and is comparable with shark occurrences in the small number of other stereo-BRUV studies in the Wider Caribbean Region. Habitat type was the most important factor driving reef-associated shark occurrences, with the highest probability of observing C. perezi and G. cirratum in soft-coral habitat. Additionally, occurrences of C. perezi were significantly influenced by the management zone, with highest probabilities of occurrence in no-fishing zones. Almost all observed reef-associated sharks (95.6%) were juveniles, indicating that the study area may be a nursery area. Overall, both species were observed in deeper waters with increasing size, indicating ontogenetic shifts in depth use. Our findings imply that protected areas should not be limited to a single habitat or depth, but focus on protecting a large area with the range of habitats and depths necessary for reef-associated sharks to complete their life cycle.

Human activities have resulted in the loss of over 90% of sharks in most ocean basins and one in four species of elasmobranch are now listed at risk of extinction by the IUCN. How this collapse will affect the ability of populations to recover in the face of continued exploitation and global climate change remains unknown. Indeed, important ecological and biological information are lacking for most shark species, particularly estimates of genetic diversity and population structure over a range of spatial scales. Using 15 microsatellite markers, we investigated genetic diversity and population structure in gray reef sharks over their Indo-Pacific range (407 specimens from 9 localities). Clear genetic differentiation was observed between the Indian and the Pacific Ocean specimens (FST = 0.145***). Further differentiation within the Pacific included a West and East cleavage as well as North-Central and South-Central Pacific clusters. No genetic differentiation was detected within archipelagos. These results highlight the legacy of past climate changes and the effects of large ocean expanses and circulation patterns on contrasting levels of connectivity at global, regional and local scales. Our results indicate a need for regional conservation units for gray reef sharks and pinpoint the isolation and vulnerability of their French Polynesian population.

In teleost fishes, catecholamine-induced increases in the activity of cation exchangers compensate for decreases in hemoglobin oxygen affinity and maximum blood oxygen carrying capacity caused by decreases in plasma pH (i.e., metabolic acidosis). The resultant red blood cell (RBC) swelling has been documented in sandbar (Carcharhinus plumbeus) and epaulette (Hemiscyllium ocellatum) sharks following capture by rod-and-reel or after a 1.5 h exposure to anoxia (respectively), although the underlying mechanisms remain unknown. To determine if RBC swelling could be documented in other elasmobranch fishes, we collected blood samples from clearnose skate (Rostroraja eglanteria), blacktip reef shark (Carcharhinus melanopterus), and sicklefin lemon shark (Negaprion acutidens) subjected to exhaustive exercise or air exposure (or both) and measured hematocrit, hemoglobin concentration, RBC count, RBC volume, and mean corpuscular hemoglobin content. We did likewise with sandbar and epaulette sharks to further explore the mechanisms driving swelling when present. We could not document RBC swelling in any species; although hematocrit increased in all species (presumably due to RBC ejection from the spleen or fluid shifts out of the vascular compartment) except epaulette shark. Our results indicate RBC swelling and associated ion shifts in elasmobranch fishes is not inducible by exercise or hypoxia, thus implying this response maybe of lesser importance for maintaining oxygen delivery during acute acidosis than in teleost fishes.

We report the first mitochondrial genome sequences for the gray reef shark, Carcharhinus amblyrhynchos. Two specimens from the British Indian Ocean Territory were sequenced independently using two different next generation sequencing methods, namely short read sequencing on the Illumina HiSeq and long read sequencing on the Oxford Nanopore Technologies’ MinION sequencer. The two sequences are 99.9% identical and are 16,705 base pairs (bp) and 16,706 bp in length. The mitogenome contains 22 tRNA genes, two rRNA genes, 13 protein-coding genes and two non-coding regions; the control region and the origin of light-strand replication (OL).

This report describes the first record of the piscicolid leech Pontobdella macrothela from Mozambique waters and the first record of P. macrothela parasitizing a Grey Reef Shark (Carcharhinus amblyrhynchos). The leech, P. macrothela, was found attached externally between the third and fourth gill slits of a shark captured off Vamizi Island, Quirimbas Archipelago, northern Mozambique.

Acoustic tagging is typically used to gather data on the spatial ecology of diverse marine taxa, informing questions about spatio-temporal attributes such as residency and home range, but detection data may also reveal unanticipated insights. Many species demonstrate predictable site fidelity, and so a sudden cessation of detections for multiple individuals may be evidence of an atypical event. During 2013 and 2014, we acoustically tagged 47 grey reef sharks (Carcharhinus amblyrhynchos) and 48 silvertip sharks (Carcharhinus albimarginatus) near reefs in the British Indian Ocean Territory (BIOT) Marine Protected Area (MPA). From March 2013 to November 2014 inclusive, tags were ‘lost’, i.e. permanently ceased to be detected within the monitoring area, at an average rate of 2.6 ± 1.0 tags per month. Between 1 and 10 December 2014, detection data suggest the near-simultaneous loss of 15 of the remaining 43 active tagged sharks, a monthly loss rate over five times higher than during the previous 21 months. Between 4 and 14 December of 2014, the BIOT patrol vessel encountered 17 vessels engaged in suspected illegal fishing in the northern BIOT MPA; such sightings averaged one per month during the previous 8 months. Two of these vessels were arrested with a total of 359 sharks on board, of which grey reef and silvertip sharks constituted 47% by number. The unusual and coincident peaks in tag loss and vessel sightings, and the catch composition of the arrested vessels, suggest illegal fishing as a plausible explanation for the unusual pattern in our detection data. A Cox proportional hazards model found that the presence of fishing vessels increased the risk of tag loss by a factor of 6.0 (95% CI 2.6–14.0, p

Underwater visual surveys represent an essential component of coastal marine research and play a crucial role in supporting the management of marine systems. However, logistical and financial considerations can limit the availability of survey data in some systems. While biologging camera tag devices are being attached to an increasing diversity of marine animals to collect behavioral information about the focal species, the ancillary imagery collected can also be used in analytical techniques developed for diver-based surveys. We illustrate this approach by extracting ancillary data from shark-borne camera tag deployments focused on the behavior of a White shark (Carcharodon carcharias) off Gansbaai, South Africa, and a Grey Reef shark (Carcharhinus amblyrhynchos) within the Chagos Archipelago. Within the giant kelp forest environment of Gansbaai we could determine the spatial density of kelp thali and underlying substrate composition. Within the coral reef environment, the animal-borne video allowed us to determine the approximate percent and type of benthic cover, as well as growth form and genus of corals down to the upper mesophotic zone. We also enumerated fish species-level abundance over reef flat and wall environments. We used established dive-survey methods to analyze video data and found the results to be broadly comparable in the two systems studied. Our work illustrates the broad applicability of ancillary animal-borne video data, which is analogous in type and quality to diver-based video data, for analysis in established marine community survey frameworks. As camera tags and associated biologging technologies continue to develop and are adapted to new environments, utilising these data could have wide-ranging applications and could maximise the overall cost–benefit ratio within biologging deployments.

Global overfishing of higher‐level predators has caused cascading effects to lower trophic levels in many marine ecosystems. On coral reefs, which support highly diverse food‐webs, the degree to which top‐down trophic cascades can occur remains equivocal. Using extensive survey data from coral reefs across the relatively unfished northern Great Barrier Reef (nGBR), we quantified the role of reef sharks in structuring coral reef fish assemblages. Using a structural equation modelling (SEM) approach, we explored the interactions between the shark abundance and teleost mesopredator and prey functional group density and biomass, whilst explicitly accounting for the potentially confounding influence of environmental variation across sites. While a fourfold difference in reef shark density was observed across our survey sites, this had no impact on either the density or biomass of teleost mesopredators or prey, providing evidence for a lack of trophic cascading across nGBR systems. Instead, many functional groups, including sharks, responded positively to environmental drivers. We found reef sharks to be positively associated with habitat complexity. In turn, physical processes such as wave exposure and current velocity were both correlated well with multiple functional groups, reflecting how changes to energetic conditions and food availability, or modification of habitat affect fish distribution. The diversity of species within coral reef food webs and their associations with bottom‐up drivers likely buffers against trophic cascading across GBR functional guilds when reef shark assemblages are depleted, as has been demonstrated in other complex ecosystems.

Sofyan JF, Ambariyanto A, Suwartimah K, Toha AHA. 2020. Relationship between the biomass of reef shark and fish in South Morotai Waters, North Maluku, Indonesia. Biodiversitas 21: 5605-5613. This study aims to determine the biomass of reef shark and fish in South Morotai Waters in North Maluku, Indonesia. The Audible Stationary Count and Underwater Visual Census method were used to collect the data of reef sharks and fish. Seven and one site locations of coral reef and shark ecosystems were surveyed and, the data were analyzed using regression analysis to obtain a correlation between the variables. The three reef sharks found were Carcharhinus melanopterus, Triaenodon obesus, and Carcharhinus amblyrhynchos. Biomass estimation of reef sharks and fish ranged from 0.59 to 19.97 kg/ha and 30.95 kg/ha to 49.92 kg/ha, consisting of 8 families. In the area of aggregations, both species were found in 7 sites, and the population of reef shark amounted to 86.96 kg/ha and fish was around 55.705 kg/ha, consisting of 9 families. There was a positive relationship between the biomass of reef shark and fish in South Morotai waters, and the index of determination was 0.8043, showing that the biomass of reef shark was influenced by that of fish. These results indicated the importance of biomass in determining the functional composition and diversity of reef shark and fish.

Ocean warming and acidification act concurrently on marine ectotherms with the potential for detrimental, synergistic effects; yet, effects of these stressors remain understudied in large predatory fishes, including sharks. We tested for behavioural and physiological responses of blacktip reef shark (Carcharhinus melanopterus) neonates to climate change relevant changes in temperature (28 and 31 °C) and carbon dioxide partial pressures (pCO2; 650 and 1050 µatm) using a fully factorial design. Behavioural assays (lateralisation, activity level) were conducted upon 7–13 days of acclimation, and physiological assays (hypoxia tolerance, oxygen uptake rates, acid–base and haematological status) were conducted upon 14–17 days of acclimation. Haematocrit was higher in sharks acclimated to 31 °C than to 28 °C. Significant treatment effects were also detected for blood lactate and minimum oxygen uptake rate; although, these observations were not supported by adequate statistical power. Inter-individual variability was considerable for all measured traits, except for haematocrit. Moving forward, studies on similarly ‘hard-to-study’ species may account for large inter-individual variability by increasing replication, testing larger, yet ecologically relevant, differences in temperature and pCO2, and reducing measurement error. Robust experimental studies on elasmobranchs are critical to meaningfully assess the threat of global change stressors in these data-deficient species.

Sharks play important functional roles in coral reef ecosystems. Studying reef shark populations' spatial ecology also contributes important data for effective conservation planning. The purpose of this study was to define the home range of neonatal blacktip reef sharks (Carcharhinus melanopterus) around Moorea, French Polynesia, and compare estimates using both mark-recapture surveys and active acoustic telemetry. Mark-recapture surveys produced a minimum convex polygon (MCP) of 0.07 km(2) that was significantly larger than the MCP derived from acoustic telemetry (0.02 km(2)). Acoustic telemetry produced 50 and 95% kernel utilization densities that were smaller (0.02 km(2)) and larger (0.14 km(2)) than home range estimates from mark-recapture surveys, respectively. Home range estimates from this study are the smallest that have been documented for neonatal blacktip reef sharks, possibly owing to the study sites' proximity to deep channels. Mark-recapture and active acoustic telemetry are complementary approaches worthy of consideration where passive telemetry is impractical.

Information on the spatial ecology of reef sharks is critical to understanding life-history patterns, yet gaps remain in our knowledge of how these species move and occupy space. Previous studies have focused on offshore reefs and atolls with little information available on the movement and space use of sharks utilising reef habitats closer to shore. Cross-shelf differences in physical and biological properties of reefs can alter regional ecosystem processes resulting in different movement patterns for resident sharks. Passive acoustic telemetry was used to examine residency, space use and depth use of 40 blacktip reef sharks, Carcharhinus melanopterus, on an inshore reef in Queensland, Australia, and assess temporal or biological influences. All sharks showed strong site-attachment to inshore reefs with residency highest among adult females. Sharks exhibited a sex-based, seasonal pattern in space use where males moved more, occupied more space and explored new areas during the reproductive season, while females utilised the same amount of space throughout the year, but shifted the location of the space used. A positive relationship was also observed between space use and size. There was evidence of seasonal site fidelity and long-distance movement with the coordinated, annual migration of two adult males to the study site during the mating season. Depth use was segregated with some small sharks occupying shallower depths than adults throughout the day and year, most likely as refuge from predation. Results highlight the importance of inshore reef habitats to blacktip reef sharks and provide evidence of connectivity with offshore reefs, at least for adult males.

Myriad anthropogenic impacts drive declines in global shark populations; yet, the consequences of a newly recognised threat, global climate change, are poorly understood. This thesis tested the hypothesis that global change stressors (ocean acidification and warming) reduce fitness in tropical reef sharks via effects on physiological performance. My specific objectives were to define thermal performance in fitness-enhancing nursery areas, physiological performance in situ, associations between thermal performance, preference, and tolerance, and physiological performance under multiple global change stressors. I found that neonatal blacktip reef sharks (Carcharhinus melanopterus) have superior growth efficiency in nursery areas relative to other habitats, but ocean acidification and warming synergistically reduce performance. This thesis suggests that global change stressors reduce fitness in tropical reef sharks by acting on physiological traits that are associated with nursery areas., Les requins sont menacés aux causes anthropiques ; mais, les conséquences qu’une menace nouveau, le changement climatique, sont mal connus. Pour ma thèse, j’ai testé l’hypothèse que le changement climatique agira sur les performances physiologiques des requins tropicaux pour réduire la valeur sélective. Mes objectives sont d’évaluer les effets des zones « nurseries » sur la performance physiologique, la performance physiologique in situ, les performances, préférences, et tolérances thermiques, et les effets du réchauffement et de l’acidification des océans sur la performance physiologique. J’ai trouvé que les requins pointes noires (Carcharhinus melanopterus) ont la croissance supérieure dans les zones nurseries mais l’acidification et le réchauffement agissent en synergie à la réduire. Mes résultats suggèrent que le changement climatique va réduire la valeur sélective des requins tropicaux par les effets sur les performances physiologiques qui sont associés aux zones nurseries.

Place: London Publisher: Nature Publishing Group WOS:000563068400001; International audience; Globally, the frequency of shark bites is rising, resulting in an increasing demand for shark deterrents and measures to lessen the impact of shark bites on humans. Most existing shark protection measures are designed to reduce the probability of a bite, but fabrics that minimise injuries when a shark bite occurs can also be used as mitigation devices. Here, we assessed the ability of the Ocean Guardian Scuba7 and Kevlar material to reduce the likelihood of blacktip reef sharks, Carcharhinus melanopterus, from feeding, and to minimise injuries from shark bites. Sharks were enticed to consume a small piece of local reef fish (bait) placed between the two Scuba7 electrodes with the deterrents randomly being turned on or kept off. In the second experiment, the bait was attached to a small pouch made of either standard neoprene or neoprene with a protective layer of Kevlar around it. The Scuba7 reduced the proportion of baits being taken by 67%, (from 100% during control trials to 33%). Sharks also took more time to take the bait when the device was active (165 +/- 20.40 s vs. 38.9 +/- 3.35 s), approached at a greater distance (80.98 +/- 1.72 cm vs. 38.88 +/- 3.20 cm) and made a greater number of approaches per trial (19.38 +/- 2.29 vs. 3.62 +/- 0.53) than when the Scuba7 was inactive. The sizes of punctures from shark bites were significantly smaller on neoprene with Kevlar compared to standard neoprene (3.64 +/- 0.26 mm vs. 5.88 +/- 0.29 mm). The number of punctures was also fewer when Kevlar was used (14.92 +/- 3.16 vs. 74.1 +/- 12.44). Overall, the Ocean Guardian Scuba7 and Kevlar reduced the impact of blacktip reef shark bites. These findings may help consumers make informed decisions when purchasing shark deterring and protective products.

International audience; While the negative effects of consumptive pressures on marine predators are well established, the effects of increasing non-consumptive activities wildlife tourism are still understudied. As such, the long-term effects of the provision of bait on shark behaviour are still unclear. Here, we assessed the effects of provisioning using a Control-Impact design on the spatial use and level of residency of the blacktip reef shark (Carcharhinus melanopterus) over a two-year period. We used effect sizes to model the differences relative changes in residency between provisioning and non-provisioning sites. Sharks showed a high degree of residency and significant changes in their habitat use which persisted overnight while the activity ceased. We suggest that provisioning activities can affect species with high level of residency such as the blacktip reef shark. Further research is needed to better understand how these changes behavioural modifications can alter the fitness of this species. It is important to adapt shark provisioning activities to limit the induced changes in habitat.

Social foraging allows groups of predators to search for, pursue, and capture prey with greater efficiency than using solitary hunting. It can vary in complexity and take many forms ranging from cooperative hunting, to social information sharing and local enhancement (Lang and Farine 2017). Theoretical and empirical studies support the advantages of group foraging, although there will be trade‐offs between benefits, such as increased prey detection or capture success, and costs such as increased competition amongst group members (Gil et al. 2017).

Fishing has had a profound impact on global reef shark populations, and the absence or presence of sharks is strongly correlated with national socio-economic conditions and reef governance. Decades of overexploitation have devastated shark populations, leaving considerable doubt as to their ecological status(1,2). Yet much of what is known about sharks has been inferred from catch records in industrial fisheries, whereas far less information is available about sharks that live in coastal habitats(3). Here we address this knowledge gap using data from more than 15,000 standardized baited remote underwater video stations that were deployed on 371 reefs in 58 nations to estimate the conservation status of reef sharks globally. Our results reveal the profound impact that fishing has had on reef shark populations: we observed no sharks on almost 20% of the surveyed reefs. Reef sharks were almost completely absent from reefs in several nations, and shark depletion was strongly related to socio-economic conditions such as the size and proximity of the nearest market, poor governance and the density of the human population. However, opportunities for the conservation of reef sharks remain: shark sanctuaries, closed areas, catch limits and an absence of gillnets and longlines were associated with a substantially higher relative abundance of reef sharks. These results reveal several policy pathways for the restoration and management of reef shark populations, from direct top-down management of fishing to indirect improvement of governance conditions. Reef shark populations will only have a high chance of recovery by engaging key socio-economic aspects of tropical fisheries.

Lay summary Elasmobranch blood haematological parameters remained stable over 3 hours of storage duration, indicating that they do not need to be measured immediately. The HemoCue haemoglobin analyser can be used to measure haemoglobin concentrations from elasmobranch blood samples with the use of a correction equation., Blood samples collected from wild-caught fishes can provide important information regarding the effects of capture (and thus post-release survival) as well as other stressors. Unfortunately, blood samples often cannot be analysed immediately upon sampling, and blood parameters (e.g. blood oxygen levels and acid–base parameters) are known to change with storage duration due to the metabolic activity of the red blood cells. We obtained blood samples from both untreated and stressed individuals of both blacktip reef shark (Carcharhinus melanopterus) and sicklefin lemon shark (Negaprion acutidens) to determine the effects of storage duration on blood pH, haematocrit and haemoglobin concentration ([Hb]). We found no significant effects after storage on ice for up to 180 minutes. Moreover, to validate the usability of a HemoCue haemoglobin analyser (a point-of-care device), we compared data from this device to [Hb] determined using the cyanomethaemoglobin method with blood samples from 10 individuals from each of the aforementioned species as well as epaulette shark (Hemiscyllium ocellatum). Values from the HemoCue consistently overestimated [Hb], and we therefore developed the necessary correction equations. The correction equations were not statistically different among the three elasmobranch species within the biologically relevant range but did differ from published corrections developed using blood from temperate teleost fishes. Although the HemoCue is useful in field situations, development of species-specific calibration equations may be necessary to ensure the reliability of inter-species comparisons of blood [Hb]. Together, these data should increase confidence in haematological stress indicators in elasmobranch fishes, measurements of which are critical for understanding the impact of anthropogenic stressors on these ecologically important species.

We present the mitochondrial genome sequence of a gray reef shark, Carcharhinus amblyrhynchos (Bleeker 1856), a coral reef associated species. This is the first mitogenome for this species from the western Indian Ocean. The mitogenome is 16,705 bp in length, has 13 protein-coding genes, 22 tRNA genes, 2 rRNA genes and a non-coding control region, and demonstrates a gene arrangement congruent with other shark and most vertebrate species. This mitogenome provides a genomic resource for assisting with population, evolutionary and conservation studies for the gray reef shark, which is increasingly under threat from fisheries.

International audience; Reef sharks are vulnerable predators experiencing severe population declines mainly due to overexploitation. However, beyond direct exploitation, human activities can produce indirect or sub-lethal effects such as behavioral alterations. Such alterations are well known for terrestrial fauna but poorly documented for marine species. Using an extensive sampling of 367 stereo baited underwater videos systems, we show modifications in grey reef shark (Carcharhinus amblyrhynchos) occurrence and feeding behavior along a marked gradient of isolation from humans across the New Caledonian archipelago (South-Western Pacific). The probability of occurrence decreased by 68.9% between wilderness areas (more than 25 hours travel time from the capital city) and impacted areas while the few individuals occurring in impacted areas exhibited cautious behavior. We also show that only large no-entry reserves (above 150 km²) can protect the behavior of grey reef sharks found in the wilderness. Influencing the fitness, human linked behavioral alterations should be taken into account for management strategies to ensure the persistence of populations. Beyond overexploitation and the global defaunation of all ecosystems on Earth 1 , humans can induce behavioral alterations on animals with potential consequences on their fitness 2,3. For instance, terrestrial wildlife like mammals 4 and birds 5 are more vigilant near humans, resulting in decreased food intake and reduced reproductive success. Similarly, marine mammals show behavioral alterations (i.e. decreased food intake and reduced reproductive success) due to nautical activities, resulting in additional energetic costs to avoid interactions or nuisance 6,7. Sharks exhibit high learning abilities similar to most terrestrial vertebrates 8. Behavior experiments demonstrate that sharks can learn discriminative tasks and retain information for long time periods 9,10. These abilities make sharks also likely to demonstrate long-lasting adaptive responses to external stimuli including interactions with human activities 11. For instance, the catchability of blacktip reef sharks (Carcharhinus melanopterus) decreases after several catches and releases 12 while bull sharks (Carcharhinus leucas) show a modified behavior related to food provisioning 13. Although human impacts on shark behavior have been documented, we still lack empirical evidence of behavioral alterations along a wide gradient of human activities from densely populated areas to the last wilderness areas. Reef sharks are emblematic species providing socioeconomic benefits through tourism while playing multiple ecological roles in coastal ecosystems 14,15. Their conservative life history traits such as slow growth rate, late sexual maturity and low fecundity, coupled with overfishing, induce severe population depletion worldwide and high extinction risk 16,17. Consequently, reef sharks are of high conservation priority 18. Marine reserves, a restrictive subset of marine protected areas 19 , are recognized as management tools to counteract human pervasive impacts by prohibiting extracting activities (no-take) or even human presence (no-entry) 20. By lessening human pressure including extraction of their prey, marine reserves may attract sharks and restore their behavior under quasi-wilderness or undisturbed conditions 21. However, only a very small portion of marine reserves are

The occurrence of sharks on coral reefs has been well documented for decades, especially since the advent of SCUBA diving. Despite this, it is only within the last decade that substantial research effort has been directed at these species. Research effort has increased in conjunction with the realisation that reef shark populations have experienced significant declines throughout their distribution. However, trends in declines have been coupled with reports of high abundance in some areas providing confusion about what healthy reef shark populations should look like. Given that coral reefs are among the most biologically diverse and productive habitats, but also are one of the most threatened by climate change due to the effects of rising temperature and declining pH, there is a need to understand reef sharks to better predict consequences for their populations. Studies of reef sharks also have the potential to provide insights into the functioning of their populations and ecosystems more broadly because of the spatially constrained nature of their distributions, and high water visibility in most locations. These aspects make studying reef shark populations integral to understanding coral reef ecosystem dynamics and resilience to pressures. This paper synthesises a number of key questions about coral reef sharks based on our experience researching this group of species over the past decade. Key research gaps and critical questions include aspects of life history, population dynamics, ecology, behaviour, physiology, energetics, and more. This synthesis also considers the methods used to date, and what new and emerging techniques may be available to improve our understanding of reef shark populations. The synthesis will highlight how even basic questions relating to reef shark population sizes, how large they should be, and what impacts do they have on reef ecosystems, remain either unanswered or highly controversial.

International audience; The evolutionary history of species is a dynamic process as they modify, expand, and contract their spatial distributions over time. Range expansions (REs) occur through a series of founder events that are followed by migration among neighboring demes. The process usually results in structured metapopulations and leaves a distinct signature in the genetic variability of species. Explicitly modeling the consequences of complex demographic events such as REs is computationally very intensive. Here we propose an an alternative approach that requires less computational effort than a comprehensive RE model, but that can recover the demography of species undergoing a RE, by combining spatially explicit modelling with simplified but realistic metapopulation models. We examine the demographic and colonization history of Carcharhinus melanopterus, an abundant reef-associated shark, as a test case. We first used a population genomics approach to statistically confirm the occurrence of a RE in C. melanopterus, and identify its origin in the Indo-Australian Archipelago. Spatial genetic modelling identified two waves of stepping-stone colonization: an eastward wave moving through the Pacific and a westward one moving through the Indian Ocean. We show that metapopulation models best describe the demographic history of this species and that not accounting for this may lead to incorrectly interpreting the observed genetic variation as signals of widespread population bottlenecks. Our study highlights insights that can be gained about demography by coupling metapopulation models with spatial modeling and underscores the need for cautious interpretation of population genetic data when advancing conservation priorities.

International audience; 1. Reef sharks are declining world-wide under ever-increasing fishing pressure, with potential consequences on ecosystem functioning. Marine protected areas (MPAs) are currently one of the management tools used to counteract the pervasive impacts of fishing. However, MPAs in which reef sharks are abundant tend to be located in remote and underexploited areas, preventing a fair assessment of management effectiveness beyond remoteness from human activities. 2. Here, we determine the conditions under which MPAs can effectively protect sharks along a wide gradient of reef accessibility, from the vicinity of a regional capital towards remote areas, using 385 records from baited remote underwater video systems and 2,790 underwater visual censuses performed in areas open to fishing and inside 15 MPAs across New Caledonia (South-Western Pacific). 3. We show that even one of the world's oldest (43 years), largest (172 km(2)) and most restrictive (no-entry) MPA (Merlet reserve) on coral reefs has between 17.3% and 45.3% fewer shark species and between 37.2% and 79.8% fewer shark abundance than remote areas in a context where sharks are not historically exploited. 4. On coral reefs situated at less than 1 hr of travel time from humans, shark populations are so low in abundance (less than 0.05 individuals per 1,000 m(2)) that their functional roles are severely limited. 5. Synthesis and applications. Remote areas are the last sanctuaries for reef sharks, providing a new baseline from which to evaluate human impacts on the species. However, there is no equivalent close to human activities even in large, old and strongly restrictive marine protected areas. As such sharks deserve strong protection efforts. The large, no-entry marine protected areas, close to humans, offer limited benefits for reef shark populations, but provide more realistic conservation targets for managers of human-dominated reefs. The exclusion of human activities on a sufficiently large area is key to protect reef shark populations. However, this strategy remains difficult to apply in many countries critically depending on reef resources for food security or livelihood.

Remote island nations face a number of challenges in addressing concerns about shark population status, including access to rigorously collected data and resources to manage fisheries. At present, very little data are available on shark populations in the Solomon Islands and scientific surveys to document shark and ray diversity and distribution have not been completed. We aimed to provide a baseline of the relative abundance and diversity of reef sharks and rays and assess the major drivers of reef shark abundance/biomass in the Western Province of the Solomon Islands using stereo baited remote underwater video. On average reef sharks were more abundant than in surrounding countries such as Fiji and Indonesia, yet below that of remote islands without historical fishing pressure, suggesting populations are relatively healthy but not pristine. We also assessed the influence of location, habitat type/complexity, depth and prey biomass on reef shark abundance and biomass. Location was the most important factor driving reef shark abundance and biomass with two times the abundance and a 43% greater biomass of reef sharks in the more remote locations, suggesting fishing may be impacting sharks in some areas. Our results give a much needed baseline and suggest that reef shark populations are still relatively unexploited, providing an opportunity for improved management of sharks and rays in the Solomon Islands.

We investigated drivers of reef shark demography across a large and isolated marine protected area, the British Indian Ocean Territory Marine Reserve, using stereo baited remote underwater video systems. We modelled shark abundance against biotic and abiotic variables at 35 sites across the reserve and found that the biomass of low trophic order fish (specifically planktivores) had the greatest effect on shark abundance, although models also included habitat variables (depth, coral cover and site type). There was significant variation in the composition of the shark assemblage at different atolls within the reserve. In particular, the deepest habitat sampled (a seamount at 70-80m visited for the first time in this study) recorded large numbers of scalloped hammerhead sharks (Sphyrna lewini) not observed elsewhere. Size structure of the most abundant and common species, grey reef sharks (Carcharhinus amblyrhynchos), varied with location. Individuals at an isolated bank were 30% smaller than those at the main atolls, with size structure significantly biased towards the size range for young of year (YOY). The 18 individuals judged to be YOY represented the offspring of between four and six females, so, whilst inconclusive, these data suggest the possible use of a common pupping site by grey reef sharks. The importance of low trophic order fish biomass (i.e. potential prey) in predicting spatial variation in shark abundance is consistent with other studies both in marine and terrestrial systems which suggest that prey availability may be a more important predictor of predator distribution than habitat suitability. This result supports the need for ecosystem level rather than species-specific conservation measures to support shark recovery. The observed spatial partitioning amongst sites for species and life-stages also implies the need to include a diversity of habitats and reef types within a protected area for adequate protection of reef-associated shark assemblages.

What did coral reef ecosystems look like before human impacts became pervasive? Early efforts to reconstruct baselines resulted in the controversial suggestion that pristine coral reefs have inverted trophic pyramids, with disproportionally large top predator biomass. The validity of the coral reef inverted trophic pyramid has been questioned, but until now, was not resolved empirically. We use data from an eight-year tag-recapture program with spatially explicit, capture-recapture models to re-examine the population size and density of a key top predator at Palmyra atoll, the same location that inspired the idea of inverted trophic biomass pyramids in coral reef ecosystems. Given that animal movement is suspected to have significantly biased early biomass estimates of highly mobile top predators, we focused our reassessment on the most mobile and most abundant predator at Palmyra, the grey reef shark (Carcharhinus amblyrhynchos). We estimated a density of 21.3 (95% CI 17.8, 24.7) grey reef sharks/km2, which is an order of magnitude lower than the estimates that suggested an inverted trophic pyramid. Our results indicate that the trophic structure of an unexploited reef fish community is not inverted, and that even healthy top predator populations may be considerably smaller, and more precarious, than previously thought.

Individuals can play different roles in maintaining connectivity and social cohesion in animal populations and thereby influence population robustness to perturbations. We performed a social network analysis in a reef shark population to assess the vulnerability of the global network to node removal under different scenarios. We found that the network was generally robust to the removal of nodes with high centrality. The network appeared also highly robust to experimental fishing. Individual shark catchability decreased as a function of experience, as revealed by comparing capture frequency and site presence. Altogether, these features suggest that individuals learnt to avoid capture, which ultimately increased network robustness to experimental catch-and-release. Our results also suggest that some caution must be taken when using capture–recapture models often used to assess population size as assumptions (such as equal probabilities of capture and recapture) may be violated by individual learning to escape recapture.

International audience; The extent of the global human footprint [ 1 ] limits our understanding of what is natural in the marine environment. Remote, near-pristine areas provide some baseline expectations for biomass [ 2, 3 ] and suggest that predators dominate, producing an inverted biomass pyramid. The southern pass of Fakarava atoll—a biosphere reserve in French Polynesia—hosts an average of 600 reef sharks, two to three times the biomass per hectare documented for any other reef shark aggregations [ 4 ]. This huge biomass of predators makes the trophic pyramid inverted. Bioenergetics models indicate that the sharks require ∼90 tons of fish per year, whereas the total fish production in the pass is ∼17 tons per year. Energetic theory shows that such trophic structure is maintained through subsidies [ 5–9 ], and empirical evidence suggests that sharks must engage in wide-ranging foraging excursions to meet energy needs [ 9, 10 ]. We used underwater surveys and acoustic telemetry to assess shark residency in the pass and feeding behavior and used bioenergetics models to understand energy flow. Contrary to previous findings, our results highlight that sharks may overcome low local energy availability by feeding on fish spawning aggregations, which concentrate energy from other local trophic pyramids. Fish spawning aggregations are known to be targeted by sharks, but they were previously believed to play a minor role representing occasional opportunistic supplements. This research demonstrates that fish spawning aggregations can play a significant role in the maintenance of local inverted pyramids in pristine marine areas. Conservation of fish spawning aggregations can help conserve shark populations, especially if combined with shark fishing bans.

Fisheries observer programs are used around the world to collect crucial information and samples that inform fisheries management. However, observer error may misidentify similar-looking shark species. This raises questions about the level of error that species misidentifications could introduce to estimates of species’ life history parameters. This study addressed these questions using the Grey Reef Shark Carcharhinus amblyrhynchos as a case study. Observer misidentification rates were quantified by validating species identifications using diagnostic photographs taken on board supplemented with DNA barcoding. Length-at-age and maturity ogive analyses were then estimated and compared with and without the misidentified individuals. Vertebrae were retained from a total of 155 sharks identified by observers as C. amblyrhynchos. However, 22 (14%) of these were sharks were misidentified by the observers and were subsequently re-identified based on photographs and/or DNA barcoding. Of the 22 individuals misidentified as C. amblyrhynchos, 16 (73%) were detected using photographs and a further 6 via genetic validation. If misidentified individuals had been included, substantial error would have been introduced to both the length-at-age and the maturity estimates. Thus validating the species identification, increased the accuracy of estimated life history parameters for C. amblyrhynchos. From the corrected sample a multi-model inference approach was used to estimate growth for C. amblyrhynchos using three candidate models. The model averaged length-at-age parameters for C. amblyrhynchos with the sexes combined were L¯∞ = 159 cm TL and L¯0 = 72 cm TL. Females mature at a greater length (l50 = 136 cm TL) and older age (A50 = 9.1 years) than males (l50 = 123 cm TL; A50 = 5.9 years). The inclusion of techniques to reduce misidentification in observer programs will improve the results of life history studies and ultimately improve management through the use of more accurate data for assessments.

Grey reef sharks (Carcharhinus amblyrhynchos) can be one of the numerically dominant high order predators on pristine coral reefs, yet their numbers have declined even in the highly regulated Australian Great Barrier Reef (GBR) Marine Park. Knowledge of both large scale and fine scale genetic connectivity of grey reef sharks is essential for their effective management, but no genetic data are yet available. We investigated grey reef shark genetic structure in the GBR across a 1200 km latitudinal gradient, comparing empirical data with models simulating different levels of migration. The empirical data did not reveal any genetic structuring along the entire latitudinal gradient sampled, suggesting regular widespread dispersal and gene flow of the species throughout most of the GBR. Our simulated datasets indicate that even with substantial migrations (up to 25% of individuals migrating between neighboring reefs) both large scale genetic structure and genotypic spatial autocorrelation at the reef scale were maintained. We suggest that present migration rates therefore exceed this level. These findings have important implications regarding the effectiveness of networks of spatially discontinuous Marine Protected Areas to protect reef sharks.

The Grey Reef Shark Carcharhinus amblyrhynchos is a moderately sized and fairly stocky species widely distributed in the Indian and Pacific Oceans from Easter Island and as far west as South Africa. In this paper, the first record of Grey reef shark (single specimen female 126cm in Total Length (TL)), caught from the Lakshadweep Sea (10o47 N and 72o09 E) by drift long line operation at 40m depth is reported.

We used acoustic telemetry to describe the patterns of vertical movement, site fidelity and residency of grey reef sharks (Carcharhinus amblyrhynchos) on the outer slope of coral reefs in Palau, Micronesia, over a period of two years and nine months. We tagged 39 sharks (mostly adult females) of which 31 were detected regularly throughout the study. Sharks displayed strong inter-annual residency with greater attendance at monitored sites during summer than winter months. More individuals were detected during the day than at night. Mean depths of tagged sharks increased from 35 m in winter to 60 m in spring following an increase in water temperature at 60 m, with maximum mean depths attained when water temperatures at 60 m stabilised around 29°C. Sharks descended to greater depths and used a wider range of depths around the time of the full moon. There were also crepuscular cycles in mean depth, with sharks moving into shallower waters at dawn and dusk each day. We suggest that daily, lunar and seasonal cycles in vertical movement and residency are strategies for optimising both energetic budgets and foraging behaviour. Cyclical patterns of movement in response to environmental variables might affect the susceptibility of reef sharks to fishing, a consideration that should be taken into account in the implementation of conservation strategies.

BACKGROUND: In recent decades, large pelagic and coastal shark populations have declined dramatically with increased fishing; however, the status of sharks in other systems such as coral reefs remains largely unassessed despite a long history of exploitation. Here we explore the contemporary distribution and sighting frequency of sharks on reefs in the greater-Caribbean and assess the possible role of human pressures on observed patterns. METHODOLOGY/PRINCIPAL FINDINGS: We analyzed 76,340 underwater surveys carried out by trained volunteer divers between 1993 and 2008. Surveys were grouped within one km2 cells, which allowed us to determine the contemporary geographical distribution and sighting frequency of sharks. Sighting frequency was calculated as the ratio of surveys with sharks to the total number of surveys in each cell. We compared sighting frequency to the number of people in the cell vicinity and used population viability analyses to assess the effects of exploitation on population trends. Sharks, with the exception of nurse sharks occurred mainly in areas with very low human population or strong fishing regulations and marine conservation. Population viability analysis suggests that exploitation alone could explain the large-scale absence; however, this pattern is likely to be exacerbated by additional anthropogenic stressors, such as pollution and habitat degradation, that also correlate with human population. CONCLUSIONS/SIGNIFICANCE: Human pressures in coastal zones have lead to the broad-scale absence of sharks on reefs in the greater-Caribbean. Preventing further loss of sharks requires urgent management measures to curb fishing mortality and to mitigate other anthropogenic stressors to protect sites where sharks still exist. The fact that sharks still occur in some densely populated areas where strong fishing regulations are in place indicates the possibility of success and encourages the implementation of conservation measures.

International audience; Commercially available enzyme immunoassay (EIA) kits were validated for measuring steroid hormone concentrations in blood plasma from three fish species: the orange clownfish Amphiprion percula, the orangefin anemonefish Amphiprion chrysopterus and the blacktip reef shark Carcharhinus melanopterus. A minimum of 5 microl plasma was required to estimate hormone concentrations with both kits. These EIA kits are a simple method requiring minimal equipment, for measuring hormone profiles under field conditions.