Carcharhinus Amblyrhynchos Research Articles

Environmental stress reduces shark residency to coral reefs

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.

Observations on an Aggregation of Grey Reef Sharks (Carcharhinus amblyrhynchos) in the Mozambique Channel Off the Coast of Nosy Be (Madagascar) and Tools for Photo-Identification-A New Aggregation Nursery Site?

Following preliminary underwater observations of about 1000 h carried out monthly between 2012 and 2023 (except the years 2021 and 2022), 23 specimens of grey reef sharks were spotted and photo-identified off the coast of Nosy Be in Madagascar, on an emerging reef called "Mokarran" at a depth between 15 and 30 m. Over 10 years of observations, eight specimens were re-sighted, identified with a non-invasive photo-identification technique of part of the first dorsal and the caudal fin, and one specimen was re-identified after 1982 days from the first sighting, i.e., after more than 5 years. In addition, six specimens of probably pregnant females were also identified in the same area. The population was entirely made up of females. The aggregation area could represent a new nursery site which, if confirmed after further investigations, will require greater protection.

Age is not just a number: How incorrect ageing impacts close-kin mark-recapture estimates of population size.

Population size is a key parameter for the conservation of animal species. Close-kin mark-recapture (CKMR) relies on the observed frequency and type of kinship among individuals sampled from the population to estimate population size. Knowledge of the age of the individuals, or a surrogate thereof, is essential for inference with acceptable precision. One common approach, particularly in fish studies, is to measure animal length and infer age using an assumed age-length relationship (a 'growth curve'). We used simulation to test the effect of misspecifying the length measurement error and the growth curve on population size estimation. Simulated populations represented two fictional shark species, one with a relatively simple life history and the other with a more complex life history based on the grey reef shark (Carcharhinus amblyrhynchos). We estimated sex-specific adult abundance, which we assumed to be constant in time. We observed small median biases in these estimates ranging from 1.35% to 2.79% when specifying the correct measurement error standard deviation and growth curve. CI coverage was adequate whenever the growth curve was correctly specified. Introducing error via misspecified growth curves resulted in changes in the magnitude of the estimated adult population, where underestimating age negatively biased the abundance estimates. Over- and underestimating the standard deviation of length measurement error did not introduce a bias and had negligible effect on the variance in the estimates. Our findings show that assuming an incorrect standard deviation of length measurement error has little effect on estimation, but having an accurate growth curve is crucial for CKMR whenever ageing is based on length measurements. If ageing could be biased, researchers should be cautious when interpreting CKMR results and consider the potential biases arising from inaccurate age inference.

Abiotic drivers of the space use and activity of gray reef sharks Carcharhinus amblyrhynchos in a dynamic tidal environment.

Predators display rhythms in behavior and habitat use, often with the goal of maximizing foraging success. The underlying mechanisms behind these rhythms are generally linked to abiotic conditions related to diel, lunar, or seasonal cycles. To understand their effects on the space use, activity, and swimming depth of gray reef sharks (Carcharhinus amblyrhynchos), we tagged 38 individuals with depth and accelerometer sensors in a French Polynesian atoll channel exposed to strong tidal flow, and monitored them over a year. C. amblyrhynchos used a larger space during nighttime and were more active at night and during outgoing currents. Shark activity also peaked during the full and new moons. The swimming depth of sharks was mostly influenced by diel cycles, with sharks swimming deeper during the day compared to nighttime. The dynamic energyscape may promote the emergence of discrete behavioral strategies in reef sharks that use the south channel of Fakarava for resting and foraging purposes. Turbulence imposed by outgoing tides induces additional foraging cost on sharks, shifting their hunting areas to the southern part of the channel, where turbulence is less pronounced. Understanding when and where sharks are active and foraging is important for our understanding of predator-prey dynamics and ecosystem dynamics. This study highlights how abiotic rhythms in a highly dynamic environment likely generate spatiotemporal heterogeneity in the distribution of predation pressure.

Environmental factors modulate the distribution of elasmobranchs in southern Mozambique

Investigating the spatiotemporal ecology of elasmobranchs is an important precursor to their effective management. Understanding long-term patterns in the movement and habitat use of threatened species can improve management plans so that they yield increased conservation benefits. We investigated the spatiotemporal and environmental drivers that underpin the abundance and distribution of elasmobranchs around reef habitats in southern Mozambique to highlight reefs that are important (“hotspots”) to the regional elasmobranch community. Visual belt transects (n = 738), supported by video recordings, were completed on 16 reef sites off the coast of southern Mozambique from 2018 to 2022. Nine elasmobranch species were encountered annually (Carcharhinus amblyrhynchos, Triaenodon obesus, Stegostoma tigrinum, Neotrygon caeruleopunctata, Pateobatis jenkinsii, Taeniurops meyeni, Mobula kuhlii, Mobula alfredi, Mobula birostris) and 11 individual environmental and spatiotemporal parameters (horizontal water visibility, tidal range and state, moon illumination, temperature on the reef, cloud cover, time of day, day of the year, transect distance from shore, transect depth, and the region that the transect occurred in) were measured. All species, (bar P. jenkinsii) were significantly more abundant around certain reefs in the sampled region. Total counts for most species were highest in the austral summer however two species’ (M. birostris and S. tigrinum) were most abundant in the winter months. The tidal state, tidal range, and moon illumination correlated significantly with the numbers of each of the nine elasmobranch species. Non-Metric Multidimensional Scaling (NMDS) indicated that species’ responses to the measured parameters grouped taxonomically. Environmental influences resulted in strong seasonal patterns of reef use by large-bodied and pelagic elasmobranch species (e.g. manta rays). The measured environmental parameters also resulted in daily, monthly, and seasonal patterns of abundance of reef-resident stingray and shark species. Banning extraction of elasmobranch species around the reefs where they aggregate and reflecting species distributions within fisheries regulations may significantly benefit the regional elasmobranch community.

Just keep swimming? Observations of resting behavior in gray reef sharks Carcharhinus amblyrhynchos (Bleeker, 1856).

Understanding the respiratory modes of sharks has important implications for studying the metabolism, energetics, and behavioral strategies of different species. Here we provide the first reported observations of resting behavior in the gray reef shark Carcharhinus amblyrhynchos, a species typically considered an obligate ram ventilator. Observations were made at several locations in the Republic of Seychelles, where sharks were found resting under reef ledges and were unresponsive to the presence of divers. These findings update our understanding of the respiratory mode of this species and have implications for future research.

Environmental DNA helps reveal reef shark distribution across a remote archipelago

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.

Characterization of 35 new microsatellite markers for the blacktip reef shark (Carcharhinus melanopterus) and cross-species amplification in eight other shark species.

Shark species are overfished at a global scale, as they are poached for the finning industry or are caught as bycatch. Efficient conservation measures require fine-scale spatial and temporal studies to characterize shark habitat use, infer migratory habits, analyze relatedness, and detect population genetic differentiation. Gathering these types of data is costly and time-consuming, especially when it requires collection of shark tissue samples. Genetic tools, such as microsatellite markers, are the most economical sampling method for collecting genetic data, as they enable the estimation of genetic diversity, population structure and parentage relationships and are thus an efficient way to inform conservation strategies. Here, a set of 45 microsatellite loci was tested on three blacktip reef shark (Carcharhinus melanopterus) populations from three Polynesian islands: Moorea, Morane and Tenararo. The set was composed of 10 previously published microsatellite markers and 35 microsatellite markers that were developed specifically for C. melanopterus as part of the present study. The 35 novel and 10 existing loci were cross-amplified on eight additional shark species (Carcharhinus amblyrhynchos, C. longimanus, C. sorrah, Galeocerdo cuvier, Negaprion acutidens, Prionacea glauca, Rhincodon typus and Sphyrna lewini). These species had an average of 69% of successful amplification, considered if at least 50% of the individual samples being successfully amplified per species and per locus. This novel microsatellite marker set will help address numerous knowledge gaps that remain, concerning genetic stock identification, shark behavior and reproduction via parentage analysis.

Persistent transboundary movements of threatened sharks highlight the importance of cooperative management for effective conservation

Migratory sharks play a key ecological role through movements within and among marine ecosystems, yet many populations are declining. Addressing the decline is especially challenging for wide-ranging species, as they may undertake movements between countries with disparate conservation priorities. To investigate the transboundary migrations of threatened sharks between neighbouring South Africa and Mozambique, we tracked 4 commonly occurring carcharhinid species (bull, blacktip, tiger and grey reef sharks). A total of 102 individuals were fitted with long-life acoustic transmitters and monitored for 4 yr (2018-2022) on an acoustic receiver network of 350 receivers. During this period, 63% of tagged bull sharks (n = 19), 87% of blacktips (n = 13), 94% of tiger (n = 16) and 25% of grey reef sharks (n = 3) undertook transboundary movements. The frequency of mean transboundary movements per year ranged between 1.3 ± 1.5 (SD) for grey reef sharks and 81 ± 35.6 for tiger sharks. Blacktip, bull and tiger sharks all undertook long-distance transboundary migrations ranging from 980 to 2256 km. These data confirm high connectivity between neighbouring countries by threatened sharks undertaking persistent transboundary movements. This study emphasizes the need for collaborative transboundary cooperation between the 2 countries and the alignment of regional management plans and interventions to address declining shark populations in this region of the Western Indian Ocean.

Like a rolling stone: Colonization and migration dynamics of the gray reef shark (Carcharhinus amblyrhynchos).

Designing appropriate management plans requires knowledge of both the dispersal ability and what has shaped the current distribution of the species under consideration. Here, we investigated the evolutionary history of the endangered gray reef shark (Carcharhinus amblyrhynchos) across its range by sequencing thousands of RADseq loci in 173 individuals in the Indo-Pacific (IP). We first bring evidence of the occurrence of a range expansion (RE) originating close to the Indo-Australian Archipelago (IAA) where two stepping-stone waves (east and westward) colonized almost the entire IP. Coalescent modeling additionally highlighted a homogenous connectivity (Nm ~ 10 per generation) throughout the range, and isolation by distance model suggested the absence of barriers to dispersal despite the affinity of C.amblyrhynchos to coral reefs. This coincides with long-distance swims previously recorded, suggesting that the strong genetic structure at the IP scale (F ST ~ 0.56 between its ends) is the consequence of its broad current distribution and organization in a large number of demes. Our results strongly suggest that management plans for the gray reef shark should be designed on a range-wide rather than a local scale due to its continuous genetic structure. We further contrasted these results with those obtained previously for the sympatric but strictly lagoon-associated Carcharhinus melanopterus, known for its restricted dispersal ability. Carcharhinus melanopterus exhibits a similar RE dynamic but is characterized by a stronger genetic structure and a nonhomogeneous connectivity largely dependent on local coral reefs availability. This sheds new light on shark evolution, emphasizing the roles of IAA as source of biodiversity and of life-history traits in shaping the extent of genetic structure and diversity.

Quantifying Catch Rates, Shark Abundance and Depredation Rate at a Spearfishing Competition on the Great Barrier Reef, Australia.

Simple SummaryCatch-per-unit-effort at a spearfishing competition in North Queensland, Australia was 1.25 fish per hour. Citizen scientists reported 358 sharks comprising of 10 species/groups, with 3.11 sharks observed per hour. Twelve percent of all sharks sighted in this study were regarded as potentially dangerous to humans. This is the first scientific study of shark depredation for spearfishing. C. amblyrhynchos were primarily responsible for depredation, with 5.9% of speared fish taken. Sixty percent of interviewees perceived that shark numbers have increased in the past 10 years.We developed and applied a method to quantify spearfisher effort and catch, shark interactions and shark depredation in a boat-based recreational spearfishing competition in the Great Barrier Reef Marine Park in Queensland. Survey questions were designed to collect targeted quantitative data whilst minimising the survey burden of spearfishers. We provide the first known scientific study of shark depredation during a recreational spearfishing competition and the first scientific study of shark depredation in the Great Barrier Reef region. During the two-day spearfishing competition, nine vessels with a total of 33 spearfishers reported a catch of 144 fish for 115 h of effort (1.25 fish per hour). A subset of the catch comprised nine eligible species under competition rules, of which 47 pelagic fish were weighed. The largest fish captured was a 34.4 kg Sailfish (Istiophorus platypterus). The most common species captured and weighed was Spanish Mackerel (Scomberomorus commerson). The total weight of eligible fish was 332 kg and the average weight of each fish was 7.1 kg. During the two-day event, spearfishers functioned as citizen scientists and counted 358 sharks (115 h effort), averaging 3.11 sharks per hour. Grey Reef Sharks (Carcharhinus amblyrhynchos) comprised 64% of sightings. Nine speared fish were fully depredated by sharks as spearfishers attempted to retrieve their catch, which equates to a depredation rate of 5.9%. The depredated fish included four pelagic fish and five reef fish. The shark species responsible were Grey Reef Shark (C. amblyrhynchos) (66%), Bull Shark (Carcharhinus leucas) (11%), Whitetip Reef Shark (Triaenodon obesus) (11%) and Great Hammerhead (Sphyrna mokarran) (11%). There were spatial differences in fish catch, shark sightings and rates of depredation. We developed a report card that compared average catch of fish, sightings of sharks per hour and depredation rate by survey area, which assists recreational fishers and marine park managers to assess spatio-temporal changes. The participating spearfishers can be regarded as experienced (average 18 days a year for average 13.4 years). Sixty percent of interviewees perceived that shark numbers have increased in the past 10 years, 33% indicated no change and 7% indicated shark numbers had decreased. Total fuel use of all vessels was 2819 L and was equivalent to 6.48 tons of greenhouse gas emissions for the competition.

Evaluation of MPA designs that protect highly mobile megafauna now and under climate change scenarios

Marine protected area (MPA) designs, including large-scale MPAs (LSMPAs; >150,000 km 2 ), mobile MPAs (fluid spatiotemporal boundaries), and MPA networks, may offer different benefits to species and could enhance protection by encompassing spatiotemporal scales of animal movement. We sought to understand how well LSMPAs could benefit nine highly-mobile marine species in the tropics now and into the future by: 1) evaluating current range overlap within a LSMPA; 2) evaluating range overlap under climate change projections; and 3) evaluating how well theoretical MPA designs benefit these nine species. We focused on Palmyra Atoll and Kingman Reef, a 2000 km 2 area within the 1.2 million km 2 U.S. Pacific Remote Islands Marine National Monument (PRIMNM) that contains marine megafauna (reef and pelagic fishes; sea turtles; seabirds; cetaceans) reflecting different behaviors and habitat use. Our approach is useful for evaluating the effectiveness of the Palmyra-Kingman MPA and PRIMNM in protecting these species, and tropical LSMPAs in general, and for informing future MPA design. Stationary MPAs provided protection at varying scales. Reef manta rays ( Mobula alfredi ), grey reef sharks ( Carcharhinus amblyrhynchos ), green sea turtles ( Chelonia mydas ), and bottlenose dolphins ( Tursiops truncatus ) had overall small ranges (<100 km from Palmyra-Kingman) and could benefit from stationary MPAs that contained heterogenous reef habitats. Yellowfin tuna ( Thunnus albacares ), sooty terns ( Onychoprion fuscatus ), red-footed boobies ( Sula sula ), great frigatebirds ( Fregata minor ), and melon-headed whales ( Peponocephala electra ) navigated complex oceanographic processes and may benefit most from mobile MPAs that shift with features including thermal fronts, cyclic regions of elevated productivity, and eddies, if relationships with these features are established and predictable. All species had capacity to travel to nearby reef systems, illustrating potential benefits of MPA networks and protected corridors. Suitable habitats will likely contract for all species as warm water expands under climate change scenarios (species habitats were predicted to decrease by 4–49% at Palmyra-Kingman) and MPAs may not protect suitable habitats into the future. Species habitat requirements and movement ecologies are critical aspects of marine spatial planning, especially with respect to dynamic ocean processes and a changing climate. • How well do large-scale MPAs protect wide-ranging megafauna they claim to protect? • We assessed scales of movement relative to MPA boundaries, designs & climate change. • Protected habitats were greatest for reef-based sharks, manta rays, sea turtles. • Tuna, seabirds, cetaceans less protected offshore or by dynamic ocean processes. • Habitats shifted away from MPAs under climate change.

Elasmobranch diversity across a remote coral reef atoll revealed through environmental DNA metabarcoding

Abstract As elasmobranchs are becoming increasingly threatened, efficient methods for monitoring the distribution and diversity of elasmobranch populations are required. Environmental DNA (eDNA) metabarcoding is a progressively applied technique that enables mass identification of entire communities and is an effective method for the detection of rare and elusive species. We performed an eDNA metabarcoding survey for fish communities around a coral reef atoll in the Chagos Archipelago (Central Indian Ocean) and assessed the diversity and distribution of elasmobranch species detected within these communities. Our eDNA survey detected 353 amplicon sequence variants (ASVs) attributed to fishes, 12 of which were elasmobranchs. There were no differences in fish communities based on the presence and absence of ASVs between sample depth (surface and 40 m) or sampling habitat, but communities based on read abundance were significantly different between habitats. The dominant elasmobranch species were grey reef (Carcharhinus amblyrhynchos) and silvertip (C. albimarginatus) sharks, and elasmobranch communities were significantly different between sampling depth and habitat. Overall, we find that eDNA metabarcoding can be used to reveal the diversity of elasmobranchs within broader taxonomic assays, but further research and development of targeted metabarcoding primers may be required before it can be integrated into a toolkit for monitoring these species.

Identifying barriers to gene flow and hierarchical conservation units from seascape genomics: a modelling framework applied to a marine predator

The ongoing decline of large marine vertebrates must be urgently mitigated, particularly under increasing levels of climate change and other anthropogenic pressures. However, characterizing the connectivity among populations remains one of the greatest challenges for the effective conservation of an increasing number of endangered species. Achieving conservation targets requires an understanding of which seascape features influence dispersal and subsequent genetic structure. This is particularly challenging for adult‐disperser species, and when distribution‐wide sampling is difficult. Here, we developed a two‐step modelling framework to investigate how seascape features drive the genetic connectivity of marine species without larval dispersal, to better guide the design of marine protected area networks and corridors. We applied this framework to the endangered grey reef shark, Carcharhinus amblyrhynchos, a reef‐associated shark distributed across the tropical Indo‐Pacific. In the first step, we developed a seascape genomic approach based on isolation‐by‐resistance models involving circuit theory applied to 515 shark samples, genotyped for 4991 nuclear single‐nucleotide polymorphisms. We show that deep oceanic areas act as strong barriers to dispersal, while proximity to habitat facilitates dispersal. In the second step, we predicted the resulting genetic differentiation across the entire distribution range of the species, providing both local and global‐scale conservation units for future management guidance. We found that grey reef shark populations are more fragmented than expected for such a mobile species, raising concerns about the resilience of isolated populations under high anthropogenic pressures. We recommend the use of this framework to identify barriers to gene flow and to help in the delineation of conservation units at different scales, together with its integration across multiple species when considering marine spatial planning.

Genomic insights into the historical and contemporary demographics of the grey reef shark.

Analyses of genetic diversity can shed light on both the origins of biodiversity hotspots, as well as the conservation status of species that are impacted by human activities. With these objectives, we assembled a genomic dataset of 14,935 single nucleotide polymorphisms from 513 grey reef sharks (Carcharhinus amblyrhynchos) sampled across 17 locations in the tropical Indo-Pacific. We analysed geographic variation in genetic diversity, estimated ancient and contemporary effective population size (Ne) across sampling locations (using coalescent and linkage disequilibrium methods) and modelled the history of gene flow between the Coral Triangle and the Coral Sea. Genetic diversity decreased with distance away from the Coral Triangle and north-western Australia, implying that C. amblyrhynchos may have originated in this region. Increases in Ne were detected across almost all sampling locations 40,000-90,000 generations ago (approximately 0.6-1.5 mya, given an estimated generation time of 16.4 years), suggesting a range expansion around this time. More recent, secondary increases in Ne were inferred for the Misool and North Great Barrier Reef sampling locations, but joint modelling did not clarify whether these were due to population growth, migration, or both. Despite the greater genetic diversity and ancient Ne observed at sites around Australia and the Coral Triangle, remote reefs around north-western New Caledonia had the highest contemporary Ne, demonstrating the importance of using multiple population size assessment methods. This study provides insight into both the past and present demographics of C. amblyrhynchos and contributes to our understanding of evolution in marine biodiversity hotspots.

Conservation implications of forage base requirements of a marine predator population at carrying capacity

SummaryPrey depletion may contribute to marine predator declines, yet the forage base required to sustain an unfished population of predatory fish at carrying capacity is unknown. We integrated demographic and physiological data within a Bayesian bioenergetic model to estimate annual consumption of a gray reef shark (Carcharhinus amblyrhynchos) population at a remote Pacific atoll (Palmyra Atoll) that are at carrying capacity. Furthermore, we estimated the proportion of the atoll's reef fish biomass production consumed by the gray reef sharks, assuming sharks either partially foraged pelagically (mean 7%), or solely within the reef environment (mean 52%). We then predicted the gray reef shark population potential of other, less remote Pacific Ocean coral reef islands, illustrating that current populations are substantially smaller than could be supported by their forage base. Our research highlights the utility of modeling how far predator population sizes are from their expected carrying capacity in informing marine conservation.

Recent expansion of marine protected areas matches with home range of grey reef sharks

Dramatic declines in reef shark populations have been documented worldwide in response to human activities. Marine Protected Areas (MPAs) offer a useful mechanism to protect these species and their roles in coral reef ecosystems. The effectiveness of MPAs notably relies on compliance together with sufficient size to encompass animal home range. Here, we measured home range of 147 grey reef sharks, Carcharhinus amblyrhynchos, using acoustic telemetry in New Caledonia. The distribution of home range was then compared to local MPA sizes. We report a home range of 12 km2 of reef for the species with strong differences between adult males (21 km2), adult females (4.4 km2) and juveniles (6.2 km2 for males, 2.7 km2 for females). Whereas local historic MPA size seemed adequate to protect reef shark home range in general, these were clearly too small when considering adult males only, which is consistent with the reported failure of MPAs to protect sharks in New Caledonia. Fortunately, the recent implementation of several orders of magnitude larger MPAs in New Caledonia and abroad show that recent Indo-Pacific MPAs are now sufficiently large to protect the home ranges of this species, including males, across its geographical range. However, protection efforts are concentrated in a few regions and cannot provide adequate protection at a global scale.

Sharks surf the slope: Current updrafts reduce energy expenditure for aggregating marine predators.

An animal's energy landscape considers the power requirements associated with residing in or moving through habitats. Within marine environments, these landscapes can be dynamic as water currents will influence animal power requirements and can change rapidly over diel and tidal cycles. In channels and along slopes with strong currents, updraft zones may reduce energy expenditure of negatively buoyant fishes that are also obligate swimmers. Despite marine predators often residing within high-current area, no study has investigated the potential role of the energetic landscape in driving such habitat selectivity. Over 500 grey reef sharks Carcharhinus amblyrhynchos reside in the southern channel of Fakarava Atoll, French Polynesia. We used diver observations, acoustic telemetry and biologging to show that sharks use regions of predicted updrafts and switch their core area of space use based on tidal state (incoming versus outgoing). During incoming tides, sharks form tight groups and display shuttling behaviour (moving to the front of the group and letting the current move them to the back) to maintain themselves in these potential updraft zones. During outgoing tides, group dispersion increases, swimming depths decrease and shuttling behaviours cease. These changes are likely due to shifts in the nature and location of the updraft zones, as well as turbulence during outgoing tides. Using a biomechanical model, we estimate that routine metabolic rates for sharks may be reduced by 10%-15% when in updraft zones. Grey reef sharks save energy using predicted updraft zones in channels and 'surfing the slope'. Analogous to birds using wind-driven updraft zones, negatively buoyant marine animals may use current-induced updraft zones to reduce energy expenditure. Updrafts should be incorporated into dynamic energy landscapes and may partially explain the distribution, behaviour and potentially abundance of marine predators.

Analysing detection gaps in acoustic telemetry data to infer differential movement patterns in fish.

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.

Aggression of Carcharhinus leucas and Carcharhinus amblyrhynchos towards humans

An important step in combating shark endangerment is lowering the stigma surrounding the species; one way to do so is to publish data on shark aggression. This paper presents findings on Carcharhinus leucas (bull shark) and Carcharhinus amblyrhynchos (grey reef shark) aggression towards humans at Beqa Adventure Divers in Shark Reef Marine Reserve, Fiji. We collected data on the number of sharks present and prods delivered - a method to redirect sharks away from tourist divers using a 2.5-meter metal stick by dive masters - during nine shark feeds. We hypothesized that grey reef sharks would receive more prods than bull sharks because grey reef sharks are typically more aggressive than bull sharks. The results supported our hypothesis, as an individual grey reef shark received 2.44 prods on average per feed, while a bull shark had an average of 0.61. Furthermore, grey reef sharks showed signs of aggression, including pectoral depressions and exaggerated swimming, throughout the feeds, while bull sharks showed none. These findings are meaningful not only to the world’s general understanding of shark aggression, but also to human protection against grey reef sharks as well as public education on bull sharks and the conservation of the species.