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Abstract and Figures

Decades of overexploitation have devastated shark populations, leaving considerable doubt as to their ecological status1,2. 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 habitats³. 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.
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Nature | Vol 583 | 30 July 2020 | 801
Article
Global status and conservation potential of
reef sharks
Decades of overexploitation have devastated shark populations, leaving considerable
doubt as to their ecological status1,2. Yet much of what is known about sharks has been
inferred from catch records in industrial sheries, whereas far less information is
available about sharks that live in coastal habitats3. 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 shing 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 shing 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 sheries.
Global demand for shark products, such as fins and meat, as well as
high levels of bycatch, have caused widespread declines in shark popu-
lations globally
1–3
, with the potential to affect the function of ocean
ecosystems
4
and jeopardize associated fishing and tourism sectors
5,6
.
However, there are large gaps in our knowledge regarding the popula-
tion status of sharks in coastal environments such as coral reefs, where
the majority of threatened species occur
1
. Scientific surveys of reef
fish typically use underwater visual census by divers, which can lead to
under- or overestimates of the abundance of large roving animals such
as sharks
7
. Although a handful of studies from remote, uninhabited or
no-access reefs have recorded exceptionally high reef shark biomass
8,9
and evidence of declines9,10, there are large differences in environmental
features
11
and sampling
7
that undermine the use of pristine remote
areas as conservation baselines for inhabited coastal environments12.
In practice, shark conservation targets for most reefs should reflect the
levels of abundance found in the best-managed places where people
are present, acknowledging the environmental and social contexts in
which people use ocean resources13.
We used baited remote underwater video systems(BRUVS) in a dedi-
cated global survey (Global FinPrint, https://globalfinprint.org) to
quantify the status of reef sharks in 58 countries, states and territories
(hereafter, nations). BRUVS footage was analysed to provide a standard-
ized index of relative shark abundance—given as the maximum number
of sharks seen in a single frame of each video set (MaxN; seeMethods)—
that has been shown to compare well with alternative methods of
estimating the relative abundance of sharks
14
(Extended Data Fig.1).
Global FinPrint surveys included sightings of 59 shark species; the vast
majority of sightings (93%) comprised species that complete their life
cycle on coral reefs or frequently visit them (seeMethods). Despite our
assumption that sharks would be present on all of the world’s coral
reefs15, they were not observed on 19% (69 out of 371) of reefs surveyed
and 63% of the 15,165 BRUVS sets in our survey did not record the pres-
ence of a shark, indicating that there has been widespread depletion
of reef sharks across much of the world’s tropical oceans (Fig.1a, b).
Evaluating the relative abundance of reef sharks
We developed a set of Bayesian hierarchical models to quantify the rela-
tive abundance of reef sharks across a range of management regimes
and to understand how the abundance of reef sharks varies globally.
We used a zero-inflated modelling approach that enabled us to examine
factors that influenced both the presence or absence of reef sharks (the
occurrence of excess zeros) and the relative abundance of sharks among
reefs, nations and regions (seeMethods). Although the conditional
mode of regional-level random effects for reef sharks was 40% higher in
the central Pacific than other regions (Fig.1c; null model), these differ-
ences disappeared under our full model, suggesting that the observed
inter-regional disparities were largely due to reef- and national-scale
effects captured by the covariates that we included (Fig.1c; full model).
In other words, although we observed strong regional differences in
our data, these were largely attributable to differences in key human
drivers of resource exploitation.
Our results show that declines in reef sharks from the coastal tropi-
cal oceans correlate with key socio-economic differences among reefs
and nations (Fig.1d). Our civil society metric (voice and accountabil-
ity) was associated with a higher likelihood of sharks being observed.
In addition, nations with larger coastal populations coincided with
sharks not being observed, whereas we found little evidence for an
effect of increased national wealth (through the human development
index).We also found that the relative abundance of reef sharks had a
negative relationship with the ‘gravitational pull’ of the closest human
settlement and any markets within 500km of each BRUVS set (our
https://doi.org/10.1038/s41586-020-2519-y
Received: 30 July 2019
Accepted: 21 May 2020
Published online: 22 July 2020
Check for updates
A list of authors and their afiliations appears at the end of the paper.
Content courtesy of Springer Nature, terms of use apply. Rights reserved
... Oceanic sharks and rays present a striking example; between 1970 and 2018, an 18-fold increase in relative fishing pressure reduced their global abundance by 71% . Sharks inhabiting coral reefs are similarly threatened, with fishing likely responsible for sharks being absent from almost 20% of reefs surveyed globally (MacNeil et al., 2020). ...
... Fisher surveys (Graham, 2007) and spatial variation in relative abundance also suggest fishing caused declines in some coastal shark populations. Notably, abundance is often highest in heavily managed Exclusive Economic Zones (EEZs; MacNeil et al., 2020), marine reserves (Bond et al., 2012;MacNeil et al., 2020), shark sanctuaries and remote areas far from human population centres (Ward-Paige et al., 2010). There are, however, signs of recent stability and/or recovery in some better-studied shark populations in the United States Peterson et al., 2017), The Bahamas (Hansell et al., 2018;Talwar et al., 2020) and Belize (Bond et al., 2017;Flowers et al., 2022), largely due to targeted management that began in the 1990s (Castro, 2013;Ward-Paige, 2017). ...
... Fisher surveys (Graham, 2007) and spatial variation in relative abundance also suggest fishing caused declines in some coastal shark populations. Notably, abundance is often highest in heavily managed Exclusive Economic Zones (EEZs; MacNeil et al., 2020), marine reserves (Bond et al., 2012;MacNeil et al., 2020), shark sanctuaries and remote areas far from human population centres (Ward-Paige et al., 2010). There are, however, signs of recent stability and/or recovery in some better-studied shark populations in the United States Peterson et al., 2017), The Bahamas (Hansell et al., 2018;Talwar et al., 2020) and Belize (Bond et al., 2017;Flowers et al., 2022), largely due to targeted management that began in the 1990s (Castro, 2013;Ward-Paige, 2017). ...
Article
Chondrichthyan fishes are among the most threatened vertebrates on the planet because many species have slow life histories that are outpaced by intense fishing. The Western Central Atlantic Ocean, which includes the Greater Caribbean, is a hotspot of chondrichthyan biodiversity and abundance, but has been characterized by extensive shark and ray fisheries and a lack of sufficient data for effective management and conservation. To inform future research and management decisions, we analysed patterns in chondrichthyan extinction risk, reconstructed catches and management engagement in this region. We summarized the extinction risk of 180 sharks, rays and chimaeras, including 66 endemic and 14 near‐endemic species, using contemporary IUCN Red List assessments. Over one‐third (35.6%) were assessed as Vulnerable, Endangered or Critically Endangered, primarily due to overfishing. Reconstructed catches from 1950 to 2016 peaked in 1992, then declined by 40.2% thereafter. The United States, Venezuela and Mexico were responsible for most catches in the region and hosted the largest proportions of the regional distributions of threatened species, largely due to having extensive coastal habitats in their Exclusive Economic Zones. The quantity and taxonomic resolution of fisheries landings data were poor in much of the region, and national‐level regulations varied widely across jurisdictions. Deepwater fisheries represent an emerging threat, although many deepwater chondrichthyans currently have refuge beyond the depths of most fisheries. Regional collaboration as well as effective and enforceable management informed by more complete fisheries data, particularly from small‐scale fisheries, are required to protect and recover threatened species and ensure sustainable fisheries.
... Similarly, nurse sharks and Caribbean reef sharks were commonly captured on longlines in Belize [22]. For rays, other studies in the Caribbean showed the high abundance of the yellow stingray in the coral reef ecosystems [11], so the data obtained here through the local ecological knowledge are supported by other studies using conventional methods [23]. ...
... The decline of shark and ray populations worldwide in the last decades is well known [1,24], particularly in the Caribbean where recent studies have provided evidence of the decline of reef shark populations [23,25]. This is concordant with the perception of the divers interviewed in our study who perceived a decline in the number sharks and rays in the Mexican Caribbean in the last decade. ...
... The reduction in shark populations in the Caribbean has recently been documented where most of the reefs in this region have been reported with a very low abundance of sharks [23,25], as was seen in the Mexican Caribbean in the present study, based on the very low number of sightings. Additionally, batoids were more frequently sighted (89%) than sharks (11%). ...
Article
Full-text available
The Mexican Caribbean is in one of the regions with the greatest diversity of elasmobranchs in the world. However, the population status of most of the shark and ray species in this region is unknown. We used a citizen science program based on divers to collect data about the diversity, abundance, and distribution of elasmobranchs in this region. We visited dive centers in six locations and performed structured interviews with divemasters, instructors, and owners of the diving centers. In total, 79 divers were interviewed, of which 69% had more than five years’ experience diving in the Mexican Caribbean. Divers could identify 24 elasmobranch species for this region. Most of the divers (82%) reported a decrease in sightings of sharks and rays. Rays were the most frequently sighted species by divers (89%), and the spotted eagle ray (A. narinari) was the most common elasmobranch species reported in the region. Citizen science was a useful approach gathering for baseline information about sharks and rays in the Mexican Caribbean, increasing our knowledge of the abundance and distribution of some species in this region. Citizen science affords the opportunity to obtain long-term data that can be useful for management and conservation.
... [30] (Heithaus, Frid, Wirsing, & Worm, 2008;MacNeil et al., 2020). However, more detailed information about population connectivity is needed to assess potential negative impacts. ...
... For example, Chin et al. (2017) proposed several hypothetical connectivity routes for hammerhead SECTION 5 -General discussion [120] sharks between Australia, Indonesia and PNG, based on an Integrated Assessment Framework which combined data from fisheries, genetics, tagging, telemetry, and biogeography. These pre-defined hypotheses provided a foundation for empirical testing by Green (2019) and Heupel et al. (2020) with genetic, parasite fauna, and telemetry tools. With the appropriate spatial scales and sample sizes the authors concluded that the connectivity between these regions likely occurs along the continental shelves with limited movement across deep water. ...
Thesis
Full-text available
Globally, elasmobranch populations (sharks and rays) are declining due to increasing anthropogenic and climate pressures. Genetic connectivity between elasmobranch populations is crucial to ensure their persistence and sustain the ecological integrity of ecosystems. Genetic connectivity implies gene flow among discrete populations occurring via the dispersal of individuals outside their population of origin, followed by reproduction — a process that can be biased between sexes (i.e. sex-biased dispersal or SBD). In this thesis, I first examine the current knowledge of population structure and SBD in elasmobranchs, and the tools that are commonly used. Next, this thesis uses novel genomic approaches (kinship, nuclear single nucleotide polymorphisms, and mitochondrial genomes) to provide insights into the patterns of (i) population structure, (ii) sex-chromosome systems, and (iii) SBD in elasmobranchs. My thesis focuses on three shark species that allow the study of dispersal patterns based on life history, local ecology, population size and different seascape features: Northern River Shark, Glyphis garricki; School Shark, Galeorhinus galeus; and Bull Shark, Carcharhinus leucas. Overall, male-biased dispersal (MBD) was observed in 25 of the 50 studied species. Population structure was found at both broad (Bull Shark) and fine (Northern River Shark) spatial scales. I demonstrated that 19 out of the 21 studied elasmobranch species contain X and Y chromosomes using the R function I developed. Combined, the sex-linked markers and kinship data supported the evidence of MBD in the Northern River Shark and the Bull Shark. My final discussion synthesised the observed dispersal patterns and examines the potential ecological and evolutionary drivers for these patterns. I critically compared the genetic and analytical approaches for the detection of population structure and SBD. Finally, potential implications of these quantitative findings for management were highlighted.
... The abundance of oceanic sharks has declined globally by 71.1% over the last half-century, with an average rate of 18.2% per decade . Overfishing has resulted in almost 20% of the surveyed reefs being without sharks (MacNeil et al., 2020). A total of 37% of sharks and rays were classified as threatened species according to the latest International Union for the Conservation of Nature (IUCN) Red List (IUCN, 2022), with 45 elasmobranchs being listed in the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) Appendix II. ...
Article
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... In this study, commercially available drones and BRUV units were used to identify the abundance and diversity of elasmobranchs within shallow-water environments adjacent to coral reefs and mangrove-associated sandflats. Surveys were conducted in the Saudi Arabian central Red Sea, an area of notable conservation concern for elasmobranchs with little information on their abundance and distribution in shallowwater habitats (Spaet et al., 2011;Dulvy et al., 2014;Spaet et al., 2016;Spaet, 2019;MacNeil et al., 2020). The main objectives of this study were to (i) quantify and compare the species composition of elasmobranchs and their relative abundance in shallow reef and sandflat habitats; and (ii) compare the species composition and abundance quantified by BRUV and UAV surveys in shallow-water environments. ...
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... In South Africa, coastal chondrichthyans are subjected to overfishing and habitat degradation, with species such as the common smoothhound shark and soupfin shark targeted by commercial shark fisheries (da Silva et al., 2015). While there remains uncertainty in the contribution that spatial protection can have in shark conservation, especially for larger and more mobile species who frequently move outside MPA boundaries (Dulvy et al., 2014;Davidson and Dulvy, 2017;MacKeracher et al., 2019), there have been some studies showing the positive effect that localised MPAs can have on shark abundance (Goetze and Fullwood, 2013;Speed et al., 2016;Bond et al., 2017;MacNeil et al., 2020). However, on a global scale, existing MPAs do not overlap in space with threatened endemic chondrichthyans (Davidson and Dulvy, 2017). ...
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1. Shark catches are common in small-scale (artisanal) and recreational fisheries; the magnitudes of these catches remain poorly known and understudied, particularly in developing countries. Data from three sources were used to assess the composition of shark landings in these fisheries in Kenya: boat-based recreational fishery tagging 1987–2016; observed landings from the Bycatch Assessment and Mitigation in the Western Indian Ocean Fisheries Project 2016–2017; and Catch Assessment Surveys landings data 2017–2020. 2. Eighteen species were identified among 1,215 sharks recorded in small-scale fisheries between June 2016 and June 2017. Most of them belonged to the families Carcharhinidae (26%), Triakidae (23%) and Sphyrnidae (14%). Longlines (41%), drift gillnets (30%) and bottom-set gillnets (21%) recorded the highest proportions of catches. 3. A total of 501 sharks comprising 16 species were recorded in the recreational tagging data between 1987 and 2016, caught using either trolling lines or rods and reels. The families Carcharhinidae (56%) and Sphyrnidae (12%) represented the highest proportion of the catch. 4. A generalized linear model was used to assess the effect of year, gear type, season and vessel propulsion mode on the variation in catches of sharks in small-scale fisheries between 2017 and 2020. Catches were significantly higher in 2017 with high catch rates observed in longlines and handlines. 5. Findings from this study highlight the importance of citizen science by recreational fishers in increasing awareness around the risks and threats to shark populations. Strengthening of existing monitoring of shark interactions with smallscale and recreational fisheries is important alongside the implementation of appropriate conservation and management measures such as reductions in fishing effort (e.g. prawn trawling) in nursery areas, prohibiting the capture of CITES protected species, and enforcing catch-and-release practices by sport fishers to ensure the long-term sustainability of both the affected shark species and the livelihoods of the fishers.
Article
1. Shark catches are common in small-scale (artisanal) and recreational fisheries; the magnitudes of these catches remain poorly known and understudied, particularly in developing countries. Data from three sources were used to assess the composition of shark landings in these fisheries in Kenya: boat-based recreational fishery tagging 1987–2016; observed landings from the Bycatch Assessment and Mitigation in the Western Indian Ocean Fisheries Project 2016–2017; and Catch Assessment Surveys landings data 2017–2020. 2. Eighteen species were identified among 1,215 sharks recorded in small-scale fisheries between June 2016 and June 2017. Most of them belonged to the families Carcharhinidae (26%), Triakidae (23%) and Sphyrnidae (14%). Longlines (41%), drift gillnets (30%) and bottom-set gillnets (21%) recorded the highest proportions of catches. 3. A total of 501 sharks comprising 16 species were recorded in the recreational tagging data between 1987 and 2016, caught using either trolling lines or rods and reels. The families Carcharhinidae (56%) and Sphyrnidae (12%) represented the highest proportion of the catch. 4. A generalized linear model was used to assess the effect of year, gear type, season and vessel propulsion mode on the variation in catches of sharks in small-scale fisheries between 2017 and 2020. Catches were significantly higher in 2017 with high catch rates observed in longlines and handlines. 5. Findings from this study highlight the importance of citizen science by recreational fishers in increasing awareness around the risks and threats to shark populations. Strengthening of existing monitoring of shark interactions with smallscale and recreational fisheries is important alongside the implementation of appropriate conservation and management measures such as reductions in fishing effort (e.g. prawn trawling) in nursery areas, prohibiting the capture of CITES protected species, and enforcing catch-and-release practices by sport fishers to ensure the long-term sustainability of both the affected shark species and the livelihoods of the fishers.
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Sharks are charismatic predators that play a key role in most marine food webs. Their demonstrated vulnerability to exploitation has recently turned them into flagship species in ocean conservation. Yet, the assessment and monitoring of the distribution and abundance of such mobile species in marine environments remain challenging, often invasive and resource-intensive. Here we pilot a novel, rapid and non-invasive environmental DNA (eDNA) metabarcoding approach specifically targeted to infer shark presence, diversity and eDNA read abundance in tropical habitats. We identified at least 21 shark species, from both Caribbean and Pacific Coral Sea water samples, whose geographical patterns of diversity and read abundance coincide with geographical differences in levels of anthropogenic pressure and conservation effort. We demonstrate that eDNA metabarcoding can be effectively employed to study shark diversity. Further developments in this field have the potential to drastically enhance our ability to assess and monitor elusive oceanic predators, and lead to improved conservation strategies.
Book
Statistical Rethinking: A Bayesian Course with Examples in R and Stan builds readers’ knowledge of and confidence in statistical modeling. Reflecting the need for even minor programming in today’s model-based statistics, the book pushes readers to perform step-by-step calculations that are usually automated. This unique computational approach ensures that readers understand enough of the details to make reasonable choices and interpretations in their own modeling work. The text presents generalized linear multilevel models from a Bayesian perspective, relying on a simple logical interpretation of Bayesian probability and maximum entropy. It covers from the basics of regression to multilevel models. The author also discusses measurement error, missing data, and Gaussian process models for spatial and network autocorrelation. By using complete R code examples throughout, this book provides a practical foundation for performing statistical inference. Designed for both PhD students and seasoned professionals in the natural and social sciences, it prepares them for more advanced or specialized statistical modeling. Web Resource The book is accompanied by an R package (rethinking) that is available on the author’s website and GitHub. The two core functions (map and map2stan) of this package allow a variety of statistical models to be constructed from standard model formulas.
Article
As luxury consumer markets in wildlife grow, many of the desired species targeted are declining, with some at elevated risk of local or even global extinction unless their trade networks are controlled and the necessary laws and regulations established and enforced. Application of the precautionary principle is strongly called for. The well-documented international trade in shark fins for high-end seafood consumption, selected as an illustrative case study, is no exception. For shark fins, and other threatened luxury wildlife, only a tiny proportion could be managed sustainably, much of the international trade is uncontrolled or currently uncontrollable, laundering and poaching are rife, criminal networks are often involved and there is scant government or consumer appetite in major demand centres pushing for sustainable and legal trade. In the case of shark fins, the sourcing of which is the major driver of shark overfishing, it is proposed to be taken off the menu before further declines in shark populations, and possible extinctions, occur, or until exploitation (including bycatch) and trade in fins is controlled to within their biological limits.