ArticlePublisher preview available

Global status and conservation potential of reef sharks

July 2020
Nature 583(7818)

DOI:10.1038/s41586-020-2519-y

Authors:

Aaron Macneil

Dalhousie University

Demian Chapman

Florida International University

Michelle R Heupel

University of Tasmania

Colin A. Simpfendorfer

James Cook University

Show all 123 authorsHide

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.

Relationships between MaxN and mean count a, Relationships for 82 BRUVS sets across the range of MaxN values observed by Global FinPrint. Linear model estimates (median (95% uncertainty intervals of the highest posterior density)) are 1.56 (1.09–2.05) (intercept) and 3.03 (2.75–3.26) (slope). The red line is the highest posterior density model fit; grey lines are 100 realizations of possible model fits given random samples from the model posteriors, showing a tight relationship to the estimated red line. b, Bootstrap-based linear model estimates (using 1,000 repetitions) from samples of 5 BRUVS sets (with replacement) from the sample of 82 BRUVS sets in a, with estimates of 1.26 (0.29–2.67) (intercept) and 3.17 (1.89–6.49) (slope), using the OLS function of the scipy statsmodels package in Python. The wider variability in b is because lines were estimated from using five data points with replacement. c, Bootstrap-based power analysis results showing the number of observations required to have a 95% probability of the estimated linear slope being >0; probabilities on the y axis are the proportion of bootstrap replicates (with replacement) for which the 95% confidence intervals of the estimated slope do not overlap zero.

…

Expected probability of sharks being observed Expected probabilities for BRUVS sets from 58 national jurisdictions surveyed by Global FinPrint. The number of reefs surveyed is indicated in parentheses. Among the eight lowest-ranked nations there is estimated to be less than 0.1% chance of a shark being present any time a BRUVS is deployed, suggesting that sharks are functionally extinct on these survey reefs. Functional extinction in this study means that reef sharks are essentially absent from the ecosystem, and have little to no functional role in structuring the surrounding reef fish community through predation or fear-based effects⁴³. Symbols are median (circles), 50% (wide horizontal lines) and 95% (thin horizontal lines) highest posterior density (credible) intervals. Note that although there are more reefs observed from jurisdictions such as Australia, the hierarchical model explicitly accounts for such imbalances.

…

Forrest plot of ZINB posterior distribution effect sizes for candidate models Left and centre columns, covariates present in the count component of the model. Right column, covariates present in the zero-inflation component of the model. The median (circles) and 95% (thin horizontal lines) highest posterior density (credible) intervals for four independent Markov chain Monte Carlo chains (left) are shown. Models with PSIS-LOO-based weights >0 include the ZINB full model (77% weight; squares), the ZINB full count model (16% weight; upwards pointing triangle) and ZINB partial null (7% weight; downwards pointing triangle); circles indicate model-weighted estimates used for inference. R-hat values for all parameters were between 1.01 and 1, suggesting no evidence that models failed to converge. Note the differences in the scale along the x axes.

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Diagnostic plots for Hamiltonian Monte Carlo analyses a–d, Bivariate plots of posterior traces (red dots) of global zero slopes versus region-level zero variances for a centred parameterization (a); global count slopes versus region-level count variances for a centred parameterization (b); global zero slopes versus region-level zero variances for a non-centred parameterization (c); and global count slopes versus region-level count variances for a non-centred parameterization (d). Green dots highlight potentially divergent transitions in NUTS samples. a, b, A high number of clustered divergent transitions are clustered, indicative of pathological parameter space. c, d, These problems are eliminated using a non-centred, re-parameterization³⁹ of the original full zero-inflated hierarchical model, which had 77% of posterior PSIS-LOO model weight.

…

Sensitivity plot for the effects of N(0, σ) or C(0, γ) prior standard deviations (σ) and scales (γ) Selected posterior parameter estimates are shown for the full zero-inflated negative binomial model (77% of PSIS-LOO weight) of reef shark MaxN observations in Global Finprint. Symbols are median (circles), 50% (wide horizontal lines) and 95% (thin horizontal lines) highest posterior density (credible) intervals. Results show minor effects of prior specification on the global rate, with small changes to Cauchy prior scale values for HDI, VOICE and POP.

…

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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

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

. 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

. 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

and sampling

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

(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

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The exploitation of sharks and the degradation of their habitats elevate the urgency to understand the factors that influence offspring survival and ultimately shark reproductive success. We monitored and sampled blacktip reef sharks (Carcharhinus melanopterus) in nursery habitats of Moorea Island (French Polynesia), to improve knowledge on shark reproductive behavior and biology. We sampled fin clips and morphometrics from 230 young-of-the-year sharks and used microsatellite DNA markers to process parentage analysis to study the reproductive philopatric behavior in female sharks and the matrotrophy within litters. These traits are driving the success of the local replenishment influencing selection through birth site and maternal reserves transmitted to pups. Parentage analysis revealed that some female sharks changed their parturition areas (inter-seasonally) while other female sharks came back to the same site for parturition, providing evidence for a plastic philopatric behavior. Morphometrics showed that there was no significant relationship between body condition indices and nursery locations. However, similarities and differences in body condition were observed between individuals sharing the same mother, indicating that resource allocation within some shark litters might be unbalanced. Our findings further our understanding of the reproductive biology and behavior that shape shark populations with the aim to introduce these parameters into future conservation strategies.

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Marine protected areas (MPAs) are increasingly advocated for the conservation and management of sharks and rays. However, substantial uncertainty remains regarding which species can benefit from MPAs. Meanwhile, area-focused protection targets have spurred recent and rapid gains in the creation of large MPAs, many of which carry vague objectives set by a diverse group of stakeholders with potentially different notions of "success." Here, we capture and critically evaluate current views on the use of MPAs for shark and ray conservation. Through interviews with scientists, MPA managers, fisheries experts, conservation practitioners, advocates and policy experts (n = 53), we demonstrate a variety of perspectives regarding: (a) the effectiveness of MPAs as a tool for shark and ray conservation; (b) which factors influence the success of MPAs for sharks and rays; and (c) the desired outcomes of these MPAs. While MPAs created specifically for sharks and rays were viewed to be slightly more effective than regular MPAs as a tool for shark and ray conservation, both were generally considered insufficient in isolation. Despite greater emphasis on social success factors (e.g., local support) over biophysical success factors (e.g., size), biological outcomes (e.g., increased abundance) were prioritized over social outcomes (e.g., livelihood benefits). We argue that a stronger focus on achieving social outcomes can enhance the potential for MPAs to benefit sharks and rays. In revealing current thinking regarding the drivers and indicators of MPA success for sharks and rays, the results of this study can inform efforts to conserve and manage these species. K E Y W O R D S effectiveness, factors, marine protected areas, outcomes, shark and ray conservation, success

Gravity of human impacts mediates coral reef conservation gains

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Significance Marine reserves that prohibit fishing are a critical tool for sustaining coral reef ecosystems, yet it remains unclear how human impacts in surrounding areas affect the capacity of marine reserves to deliver key conservation benefits. Our global study found that only marine reserves in areas of low human impact consistently sustained top predators. Fish biomass inside marine reserves declined along a gradient of human impacts in surrounding areas; however, reserves located where human impacts are moderate had the greatest difference in fish biomass compared with openly fished areas. Reserves in low human-impact areas are required for sustaining ecological functions like high-order predation, but reserves in high-impact areas can provide substantial conservation gains in fish biomass.

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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.

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Jan 2018

Richard McElreath

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.

Out of control means off the menu: The case for ceasing consumption of luxury products from highly vulnerable species when international trade cannot be adequately controlled; shark fin as a case study

Article

Sep 2018
MAR POLICY

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.