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Overfishing is the primary cause of marine defaunation, yet declines in and increasing extinction risks of individual species are difficult to measure, particularly for the largest predators found in the high seas1–3. Here we calculate two well-established indicators to track progress towards Aichi Biodiversity Targets and Sustainable Development Goals4,5: the Living Planet Index (a measure of changes in abundance aggregated from 57 abundance time-series datasets for 18 oceanic shark and ray species) and the Red List Index (a measure of change in extinction risk calculated for all 31 oceanic species of sharks and rays). We find that, since 1970, the global abundance of oceanic sharks and rays has declined by 71% owing to an 18-fold increase in relative fishing pressure. This depletion has increased the global extinction risk to the point at which three-quarters of the species comprising this functionally important assemblage are threatened with extinction. Strict prohibitions and precautionary science-based catch limits are urgently needed to avert population collapse6,7, avoid the disruption of ecological functions and promote species recovery8,9.

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.

Small-bodied coastal sharks are often caught as by-catch in fishing operations. Life-history information for these sharks is needed to ascertain the level at which these populations are potentially affected by fishing. This study determined the age, growth and maturity of Carcharhinus coatesi captured by prawn trawlers in the Gulf of Papua. Using vertebral aging and an information-theoretic multimodel approach, the von Bertalanffy growth model fit the data best; parameters were L0=40.6cm±0.8, L∞=74.8cm±2.1, k=0.33year−1±0.06. Length-at-maturity analysis indicated that males reach maturity at L50=66.3cm (CI: 63.8, 71.4) and L95=71.6 (CI: 64.6, 74.2) cm while females matured at L50=71.4cm (CI: 61.5, 72.01) and L95=72.5cm (CI: 62.7, 74.0). Age-at-maturity estimates showed that both males (A50=5.1 years (CI: 4.6, 7.1), A95=6.4 years (CI: 5.1, 7.2)) and females (A50=5.3 years (CI: 3.5, 8.7), A95=7.4 years (CI: 3.6, 8.8)) reach maturity at about the same age. This study addresses the need for specific life-history information for a data-deficient species in a region that has remained relatively under-researched but in need of ongoing improvements to fisheries management.

The application of genome-wide cytonuclear molecular data to identify management and adaptive units at various spatio-temporal levels is particularly important for overharvested large predatory organisms, often characterized by smaller, localized populations. Despite being “near threatened”, current understanding of habitat use and population structure of Carcharhinus galapagensisis limited to specific areas within its distribution. We evaluated population structure and connectivity across the Pacific Ocean using genome-wide single-nucleotide polymorphisms (~7200 SNPs) and mitochondrial control region sequences (945 bp) for 229 individuals. Neutral SNPs defined at least two genetically discrete geographic groups: an East Tropical Pacific (Mexico, east and west Galapagos Islands), and another central-west Pacific (Lord Howe Island, Middleton Reef, Norfolk Island, Elizabeth Reef, Kermadec, Hawaii and Southern Africa). More fine-grade population structure was suggested using outlier SNPs: west Pacific, Hawaii, Mexico, and Galapagos. Consistently, mtDNA pairwise ΦSTdefined three regional stocks: east, central and west Pacific. Compared to neutral SNPs (FST= 0.023–0.035), mtDNA exhibited more divergence (ΦST= 0.258–0.539) and high overall genetic diversity (h= 0.794 ± 0.014; π= 0.004 ± 0.000), consistent with the longstanding eastern Pacific barrier between the east and central–west Pacific. Hawaiian and Southern African populations group within the west Pacific cluster. Effective population sizes were moderate/high for east/west populations (738 and 3421, respectively). Insights into the biology, connectivity, genetic diversity, and population demographics informs for improved conservation of this species, by delineating three to four conservation units across their Pacific distribution. Implementing such conservation management may be challenging, but is necessary to achieve long-term population resilience at basin and regional scales.

Shark nets have been set off the beaches of KwaZulu?Natal, South Africa, since 1952 to reduce the risk of shark attack. The nets fish in fixed localities 400 m from shore and both directly affect local shark populations and act as fisheries-independent monitoring devices. Reliable catch information at the species level was available for the period 1978?2003. Trends in catch rate and size were used to assess the population status of 14 commonly caught shark species. In addition, a demographic modelling approach was used in conjunction with the catch information to assess the potential effect of the nets on populations. Catch rates of four species (Carcharhinus leucas, C. limbatus, Sphyrna lewini and S. mokarran) showed a significant decline, as did the mean or median length of three species (Carcharhinus amboinensis, C. limbatus and female Carcharodon carcharias). For three species that showed declining catch rates or length the potential effect of the shark nets was assessed to be low, suggesting that other sources of catch were responsible for the declining status. The potential effect of the shark nets was assessed to be high for two species (Carcharhinus obscurus and Carcharias taurus, neither of which showed declines in catch rate or length), because of very low intrinsic rates of population increase.

Increasing fishing pressure on sharks stocks over recent decades has resulted in declines of many populations and led to increasing concerns for their conservation. The extent of these declines, however, has been highly variable—the result of the level of fishing, ocean conditions, and the life history of individual species. Two recent articles have described the collapse and possible extirpation of shark populations in the northwest Atlantic Ocean and Gulf of Mexico. Herein, we examine the results of these two papers commenting on the data sets used, comparing them to other available data sets, and critically evaluating the analyses and conclusions. We argue that these conclusions have been overstated because: (1) the analyses were based on a limited number of data sets, (2) the data sets themselves are inadequate to describe the status of all shark populations in the northwest Atlantic Ocean and Gulf of Mexico reported in these studies, (3) available data sets that could produce different conclusions were not utilized, (4) some factors were not taken into account that could have biased the results, (5) there were no alternate hypotheses presented evaluating other causes of the perceived decline, and (6) the authors did not consider any current stock assessments, which in several cases report the status of sharks to be considerably healthier than asserted.

Abstract: The distribution and salinity preference of immature bull sharks (Carcharhinus leucas) were examined based on the results of longline surveys in three adjacent estuarine habitats in southwest Florida: the Caloosahatchee River, San Carlos Bay, and Pine Island Sound. Mean sizes were significantly different between each of these areas indicating the occurrence of size-based habitat partitioning. Neonate and young-of-the-year animals occurred in the Caloosahatchee River and juveniles older than 1 year occurred in the adjacent embayments. Habitat partitioning may reduce intraspecific predation risk and increase survival of young animals. Classification tree analysis showed that both temperature and salinity were important factors in determining the occurrence and catch per unit effort (CPUE) of immatureC. leucas. The CPUE of <1 year oldC. leucas was highest at temperatures over 29°C and in areas with salinities between 7‰ and 17.5‰ Although they are able to osmoregulate in salinities from fresh to fully marine, youngC. leucas may have a salinity preference. Reasons for this preference are unknown, but need to be further investigated.

Age and growth of Carcharhinus obscurus was studied by vertebral ageing techniques. The annual formation of growth bands in the vertebrae was validated with oxytetracycline-injected individuals at liberty up to 1481 days. Growth-band counts of vertebral samples from 305 individuals ranging in size from 63 to 282 cm fork length were used to construct growth curves. Assuming a mean size at birth of 75.3 cm, estimates of von Bertalanffy growth parameters for males were: L∞ = 336.5 cm and K = 0.045 year–1. Growth parameters for females were: L∞= 354.4 cm and K = 0.043 year–1. The age at maturity was estimated to be 17–22 years for females (220–250 cm), and 20 – 23 years for males (230–243 cm). The oldest animal aged was a 32-year-old female 274 cm in length. Growth-rate estimates based on vertebral ageing for animals up to 5 years of age are similar to those from tag–recapture studies on the same population. The results of the present study were similar to those for C. obscurus from the south-western Indian Ocean and the western North Atlantic Ocean, indicating that age and growth are similar among these populations. Extra keywords: age validation, micro-radiography, elasmobranch, Indian Ocean

The diets of dusky (Carcharhinus obscurus), whiskery (Furgaleus macki) and gummy (Mustelus antarcticus) sharks from south-western Western Australia were examined by analysis of stomach contents. The majority of samples were obtained from catches of commercial gill-net fishers. Carcharhinus obscurus had a diverse diet dominated by pelagic teleosts and cephalopods. A wide range of demersal and benthic prey were also consumed, but represented only a small portion of the diet. As body size increased, importance of elasmobranchs in the diet of C. obscurus increased, while most other groups remained at similar levels. Furgaleus macki had a specialized diet, feeding almost exclusively on octopus and other cephalopods. The diet of M. antarcticus was dominated by benthic and epibenthic prey, including crabs, lobsters, tetraodontid fishes and octopus. As M. antarcticus increased in size there was an increase in the occurrence of rock lobster and a decrease in the occurrence of crabs in the diet. Differences in the diet were also noted between male and female M. antarcticus, but were confounded with differences between size classes. SW Australia, ontogenetic shift, dusky shark, whiskery shark, gummy shark, Carcharhinus obscurus, Furgaleus macki, Mustelus antarcticus

An Amendment to this paper has been published and can be accessed via a link at the top of the paper.

Thermal dependence of growth and metabolism can influence thermal preference and tolerance in marine ectotherms, including threatened and data-deficient species. Here, we quantified the thermal dependence of physiological performance in neonates of a tropical shark species (blacktip reef shark, Carcharhinus melanopterus) from shallow, nearshore habitats. We measured minimum and maximum oxygen uptake rates (ṀO2), calculated aerobic scope, excess post-exercise oxygen consumption and recovery from exercise, and measured critical thermal maxima (CTmax), thermal safety margins, hypoxia tolerance, specific growth rates, body condition and food conversion efficiencies at two ecologically relevant acclimation temperatures (28 and 31°C). Owing to high post-exercise mortality, a third acclimation temperature (33°C) was not investigated further. Acclimation temperature did not affect ṀO2 or growth, but CTmaxand hypoxia tolerance were greatest at 31°C and positively associated. We also quantified in vitrotemperature (25, 30 and 35°C) and pH effects on haemoglobin–oxygen (Hb–O2) affinity of wild-caught, non-acclimated sharks. As expected, Hb–O2affinity decreased with increasing temperatures, but pH effects observed at 30°C were absent at 25 and 35°C. Finally, we logged body temperatures of free-ranging sharks and determined that C. melanopterusneonates avoided 31°C in situ. We conclude that C. melanopterusneonates demonstrate minimal thermal dependence of whole-organism physiological performance across a seasonal temperature range and may use behaviour to avoid unfavourable environmental temperatures. The association between thermal tolerance and hypoxia tolerance suggests a common mechanism warranting further investigation. Future research should explore the consequences of ocean warming, especially in nearshore, tropical species.

The whiskery shark, Furgaleus macki, is important in the commercial shark fishery off southern Western Australia. The reproductive biology was studied with the aid of 88 specimens collected by commercial gill-net vessels off south-western Australia between March 1994 and August 1996. The size selectivity of the gill-nets resulted in only mature and large juvenile specimens being caught. Size at birth was estimated to be 22–27 cm total length. Size (fork length) at maturity was 107 cm for males and 112 cm for females. There is a seasonal reproductive cycle. Mating is most likely to occur from August to September, with females storing spermatozoa until ovulation in late January to early April. Gestation lasts 7–9 months, with parturition from August to October. Litter sizes vary from 4 to 28 (mean 19). There was a significant linear relationship between litter size and maternal length. Mature males mate each year, but females produce litters every second year. Mature non-pregnant females develop large yolky ova from March to October, with development of the ova completed several months before ovulation. The results are compared with previous records and with data for other species of the family Triakidae that occur off southern Australia. Extra keywords: Triakidae, reproduction

The shark fishery in the southern half of Western Australia fishery began in 1941 and developed slowly until the mid 1970s when the fishery began to expand rapidly. A management plan incorporating limited entry, gear specifications and effort controls was introduced in 1988 in response to concerns about the status of the stocks. Research has focused on the assessment of stocks, and has involved the collection of catch-and-effort data since 1975, and tactical research projects to gather data on biology; this has allowed the implementation of stock assessment, modelling and forecasting techniques. The fishing industry is involved in the development of the research projects and in the decision-making process of management. This, together with regular reporting of research results, assists in maintaining industry support and acceptance of results. A potential disadvantage of the involvement of industry is the delay in implementation that may be caused by the complexity of the system of consultation or by the conflict between regulation of the fishery and the present livelihood of fishers. However, this is minimized by the creation of clear quantitative targets for management. Extra keywords: fisheries research, fisheries management, stock assessment, dusky shark, gummy shark, whiskery shark

Stomachs from 433 specimens of Rhizoprionodon taylori caught by gill-nets and otter trawls in Cleveland Bay, north Queensland, were examined. At least 5.3% of specimens examined had regurgitated. Of the remaining 410 specimens 59.0% had empty stomachs and only 19.3% contained food items identifiable to the family level. The diet comprised mostly small teleosts from the families Leiognathidae, Clupeidae, Teraponidae and Engraulidae. Penaeid prawns and loliginid squid were also important in the diet. Average weight of individual recently ingested food items was 28.5 g, which represented 2.3% of body weight. The high diversity of potential prey groups, high rate of regurgitation, and high proportion of empty stomachs meant that although a large number of specimens were examined the sample size was probably insufficient to provide a thorough analysis of the diet of R. taylori in Cleveland Bay.

Gill-net mesh selectivity parameters were estimated for C. obscurus and F. macki from catches of an experimental net composed of seven panels of mesh ranging in size from 12.7 cm to 21.6 cm. A maximum-likelihood model was used to fit a gamma distribution to length data from each of the different-sized meshes. Mesh selectivity parameters for the gilled-only data set were θ1 =126.90 and θ1 =20253, and for the data set including gilled and non-gilled sharks θ1 =130.13 and θ1 =29237. The advantages of the parameters from each of these data sets are discussed. Mesh-selectivity parameters for F. macki were θ1 =173.70 and θ2 =26415. The exclusion of data from the two largest mesh sizes, which had much lower catch rates than the other mesh sizes, did not significantly change the parameter estim- ates. Catch rates from the panels in the experimental net that correspond to those used by the commercial gill-net fishery in south-western Australia were compared and no significant differences were found.