Search

Identifying critical habitat for highly mobile species such as sharks is difficult, but essential for effective management and conservation. In regions where baseline data are lacking, non‐traditional data sources have the potential to increase observational capacity for species distribution and habitat studies. In this study, a research and education organization conducted a 5‐year (2013–2018) survey of shark populations in the coastal waters of west‐central Florida, an area where a diverse shark assemblage has been observed but no formal population analyses have been conducted. The objectives of this study were to use boosted regression tree (BRT) modeling to quantify environmental factors impacting the distribution of the shark assemblage, create species distribution maps from the model outputs, and identify spatially explicit hot spots of high shark abundance. A total of 1036 sharks were captured, encompassing eleven species. Abundance hot spots for four species and for immature sharks (collectively) were most often located in areas designated as “No Internal Combustion Engine” zones and seagrass bottom cover, suggesting these environments may be fostering more diverse and abundant populations. The BRT models were fitted for immature sharks and five species where n > 100: the nurse shark (Ginglymostoma cirratum), blacktip shark (Carcharhinus limbatus), blacknose shark (C. acronotus), Atlantic sharpnose shark (Rhizoprionodon terraenovae), and bonnethead (Sphyrna tiburo). Capture data were paired with environmental variables: depth (m), sea surface temperature (°C), surface, middle, and bottom salinity (psu), dissolved oxygen (mg/L), and bottom type (seagrass, artificial reef, or sand). Depth, temperature, and bottom type were most frequently identified as predictors with the greatest marginal effect on shark distribution, underscoring the importance of nearshore seagrass and barrier island habitats to the shark assemblage in this region. This approach demonstrates the potential contribution of unconventional science to effective management and conservation of coastal sharks., Identifying critical habitat for highly mobile species such as sharks is difficult, but essential for effective management and conservation. Using shark capture data collected by a research and education group in Florida, this study used boosted regression tree (BRT) modeling to quantify environmental factors impacting the distribution of the shark assemblage, create species distribution maps from the model outputs, and identify spatially explicit hot spots of high shark abundance. This approach for identifying spatially explicit areas and environmental conditions suited for shark abundance demonstrates the potential contribution of unconventional science to effective management and conservation of coastal sharks.

Energetic condition is one of the most important factors that influence fitness and reproductive performance in vertebrates. Yet, we lack evidence on how energetic states change in response to reproduction in large marine vertebrates. In the present study, we used a non-lethal approach to assess relationships among reproductive stage, circulating steroid hormones (testosterone and relative corticosteroid levels), plasma fatty acids, and the ketone body β-hydroxybutyrate in male sharks of two species with divergent ecologies, the benthic nurse shark (Ginglymostoma cirratum) and the epipelagic blacktip shark (Carcharhinus limbatus). We found higher relative corticosteroid levels in adult nurse sharks during the pre-mating period and in blacktip sharks during the mating period. Higher levels of β-hydroxybutyrate were found in adult nurse sharks during the mating period, but concentrations of this ketone body did not significantly vary across reproductive stages in blacktip sharks. We also detected reduced percentages of essential fatty acids during the mating period of both nurse and blacktip sharks. Taken together, our findings suggest that nurse and blacktip sharks differ in their energetic strategy to support reproduction, however, they likely rely on physiologically important fatty acids during mating, to support spermatogenesis.

Despite indications that sharks have an exceptional capacity to heal from traumatic injuries, no detailed microscopic observations of integumental wounds have been reported for sharks. This study details the histopathological features of such wounds in a free-ranging shark. An adult male Blacktip Shark Carcharhinus limbatus was collected in 2017 during fisheries-independent sampling efforts in the coastal southeastern U.S. Atlantic. The shark had numerous lesions on his head, torso, and left pectoral fin that were compatible with shark bites. Representative samples from two wounds on the head were examined by light microscopy. The epidermal changes included hyperplasia and hypertrophy; intracellular edema; and the absence of goblet cells, denticles, and chromatophores. In the dermis, fibrinocellular exudation, granulation tissue, and marked skeletal muscle necrosis and regeneration were observed. The above features were comparable to wound healing in bony fish, albeit minor differences were found. Although this case documents exceptionally good regeneration of skeletal muscle in the shark, we found no evidence of unique morphological healing patterns. Further studies on wound healing are needed because recent molecular and genetic findings do suggest evolutionary adaptations enhancing healing in sharks.

The integration of eDNA analysis into the population assessment and monitoring of sharks could greatly improve temporal and spatial data used for management purposes. This study aimed to compare eDNA detection against well-established seasonal changes in blacktip shark (Carcharhinus limbatus) abundance in Terra Ceia Bay (FL, USA). We used a species-specific real-time PCR approach to detect C. limbatus eDNA in the bay on a near monthly basis from spring through mid-fall in 2018 and 2019. Previous studies have shown that C. limbatus give birth in the bay in early summer and immature sharks occur there until late fall, when decreasing water temperatures cause them to move offshore and southwards. Water samples (2 L) were collected (4–6 per month) and filtered in the field, with each then being subjected to real-time PCR. Carcharhinus limbatus ‘positive’ filters were significantly more commonly collected during the April-July sampling period than during the August-October sampling period. While following the predicted pattern, eDNA concentration was generally too low for accurate quantification. Our results show that C. limbatus eDNA detection follows known seasonal residency patterns consistently over 2 years of monitoring. Species-specific eDNA analysis using real-time PCR could therefore represent a cost-effective, scalable sampling tool to facilitate improved shark population monitoring in semi-enclosed marine habitats.

The stomach contents of 90 Blacktip Shark (Carcharhinus limbatus) specimens caught in the northwestern Gulf of Mexico (GOM) were examined. Stomach contents were identified to the lowest possible taxon, and quantified using percent weight, percent number, percent frequency of occurrence, and percent index of relative importance (IRI). Teleosts were the dominant prey group (98.95% IRI), although most were unidentified (61.70% IRI). Of identified teleost species, Atlantic Croaker (Micropogonias undulatus) (28.43% IRI), and Gulf Menhaden (Brevoortia patronus) (2.31% IRI) were the most important. Crustaceans (0.65% IRI), mollusks (0.46% IRI), and elasmobranchs (0.03% IRI) formed a minor component of the diet. Suspected shrimp fishery discards were found in 11% of stomachs, highlighting the potential importance of this food source for the Blacktip Shark. Diet composition did not differ between male and female sharks, but did between juveniles and adults. Juvenile shark diets had greater proportions of unidentified teleost, Clupeidae and Penaeidae, while adult diets had greater proportions of Sciaenidae, Ariidae and cephalopods. Our results were similar, although not identical to, other studies of Blacktip Shark diets in the northwestern GOM. Of note is finding of the mantis shrimp Squilla empusa, a species previously unreported in Blacktip Shark stomach contents. This new finding, the high importance of unidentified teleosts, and the lack of asymptote in the prey accumulation curve emphasize the need for further study of the Blacktip Shark diet in the northwestern GOM.

The blacktip shark Carcharhinus limbatus is a cosmopolitan species found in warm-temperate, subtropical and tropical waters around the world. The research here aimed to assess whether multiple pate...

Few studies have assessed whether patterns of resource partitioning among sympatric marine predators vary between different habitats. This type of data is important for understanding food web functioning as well as for supporting an ecosystem-based fisheries management plan. In this study, we used δ13C and δ15N stable isotope analysis to assess the relative isotopic niche area (i.e., which species has the largest and smallest isotopic niche area within a study site) and core isotopic niche overlap between 299 sharks of 11 co-occurring shark species that vary in size, diet and behavior in three coastal study areas in South Florida. Overall, results show that the relative isotopic niche area and patterns of core isotopic niche overlap between some sympatric shark species varied between sites, suggesting plasticity in resource use and competitive interactions between shark species (e.g., 85% of blacktip shark Carcharhinus limbatus standard ellipse area overlapped with the blacknose shark C. acronotus ellipse in Biscayne Bay, but there was no overlap between these species’ ellipses in nearby Florida Bay). Therefore, patterns of resource use and trophic interactions documented among species from one site may not be applicable to a neighboring location. These findings may have implications for food web models that incorporate trophic data from different areas for a species when local data are unavailable.

We tagged 12 Carcharhinus limbatus with acoustic transmitters and monitored their presence at five piers along the north-east coast of South Carolina, USA in 2016 and four piers in 2017 using acoustic receivers. Data were analysed with pier association indices (PAI), mixed models and fast Fourier transformation analyses to identify potential factors related to residence time and presence at piers and any cyclical patterns in visits to piers. While the majority of monitored C. limbatus were infrequently detected at piers, three (25.0%) were highly associated with piers (PAI ≥ 0.50). Of the C. limbatus that were detected after initial capture, three (25.0%) recorded detection events only at the pier where they were tagged and two individuals (16.7%) recorded at least one detection event at all monitored piers. The best-fit model explaining C. limbatus residence time at piers included terms for pier location and diel cycle (wi = 0.88), whereas the best fit model explaining presence-absence of C. limbatus at piers included terms for tidal height, diel cycle, barometric pressure and angler count (wi = 0.98). Carcharhinus limbatus did not appear to display cyclical patterns in their visits to piers. Along the north-east coast of South Carolina, association of C. limbatus with piers is a phenomenon for a proportion of mature individuals, but continued research is necessary to understand if this behaviour is driven by attraction to and feeding on angler discards or increased foraging opportunities resulting from the attraction of potential prey to the physical structure provided by piers.

We herein redescribe the enigmatic Cathariotrema selachii (MacCallum, 1916) Johnston and Tiegs, 1922 based on the holotype, paratypes, and newly collected specimens infecting the olfactory organ of 5 shark species from the Gulf of Mexico (all new host records): scalloped hammerhead shark, Sphyrna lewini (Griffith and Smith, 1834) (Carcharhiniformes: Sphyrnidae); great hammerhead shark, Sphyrna mokarran (Ruppell, 1837); blacktip shark, Carcharhinus limbatus (Muller and Henle, 1839) (Carcharhiniformes: Carcharhinidae); spinner shark, Carcharhinus brevipinna (Muller and Henle, 1839); and Atlantic sharpnose shark, Rhizoprionodon terraenovae (Richardson, 1836) (Carcharhinidae). These specimens were morphologically indistinguishable from each other and from MacCallum's holotype and paratypes. Those sequenced had identical first internal transcribed spacer (ITS1) and large subunit ribosomal DNA (28S) nucleotide sequences. As such, C. selachii infects sharks of 2 orders (Carcharhiniformes, Lamniformes) and 3 families (Carcharhinidae, Sphyrnidae, Lamnidae) in the Northwestern Atlantic Ocean (type locality) and Gulf of Mexico (new records herein). This report is the first of new specimens of C. selachii in the Atlantic Ocean Basin in 95 yr and corrects long-standing error cascades and ambiguities concerning the morphology and systematic placement of C. selachii. Considering morphology and nucleotide-based phylogenetic evidence (28S, Bayesian analysis), we herein emend monotypic CathariotremaJohnston and Tiegs, 1922 and propose Cathariotrematinae Bullard n. subfam. for it and 4 other genera (all formerly assigned to Merizocotylinae Johnston and Tiegs, 1922). These genera comprise species infecting only the nose of sharks (monotypic Cathariotrema, SqualotremaKearn and Green, 1983 and SeptitremaKheddam, Chisholm, and Tazerouti, 2020 plus 3 species of TriloculotremaKearn, 1993) and nose of a chimaera (monotypic HolocephalocotyleDerouiche, Neifar, Gey, Justine, and Tazerouti, 2019). Cathariotrematinae differs from Merizocotylinae by having a 3-part attachment organ and by lacking open loculi that symmetrically encircle a cluster of >2 loculi in the center of the haptor. Monophyletic Cathariotrematinae (with sequences representing species of Cathariotrema, Triloculotrema, and Holocephalocotyle only) was sister to monophyletic Merizocotylinae, which together were sister to monophyletic Calicotylinae Monticelli, 1903. These subfamilies comprise a monophyletic group of monocotylids that have a double vagina and infect extrabranchial, enclosed niches (urogenital system, body cavity, olfactory chamber/nose) on their shark, ray, and chimaera hosts (all other monocotylids have a single vagina and infect the gill or body surfaces of rays only). Monocotylinae Taschenberg, 1879 and Decacotylinae Chisholm, Wheeler, and Beverley-Burton, 1995 were recovered as monophyletic. Heterocotylinae Chisholm, Wheeler, and Beverley-Burton, 1995 remained paraphyletic. We accept ParacalicotyleSzidat, 1970.

Brain size varies dramatically, both within and across species, and this variation is often believed to be the result of trade-offs between the cognitive benefits of having a large brain for a given body size and the energetic cost of sustaining neural tissue. One potential consequence of having a large brain is that organisms must also meet the associated high energetic demands. Thus, a key question is whether metabolic rate correlates with brain size. However, using metabolic rate to measure energetic demand yields a relatively instantaneous and dynamic measure of energy turnover, which is incompatible with the longer evolutionary timescale of changes in brain size within and across species. Morphological traits associated with oxygen consumption, specifically gill surface area, have been shown to be correlates of oxygen demand and energy use, and thus may serve as integrated correlates of these processes, allowing us to assess whether evolutionary changes in brain size correlate with changes in longer-term oxygen demand and energy use. We tested how brain size relates to gill surface area in the blacktip shark Carcharhinus limbatus. First, we examined whether the allometric slope of brain mass (i.e., the rate that brain mass changes with body mass) is lower than the allometric slope of gill surface area across ontogeny. Second, we tested whether gill surface area explains variation in brain mass, after accounting for the effects of body mass on brain mass. We found that brain mass and gill surface area both had positive allometric slopes, with larger individuals having both larger brains and larger gill surface areas compared to smaller individuals. However, the allometric slope of brain mass was lower than the allometric slope of gill surface area, consistent with our prediction that the allometric slope of gill surface area could pose an upper limit to the allometric slope of brain mass. Finally, after accounting for body mass, individuals with larger brains tended to have larger gill surface areas. Together, our results provide clues as to how fishes may evolve and maintain large brains despite their high energetic cost, suggesting that C. limbatus individuals with a large gill surface area for their body mass may be able to support a higher energetic turnover, and, in turn, a larger brain for their body mass.

Increasing fishing effort, including bycatch and discard practices, are impacting marine biodiversity, particularly among slow-to-reproduce taxa such as elasmobranchs, and specifically sharks. While some fisheries involving sharks are sustainably managed, collateral mortalities continue, contributing towards > 35% of species being threatened with extinction. To effectively manage shark stocks, life-history information, including resource use and feeding ecologies is pivotal, especially among those species with wide-ranging distributions. Two cosmopolitan sharks bycaught off eastern Australia are the common blacktip shark (Carcharhinus limbatus; globally classified as Near Threatened) and great hammerhead (Sphyrna mokarran; Critically Endangered). We opportunistically sampled the digestive tracts of these two species (and also any whole prey; termed the ‘Russian-doll’ approach), caught in bather-protection gillnets off northern New South Wales, to investigate the capacity for DNA metabarcoding to simultaneously determine predator and prey regional feeding ecologies. While sample sizes were small, S. mokkaran fed predominantly on stingrays and skates (Myliobatiformes and Rajiformes), but also teleosts, while C. limbatus mostly consumed teleosts. Metabarcoding assays showed extensive intermixing of taxa from the digestive tracts of predators and their whole prey, likely via the predator’s stomach chyme, negating the opportunity to distinguish between primary and secondary predation. This Russian-doll effect requires further investigation in DNA metabarcoding studies focussing on dietary preferences and implies that any outcomes will need to be interpreted concomitant with traditional visual approaches.

Recent work showed that two species of hammerhead sharks operated as a double oscillating system, where frequency and amplitude differed in the anterior and posterior parts of the body. We hypothesized that a double oscillating system would be present in a large, volitionally swimming, conventionally shaped carcharhinid shark. Swimming kinematics analyses provide quantification to mechanistically examine swimming within and among species. Here, we quantify blacktip shark (Carcharhinus limbatus) volitional swimming kinematics under natural conditions to assess variation between anterior and posterior body regions and demonstrate the presence of a double oscillating system. We captured footage of 80 individual blacktips swimming in the wild using a DJI Phantom 4 Pro aerial drone. The widespread accessibility of aerial drone technology has allowed for greater observation of wild marine megafauna. We used Loggerpro motion tracking software to track five anatomical landmarks frame by frame to calculate tailbeat frequency, tailbeat amplitude, speed, and anterior/posterior variables: amplitude and frequency of the head and tail, and the body curvature measured as anterior and posterior flexion. We found significant increases in tailbeat frequency and amplitude with increasing swimming speed. Tailbeat frequency decreased and tailbeat amplitude increased as posterior flexion amplitude increased. We found significant differences between anterior and posterior amplitudes and frequencies, suggesting a double oscillating modality of wave propagation. These data support previous work that hypothesized the importance of a double oscillating system for increased sensory perception. These methods demonstrate the utility of quantifying swimming kinematics of wild animals through direct observation, with the potential to apply a biomechanical perspective to movement ecology paradigms.

The determination of the period of parturition and identification of nursery areas are fundamental for the management and conservation of a fishing resource. Through combination of monthly abundance, length measurements, and development time of embryos we inferred about parturition time of the Blacktip sharks, Carcharhinus limbatus, from Southwestern Atlantic. Specimens of C. limbatus were caught from 2002 and 2008 by artisanal fleets from three different locations along the Brazilian coast using gillnets (mesh width ranging between 30 and 120 mm between opposite knots and operating from 9 to 120 m in depth), handlines (19 to 140 m depth) and longlines (6 to 90 m depth). Through a comparative analysis of fisheries landing data collected, we have verified neonatal (< 90 cm) catch peaks at specific times of the year in different locations and, matching with birth prediction of embryos, propose that C. limbatuspresents a well-defined parturition time in late spring-early summer in Southwestern Atlantic. Moreover, we indicate supposed nursery grounds for the species along the Brazilian coast. This information will be crucial for stock assessments of the species and may serve as a basis for determining fisheries management measures. Resumo A determinação do período de parto e identificação de áreas de berçário são fundamentais para o manejo e conservação da espécie. Através da combinação de abundância mensal, comprimentos e tempo de desenvolvimento de embriões nós inferimos sobre a época de parto do tubarão galha preta, Carcharhinus limbatus, no Sudoeste Atlântico. Espécimes de C. limbatus foram capturados entre 2002 e 2008 por frotas artesanais de três locais diferentes ao longo da costa brasileira usando redes de emalhar (largura de malha variando entre 30 e 120 mm entre nós opostos e operando de 9 a 120 m de profundidade), linhas de mão (19 a 140 m de profundidade) e espinhel (6 a 90 m de profundidade). Através de análise comparativa de dados de desembarque pesqueiro, verificamos picos de captura de neonatos (< 90 cm) em períodos específicos do ano em diferentes localidades e, combinando com a previsão de nascimento de embriões, propomos que C. limbatus apresenta um período de parto bem definido no final da primavera e início do verão no Sudoeste do Atlântico. Além disto, nós indicamos prováveis áreas de berçário ao longo da costa brasileira. Estas informações serão cruciais para avaliações de estoque da espécie e poderão servir de base para determinação de medidas de manejo de pescarias.

Proportional survival (S) is a crucial life-history parameter in population dynamics, natural selection, and management of harvested stocks; variations in survival due to age, sex, or geographic region may have large effects on the success of managing fish stocks. The blacktip shark, Carcharhinus limbatus, is the most abundantly harvested shark species in American fisheries. Direct estimates of survival are preferred, but all current survival estimates for this species are either focused on young-of-the-year (YOY) or based on indirect methods. The objectives of this study were to determine whether age, sex, or geographic grouping affects survival and to generate direct survival estimates based on tag-recovery data. As a byproduct of this analysis, distribution maps and descriptive data summarizing captures were included. The U.S. National Marine Fisheries Service has been collecting tag-recovery data since 1962 through the Cooperative Shark Tagging Program (CSTP). Models were generated from this database with program MARK, ranked in order of parsimony according to Akaike’s Information Criterion, and tested for significance of effects with likelihood ratio tests. No movement has been observed to date between the west Gulf of Mexico, east Gulf of Mexico, and U.S. Atlantic, but 2 sharks tagged in the U.S. Virgin Islands were recaptured off Florida and Georgia (displacement= 1049 and 1183 n. mi., respectively). Survival did not differ significantly for males vs. females (P=0.761), east vs. west Gulf of Mexico (P=0.654), or U.S. Atlantic vs. Gulf of Mexico (P=0.243). However, significant differences were found for survival of YOY (0.580) and post-YOY (0.725) within the Gulf of Mexico (P=0.0003). These results demonstrate that survival can be modeled effectively for species in the CSTP with relatively small sample sizes. Future analyses may benefit from a length-based model, due to the difficulty in assigning life stages based on size.

As environmental change persists, understanding resource use patterns is of value to predict the consequences of shifting trophic structures. While many sharks are opportunistic predators, some exhibit prey selectivity, putting them at higher risk compared to species with greater trophic plasticity. In the Gulf of Mexico (GOM), Clupeids and Sciaenids comprise 69% of blacktip shark (Carcharhinus limbatus) diets, which is consequential considering potential responses of these prey groups to disturbance and over harvesting. We assessed if blacktips exhibit selectivity for Clupeids and Sciaenids in the western GOM based on stomach contents from sharks in coastal Texas. Clupeids comprised2% of diets, while striped mullet (Mugil cephalus) and red drum (Sciaenops ocellatus) comprised70% of identifiable prey. Ontogenetic shifts from smaller (Clupeids, small Sciaenids) to larger, higher trophic level (Ariidae, Elasmobranchii) prey fits our understanding of foraging among coastal sharks, and suggests our regional understanding of blacktip trophic ecology may be limited by the sizes of sampled sharks. Observed increases in blacktip densities coupled with declines in prey (Mugilids, Sciaenids) is concerning if blacktips have limited diet plasticity. Yet GOM blacktips may be more generalized than previously thought, which is promising for conservation and management.

Lay summary This study examines the stress associated with recreational rod and reel capture of blacktip sharks and estimates how many sharks survive once released in the Gulf of Mexico. A post-release mortality rate of 23% was determined, with some sharks dying immediately while others experienced a delayed mortality after 12 h., Understanding the stress responses of sharks to recreational catch and release fishing has important management and conservation implications. The blacktip shark Carcharhinus limbatus is a popular recreational species targeted throughout the western, central and eastern Gulf of Mexico (Gulf) yet it is unclear what levels of physiological stress result from catch-release fishing practices with hook and line gear and if the stress levels result in post-release mortality. This study correlates physiological response to stress through blood chemistry analysis and examines post-release behaviour of adult blacktip sharks caught to determine post-release mortality rates. Release behaviour was determined by pop-up satellite archival transmitting (PSAT) tags that record temperature, depth and light level data. To quantify physiological stress levels, blood samples were collected from 52 blacktip sharks and a suite of metabolic and osmotic markers were measured. Thirty-six of those blacktip sharks were also outfitted with a PSAT tag yielding time-at-large from 3 to 180 days. Of the 36 tags, 22 (61%) provided sufficient data to confirm post-release fate and 11 (31%) were recovered providing high-resolution data. Tag data suggests a post-release morality rate of 22.7% (95% confidence interval 7.8–45.4%), with mortality occurring within minutes (immediate mortality) to over 12 h post-release (delayed mortality). Compared to survivors, immediate mortalities exhibited significantly higher lactate (median 2.8 mmol/Lsurvivor vs 5.9 mmol/Limmediate mortality) and significantly lower hematocrit (median 24.4% survivor vs 14% immediate mortality) levels, but no difference was detected between survivors and delayed mortalities. Higher mortality in the western (30%) compared to the central (20%) Gulf may be due to shark handling. All PSATs from mortalities (N = 5) were recovered, and archived data revealed evidence of tag ingestion by predators. Results suggest reduced fight time, decreased handling time and limited air exposure provide blacktip sharks the best survival chances after release by recreational anglers.

The blacktip shark (Carcharhinus limbatus) is a common coastal species in tropical and subtropical waters around the world. To examine the fine-scale movement ecology of this species in a Caribbean nursery, 17 neonate blacktip sharks were acoustically tagged in May, 2012 and tracked for one year in Coral Bay, St John, USVI. By quantifying linear movement and shifts in position from a fixed Inner harbor location, we identified a diel movement pattern where blacktip sharks spend daylight hours within core habitat of Inner Coral Harbor and move each night to the central and outer portions of the Bay, a linear shift of 174–934 m. When compared to standard home range calculations applied to the overall movement data, these nightly positions were outside of the 95% activity space and, therefore, undetected using traditional space utilization methods, despite their predictable daily occurrence. Cluster analysis and nonmetric multidimensional scaling indicated distinct movement periods and locations: daytime (sun up) within Inner Coral Harbor; nighttime (sun down) in the center of the Bay; and brief periods approximately six hours after sunset at the mouth of the Bay. This diel shift in habitat use is likely associated with nocturnal foraging because it coincides with similar shifts in potential prey species. Habitat and resource management that incorporates the blacktip shark movements described herein is critical to the protection of these vulnerable life stages. The findings of this paper advance the understanding of blacktip shark behavior and acoustic telemetry experimental design.

Key methodological assumptions regarding the degree of natural variability and influence of sample handling and storage of elasmobranch vertebral chemistry were assessed using laser-ablation inductively coupled plasma mass spectrometry. Vertebral chemistry of juvenile blacktip sharks Carcharhinus limbatus was examined to identify whether differences existed among different regions of the vertebral column, between thoracic vertebrae of individual fish or within individual vertebrae. Additionally, the effects of bleach exposure and storage in ethanol on vertebral chemistry were compared. No significant variation in vertebral chemistry was found among different regions of the vertebral column or between thoracic vertebrae, but significant differences among transect locations within individual vertebrae were observed. The variation at all three levels appears comparable with published data on sagittal otoliths of bilaterally symmetrical teleost fishes. The experimental assessment of potential treatment effects indicated vertebral chemistry was not significantly affected by bleach or exposure to ethanol. Taken together, these results support the idea that vertebrae taken from the same region of the vertebral column can be treated as equivalent and at least certain elements remain robust to exposure to bleach and ethanol.

Information on how shark populations respond to fishing mortality ( F ) is critical to developing successful management and conservation strategies. However, data on catch, fishing effort and species abundance are often lacking for shark populations – preventing stock assessments from being conducted. Static demographic models circumvent this issue as they only require life history parameters. Age-structured Leslie Matrix models were developed and applied for silvertip shark Carcharhinus albimarginatus and common blacktip shark Carcharhinus limbatus sampled from heavily fished populations in the Indo-Pacific. Stochasticity was introduced to these models by varying vital rates though Monte Carlo simulations. Varying levels of F were introduced to the analyses to determine how both species respond to fishing pressure. Management scenarios were further developed to determine strategies that could facilitate sustainable harvesting. The demographic estimates demonstrated that without fishing both species would have increasing populations ( λ = 1.06 yr −1 for C. albimarginatus and 1.05 yr −1 for C. limbatus ) until density dependent effects occur. However, both populations would decline when low levels of F (>0.1 yr −1 ) were applied to all age-classes. The matrix elasticities revealed that changes to fertility elements had little effect on λ, while changes in juvenile survival led to the largest changes. However, age-at-first-capture analysis suggests protecting the juvenile life stage of both species would be an ineffective management strategy as both species mature at old ages. An age-at-last-capture analysis suggests these species could be harvested while maintaining increasing populations through a gauntlet fishery. This required F to be restrained to individuals C. limbatus and C. albimarginatus, respectively, until density dependent effects begin to manifest. However, this strategy depends on the ability to successfully protect all other age-classes from fishing – a strategy that may not be pragmatic in developing nations where little regulation occurs.

We analysed the stomach contents of 69 silky sharks Carcharhinus falciformis, 44 blacktip sharks Carcharhinus limbatus and 24 whitenose sharks Nasolamia velox caught in the Ecuadorian Pacific from August 2003 to December 2004. Prey included bony fishes, elasmobranchs, molluscs, crustaceans and turtles, with bony fishes being the most important to the diets of all three sharks, suggesting they are piscivorous predators. Based on the index of relative importance, the C. falciformis diet includes Thunnus albacares, Thunnus sp. and Auxis thazard, as well as some squid, fish and turtles. Similarly, the C. limbatus diet was dominated by T. albacares, Exocoetus monocirrhus, A. thazard, Katsuwonus pelamis, members of the Ophichthidae family and other elasmobranchs. Meanwhile, N. velox consumed mainly Dosidicus gigas, Larimus argenteus, Cynoscion sp. and Lophiodes spilurus. There is little competition for food between these tertiary carnivores: C. limbatus prefers prey from coastal-oceanic habitats; C. falciformis consumes mostly oceanic prey and N. velox focuses on prey from coastal habitats. The lack of information on the biology of sharks in Ecuador hinders the development of appropriate management and conservation plans to protect shark resources. This study increases our knowledge and understanding of sharks in Ecuador, thus contributing to their conservation.

Recent studies on shark assemblages on the northeast Florida and southeast Georgia coast (hereafter referred to collectively as the "First Coast") have demonstrated differences in species and age-class composition of catch from previously characterized estuaries and newly surveyed area beaches, demonstrating that these regions may provide a critical habitat to different segments (i.e., life stages) of local shark populations. In this study, carbon and nitrogen stable isotopes (δ13 C and δ15 N) from muscle tissue and blood plasma were used to examine trophic dynamics (and temporal variability thereof) of the three dominant co-occurring species found along First Coast beaches (the Atlantic Sharpnose shark Rhizoprionodon terraenovae, Blacknose shark Carcharhinus acronotus and Blacktip shark Carcharhinus limbatus) to determine if they exhibit overlap in resource use along with spatial and temporal habitat use. Although considered spatially segregated from the beach species, a dominant, age-class species found in First Coast estuaries (juvenile Sandbar sharks Carcharhinus plumbeus) was also included in this analysis for comparison. Temporal variability of resource-use characteristics was detected at the species level. Resource-use overlap among species varied by tissue type and was generally higher for blood plasma, suggesting greater resource sharing over more recent time periods. Over longer time periods Atlantic Sharpnose and Blacktip sharks exhibited resource-use expansion, whereas Blacknose sharks exhibited a narrowing in resource use, suggesting a more specialized foraging strategy compared to the other species. The resource-use breadth of Sandbar sharks also expanded between blood plasma and muscle tissue. Significant size relationships were detected in Blacktip and Sandbar sharks, indicating ontogenetic resource shifts for both species. A diversity of highly productive resource pools likely support shark populations along the First Coast such that resource-use differentiation is not required to facilitate species co-occurrence. This work may shed light on understanding patterns of species co-occurrence as well as aid in future conservation efforts.

Pakistan was listed as eighth globally in its landings of sharks and other elasmobranchs during the 1990s. Balochistan occupies over three-quarters of the coast of Pakistan yet the nature of the elasmobranch fishery there remains undocumented. Landings of elasmobranchs at landing sites were surveyed; the main species recorded were blacktip shark ( Carcharhinus limbatus ), bull shark ( C. leucas ) and spot-tail shark ( C. sorrah ). Altogether 25 shark species were identified, of which nine are regionally vulnerable, eight endangered, and one (the sand tiger shark, Carcharias taurus ) critically endangered. Of the thirteen other elasmobranchs recorded, five are regionally vulnerable, two are endangered and one (the sawfish, Pristis pristis ) critically endangered. Local fishers and processors were interviewed about their industry. Sharks were caught using both long-lines and nets, largely in May – July. The fishers retained some meat (for consumption) or liver (for the oil used for waterproofing boats), but did not process the sharks themselves, instead selling them to agents of companies that exported fins and other elasmobranch products. Results showed that recorded landings in both Balochistan and the neighbouring Sindh Province have declined to a tenth or less of peak catch. Meanwhile, the numbers of registered fishermen continued to increase, a persistent threat to elasmobranchs stocks. It is recommended that a realistic national plan of action and widespread public awareness programme, with support to fishers and processors would help to alleviate this critical situation.

The blacktip shark (Carcharhinus limbatus) is a placental shark that uses coastal nursery areas in the Galapagos Marine Reserve, where juveniles live during their early life stages. To better understand the feeding behavior of early juvenile blacktip sharks, we conducted stable isotope analysis on 57 samples from three coastal nurseries around the archipelago. We evaluated the intraspecific variation in trophic shifts of four size classes (

Multivariate and machine-learning methods were used to develop field identification techniques for two species of cryptic blacktip shark. From 112 specimens, precaudal vertebrae (PCV) counts and molecular analysis identified 95 Australian blacktip sharks Carcharhinus tilstoni and 17 common blacktip sharks Carcharhinus limbatus. Molecular analysis also revealed 27 of the 112 were C. tilstoni × C. limbatus hybrids, of which 23 had C. tilstoni PCV counts and four had C. limbatus PCV counts. In the absence of further information about hybrid phenotypes, hybrids were assigned as either C. limbatus or C. tilstoni based on PCV counts. Discriminant analysis achieved 80% successful identification, but machine-learning models were better, achieving 100% successful identification, using six key measurements (fork length, caudal-fin peduncle height, interdorsal space, second dorsal-fin height, pelvic-fin length and pelvic-fin midpoint to first dorsal-fin insertion). Furthermore, pelvic-fin markings could be used for identification: C. limbatus has a distinct black mark >3% of the total pelvic-fin area, while C. tilstoni has markings with diffuse edges, or has smaller or no markings. Machine learning and pelvic-fin marking identification methods were field tested achieving 87 and 90% successful identification, respectively. With further refinement, the techniques developed here will form an important part of a multi-faceted approach to identification of C. tilstoni and C. limbatus and have a clear management and conservation application to these commercially important sharks. The methods developed here are broadly applicable and can be used to resolve species identities in many fisheries where cryptic species exist.

Globally, many gillnet fisheries have excessive discards which can comprise charismatic megafauna threatened with extinction, including numerous elasmobranchs. Very few discarded elasmobranch species have been assessed for their associated fates. Here we describe the immediate mortality of several threatened species discarded from gillnets deployed off eastern Australia to target key carcharhinids and white sharks, Carcharodon carcharias (within an objective of protecting bathers) and causal factors for abundant elasmobranchs. In total, 420 animals comprising at least 22 species were gillnetted with a total immediate mortality of 49 % and group-specific estimates of 100 % for dolphins (n = 8), 100 % for teleosts (n = 16), 86 % for sharks (n = 75), 45 % for turtles (n = 20) and 36 % for rays (n = 301). Among elasmobranchs, species that were obligate ram-ventilating (e.g. great hammerhead, Sphyrna mokarran and common blacktip shark, Carcharhinus limbatus) had greater mortality (>95 %) than those species with spiracles, and likely capable of some stationary respiration (e.g. whitespotted guitar fish, Rhynchobatus australiae, whitespotted eagle ray, Aetobatus ocellatus, Australian cownose ray, Rhinoptera neglecta and to a lesser extent, pygmy devilray, Mobula kuhlii cf. eregoodootenkee) (16–74 % mortality). Mortalities among Australian cownose rays increased across longer soaks. The effect of soak time was further assessed to determine an optimal to maintain gillnet efficiency for target sharks, but minimise the absolute mortality of abundant rays and was estimated at up to three or four fishing nights (72–96 h). Other potential operational modifications, including using light and better retroactive deterrents to reduce turtle and dolphin bycatches and mortality, warrant investigation.

The increase in demand for shark meat and fins has placed shark populations worldwide under high fishing pressure. In the Arabian region, the spot‐tail shark Carcharhinus sorrah and the Blacktip shark Carcharhinus limbatus are among the most exploited species. In this study, we investigated the population genetic structure of C. sorrah (n = 327) along the coasts of the Arabian Peninsula and of C. limbatus (n = 525) along the Arabian coasts, Pakistan, and KwaZulu‐Natal, South Africa, using microsatellite markers (15 and 11 loci, respectively). Our findings support weak population structure in both species. Carcharhinus sorrah exhibited a fine structure, subdividing the area into three groups. The first group comprises all samples from Bahrain, the second from the UAE and Yemen, and the third from Oman. Similarly, C. limbatus exhibited population subdivision into three groups. The first group, comprising samples from Bahrain and Kuwait, was highly differentiated from the second and third groups, comprising samples from Oman, Pakistan, the UAE, and Yemen; and South Africa and the Saudi Arabian Red Sea, respectively. Population divisions were supported by pairwise FST values and discriminant analysis of principal components (DAPC), but not by STRUCTURE. We suggest that the mostly low but significant pairwise FST values in our study are suggestive of fine population structure, which is possibly attributable to behavioral traits such as residency in C. sorrah and site fidelity and philopatry in C. limbatus. However, for all samples obtained from the northern parts of the Gulf (Bahrain and/or Kuwait) in both species, the higher but significant pairwise FST values could possibly be a result of founder effects during the Tethys Sea closure. Based on DAPC and FST results, we suggest each population to be treated as independent management unit, as conservation concerns emerge.

The present work represents a comprehensive study of in utero maternal transfer of legacy and emerging flame retardants (FRs) in marine predators. We analyzed liver tissues from pregnant sharks of five viviparous species, including blacknose shark ( Carcharhinus acronotus; n = 12), blacktip shark ( Carcharhinus limbatus; n = 2), bonnethead ( Sphyrna tiburo; n = 2), Atlantic sharpnose shark ( Rhizoprionodon terraenovae; n = 2), and spinner shark ( Carcharhinus brevipinna; n = 2), as well as their embryos ( n = 84 in total from five species), collected from the western North Atlantic Ocean. Concentrations of frequently detected emerging FRs in adult female blacknose sharks were determined to be 6.1-83.3 ng/g lipid weight (lw) for dechlorane analogues, 2.5-29.8 ng/g lw for tetrabromo- o-chlorotoluene, and nondetection -32.6 ng/g lw for hexabromobenzene. These concentrations were 1-2 orders of magnitude lower than those of legacy polybrominated diphenyl ether flame retardants (85.7-398 ng/g lw). Similar contamination profiles were also found in the other four species, although FR concentrations varied in different species. A total of 21 FRs were commonly found in developing embryos of female sharks from five species, demonstrating maternal transfer in utero. The maternal transfer ratio (i.e., ratio of the mean litter concentration to their mother's concentration) determined in blacknose shark mother/embryo groups for each FR chemical was negatively associated with its octanol-water partition coefficient. Our work lays a solid foundation for future investigation of the underlying mechanisms of in utero transfer and additional physical or chemical factors that affect maternal transfer.

A longline survey was conducted from 2004 to 2014 to investigate the demographic population structure and seasonal abundance of the blacktip shark Carcharhinus limbatus in the Bimini Islands, the Bahamas. All individuals sampled (n = 242) were sub-adult or adults [70·1-145·1 cm pre-caudal length (LPC) range] with no neonates or YOY recorded in Bimini. Carcharhinus limbatus abundance peaked in September, coincident with the largest ratio of female to male sharks and a peak in fresh mating wounds on females. Mitochondrial control region (mtCR) DNA sequences were obtained from C. limbatus at Bimini to test whether Bimini C. limbatus are most closely related to geographically proximate populations sampled on the south-eastern coast of the U.S.A., the closest known nursery areas for this species. Nine mtCR haplotypes were observed in 32 individuals sampled at Bimini [haplotype diversity (h) = 0·821, nucleotide diversity (π) = 0·0015]. Four haplotypes observed from Bimini matched those previously found in the northern Yucatan (Mexico)-Belize and two matched a haplotype previously found in the U.S.A. Four haplotypes were novel but were closely related to the northern Yucatan-Belizean haplotypes. Pair-wise ΦST analysis showed that Bimini was significantly differentiated from all of the populations previously sampled (U.S.A. Atlantic, U.S.A. Gulf of Mexico, northern Yucatan, Belize and Brazil). This indicates that C. limbatus sampled from Bimini are unlikely from the described, proximate U.S.A. nurseries.

Age and growth estimates from length‑at‑age data were produced for the common blacktip shark Carcharhinus limbatus from Indonesia. Back‑calculation techniques were used due to a low sample size (n = 30), which was dominated by large, mature sharks. A multi‑model approach incorporating Akaike's information criterion with a bias correction (AICc) was used to estimate growth rates, with the von Bertalanffy growth function (VBGF) providing the best fit for the separate sexes using the back‑calculation data. These age and growth estimates were then compared to those of other populations of C. limbatus from the USA and South Africa using a combination of VBGF parameters and instantaneous (empirical) growth rates at birth (dL/dt0) and maturity (dL/dtmat). In comparison with populations from the USA, C. limbatus from Indonesia grow substantially larger and are more similar to South African populations. Differences in empirical growth rates were also determined between the populations, although this was not detected by the VBGF parameter k. This occurred because the parameter k is a measure of the rate at which a population reaches asymptotic length (L∞) and is not a measure of growth. This study demonstrated that the use of dL/dt0 and dL/dtmat to measure and compare empirical growth rates can be a useful addition to life‑history studies.