Assessment of multiple paternity in single litters from three species of carcharhinid sharks in Hawaii

We tested for presence or absence of multiple paternity in single litters from each of three congeneric shark species in Hawaii: the sandbar shark, Carcharhinus plumbeus, bignose shark, Carcharhinus altimus, and Galapagos shark, Carcharhinus galapagensis. Based on eight polymorphic microsatellite loci, we excluded paternity by a single sire in sandbar and bignose sharks, but could not exclude a single sire for the litter from the Galapagos shark. This study doubles the number of shark species tested for multiple paternity, and is the first demonstration of multiple paternity in sandbar and bignose sharks.     

Utility of mesohabitat features for determining habitat associations of subadult sharks in Georgia’s estuaries

We examined the affects of selected water quality variables on the presence of subadult sharks in six of nine Georgia estuaries. During 231 longline sets, we captured 415 individuals representing nine species. Atlantic sharpnose shark (Rhizoprionodon terranovae), bonnethead (Sphyrna tiburo), blacktip shark (Carcharhinus limbatus) and sandbar shark (C. plumbeus) comprised 96.1% of the catch. Canonical correlation analysis (CCA) was used to assess environmental influences on the assemblage of the four common species. Results of the CCA indicated Bonnethead Shark and Sandbar Shark were correlated with each other and with a subset of environmental variables. When the species occurred singly, depth was the defining environmental variable; whereas, when the two co-occurred, dissolved oxygen and salinity were the defining variables. Discriminant analyses (DA) were used to assess environmental influences on individual species. Results of the discriminant analyses supported the general CCA findings that the presence of bonnethead and sandbar shark were the only two species that correlated with environmental variables. In addition to depth and dissolved oxygen, turbidity influenced the presence of sandbar shark. The presence of bonnethead shark was influenced primarily by salinity and turbidity. Significant relationships existed for both the CCA and DA analyses; however, environmental variables accounted for <16% of the total variation in each. Compared to the environmental variables we measured, macrohabitat features (e.g., substrate type), prey availability, and susceptibility to predation may have stronger influences on the presence and distribution of subadult shark species among sites.     

Structure and Dynamics of the Shark Assemblage off Recife,Northeastern Brazil

Understanding the ecological factors that regulate elasmobranch abundance in nearshore waters is essential to effectively manage coastal ecosystems and promote conservation. However, little is known about elasmobranch populations in the western South Atlantic Ocean. An 8-year, standardized longline and drumline survey conducted in nearshore waters off Recife, northeastern Brazil, allowed us to describe the shark assemblage and to monitor abundance dynamics using zero-inflated generalized additive models. This region is mostly used by several carcharhinids and one ginglymostomid, but sphyrnids are also present. Blacknose sharks, Carcharhinus acronotus, were mostly mature individuals and declined in abundance throughout the survey, contrasting with nurse sharks, Ginglymostoma cirratum, which proliferated possibly due to this species being prohibited from all harvest since 2004 in this region. Tiger sharks, Galeocerdo cuvier, were mostly juveniles smaller than 200 cm and seem to use nearshore waters off Recife between January and September. No long-term trend in tiger shark abundance was discernible. Spatial distribution was similar in true coastal species (i.e. blacknose and nurse sharks) whereas tiger sharks were most abundant at the middle continental shelf. The sea surface temperature, tidal amplitude, wind direction, water turbidity, and pluviosity were all selected to predict shark abundance off Recife. Interspecific variability in abundance dynamics across spatiotemporal and environmental gradients suggest that the ecological processes regulating shark abundance are generally independent between species, which could add complexity to multi-species fisheries management frameworks. Yet, further research is warranted to ascertain trends at population levels in the South Atlantic Ocean.     

Histological and morphological aspects of reproduction in the sandbar shark Carcharhinus plumbeus in the U.S. south‐eastern Atlantic Ocean and Gulf of Mexico

The reproduction of the sandbar shark Carcharhinus plumbeus in the U.S. south‐eastern Atlantic Ocean including the Gulf of Mexico was examined using a combination of histological and morphological characteristics of C. plumbeus collected through fishery‐dependent and ‐independent sampling programmes (n = 1,567). Indices of maturity were constructed using measurements of gonads, reproductive tracts and claspers, and sandbar sharks exhibited 50% maturity sizes of 140 and 148 cm fork length for males and females respectively. Gonado‐somatic indices and variation in reproductive tract condition were used to determine seasonal trends in reproduction of mature C. plumbeus. Sandbar sharks have discrete seasonal reproductive cycles in which males produce sperm from January to May with a peak in May and females develop eggs from January to May with ovulation occurring in June. Females were shown to exhibit a >2 year reproductive cycle. Embryonic development was assessed through measurements of masses and lengths of uterine contents. Gestation was 12 months, from July to the following June, with parturition in late June. This research highlights a difference from previously reported data on the periodicity of female reproduction in C. plumbeus in the U.S. south‐eastern Atlantic Ocean and Gulf of Mexico, which may have major effects on future C. plumbeus stock management.     

Evidence for dynamic resource partitioning between two sympatric reef shark species within the British Indian Ocean Territory

Stable-isotope analyses (δ¹³C, δ¹⁵N and δ³⁴S) of multiple tissues (fin, muscle, red blood cells and plasma), revealed ontogenetic shifts in resource use by grey reef sharks Carcharhinus amblyrhynchos and resource partitioning with silvertip sharks Carcharhinus albimarginatus within the British Indian Ocean Territory marine protected area (MPA). Resource partitioning varied temporally, with C. albimarginatus feeding on more pelagic prey during October to January, potentially attributable to an influx of pelagic prey from outside the MPA at that time. Reef sharks may therefore be affected by processes outside an MPA, even if the sharks do not leave the MPA.     

Trophic ecology of juvenile bull sharks (Carcharhinus leucas) in the Coyote estuary,Costa Rica

Bull shark (Carcharhinus leucas) is a near-threatened elasmobranch species capable of moving between the fresh and salty waters of tropical and subtropical coastal areas, for which we still lack important ecological information. During their first years of life, bull sharks use estuarine systems as nursery areas, making them highly susceptible to environmental and anthropogenic pressures. We studied the trophic ecology of juveniles found in the Coyote estuary, a potential nursery area in Costa Rica, to understand the potential impact of further bull shark declines and gain knowledge that could aid in their conservation. We analysed the trophic ecology of juvenile bull sharks [81–103 cm total length (TL)] in the Coyote estuary, Costa Rica, using stable isotopes of δ¹⁵N and δ¹³C. Since one problem using this technique in juveniles is the confounding effect of the maternal signature, we sampled different tissues (muscle and plasma), verified the status of the shark's umbilical scar and identified the size at which the isotope signature is a result of the animal's current diet. The isotopic values of the muscle tissue reflected the maternal isotopic signature. In contrast, plasma values reflected the diet of juvenile bull sharks >95 cm TL and with a closed umbilical scar. Juvenile bull sharks fed primarily on teleost fishes of the order Anguilliformes and Siluriformes, and have a high trophic position (≥4.0) in the Coyote estuary. Our findings suggest that this estuary is an important feeding site for juvenile bull sharks of the Pacific of Costa Rica. Thus, the protection of essential habitats such as the Coyote estuary will benefit not only bull shark conservation, but also the conservation of an array of fish species that also use this habitat as a rookery, many of which are of commercial interest.     

Ontogenetic changes in the diet of the sandbar shark, <Emphasis Type="Italic">Carcharhinus plumbeus</Emphasis>, in lower Chesapeake Bay and Virginia (USA) coastal waters

This study describes the diet of the sandbar shark, Carcharhinus plumbeus, highlighting differences in diet within various regions of the Virginia (USA) nursery area, as well as ontogenetic changes in diet. Stomach samples were obtained in 2001 and 2002 from 232 sharks caught by gillnets or longlines. Historical data from the Virginia Institute of Marine Science (VIMS) Shark Ecology Program were also analyzed. Ontogenetic changes in diet were evident, with crustacean prey decreasing in frequency with increasing shark size, and elasmobranch prey importance increasing with increasing shark size. Whereas previous research in Chincoteague Bay, VA showed the blue crab, Callinectes sapidus, was the dominant crustacean in sandbar shark diet, the mantis shrimp, Squilla empusa, dominated the crustacean portion of the diet in this study. Differences in diet of sharks were observed among locations within the study area. Small juveniles (≤80 cm precaudal length) in the lower Chesapeake Bay ate more fishes, whereas Eastern Shore juveniles ate more crustaceans. Crustacean prey items varied among locations along the Eastern Shore, with more portunid crabs consumed in waters near Wachapreague and more mantis shrimp consumed near Sand Shoal Inlet. Our study showed that Carcharhinus plumbeus is a generalist predator and is thus unlikely to strongly impact the population of any particular prey species, and in turn is not likely to be strongly affected by fluctuations in abundance of a single prey species.     

Size,sex and seasonal patterns in the assemblage of Carcharhiniformes in a sub‐tropical bay

Size, sex and seasonal patterns among Carcharhiniformes were examined in shallow regions of Moreton Bay, Queensland, Australia. A total of 1259 sharks were caught, comprising 13 species. The Australian sharpnose shark Rhizoprionodon taylori and the blacktip complex Carcharhinus limbatus–Carcharhinus tilstoni comprised 55% of all shark individuals. Neonates were observed for five species including the dusky shark Carcharhinus obscurus, which contrary to previous reports was relatively abundant in shallow, predominantly estuarine waters. Three contrasting patterns of occurrence were observed: smaller species were abundant and present throughout much of their ontogeny, larger species were mainly caught as neonates or juveniles and vagrant species were only caught during the warmer months. The shark assemblage differed significantly among seasons. While many species were observed during the warmer months, species diversity was lower in winter when C. obscurus comprised 43% of the catch. Overall, the results indicated that spatial and temporal distribution patterns were not synchronous for all species. The capture of small numbers of neonate C. obscurus in late autumn and winter demonstrates that parturition among Carcharhiniformes is not confined to spring and summer in sub‐tropical waters.     

Big catch,little sharks: Insight into Peruvian small‐scale longline fisheries

Shark take, driven by vast demand for meat and fins, is increasing. We set out to gain insights into the impact of small‐scale longline fisheries in Peru. Onboard observers were used to document catch from 145 longline fishing trips (1668 fishing days) originating from Ilo, southern Peru. Fishing effort is divided into two seasons: targeting dolphinfish (Coryphaena hippurus; December to February) and sharks (March to November). A total of 16,610 sharks were observed caught, with 11,166 identified to species level. Of these, 70.6% were blue sharks (Prionace glauca), 28.4% short‐fin mako sharks (Isurus oxyrinchus), and 1% were other species (including thresher (Alopias vulpinus), hammerhead (Sphyrna zygaena), porbeagle (Lamnus nasus), and other Carcharhinidae species (Carcharhinus brachyurus, Carcharhinus falciformis, Galeorhinus galeus). Mean ± SD catch per unit effort of 33.6 ± 10.9 sharks per 1000 hooks was calculated for the shark season and 1.9 ± 3.1 sharks per 1000 hooks were caught in the dolphinfish season. An average of 83.7% of sharks caught (74.7% blue sharks; 93.3% mako sharks) were deemed sexually immature and under the legal minimum landing size, which for species exhibiting k‐selected life history traits can result in susceptibility to over exploitation. As these growing fisheries operate along the entire Peruvian coast and may catch millions of sharks per annum, we conclude that their continued expansion, along with ineffective legislative approaches resulting in removal of immature individuals, has the potential to threaten the sustainability of the fishery, its target species, and ecosystem. There is a need for additional monitoring and research to inform novel management strategies for sharks while maintaining fisher livelihoods.     

Natural or Artificial? Habitat-Use by the Bull Shark,Carcharhinus leucas

Background

Despite accelerated global population declines due to targeted and illegal fishing pressure for many top-level shark species, the impacts of coastal habitat modification have been largely overlooked. We present the first direct comparison of the use of natural versus artificial habitats for the bull shark, Carcharhinus leucas, an IUCN ‘Near-threatened’ species - one of the few truly euryhaline sharks that utilises natural rivers and estuaries as nursery grounds before migrating offshore as adults. Understanding the value of alternate artificial coastal habitats to the lifecycle of the bull shark is crucial for determining the impact of coastal development on this threatened but potentially dangerous species.

Methodology/Findings

We used longline surveys and long-term passive acoustic tracking of neonate and juvenile bull sharks to determine the ontogenetic value of natural and artificial habitats to bull sharks associated with the Nerang River and adjoining canals on the Gold Coast, Australia. Long-term movements of tagged sharks suggested a preference for the natural river over artificial habitat (canals). Neonates and juveniles spent the majority of their time in the upper tidal reaches of the Nerang River and undertook excursions into adjoining canals. Larger bull sharks ranged further and frequented the canals closer to the river mouth.

Conclusions/Significance

Our work suggests with increased destruction of natural habitats, artificial coastal habitat may become increasingly important to large juvenile bull sharks with associated risk of attack on humans. In this system, neonate and juvenile bull sharks utilised the natural and artificial habitats, but the latter was not the preferred habitat of neonates. The upper reaches of tidal rivers, often under significant modification pressure, serve as nursery sites for neonates. Analogous studies are needed in similar systems elsewhere to assess the spatial and temporal generality of this research.     

Spatial separation without territoriality in shark communities

Spatial separation within predator communities can arise via territoriality but also from competitive interactions among and within species. However, linking competitive interactions to predator distribution patterns is difficult and theoretical models predict different habitat selection patterns dependent on habitat quality and how competition manifests itself. While models generally consider competitors to be either equal in ability, or for one phenotype to have a fixed advantage over the other, few studies consider that an animal may only have a competitive advantage in specific habitats. We used  10 years of telemetry data, habitat surveys and behavioral experiments, to show spatial partitioning between and within two species of reef shark (grey reef Carcharhinus amblyrhinchos and blacktip reef sharks C. melanopterus) at an unfished Pacific atoll. Within a species, sharks remained within small ‘sub‐habitats’ with very few movements of individuals between sub‐habitats, which previous models have suggested could be caused by intra‐specific competition. Blacktip reef sharks were more broadly distributed across habitat types but a greater proportion used lagoon and backreef habitats, while grey reef sharks preferred forereef habitats. Grey reef sharks at a nearby atoll where blacktip reef sharks are absent, were distributed more broadly between habitat types than when both species were present. A series of individual‐based models predict that habitat separation would only arise if there are competitive interactions between species that are habitat‐specific, with grey reefs having a competitive advantage on the forereefs and blacktips in the lagoons and backreef. We provide compelling evidence that competition helps drive distribution patterns and spatial separation of a marine predator community, and highlight that competitive advantages may not be constant but rather dependent on habitats.     

Escaped bait: Bull shark Carcharhinus leucas with an intentionally attached harness rig

A 193 cm total length female bull shark Carcharhinus leucas was captured in Florida bearing intentionally attached materials which resembled a harness. Harness-type live bait rigs are commonly used for small baitfish; some anglers use such devices with small sharks when targeting large sharks and bony fish. Biofouling on the apparatus and the extent of the injuries indicated the material had likely been on the shark for several years. This case highlights the dangers of using these types of devices on juveniles of long-lived species that attain a large body size.     

Population structure,connectivity, and demographic history of an apex marine predator,the bull shark Carcharhinus leucas

Knowledge of population structure, connectivity, and effective population size remains limited for many marine apex predators, including the bull shark Carcharhinus leucas. This large‐bodied coastal shark is distributed worldwide in warm temperate and tropical waters, and uses estuaries and rivers as nurseries. As an apex predator, the bull shark likely plays a vital ecological role within marine food webs, but is at risk due to inshore habitat degradation and various fishing pressures. We investigated the bull shark's global population structure and demographic history by analyzing the genetic diversity of 370 individuals from 11 different locations using 25 microsatellite loci and three mitochondrial genes (CR, nd4, and cytb). Both types of markers revealed clustering between sharks from the Western Atlantic and those from the Western Pacific and the Western Indian Ocean, with no contemporary gene flow. Microsatellite data suggested low differentiation between the Western Indian Ocean and the Western Pacific, but substantial differentiation was found using mitochondrial DNA. Integrating information from both types of markers and using Bayesian computation with a random forest procedure (ABC‐RF), this discordance was found to be due to a complete lack of contemporary gene flow. High genetic connectivity was found both within the Western Indian Ocean and within the Western Pacific. In conclusion, these results suggest important structuring of bull shark populations globally with important gene flow occurring along coastlines, highlighting the need for management and conservation plans on regional scales rather than oceanic basin scale.     

Shark attacks in Dakar and the Cap Vert Peninsula, Senegal: low incidence despite high occurrence of potentially dangerous species

Background

The International Shark Attack File mentions only four unprovoked shark attacks on the coast of West Africa during the period 1828–2004, an area where high concentrations of sharks and 17 species potentially dangerous to man have been observed. To investigate if the frequency of shark attacks could be really low and not just under-reported and whether there are potentially sharks that might attack in the area, a study was carried out in Dakar and the Cap Vert peninsula, Senegal.

Methodology/Principal Findings

Personnel of health facilities, administrative services, traditional authorities and groups of fishermen from the region of Dakar were interviewed about the occurrence of shark attacks, and visual censuses were conducted along the coastline to investigate shark communities associated with the coasts of Dakar and the Cap Vert peninsula. Six attacks were documented for the period 1947–2005, including two fatal ones attributed to the tiger shark Galeocerdo cuvieri. All attacks concerned fishermen and only one occurred after 1970. Sharks were observed year round along the coastline in waters 3–15 m depth. Two species potentially dangerous for man, the nurse shark Ginglymostoma cirratum and the blacktip shark Carcharhinus limbatus, represented together 94% of 1,071 sharks enumerated during 1,459 hours of observations. Threatening behaviour from sharks was noted in 12 encounters (1.1%), including 8 encounters with C. limbatus, one with Galeocerdo cuvieri and 3 with unidentified sharks.

Conclusions/Significance

These findings suggest that the frequency of shark attacks on the coast of West Africa is underestimated. However, they also indicate that the risk is very low despite the abundance of sharks. In Dakar area, most encounters along the coastline with potentially dangerous species do not result in an attack. Compared to other causes of water related deaths, the incidence of shark attack appears negligible, at least one thousand fold lower.     

Long-Term Changes in Species Composition and Relative Abundances of Sharks at a Provisioning Site

Diving with sharks, often in combination with food baiting/provisioning, has become an important product of today’s recreational dive industry. Whereas the effects baiting/provisioning has on the behaviour and abundance of individual shark species are starting to become known, there is an almost complete lack of equivalent data from multi-species shark diving sites. In this study, changes in species composition and relative abundances were determined at the Shark Reef Marine Reserve, a multi-species shark feeding site in Fiji. Using direct observation sampling methods, eight species of sharks (bull shark Carcharhinus leucas, grey reef shark Carcharhinus amblyrhynchos, whitetip reef shark Triaenodon obesus, blacktip reef shark Carcharhinus melanopterus, tawny nurse shark Nebrius ferrugineus, silvertip shark Carcharhinus albimarginatus, sicklefin lemon shark Negaprion acutidens, and tiger shark Galeocerdo cuvier) displayed inter-annual site fidelity between 2003 and 2012. Encounter rates and/or relative abundances of some species changed over time, overall resulting in more individuals (mostly C. leucas) of fewer species being encountered on average on shark feeding dives at the end of the study period. Differences in shark community composition between the years 2004–2006 and 2007–2012 were evident, mostly because N. ferrugineus, C. albimarginatus and N. acutidens were much more abundant in 2004–2006 and very rare in the period of 2007–2012. Two explanations are offered for the observed changes in relative abundances over time, namely inter-specific interactions and operator-specific feeding protocols. Both, possibly in combination, are suggested to be important determinants of species composition and encounter rates, and relative abundances at this shark provisioning site in Fiji. This study, which includes the most species from a spatially confined shark provisioning site to date, suggests that long-term provisioning may result in competitive exclusion among shark species.     

Possible range extension in the endangered and understudied winghead shark (Eusphyra blochii)

Here the authors report on a possible range extension in the rare and understudied winghead shark (Eusphyra blochii). A specimen was captured by recreational fishermen in Moreton Bay, Queensland, Australia, c. 800 km south of its current distribution. As winghead sharks show a clumped distribution in Australia associated with river outflow, Moreton Bay, with its large catchment area, may represent a suitable habitat for the species and previous occurrence may have gone undetected. Alternatively, climate change may have caused a recent southward shift in winghead sharks, as observed in other elasmobranch species along Australia's East Coast.     

Shark bite wounds of the valvular intestine: The cause of an acute mortality syndrome of captive blacktip reef sharks,Carcharhinus melanopterus

During 1987–1990 nine cases of mortality in captive blacktip reef sharks (Carcharhinus melanopterus) were attributed to bite wounds to the valvular intestine. Necropsies revealed lacerations, amputation of the scroll valve folds, or complete severance of the valvular intestine. Wounding resulting from attack by tankmates occurred during episodes of intestinal eversion, an apparently regular event of unclear physiological function in sharks with a scroll valve intestine. This paper adds intestinal biting as a putative cause of mortality in captive shark species having a scroll valve intestine.     

Rapid and Simultaneous Identification of Body Parts from the Morphologically Similar Sharks Carcharhinus obscurus and Carcharhinus plumbeus (Carcharhinidae) Using Multiplex PCR

Many commercially exploited carcharhinid sharks are difficult to identify to species owing to extensive morphological similarities. This problem is severely exacerbated when it comes to identifying detached shark fins, and the finless and headless shark carasses typically sold in markets. To assist in the acquisition of urgently needed conservation and management data on shark catch and trade, we have developed a highly streamlined approach based on multiplex polymerase chain reaction (PCR) that uses species-specific primers derived from nuclear ribosomal ITS2 sequences to achieve rapid species identification of shark body parts. Here we demonstrate the utility of this approach for identifying fins and flesh from two globally distributed, morphologically very similar carcharhinid sharks (Carcharhinus obscurus and Carcharhinus plumbeus) intensively targeted in fisheries worldwide, and often confused for each other even as whole animals. The assay is conducted in a 4-primer multiplex format that is structured to simultaneously achieve the following efficiency and cost-reduction objectives: it requires only a single-tube amplification reaction for species diagnosis, it incorporates an internal positive control to allow detection of false-negative results, and it is novel in that it allows species identification even when DNAs from two species are combined in the same tube during the PCR reaction. The latter innovation reduces the required effort for screening a set of unknown samples by 50%. The streamlined approach illustrated here should be amenable for use in a shark conservation and management context where large numbers of samples typically need to be screened; the approach shown may also provide a model for a rapid diagnostic method applicable to species identification in general. Received September 15, 2000; accepted December 15, 2000     

Detection of interspecies hybridisation in Chondrichthyes: hybrids and hybrid offspring between Australian (Carcharhinus tilstoni) and common (C. limbatus) blacktip shark found in an Australian fishery

Interspecies hybridisation in nature is a well-studied phenomenon, but it has not been analysed using genetic markers in the class Chondrichthyes (sharks, rays and chimeras). Two black-tip whaler shark species (Australian, Carcharhinus tilstoni; Common, C. limbatus) have overlapping distributions in Australia, distinct mitochondrial DNA sequence (ND4, COI, control region) and distinct morphological features such as length at sexual maturity, length at birth and number of vertebrae. A mismatch was observed between species identification using mtDNA sequence and species identification using morphological characters. To test whether hybridisation between the two species was responsible, a nuclear gene with species-specific mutations was sequenced. Extensive interspecies hybridisation was found to be occurring. Hybrids were found from five locations on the eastern Australian coastline, spanning 2,000 km. If hybrid fitness is low and hybrids are common, then fisheries recruitment may be overestimated and the productivity of the black-tip shark fishery may be well below that required to support commercial exploitation. To guard against identification errors, the likelihood of hybridisation and subsequent introgression should be assessed prior to using mtDNA (e.g. barcoding) to identify shark species. The C. limbatus–C. tilstoni species complex provides a unique opportunity to investigate the ability of sharks to adapt to environmental change, in particular, the impact of hybridization on species distributions which favour C. tilstoni along the north and C. limbatus along the south eastern Australian coastline.