Shark Carcharhinus Research Articles

Length–weight relationships of three species of pelagic sharks from southeastern Arabian Sea

Length–weightrelationships (LWRs) of three species of pelagic sharks belonging to the family Carcharhinidae viz., the silky shark Carcharhinus falciformis and the graceful shark C. amblyrhynchoides and Alopidae viz, the pelagic thresher Alopias pelagicus, were estimated based on 525 samples collected from the longline and gillnet fishery of southeastern Arabian Sea from January 2016 to November 2018. Two major fish landing centres of Kerala, southwest coast of India were selected for sampling; Cochin (Lat. 09∘56′327′′N, Long. 76∘15′764′′E) and Munambam (Lat. 10∘10′965′′N, Long. 76∘10′258′′E). The estimated allometric coefficient b value ranged from 2.687 (A. pelagicus, N = 122) to 3.11 (C. falciformis, N = 295). Coefficient of determination scores ranged from 0.901 (A. pelagicus) to 0.984 (C. amblyrhynchoides, N = 108), indicating robustness of the samples analysed. Maximum total length presented for C. amblyrhynchoides in this study was a new record. The study reports first estimates of length–weight relationships for three important species of pelagic sharks which contribute to most of the shark catches in the region, and the generated parameters will be useful for the assessment of stocks and sustainable management of pelagic sharks in southeastern Arabian Sea.

Multi-tissue stable isotope analysis reveals resource partitioning and trophic relationships of large reef-associated predators

Defining the role of reef predators is particularly important given the rapid rate at which some species are declining, yet knowledge of trophic relationships is often lacking, particularly for large wide-ranging species that may use coral reefs seasonally or opportunistically. We used a multi-tissue stable isotope approach to investigate the trophic ecology of common reef predators in the central Great Barrier Reef of Australia. Our study revealed significant trophic separation among reef predators, especially when considering isotopic data from muscle, a slower turnover tissue. Based on muscle data, the bull shark Carcharhinus leucas, a large wide-ranging coastal species, occupied a higher trophic position and had a larger isotopic niche breadth (19.12) relative to smaller predators, including resident sharks (4.5 ± 0.72) and large-bodied teleosts (4.4 ± 1.82). Spanish mackerel Scomberomorus commerson and bull sharks had the largest unique total areas of isotopic niche space (expressed as a percentage) that did not overlap with any other species, 95% and 69.4%, respectively, which means they had lower isotopic overlap. In general, faster turnover tissues such as whole blood and plasma showed higher isotopic overlap and smaller niche breadth for all reef predators. These results suggest that bull sharks use similar prey resources to large and small resident predators, at least during short periods. Our findings highlight the importance of investigating dietary changes in faster-turnover tissues of reef predators, particularly large wide-ranging species, which may have key roles in coral reef food webs through direct predation and competition.

Survival of a grey reef shark Carcharhinus amblyrhynchos without a dorsal fin.

An adult, female grey reef shark Carcharhinus amblyrhnchos was observed missing its first dorsal fin in 2014. The same individual was re-photographed 4 years later indicating that this numerically dominant reef shark can survive total loss of its first dorsal fin. While this disability may impair the shark's ability to undertake pursuit predation, the species has a diversity of foraging modes that probably facilitates survival.

Spatial distribution, seasonal abundance and exploitation status of shark species in Kenyan coastal waters

Efforts to conserve and manage shark populations are often hampered by a lack of basic data, such as species-specific landings and distribution ranges. We bridge this gap in coastal East Africa by providing data on the distributions, catch rates, morphometrics, and exploitation status of shark species in Kenyan coastal waters. Data were collected from artisanal fishers and from bycatch taken by shallow-water (10–50 m) prawn trawlers from Malindi-Ungwana Bay and demersal research trawlers (10–150-m depth) along the ∼640-km coastline, over a 12-month period (June 2012 to May 2013). A total of 1 893 individual sharks (representing 20 species and 11 families) were sampled from the artisanal fishery (n = 1 610) and the trawlers (n = 283). The demersal trawl bycatches were dominated by the African angelshark Squatina africana (2.39 kg h–1), shortnose spurdog shark Squalus megalops (1.48 kg h–1) and African spotted catshark Holohalaelurus punctatus (0.11 kg h–1). Catches of the scalloped hammerhead shark Sphyrna lewini (0.73 kg h–1), smooth hammerhead shark Sphyrna zygaena (0.60 kg h–1) and grey reef shark Carcharhinus amblyrhynchos (0.77 kg h–1) dominated in the prawn trawlers. Only a few species (S. lewini, C. amblyrhynchos, and blacktip reef shark Carcharhinus melanopterus) showed a coast-wide distribution in the artisanal fishery. Artisanal fishers harvested mostly immature specimens of S. lewini, C. melanopterus and blacktip shark Carcharhinus limbatus, suggesting that the fishery might be unsustainable in the long-term. The Endangered S. lewini is the most vulnerable to overexploitation on the Kenyan coast, with most specimens landed (>90%) being below the size at maturity. Data are also presented on morphometric relationships and observed or estimated exploitation reference points (maximum observed length Lmax, asymptotic length L∞, mean length at first maturity Lm, and optimum length Lopt) for the commonly landed species. A more comprehensive coast-wide National Plan of Action is recommended for the management of shark populations in Kenya.

Intraspecific differences in relative isotopic niche area and overlap of co-occurring sharks

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.

New host and distribution record of Pontobdella macrothela (Schmarda, 1861) (Annelida, Hirudinea) from a Grey Reef Shark, Carcharhinus amblyrhynchos (Bleeker, 1856), in Mozambique, Western Indian Ocean

This report describes the first record of the piscicolid leech Pontobdella macrothela from Mozambique waters and the first record of P. macrothela parasitizing a Grey Reef Shark (Carcharhinus amblyrhynchos). The leech, P. macrothela, was found attached externally between the third and fourth gill slits of a shark captured off Vamizi Island, Quirimbas Archipelago, northern Mozambique.

Trophic niches determined from fatty acid profiles of sympatric coral reef mesopredators

Previous research has identified similar trophic levels in a wide range of coral reef sharks and large teleost fishes but has been unable to resolve the extent of dietary overlap and resource sharing that lead to interpretation of functional roles and, hence, adequately describe interaction strengths in food webs. We used fatty acid (FA) profiles of muscle and plasma reflecting the diet of several mesopredators to better understand the trophic structure and functioning of Great Barrier Reef ecosystems. Multivariate analysis identified significant dietary overlap between 2 shark species (whitetip reef shark Triaenodon obesus and grey reef shark Carcharhinus amblyrhynchos), but whitetip reef sharks occupied a smaller niche area than grey reef sharks. We also found clear niche separation between sharks and teleost fishes (red throat emperor, coral trout, and grass emperor). All mesopredators sampled had high proportions of polyunsaturated FAs linking them to carnivory, but differences among species in trophic biomarker ratios suggest distinctive dietary sources. However, based on a degree of niche overlap of species sharing common diet, reef-dwelling teleost fishes such as coral trout also derive nutrients from pelagic environments. A faster analytical method to determine FA profiles, proven highly useful in tuna, was tested and deemed viable for muscle tissue of tropical teleost fishes and sharks, while questions remain for use of FA extraction of plasma in ecological studies. These results demonstrate that fine-scale inter-specific differences in diet exist between mesopredators in coral reef ecosystems. Our results raise important questions about the ecological influences of these species and energy flow among mesopredators in coral reef ecosystems.

Pola Pertumbuhan dan Faktor Kondisi Hiu Merak Bulu Carcharhinus brevipinna di Perairan Selatan Nusa Tenggara

<strong>Growth Pattern and Condition Factor of Spinner Shark Carcharhinus brevipinna in Southern Nusa Tenggara Waters.</strong> The spinner sharks are one of shark commodities that are utilized optimally from the southern of Nusa Tenggara waters, however, the scientific information is still limited. This study aimed to assess the growth pattern and condition factors of the spinner shark, <em>Carcharhinus brevipinna</em> in southern Nusa Tenggara waters. The research was conducted from January to December 2016 using direct recording of the catch data. Information on length, weight and sex determinations were collected by the enumerator. The data was analysed descriptively with the shark condition was determined based on the relative condition factor obtaining from the length-weight relationship. The results showed that growth patterns of the spinner shark, both males and females, were allometrically negative. The relative condition factors of the catch of male sharks were ranged from 0.206 – 2.225 and females from 0.237 – 3.361. The condition factors of spinner sharks were not different between sexes and time of fishing, but they were different in length classes.

Tubbataha Reefs Natural Park: the first comprehensive elasmobranch assessment reveals global hotspot for reef sharks

Elasmobranchs are experiencing population declines worldwide because of anthropogenic stressors. Marine protected areas (MPAs) have been shown to benefit mesopredatory species of shark. The Tubbataha Reefs Natural Park (TRNP) is a United Nations Educational, Scientific and Cultural Organization World Heritage Site and the largest no-take MPA in the Philippines, yet information on the current status of elasmobranchs in the park is limited. Here, we investigate the abundance and biodiversity of elasmobranchs in TRNP using underwater visual census (UVC) and baited remote underwater video (BRUV) surveys. Twenty UVC were conducted in May 2015 (n = 10) and June 2016 (n = 10), documenting eight species of elasmobranchs with an average density of 7.18 sharks ha-1 for both years. A total of 113 successful BRUV surveys were conducted in TRNP between March–June 2015 (n = 47) and April–June 2016 (n = 66). The BRUV surveys recorded 14 species with an average catch per unit effort of 2.09 elasmobranchs per hour for both years. Both survey methods report some of the highest densities of the grey reef shark Carcharhinus amblyrhynchos and the whitetip reef shark Triaenodon obesus documented worldwide. This study underlines the importance of large, well-managed MPAs such as TRNP for the conservation of reef-associated elasmobranchs.

New polymorphic microsatellite loci revealed for the dusky shark Carcharhinus obscurus through Ion Proton double-digest RAD sequencing.

The non-model shark species, dusky shark Carcharhinus obscurus, is a bio-economically and recreationally important shark in many areas of its range. Despite of the fishery importance of C. obscurus few genetic resources are currently available for the species. Here, we report on the isolation of eight novel microsatellite loci from C. obscurus using a double-digest restriction site associated DNA (RAD) sequencing approach on the Ion Proton semiconductor platform (ddRADseq-ion). We characterised the loci in 26 individuals and all loci were polymorphic, exhibiting 5-10 alleles (average 6.6), and observed and expected heterozygosities of 0.385-0.962 and 0.479-0.847, respectively. We found that all pairs of loci were in linkage equilibrium and conformed to Hardy-Weinberg expectations. The loci reported in this study are only the second set of microsatellite loci ever characterized for C. obscurus and will be valuable for molecular ecology studies for this vulnerable species.

Life after death: behaviour of multiple shark species scavenging a whale carcass

A drone was used to study the richness and behaviour of scavengers attracted to the carcass of a sperm whale (Physeter microcephalus) in Seychelles, Western Indian Ocean. Over 30 sharks of 3 species (tiger shark Galeocerdo cuvier, bull shark Carcharhinus leucas and tawny nurse shark Nebrius ferrugineus) were recorded scavenging on the carcass. Tiger sharks dominated the scavenging, but with no evidence of agonistic behaviours or hierarchy, and may have facilitated scavenging by other species. Drone use allowed the analysis of such behaviours without observer influence, providing new insights into communal scavenging events.

Weak population structure of the Spot-tail shark Carcharhinus sorrah and the Blacktip shark C.limbatus along the coasts of the Arabian Peninsula, Pakistan, and South Africa.

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.

Metabolic rates and the energetic cost of external tag attachment in juvenile blacktip sharks Carcharhinus limbatus.

This study reports on the metabolic rate of the blacktip shark Carcharhinus limbatus and the energetic costs of external tag attachment. Metabolic rates, swimming speed and tail-beat (BT ) frequency were measured in a static respirometer with untagged animals and animals equipped with a small data logger. Tagged sharks showed significantly higher routine oxygen consumption and lower swimming speeds than untagged animals, indicating that tagging significantly affected the swimming efficiency and energetic requirements in these small sharks, and that these effects must be accounted for when interpreting telemetry data from free-ranging individuals.

Alligator Gars in Texas Coastal Bays: Long‐Term Trends and Environmental Influences

AbstractRecognition of the recreational and conservation value of the Alligator Gar Atractosteus spatula has spurred efforts to understand its biology, ecology, and status in river–reservoir systems throughout their range. However, Alligator Gars have also been found in the coastal bays that link these rivers to the Gulf of Mexico. Little is known about the species in saltwater environments or their potential role in coastal fish communities. For that reason, we analyzed 37 years (1975–2012) of standardized gill‐net catches of Alligator Gars in 10 Texas bays to characterize spatiotemporal trends in abundance and size structure and identify influential environmental covariates. Over 28,000 Alligator Gars (113–2,040 mm TL) were collected during 24,756 net‐nights of effort. The species was generally omnipresent both spatially and temporally; however, catch per unit effort varied by two orders of magnitude and was both cyclical and synchronous among bays. Trends in catch and size structure were consistent in most bays, which is indicative of stable abundance and dynamic rates. Current salinity and freshwater inflows, as well as catch rates, salinity, and freshwater inflows during the previous sampling year, explained significant proportions of the variation in catch in all but one system. Generally, periods of increased catch were associated with lower salinity and higher freshwater inflow, though Alligator Gars continued to be collected at salinities over 35‰. Analyses revealed that Alligator Gar catches in Texas bays were similar to or considerably greater than those of Bull Sharks Carcharhinus leucas, which are considered the most abundant top predator in these systems. The broad distribution of the Alligator Gars along the coast, coupled with its high relative abundance, highlight the need to understand the species’ role in coastal food webs and ecosystem function, with the ultimate goal of improving our ability to manage both Alligator Gar populations and coastal communities.

Environmental and anthropogenic factors affecting the increasing occurrence of shark-human interactions around a fast-developing Indian Ocean island

Understanding the environmental drivers of interactions between predators and humans is critical for public safety and management purposes. In the marine environment, this issue is exemplified by shark-human interactions. The annual shark bite incidence rate (SBIR) in La Réunion (Indian Ocean) is among the highest in the world (up to 1 event per 24,000 hours of surfing) and has experienced a 23-fold increase over the 2005–2016 period. Since 1988, 86% of shark bite events on surfers involved ocean-users off the leeward coast, where 96% of surfing activities took place. We modeled the SBIR as a function of environmental variables, including benthic substrate, sea temperature and period of day. The SBIR peaked in winter, during the afternoon and dramatically increased on coral substrate since the mid-2000s. Seasonal patterns of increasing SBIR followed similar fluctuations of large coastal shark occurrences (particularly the bull shark Carcharhinus leucas), consistent with the hypothesis that higher shark presence may result in an increasing likelihood of shark bite events. Potential contributing factors and adaptation of ocean-users to the increasing shark bite hazard are discussed. This interdisciplinary research contributes to a better understanding of shark-human interactions. The modeling method is relevant for wildlife hazard management in general.

Life history characteristics of the silky shark Carcharhinus falciformis from the central west Pacific

In the central west Pacific region, silky sharks (Carcharhinus falciformis) are commonly taken in fisheries, forming up to 95% of incidental elasmobranch bycatch. The present study examined the life history of silky sharks (n=553) from Papua New Guinean waters. Age was analysed using sectioned vertebrae, and a multimodel approach was applied to the length-at-age data to fit growth models. Females ranged in length from 65.0- to 253.0-cm total length (TL), with the oldest estimated at 28 years. Males ranged in length from 68.4 to 271.3cm TL and were aged to a maximum of 23 years. The logistic model provided the best fitting growth parameter estimates of length at birth L0=82.7cm TL, growth coefficient g=0.14year–1 and asymptotic length L∞=261.3cm TL for the sexes combined. Females reached sexual maturity at 204cm TL and 14.0 years, whereas males reached maturity at 183cm TL and 11.6 years. The average litter size from 28 pregnant females was 8 (range of 3–13). The growth parameters and late ages of sexual maturation for silky sharks in the central west Pacific suggest a significant risk from fisheries exploitation without careful population management.

Analysis of bycatch in the South African midwater trawl fishery for horse mackerel Trachurus capensis based on observer data

The South African midwater trawl fishery targets adult horse mackerel Trachurus capensis. The bulk of the catch is taken by a single freezer-trawler, the biggest fishing vessel operating in South African waters. As fishing takes place off the south coast in ecologically sensitive areas, there are concerns about the potential impacts of this fishing operation on non-target species. Fishing behaviour and bycatch of this fishery from 2004 to 2014 were investigated by analysing observer records with regard to catch composition, volume and temporal and spatial patterns. The midwater trawl fishery was estimated to have caught 25 415 tonnes annually, with a bycatch of 6.9% of the total catch, by weight. There are species overlaps with various fisheries, namely the demersal trawl, small-pelagic, line, shark longline and squid fisheries, yet the total bycatch estimates from this fishery are generally small relative to catches taken in the target fisheries. Bycatch species with the highest average annual catches were chub mackerel Scomber japonicus, redeye roundherring Etrumeus whiteheadi, ribbonfish Lepidopus caudatus and hake Merluccius spp. Large-fauna bycatch species included sunfish Mola mola as well as a number of CITES II- and IUCN-listed species, such as Cape fur seal Arctocephalus pusillus, dusky shark Carcharhinus obscurus, smooth hammerhead shark Sphyrna zygaena and thresher sharks Alopias spp. The 97.9% observer coverage is high and the 6.9% bycatch rate low compared to other South African fisheries; however, due to the large size of the individual hauls (average of 46.3 t), the average sampling rate of 1.56% is low. Our analyses suggest that bycatch in the South African midwater trawl fishery has been lower than in other South African fisheries and similar fisheries elsewhere, but due to the combination of high catch volumes and low sampling rates, estimation errors for rare species are high and there is a substantial risk of incidental unmonitored bycatch of rare large fauna and aggregations of small fauna. This could be mitigated by spatio-temporal management of this fishery, to avoid fishing in high-risk areas, and the introduction of an electronic monitoring programme.

A major range extension for Carcharhinus altimus (Springer, 1950) in the South Pacific Ocean

The occurrence of the bignose shark Carcharhinus altimus at Ducie Island, Pitcairn Islands is reported for the first time based upon re-identification of four museum voucher specimens. The presence of a pregnant female and a juvenile confirms C. altimus breeds at Ducie Island and should be considered resident there. The specimens had been misidentified as the grey reef shark, C. amblyrhynchos, emphasising the ongoing value of collecting museum voucher specimens of supposedly common species. The occurrence of C. altimus at Ducie Island suggests resident populations of C. altimus are likely to occur throughout the tropical and subtropical Pacific Ocean but may have been overlooked due to their preference for deep demersal habitats during the day, or misidentification as other Carcharhinus species.

Oxidative stress and RNA DNA ratio following longline capture in the silky shark Carchahrinus falciformis (Muller and Henle, 1839)

This study used transcriptomics tracers and measurements of oxidative stress to examine the effect of capture by longline on the silky shark Carcharhinus falciformis . From February 2010 to May 2012, C. falciformis individuals captured in the eastern Pacific Ocean were initially examined for sex, degree of maturity, and morphometric data. Samples were then taken from muscle tissue to determine RNA, DNA, and protein concentrations as well as RNA/DNA and protein/DNA ratios. The levels of three oxidative stress indicators [superoxide dismutase (SOD), glutathione (GSH), and malondialdehyde (MDA)] were determined in muscle. The analysis of the 159 specimens (93 juveniles and 66 adults) showed low RNA/DNA ratios at the different stages of maturity. This basal level of transcriptomics capacity may have been caused by the physiological stress resulting from capture. In juveniles, the low RNA/DNA and protein/DNA ratios were associated with low levels of cellular damage related to oxidative stress, whereas in adults the level of cell damage due to oxidative stress was high, especially in older females. This pattern indicated decreased antioxidant response capacity with increasing age in elasmobranchs

Development of Optimized Edible Packaging Based on White-cheek Shark (Carcharhinus dussumieri) Skin Gelatin Biopolymer: Mechanical, Water Vapor Permeability, and Structural Properties

ABSTRACTWhite-cheek shark skin gelatin (WSG) edible films were optimized with respect to their barrier, mechanical, and structural properties using response surface methodology (RSM), with the introduction of gelatin concentrations (1–4% w/v based on film forming solution) and glycerol contents (15–35% w/w based on gelatin weight) as the independent variables. WSG films with moderate levels of glycerol had higher tensile strength (TS), elastic modulus (EM), and elongation at break (E@b) depending more significantly on gelatin content. Adding higher levels of glycerol, film roughness was observed to decrease exponentially but water vapor permeability (WVP) increased linearly. Structural analysis using Fourier transform infrared spectroscopy (FTIR) showed more aggregated protein structures at higher gelatin content, which could be strongly related to the notable increase of contact angle. According to the model, the optimum conditions for the WSG film formula were found to be 3.3% WSG and 19.3% glycerol, giving a predicted maximum E@b, TS, EM, roughness, and contact angle of 183%, 4.4 MPa, 92 MPa, 4.93 nm, and 101°, respectively, with a predicted minimum WVP of 11.8 g.mm/day.m2.kPa.