Smooth Hammerhead Shark Sphyrna Zygaena Research Articles

Local adaptive variation in a highly migratory fish: The smooth hammerhead shark Sphyrna zygaena

Populations of highly mobile species that undertake long distance migrations are typically considered to be panmictic. Nonetheless, mechanisms related to behavior or local environmental conditions promote genetic isolation in the absence of physical barriers. Highly migratory shark species exhibit varying levels of fidelity to specific regions, shaping the genetic architecture of different populations and resulting in geographically based genetic variation with potential adaptive value. An understanding of the genetic variation of highly migratory species is needed to develop effective conservation strategies. This study aimed to assess the neutral and adaptive variation of the smooth hammerhead shark (Sphyrna zygaena) in the northern Mexican Pacific (NMP) via single nucleotide polymorphisms (SNPs). We analyzed 1480 SNPs in 92 individuals from four geographic regions in the NMP, of which 1469 SNPs were neutral loci (n-SNP), and 11 were putatively under selection (o-SNP) using four genoma scan methods. Genetic diversity was geographically similar among regions (Ho = 0.275). The neutral variation showed panmixia (n-SNPs; FST = 0.0012, p = 0.44), which may be associated with the high dispersal capacity of S. zygaena. A pattern of adaptive variation between individuals from the Gulf of California and Pacific coast was revealed using o-SNPs FST-based methods (24 oSNPs; FST = 0.061, p < 0.001), which may be promoted by individual preferences based on physiological limitations. The estimated effective population size (Ne) of S. zygaena was 1390 individuals, which is theoretically optimal for the population to persist over time.

Foraging depth depicts resource partitioning and contamination level in a pelagic shark assemblage: Insights from mercury stable isotopes

The decline of shark populations in the world ocean is affecting ecosystem structure and function in an unpredictable way and new ecological information is today needed to better understand the role of sharks in their habitats. In particular, the characterization of foraging patterns is crucial to understand and foresee the evolution of dynamics between sharks and their prey. Many shark species use the mesopelagic area as a major foraging ground but the degree to which different pelagic sharks rely on this habitat remains overlooked. In order to depict the vertical dimension of their trophic ecology, we used mercury stable isotopes in the muscle of three pelagic shark species (the blue shark Prionace glauca, the shortfin mako shark Isurus oxyrinchus and the smooth hammerhead shark Sphyrna zygaena) from the northeastern Pacific region. The Δ199Hg values, ranging from 1.40 to 2.13‰ in sharks, suggested a diet mostly based on mesopelagic prey in oceanic habitats. We additionally used carbon and nitrogen stable isotopes (δ13C, δ15N) alone or in combination with Δ199Hg values, to assess resource partitioning between the three shark species. Adding Δ199Hg resulted in a decrease in trophic overlap estimates compared to those based on δ13C/δ15N alone, demonstrating that multi-isotope modeling is needed for accurate trophic description of the three species. Mainly, it reveals that they forage at different average depths and that resource partitioning is mostly expressed through the vertical dimension within pelagic shark assemblages. Concomitantly, muscle total mercury concentration (THg) differed between species and increased with feeding depth. Overall, this study highlights the key role of the mesopelagic zone for shark species foraging among important depth gradients and reports new ecological information on trophic competition using mercury isotopes. It also suggests that foraging depth may play a pivotal role in the differences between muscle THg from co-occurring high trophic level shark species.

NEW RECORDS AND PRELIMINARY REPRODUCTIVE ASPECTS OF ELASMOBRANCHS OF THE TRINDADE-MARTIN VAZ INSULAR COMPLEX

Here we presented records of elasmobranchs obtained during a longline pelagic fishing monitoring around the Trindade-Martin Vaz insular complex, including one new record. New information on the reproductive aspects of the blue shark (Prionace glauca) and pelagic stingray (Pteroplatytrygon violacea) suggest the use of this area as mating and parturition ground for these species. The most abundant species was the P. glauca (N = 65), followed by shortfin mako shark Isurus oxyrinchus (N = 12). We also recorded for the first time the smooth hammerhead shark Sphyrna zygaena (N = 3) and the bigeye thresher shark Alopias superciliosus (N = 2), and other shark species previous reported for the insular complex, including the oceanic whitetip Carcharhinus longimanus (N = 2), and the tiger shark Galeocerdo cuvier (N = 1). Our findings highlight the importance of further studies at the Trindade-Martin Vaz insular complex considering the pelagic biodiversity and reproductive aspects of elasmobranchs.

Diet, trophic interactions and possible ecological role of commercial sharks and batoids in northern Peruvian waters.

The Peruvian sea represents one of the most productive ocean ecosystems and possesses one of the largest elasmobranch fisheries in the Pacific Ocean. Ecosystem-based management of these fisheries will require information on the trophic ecology of elasmobranchs. This study aimed to understand the diet, trophic interactions and the role of nine commercial elasmobranch species in northern Peru through the analysis of stomach contents. A total of 865 non-empty stomachs were analysed. Off northern Peru, elasmobranchs function as upper-trophic-level species consuming 78 prey items, predominantly teleosts and cephalopods. Two distinctive trophic assemblages were identified: (a) sharks (smooth hammerhead shark Sphyrna zygaena, thresher shark Alopias spp. and blue shark Prionace glauca) that feed mainly on cephalopods in the pelagic ecosystem; and (b) sharks and batoids (Chilean eagle ray Myliobatis chilensis, humpback smooth-hound Mustelus whitneyi, spotted houndshark Triakis maculata, Pacific guitarfish Pseudobatos planiceps, copper shark Carcharhinus brachyurus and school shark Galeorhinus galeus) that feed mainly on teleosts and invertebrates in the benthonic and pelagic coastal ecosystem. This study reveals for the first time the diet of T. maculata and the importance of elasmobranchs as predators of abundant and commercial species (i.e., jumbo squid Dosidicus gigas and Peruvian anchovy Engraulis ringens). The results of this study can assist in the design of an ecosystem-based management for the northern Peruvian sea and the conservation of these highly exploited, threatened or poorly understood group of predators in one of the most productive marine ecosystems.

Interannual variability of distribution, abundance and population dynamics of the smooth hammerhead Sphyrna zygaena (Linnaeus, 1758) in the central‐southeast Pacific Ocean

This study provides information on the population dynamics of the smooth hammerhead shark Sphyrna zygaena in the central-southeast Pacific Ocean. The samples were obtained from artisanal fisheries from 2008 to 2013 to analyse cohorts, growth, mortality, abundance and recruitment using methods based on length and relative age and an inference multi-model. According to the von Bertalanffy growth model, the results indicated that the species showed from three to seven cohorts annually and slow growth (Wi = 57.20%): L∞ = 292.86 cm, K = 0.141 per year and t0 = -2.26. Length of sexual maturity was 239.30 and 209.10 cm at first capture; the average natural mortality value was 0.231 per year, by fishing 0.051 per year with a total mortality of 0.26 1 per year and exploitation rate of 0.15. Interannual variability was recorded in biomass with an average of 7367.24 tonnes, as well as a spatial permanence pattern. The results suggest that the La Niña event biologically favours the population in the area. The information from this study is essential for better understanding the species population and serves as a basis for future actions of management and conservation of S. zygaena.

Spatio-temporal genetic variation of juvenile smooth hammerhead sharks in South Africa

ABSTRACTThe smooth hammerhead shark Sphyrna zygaena is known to display a high degree of site fidelity to specific areas along the South African coastline. This species is vulnerable to overfishing and steep declines have been noted globally. In this study, a dual marker approach was used to investigate the spatial and temporal genetic variation of juvenile S. zygaena across the south and east coast of South Africa. Kinship of individuals was determined within and across cohorts from juvenile S. zygaena sampled in Mossel Bay, a putative pupping ground. Significant genetic differentiation (supported by F-statistics, multivariate analysis and Bayesian clustering) was evident between individuals sampled from the warm temperate south coast and the subtropical east coast, with asymmetric gene flow between regions. Distinct genetic composition was also revealed between sampling years, with different levels of relatedness within and among cohorts, indicating different temporal populations utilizing Mossel Bay annually. Based on these results, further investigation of S. zygaena populations across a wider region is warranted in order to make recommendations regarding conservation management of this vulnerable coastal shark.

Population genetic divergence as consequence of past range expansion of the smooth hammerhead shark Sphyrna zygaena

The Smooth Hammerhead Shark Sphyrna zygaena is a species with anti-tropical distribution in the eastern Pacific from California USA, to southern Chile with a remarkably gap in abundance in equatorial areas between southern Mexico and Central America. For temperate species, warm waters can represent oceanographic barriers limiting the gene flow between adjacent populations. The mtDNA control region was used to infer the phylogeography and historical demography of the Smooth Hammerhead in the eastern Pacific. An AMOVA comparing locations grouped into northern and southern eastern Pacific equatorial areas indicated significant genetic differentiation. Each population had a common haplotype separated by two mutational differences suggesting that divergence occurred very recently. The pattern of genetic differences matches with the anti-tropical distribution of S. zygaena in the eastern Pacific. Gene diversity was lower in the northern population when compared to the southern area, a pattern indicative of a recent founder event. Broadly, this pattern suggests a recent population expansion occurring 25,000–10,000 years. bp, during the last glacial cycle, when the drop of surface sea temperatures created favorable conditions at equatorial latitudes to cross the thermal barrier imposed by warm waters at equatorial latitudes. These genetically distinct population groups might lead to delineate a management plan that considers two separated stocks in the eastern Pacific.

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.

Possible female philopatry of the smooth hammerhead shark Sphyrna zygaena revealed by genetic structure patterns.

We assessed the spatial pattern of genetic structure of smooth hammerhead shark Sphyrna zygaena in 10 localities from the Northern Mexican Pacific. A total of 35 haplotypes were identified in 129 sequences of the mtDNA control region. The results showed slight but significant genetic structure among localities (ΦST = 0.044, P < 0.001). In addition, the localities with highest number of juveniles were genetically different (ΦST = 0.058, P < 0.024), which may be representative of nursery areas. The genetic differentiation pattern can be associated to female philopatry and preference for particular birthing sites. Finally, historical demography shows that S. zygaena populations present a recent demographic expansion that occurred during glacial events in the late Pleistocene to early Holocene.

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.

Age and growth of the smooth hammerhead shark,Sphyrna zygaena, in the Eastern Equatorial Atlantic Ocean, using vertebral sections

The smooth hammerhead shark Sphyrna zygaena (Sphyrnidae) is regularly caught as bycatch in pelagic longline fisheries, but is one of the least studied of all pelagic sharks. Recently, ICCAT (International Commission for the Conservation of Atlantic Tunas) issued recommendations underlining the need for more studies on the life history parameters of this and other pelagic shark species. To this end, the age and growth of S. zygaena were studied in the Eastern Equatorial Atlantic Ocean, in an area where growth parameters were not yet available for this species. Data from 139 specimens, caught between June and September 2009, ranging in size from 136 to 233 cm fork length (FL), were analysed. Preliminary trials were carried out to assess the most efficient growth band enhancement technique. These indicated that sectioning the vertebrae into 500 μ m sections followed by staining with crystal violet produced the best results. Growth models were fitted using the traditional von Bertalanffy growth equation and a modification of this equation using a known size at birth. Growth models were compared using the Akaike information criterion (AIC). The von Bertalanffy growth equation seemed to be the most adequate model to describe growth in this species, with resulting growth parameters of L inf = 272 cm FL, k = 0.06 year for males and L inf = 285 cm FL, k = 0.07 year for females. In the first four years of life, S. zygaena grows 25 cm per year on average, but its growth slows down in later life. Future stock assessment models should incorporate these age and growth parameters for species management and conservation.

Shark predation of sea snakes (Reptilia: Elapidae) in the shallow waters around the Yaeyama Islands of the southern Ryukyus, Japan

Stomach contents of 845 wild-captured elasmobranchs (representing 21 species of more than 13 genera) from the Yaeyama area, southern Ryukyus, were examined for the elapid sea snakes. These specimens included 496 tiger sharks Galeocerdo cuvier , of which 343 had prey remains and 48 had sea snakes. Of the others, one of the 29 nurse sharks Nebrius concolor (including 23 with some items in stomachs) and the one smooth hammerhead shark Sphyrna zygaena also had sea snakes, whereas the remainder, including 187 with some prey remains in stomachs, had no sea snakes. Six of the seven sea snake species hitherto recorded from the Yaeyama area, including Pelamis platura , for which very little is known about its predator, were found from the tiger shark stomachs.