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A minimally invasive technique to assess several life-history characteristics of the endangered great hammerhead shark Sphyrna mokarran
Journal of Fish Biology
Abstract
A dorsal-fin photo-identification technique paired with a non-invasive parallel laser photogrammetry technique was used to non-invasively identify individual Sphyrna mokarran over time. Based on the data collected over a duration of 59 days, 16 different S. mokarran (mean±s.d. pre-caudal length: 220⋅82±13⋅66 cm; mean±s.d. cephalofoil width: 71⋅38±7⋅94 cm) were identified using dorsal-fin photo-identification, with a mean±s.d. shark re-sighting frequency of 4⋅05±3⋅06 at-sea days. The results illustrate a high S. mokarran sighting rate and therefore, the utilization of parallel laser pho- togrammetry and dorsal-fin photo-identification may be a plausible multi-year approach to aid in non-invasively determining the growth rate and inter-annual site fidelity of these animals.
... Great hammerhead sharks also suffer from high atvessel and post-release mortality due to an elevated stress response to capture (Morgan and Burgess, 2007;Gallagher et al., 2014). High likelihood of post-release mortality and slow life-history traits combined with increasing anthropogenic pressures has led to population decline across the species range and to great hammerhead sharks being globally listed as Critically Endangered on the IUCN (International Union for Conservation of Nature) Red List of Threatened Species (Rigby et al., Most studies on great hammerhead sharks have been conducted in the northwest Atlantic Ocean (Piercy et al., 2010;O'Connell and Leurs, 2015;Guttridge et al., 2017;Doan and Kajiura, 2020;Heim et al., 2021;Macdonald et al., 2021), southwest Pacific Ocean (Stevens and Lyle, 1989;Harry et al., 2011;Raoult et al., 2019), and to a lesser extent in the Western Indian Ocean (Cliff, 1995) and in the Arabian Gulf (Hsu et al., 2020). Extensive industrial fishing in those regions over the past half-century has resulted in an unprecedented increase in the risk of extinction of sharks and in particular of pelagic sharks (Pacoureau et al., 2021;Walls and Dulvy, 2021). ...
... Photographs of great hammerhead sharks were taken to catalog all sharks sighted on the day. Individual great hammerhead sharks were identified using natural markings (e.g., spots, and dark patterns on the body and fintips), fins morphology (e.g., notches, outgrowth, shortened), scars (e.g., wounds, scratches, depigmentation marks), and ventral pigmentation (Domeier and Nasby-Lucas, 2007;O'Connell and Leurs, 2015;Guttridge et al., 2017) (Figure 2; Table 1). Given the fast-healing capacity of elasmobranchs (Chin et al., 2015), we only used scars for intra-annual identification (within a season). ...
... The results from the historic dataset suggest that males are essentially observed between August and October, which supports our lack of males sighted during the austral summers of 2019 -2020 and 2020 -2021. Sexual segregation in great hammerhead sharks has not been reported in fishery-based studies (Stevens and Lyle, 1989;Cliff, 1995;Piercy et al., 2010;O'Connell and Leurs, 2015;Raoult et al., 2019;Hsu et al., 2020), unlike other hammerhead species, such as the scalloped hammerhead sharks (Sphyrna lewini, Griffith and Smith, 1834) (Klimley, 1987;Stevens and Lyle, 1989;Harry et al., 2011;Noriega et al., 2011); or other marine species (Sims, 2005;Wearmouth and Sims, 2008). Yet, all three studies conducted on the great hammerhead sharks in Bimini, Bahamas, show a female-dominated population (Guttridge et al., 2017;Heim et al., 2021;Guttridge et al., 2022). ...
The occurrence and seasonality of the Critically Endangered great hammerhead shark, Sphyrna mokarran, is data deficient in the Central Pacific region. Using photo-identification and laser-photogrammetry, we describe the seasonal population of great hammerhead sharks in the Tiputa pass (Rangiroa atoll) and Tuheiava pass (Tikehau atoll) in the Tuamotu archipelago of French Polynesia. During the austral summer of 2020 and 2021, we recorded a female-biased aggregation of at least 55 individuals (54 females; 1 unknown sex), representing an unprecedented number of S. mokarran in one study. All measured sharks were likely mature with pre-caudal lengths ranging from 147 cm to 297 cm (n = 35). Videos from citizen scientists recorded over a 15-year period enable us to identify 30 additional individuals between 2006 and 2019, with strong evidence of sexual segregation during the year. Our findings revealed seasonal residency (n = 32) of up to 6 days/month and for up to 5 months at the study site during the austral summer. We also demonstrated site fidelity with 32 individuals returning to the same atoll for up to 12 years between the first and last sighting, and with limited evidence of connectivity between the two sites. Our analysis also provides preliminary insight into the environmental factors driving S. mokarran aggregation in the Tiputa pass, suggesting a marked influence of the lunar cycle and of ocellated eagle rays (Aetobatus ocellatus) abundance. This study, conducted in situ with non-invasive methods offers a unique opportunity to study a great hammerhead shark population in a near-pristine ecosystem and provides important life-history elements for the Central Pacific region.
... The lasers were pre-calibrated to 0.33 m to ensure technique accuracy. As implemented in previous studies, lasers were properly positioned onto the side of the focal animal by maneuvering the ROV and post-hoc image analysis was implemented to obtain shark total length (TL) estimates (see Figure 2) [35,36]. Using these TL estimates, sharks were then categorized as YOY (i.e., <1.75 m TL), juveniles (i.e., 1.75-3.79 ...
... The lasers were pre-calibrated to 0.33 m to ensure techniq accuracy. As implemented in previous studies, lasers were properly positioned onto side of the focal animal by maneuvering the ROV and post-hoc image analysis was imp mented to obtain shark total length (TL) estimates (see Figure 2) [35,36]. Using these estimates, sharks were then categorized as YOY (i.e., <1.75 m TL), juveniles (i.e., 1.75-3 and 1.75-4.5 m TL for males and females, respectively), or mature adults (i.e., >3.79 a >4.5 m TL for males and females, respectively [10,17,19,37]) ...
The white shark (Carcharodon carcharias) is a globally distributed top predator. Due to its ecological importance, increasing knowledge through continued research can enhance management measures. One such facet of biological knowledge is the identification of shark nursery areas, as protection of these regions is critical to species survival. Presently, there are two known C. carcharias nursery areas in association with the Eastern Australian subpopulation; however, a nursery area associated with the Southern-Western Australian C. carcharias subpopulation has yet to be identified. Herein, we report opportunistic laser photogrammetry, stereo-photogrammetry, and baited remote underwater video systems (BRUVS) data that resulted in the identification of sixteen young-of-the-year (YOY)-juvenile C. carcharias from two separate regions (i.e., Salisbury Island and Daw Island) in South-West Australia. Additionally, anecdotal bycatch data associated with two YOY C. carcharias (i.e., 1.40 and 1.70 m total length) from another location within the Great Australian Bight are reported. While it is premature to conclude that these sites represent discrete or an expansive interconnected nursery area, the sightings success in this study is indicative that future research may want to consider implementing a study during a similar time period (i.e., February–March) while using a similar attractant methodology (i.e., bottom-set BRUVS baited with squid [Sepioteuthis australis]) to help elucidate the unique life-history characteristics of this C. carcharias subpopulation.
... Interestingly, a handful of observations have documented great hammerhead sharks' use of extreme shallow water (< 2 m) habitats in Andros Island and Eleuthera (Roemer et al., 2016), as well as two predation events in Bimini on a southern stingray (Dasyatis americana) and spotted eagle ray (Aetobatus narinari) (Strong et al., 1990;Chapman and Gruber, 2002). However, more recently, great hammerheads have been reliably encountered off the west of Bimini, in shallow sand bottom waters during the winter months (O'Connell and Leurs, 2016). ...
... At both sites, it was possible to capture and equip great hammerhead sharks with acoustic, satellite and conventional tags, however Bimini's excellent underwater conditions enabled our team to use additional methods, such as breath-hold diving to attach tags, photo-identification and laser photogrammetry to supplement tracking data (O'Connell and Leurs, 2016). Methods varied based on study site and available personnel and gear, and these, and the associated monitoring periods, are summarized in Table 1. ...
A thorough understanding of movement patterns of a species is critical for designing effective conservation and management initiatives. However, generating such information for large marine vertebrates is challenging, as they typically move over long distances, live in concealing environments, are logistically difficult to capture and, as upper-trophic predators, are naturally low in abundance. Large-bodied, broadly distributed tropical shark typically restricted to coastal and shelf habitats, the great hammerhead shark Sphyrna mokarran epitomizes such challenges. Highly valued for its fins (in target and incidental fisheries), it suffers high bycatch mortality coupled with fecundity conservative life history, and as a result, is vulnerable to over-exploitation and population depletion. Although there are very little species-specific data available, the absence of recent catch records give cause to suspect substantial declines across its range. Here, we used biotelemetry techniques (acoustic and satellite), conventional tagging, laser-photogrammetry, and photo-identification to investigate the level of site fidelity/residency for great hammerheads to coastal areas in the Bahamas and U.S., and the extent of movements and connectivity of great hammerheads between the U.S. and Bahamas. Results revealed large-scale return migrations (3030 km), seasonal residency to local areas (some for 5 months), site fidelity (annual return to Bimini and Jupiter for many individuals) and numerous international movements. These findings enhance the understanding of movement ecology in great hammerhead sharks and have potential to contribute to improved conservation and management.
... Furthermore, it has been used for a wide variety of taxa, including ungulates (Bergeron, 2007;Kumar et al., 2012;Weisgerber et al., 2015), fishes and marine mammals (Durban & Parsons, 2006;Leurs et al., 2015;O'Connell & Leurs, 2016;Rizzo et al., 2017;Rogers et al., 2017;Rohner et al., 2011;Webster et al., 2010), and both terrestrial and arboreal primates (Anzà et al., 2022;Barrickman et al., 2015;Brown et al., 2022;Chappell et al., 2015;Galbany et al., 2016Galbany et al., , 2017Lu et al., 2016;Richardson et al., 2022;Rothman et al., 2008;Sandel et al., 2022;Schuppli et al., 2016;Wright et al., 2019Wright et al., , 2020. ...
Parallel laser photogrammetry (PLP), which consists of attaching two or three parallel laser beams at a known inter-beam distance to a camera, can be used to collect morphological measurements of organisms noninvasively. The lasers project onto the photo being taken, and because the inter-beam distance is known, they act as a scale for image analysis programs like ImageJ. Traditionally, this method has been used to measure larger morphological traits (e.g., limb length, crown-rump length) to serve as proxies for overall body size, whereas applications to smaller anatomical features remain limited. To that end, we used PLP to measure the testes of 18 free-living mantled howler monkeys (Alouatta palliata) at La Selva Biological Station, Costa Rica. We tested whether this method could reliably measure this relatively small and globular morphology, and whether it could detect differences among individuals. We tested reliability in three ways: within-photo (coefficient of variation [CV] = 4.7%), between-photo (CV = 5.5%), and interobserver (intraclass correlation = 0.92). We found an average volume of 36.2 cm3 and a range of 16.4-54.4 cm3 , indicating variation in testes size between individuals. Furthermore, these sizes are consistent with a previous study that collected measurements by hand, suggesting that PLP is a useful method for making noninvasive measurements of testes.
... Size segregation is known to occur in sea turtles (Bowen et al., 1992), and it is therefore possible to infer the developmental stage of individuals based on size. Paired-laser photogrammetry (photogrammetry) has been used to accurately measure and determine life-history parameters of marine mammals (Webster et al., 2010) and elasmobranches (Deakos, 2010;O'Connell and Leurs, 2016;Rohner et al., 2015). Photogrammetry removes the need to physically capture an animal in order to measure it, and has been suggested as a suitable method to measure sea turtles (Williams et al., 2015). ...
There is global awareness that many species of elasmobranchs (sharks and rays) have life history characteristics that make them susceptible to overexploitation. The study of these animals is critical, as it contributes to increasing knowledge of these specimens and aids in their conservation. In particular, growth rate, age, fecundity, and size at maturity are key parameters for defining management and conservation strategies in elasmobranchs. Biometric data collection allows these parameters to be determined and considered in the evaluation of population demography. Over the last decades, several methodologies for measuring elasmobranch size have evolved, progressing from traditional capture-based methods to sophisticated, non-intrusive photographic techniques. The present review aims to understand and analyse all the existing non-invasive techniques that currently allow the collection of zoometric data in elasmobranchs and, later, to highlight the advantages and limitations of each technique, with comments on their application to fieldwork. To this end, 49 articles were selected, encompassing seven measurement techniques: photogrammetry using distance to the individual, bar photogrammetry, laser photogrammetry, stereo-DOV, stereo-BRUV, stereo-ROV, and aerial photogrammetry. Globally, the last four techniques are excellent alternatives to methods that involve animal capture or death, as they are practical, simple to use, minimally invasive, and potentially highly accurate. Each technique's requirements related to equipment and cost, limitations, and distinctive features are presented here and summarized to guide researchers on what's available and how to select the most appropriate for their studies.
A new observation of a great hammerhead shark (Sphyrna mokarran) natural predation on a southern stingray (Hypanus americana) is described in combination with two novel field experiments that investigate both: (1) the sensory cues that elicit S. mokarran foraging responses, as well as (2) the unique prey handling techniques that make this hammerhead such a specialized predator. This study is the first to demonstrate the importance of electro-sensory cues at close ranges for prey detection in S. mokarran. In addition, both the observed natural predation and field experiments provided visual evidence of the repeated prey manipulation technique, termed ‘lateral headshake repositioning’, that may maximize prey handling and foraging success. Further research using more accurate chemo-sensory cues (e.g. those from southern stingrays – Hypanus americana) or varying decoy manipulations (e.g. a buried decoy with active electrodes) is warranted to enhance our understanding of the sensory allocation and prey handling behavioral patterns in relation to the foraging success of this endangered predator.
Whale sharks Rhincodon typus are globally threatened, but a lack of biological and demographic information hampers an accurate assessment of their vulnerability to further decline or capacity to recover. We used laser photogrammetry at two aggregation sites to obtain more accurate size estimates of free-swimming whale sharks compared to visual estimates, allowing improved estimates of biological parameters. Individual whale sharks ranged from 432–917 cm total length (TL) (mean ± SD = 673 ± 118.8 cm, N = 122) in southern Mozambique and from 420–990 cm TL (mean ± SD = 641 ± 133 cm, N = 46) in Tanzania. By combining measurements of stranded individuals with photogrammetry measurements of free-swimming sharks, we calculated length at 50% maturity for males in Mozambique at 916 cm TL. Repeat measurements of individual whale sharks measured over periods from 347–1,068 days yielded implausible growth rates, suggesting that the growth increment over this period was not large enough to be detected using laser photogrammetry, and that the method is best applied to estimating growth rates over longer (decadal) time periods. The sex ratio of both populations was biased towards males (74% in Mozambique, 89% in Tanzania), the majority of which were immature (98% in Mozambique, 94% in Tanzania). The population structure for these two aggregations was similar to most other documented whale shark aggregations around the world. Information on small (<400 cm) whale sharks, mature individuals, and females in this region is lacking, but necessary to inform conservation initiatives for this globally threatened species.
The great hammerhead shark Sphyrna mokarran is an endangered species that is exposed to several sources of anthropogenic mortality, including beach nets. Although not a major contributor to S. mokarran mortality, beach nets are utilized in several locations to minimize the potential harmful interaction between sharks and beachgoers. To address this mortality, permanent magnets have been employed to determine if these materials can deter sharks away from netted areas. The present study examined the effects of barium-ferrite (BaFe12O19) permanent magnets on S. mokarran behavior under several environmental and biological conditions. In the bait experiment, feeding frequency significantly decreased and avoidance frequency significantly increased with the magnet treatment, with exposure quantity yielding an increase in feeding frequency, although this effect was not statistically significant. For the barrier experiment, entrance frequency significantly decreased and avoidance and pass-around frequencies significantly increased with the magnet treatment, with heterospecific density also being a significant pre - dictor of entrance frequency. The findings demonstrate how permanent magnets can modify S. mokarran behavior and how this behavior is modified based on situational context. Since several other sphyrnid species are caught in beach nets more frequently than S. mokarran (e.g. scalloped hammerheads S. lewini), the present results may serve as a model for these other sphyrnid species and illustrate the potential conservation implications of future magnetic deterrent barrier technologies.
The great white shark, Carcharodon carcharias, is the most widely protected elasmobranch in the world, and is classified as Vulnerable by the IUCN and listed on Appendix III of CITES. Monitoring of trade in white shark products and enforcement of harvest and trade prohibitions is problematic, however, in large part due to difficulties in identifying marketed shark parts (e.g., dried fins, meat and processed carcasses) to species level. To address these conservation and management problems, we have developed a rapid, molecular diagnostic assay based on species-specific PCR primer design for accurate identification of white shark body parts, including dried fins. The assay is novel in several respects: It employs a multiplex PCR assay utilizing both nuclear (ribosomal internal transcribed spacer 2) and mitochondrial (cytochrome b) loci simultaneously to achieve a highly robust measure of diagnostic accuracy; it is very sensitive, detecting the presence of white shark DNA in a mixture of genomic DNAs from up to ten different commercially fished shark species pooled together in a single PCR tube; and it successfully identifies white shark DNA from globally distributed animals. In addition to its utility for white shark trade monitoring and conservation applications, this highly streamlined, bi-organelle, multiplex PCR assay may prove useful as a general model for the design of genetic assays aimed at detecting body parts from other protected and threatened species.
The great hammerhead shark, Sphyrna mokarran, is a cosmopolitan species that is caught in a variety of fisheries throughout much of its range. The apparent decline of great hammerhead shark populations has reinforced the need for accurate biological data to enhance fishery management plans. To this end, age and growth estimates for the great hammerhead were determined from sharks (n = 216) ranging in size from 54- to 315-cm fork length (FL), captured in the Gulf of Mexico and north-western Atlantic Ocean. Growth curves were fitted using multiple models and evaluated using Akaike’s information criterion. The von Bertalanffy growth model was the best fitting model, with resulting growth parameters of L∞ = 264.2-cm FL, k = 0.16 year–1, t0 = –1.99 year for males, and L∞ = 307.8-cm FL, k = 0.11 year–1, t0 = –2.86 year for females. Annual band pair deposition was confirmed through marginal-increment analysis and a concurrent bomb radiocarbon validation study. Great hammerheads have one of the oldest reported ages for any elasmobranch (44 years) but grow at relatively similar rates (on the basis of von Bertalanffy k value) to other large hammerhead species from this region. The present study is the first to provide vertebral ages for great hammerheads.
The white shark Carcharodon carcharias was one of the first elasmobranch species where photo identification was used to identify unique individuals. In this study, we propose guidelines that improve the current photo identification technique for white sharks by presenting a novel categorisation system. Using this method, a high-resolution photograph of the dorsal fin is placed on a standardised three-section grid. Notches associated with the trailing edge of the fin are counted and scored within each grid section. The number of notches in each grid section is then used to produce a three-part code that is utilised to systematically organise the individuals into a database. Our proposed system was tested on sharks photographed in the sampling area over a 27-month period (4 398 photographs) and the method significantly reduced the search time associated with identifying resighted individuals. Using the notches code, we were able to identify 426 different C. carcharias within the Dyer Island Nature Reserve (South Africa). Due to the inherent ease of use, the accuracy associated with this method, and the ability to ‘resight’ individuals rapidly within a large photographic database, this non-invasive technique presents a validated and feasible alternative for future white shark photo identification studies.
The hammerhead sharks Eusphyra blochii, Sphyrna mokarran and S. lewini form part of the incidental catch of a commercial gill-net fishery off northern Australia. Of the specimens sampled between June 1980 and December 1986, 46% of S. mokarran, 41% of E. blochii and 31% of S. lewini were females. Few adult female S. lewini were caught and it is suggested that these occur offshore of the study area. In northern Australia, the usual size at maturity of male E. blochii, S. lewini and S. mokarran is 108, 150 and 225 cm total length (TL), ,and of females is 120, 200 and 210 cm TL, respectively. S. mokarran and E. blochii gave birth in January and February/March, respectively, after a gestation period of 10-11 months. S. Iewini appears to have a more extended seasonal cycle: the young are born between October and January after 9-10 months gestation. Size at birth is about 45-50 cm TL in E. blochii and S. lewini, and 65 cm TL in S. mokarran. Mean litter size is 12 in E. blochii, 15 in S. mokarran and 17 in S. lewini. Individual E. blochii females breed every year, whereas S. mokarran females probably breed every other year. Fish are an important component of the diet of all three species as, to a lesser extent, are cephalopods for S. lewini and crustaceans for S. mokarran and E. blochii.
Morphometrics are useful for describing and managing animal populations, but measurements
can be difficult to obtain, especially on large free-ranging aquatic animals. The accuracy
and precision of paired-laser photogrammetry were tested as a simple and non-invasive remote sensing
system for measuring the body size of free-ranging, resident manta rays Manta alfredi, a newly
described species that is poorly understood. Based on repeated measurements of a pipe of known
size, the paired-laser system proved accurate (mean error of 0.39%) and precise (CV = 0.54%).
Repeated measurements on 154 different manta rays visiting a cleaning station off Maui, Hawaii,
produced a mean CV of 1.46%. Disc length (DL) measurements were more precise than disc width
(DW) measurements, and an empirically derived disc ratio (DR) function was applied to convert DL to
DW measurements for standard comparison with other studies. Sexual dimorphism was present with
the largest female (3.64 m DW) 18% larger than the largest male (3.03 m DW). Sexual maturity in
females, based on evidence of pregnancy and mating scars, was conservatively determined to be
3.37 m DW. The DW at which 50% of the males were likely to be mature (based on clasper length)
was between 2.7 and 2.8 m. The absence of individuals < 2.5 m DW suggests that age class segregation
occurs in this population. Paired-laser photogrammetry proved to be a simple, non-invasive,
accurate, and precise method for sizing free-ranging manta rays. Repeated measurements on known
individuals over time could provide population growth parameters needed for adequate management
of this poorly understood species.
The ability to measure and age individuals within a population has many important applications, for example, for examining growth and determining size class. We developed a simple photogrammetric system using two parallel lasers and a digital camera, in order to measure dorsal fin dimensions of free-ranging Hector's dolphins. Laser dots were projected onto the fin, providing scale, thus allowing measurement as well as simultaneous photo-ID of 34 individuals from fin nicks and other marks. Multiple measurements (≥5) were available for six individuals; these resulted in mean CVs of 3.71% for fin length and 3.76% for fin height. Errors due to variations in angle and measurement were quantified via photography of a fiberglass Hector's dolphin model. Allometric measurements and age data were collated from 233 autopsied Hector's dolphins. Using these data, fin length was found to be a better predictor of total length (females r2= 0.732, males r2= 0.678) than fin height. Gompertz age/length growth curves were fitted to these individuals. Linear regressions were used to estimate total length for 34 individuals from laser-metrically estimated fin base length. Individuals were then assigned one of three age categories. This system shows promise as a noninvasive way of measuring individuals, while allowing simultaneous photographic identification.
A bstract
We estimated adult survival probabilities for the endangered Florida manatee ( Trichechus manatus latirostris ) in four regional populations using photoidentification data and open‐population capture‐recapture statistical models. The mean annual adult survival probability over the most recent 10‐yr period of available estimates was as follows: Northwest ‐ 0.956 (SE 0.007), Upper St. Johns River ‐ 0.960 (0.011), Atlantic Coast ‐ 0.937 (0.008), and Southwest ‐ 0.908 (0.019). Estimates of temporal variance independent of sampling error, calculated from the survival estimates, indicated constant survival in the Upper St. Johns River, true temporal variability in the Northwest and Atlantic Coast, and large sampling variability obscuring estimates for the Southwest. Calf and subadult survival probabilities were estimated for the Upper St. Johns River from the only available data for known‐aged individuals: 0.810 (95% CI 0.727–0.873) for 1st year calves, 0.915 (0.827–0.960) for 2nd year calves, and 0.969 (0.946–0.982) for manatee 3 yr or older. These estimates of survival probabilities and temporal variance, in conjunction with estimates of reproduction probabilities from photoidentification data can be used to model manatee population dynamics, estimate population growth rates, and provide an integrated measure of regional status.
A systematic, reliable method for identifying white sharks, Carcharodon carcharias Linnaeus, from underwater photographs was developed and applied to examine site fidelity at Guadalupe Island, Mexico (29˚N,
118˚W). The most reliable features for repeat identification in multiple years were the pigment patterns on the gill flaps,
pelvic fins, and caudal fins. Pigment patterns in all three regions were asymmetrical on the right and left sides making it
necessary to photograph both sides to catalog each individual. However, once cataloged, an individual could be re-identified
using a partial body image. Using this method, 73 individuals were identified between 2001 and 2005. Site fidelity was indicated
through repeated annual sightings of individuals with 78% of the identified sharks observed over at least 2years. Males were
found to arrive at Guadalupe Island as early as July and females in September. Peak abundances at the site occurred August–December.
The sex ratio was not significantly different from unity in 2002, 2004, and 2005. This monitoring technique has shown Guadalupe
Island to be an important white shark aggregation site in the eastern Pacific.
The future status of sharks is an issue of widespread conservation concern due to declines in many species in the face of
high levels of exploitation to satisfy market demands for products, especially fins. Substantial declines in the large-bodied
hammerhead sharks, Sphyrna lewini, S. mokarran and S. zygaena, even in regions where some management occurs, indicate that informed conservation measures are warranted for these circumglobally
distributed species. Despite the importance of assessing shark catch and trade on a species-specific basis to detect potential
overexploitation of individual species, achieving this goal for hammerheads has proven elusive due to difficulties in identification
of their products. Here, we present the development and application of a diagnostic, streamlined, five-primer multiplex polymerase
chain reaction assay utilizing species-specific primers based on nuclear ribosomal ITS2 for the three hammerhead species throughout
their global distribution. Application of this assay to investigations of the fin market confirmed the presence of hammerhead
fins in the international trade. A study of the world’s largest fin market in Hong Kong revealed a high concordance between
specific Chinese-name trade categories and fins from these three species (“Bai Chun” with S. lewini, “Gui Chun” with S. zygaena and “Gu Pian” with S.␣mokarran), and clear species preferences. This concordance information allows the use of market records for monitoring species-specific
trends in trade and exploitation rates. The assay is also proving useful for identification of shark body parts in U.S. fisheries
law-enforcement activities. Screening of morphologically identified “ S. lewini” from globally distributed areas using this assay with subsequent whole ITS2 sequencing suggests a cryptic species closely
related to S. lewini occurs off the SE USA coast.
Mark-recapture techniques can be used to estimate white shark (Carcharodon carcharias) population abundance. These frameworks are based on assumptions that marks are conserved and animals are present at the
sampling location over the entire duration of the study. Though these assumptions have been validated across short-time scales
for white sharks, long-term studies of population trends are dependent on these assumptions being valid across longer periods.
We use 22years of photographic data from aggregation sites in central California to support the use of dorsal fin morphology
as long-term individual identifiers. We identified five individuals over 16–22years, which support the use of dorsal fins
as long-time individual identifiers, illustrate strong yearly site fidelity to coastal aggregation sites across extended time
periods (decades), and provide the first empirical validation of white shark longevity >22years. These findings support the
use of fin morphology in mark-recapture frameworks for white sharks.
Visual identification of naturally acquired marks has been a popular if subjective method of animal identification and population
estimation over the last 40years. Molecular genetics has also independently developed objective individual marking techniques
during the same period. Here, we assess the concordance of individual great white shark (Carcharodon carharias) dorsal fin recognition and identification, using seven microsatellite loci as the independent unbiased arbiter, over a period
of 5years. As a monitoring technique, fin photographs offer a very good individual identification key for white sharks over
a relatively short period of time (5years), matching with genetic data in about 85% of cases, whilst caution and a continuously
updated database is required for animal recognition over a longer period.
The decline of sharks in the global oceans underscores the need for careful assessment and monitoring of remaining populations. The northeastern Pacific is the home range for a genetically distinct clade of white sharks (Carcharodon carcharias). Little is known about the conservation status of this demographically isolated population, concentrated seasonally at two discrete aggregation sites: Central California (CCA) and Guadalupe Island, Mexico. We used photo-identification of dorsal fins in a sequential Bayesian mark-recapture algorithm to estimate white shark abundance off CCA. We collected 321 photographs identifying 130 unique individuals, and estimated the abundance off CCA to be 219 mature and sub-adult individuals ((130, 275) 95% credible intervals), substantially smaller than populations of other large marine predators. Our methods can be readily expanded to estimate shark population abundance at other locations, and over time, to monitor the status, population trends and protection needs of these globally distributed predators.
This study employed a non-lethal measurement tool, which combined an existing photo-identification technique with a surface, parallel laser photogrammetry technique, to accurately estimate the size of free-ranging white sharks Carcharodon carcharias. Findings confirmed the hypothesis that surface laser photogrammetry is more accurate than crew-based estimations that utilized a shark cage of known size as a reference tool. Furthermore, field implementation also revealed that the photographer's angle of reference and the shark's body curvature could greatly influence technique accuracy, exposing two limitations. The findings showed minor inconsistencies with previous studies that examined pre-caudal to total length ratios of dead specimens. This study suggests that surface laser photogrammetry can successfully increase length estimation accuracy and illustrates the potential utility of this technique for growth and stock assessments on free-ranging marine organisms, which will lead to an improvement of the adaptive management of the species.
© 2015 The Fisheries Society of the British Isles.
Beach nets are preventative devices that are utilized to minimize the potential interaction between a beachgoer and a predatory shark. One species, the great white shark (Carcharodon carcharias), the focal species for the present study and a protected species in South African waters, is often killed in beach nets within the KwaZulu-Natal (KZN) region. To address the issue of C. carcharias capture in beach nets and to reduce mortality of this species, two related experiments were carried out: the bait experiment and the magnetic-control barrier experiment. Both experiments were aimed to determine the effect of permanent magnets on C. carcharias. During the bait experiment, a total of twenty C. carcharias interacted with the control and magnetic apparatuses. The results indicate that avoidance and feeding behaviors were significantly associated with treatment type, suggesting that permanent magnets had C. carcharias deterrent capabilities. In addition, it was demonstrated that the likelihood of an avoidance behavior on the magnet-associated baits was not significantly correlated with water visibility or conspecific density. For the second experiment, results from stage I of the magnetic-control barrier experiment indicate that behavior was not associated with treatment zone; however, stage II indicated that behavior was significantly associated with treatment type. Results from the magnetic-control barrier experiment clearly demonstrate that although a visual barrier, such as the procedural control barrier, may be sufficient to deter C. carcharias from an area, the addition of permanent magnets provide additional successful deterrence of C. carcharias. This study demonstrates that C. carcharias are sensitive to strong permanent magnetic fields; therefore a large-scale experiment with a substantially greater sample size is warranted to investigate the potential of a non-invasive magnetic barrier to replace detrimental beach nets in KwaZulu-Natal, South Africa.
We provide pilot data from a satellite-tracked great hammerhead shark Sphyrna mokarran in the Atlantic, representing the first such data on this species in the literature. The 250 cm shark was tagged off the coast of the middle-Florida Keys (USA) and transmitted for 62 d. During this time it migrated a minimum distance of ~1200 km northeast from the coast of Florida, into pelagic international waters of the Northwest Atlantic. When compared to the primary literature, this migration represented a northeasterly range extension for this species off the continental slope in the Atlantic. The significance of this range extension is discussed in terms of the vulnerability of S. mokarran to target and non-target fisheries.
Between 1978 and 1993, 209 great hammerhead sharks Sphyrna mokarran were caught in the shark nets which protect the swimming beaches of KwaZulu-Natal. This species constituted 0,97% of the total shark catch, with a mean annual catch of 13. Catch rates showed a significant decline during the period under review, from 0,66 to 0,09 sharks·km-net−1·year−1. Most sharks were caught in the north of the netted region between January and May. Catches at Mzamba, the southern extremity of the netted region, represent the southernmost records of this species on the east African coast. The males ranged in size from 106 cm precaudal length (18 kg) to 264 cm (220 kg) and females from 140 cm (35 kg) to 326 cm (400 kg). Males matured at about 217 cm and females at 237 cm. Very few sharks were found in mating condition. Elasmobranchs were found in 82% of non-empty stomachs. There was a high incidence of stingrays (Dasyatidae), guitarfish (Rhinobatidae) and other bottom-dwelling fish in the diet.
Laser photogrammetry was found to be a promising new cost-effective technique for measuring free-swimming whale sharks Rhincodon typus. Photogrammetric measurements were more precise than visual size estimates by experienced researchers, with results from the two methods differing by 9· 8 ± 1· 1% (mean ±s.e.). A new metric of total length and the length between the fifth gill and first dorsal fin (r² = 0· 93) is proposed to facilitate easy, accurate length measurements of whale sharks in the field.
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1. Precise estimates of demographic rates are key components of population models used to predict the effects of stochastic environmental processes, harvest scenarios and extinction probability. 2. We used a 12-year photographic identification library of whale sharks from Ningaloo Reef, Western Australia to construct Cormack-Jolly-Seber (CJS) model estimates of survival within a capture-mark-recapture (CMR) framework. Estimated survival rates, population structure and assumptions regarding age at maturity, longevity and reproduction frequency were combined in a series of age-classified Leslie matrices to infer the potential trajectory of the population. 3. Using data from 111 individuals, there was evidence for time variation in apparent survival (phi) and recapture probability (p). The null model gave a phi of 0.825 (95% CI: 0.727-0.893) and p = 0.184 (95% CI: 0.121-0.271). The model-averaged annual phi ranged from 0.737 to 0.890. There was little evidence for a sex effect on survival. 4. Using standardized total length as a covariate in the CMR models indicated a size bias in phi. Ignoring the effects of time, a 5-m shark has a phi = 0.59 and a 9 m shark has phi = 0.81. 5. Of the 16 model combinations considered, 10 (63%) indicated a decreasing population (lambda < 1). For models based on age at first reproduction (alpha) of 13 years, the mean age of reproducing females at the stable age distribution (A) ranged from 15 to 23 years, which increased to 29-37 years when alpha was assumed to be 25. 6. All model scenarios had higher total elasticities for non-reproductive female survival [E(s(nr))] compared to those for reproductive female survival [E(s(r))]. 7. Assuming relatively slow, but biologically realistic, vital rates (alpha = 25 and biennial reproduction) and size-biased survival probabilities, our results suggest that the Ningaloo Reef population of whale sharks is declining, although more reproductive data are clearly needed to confirm this conclusion. Combining relatively precise survival estimates from CMR studies with realistic assumptions of other vital rates provides a useful heuristic framework for determining the vulnerability of large oceanic predators for which few direct data exist.
The formulation of conservation policy for species that are rare and migratory requires broad cooperation to ensure that adequate levels of standardized data collection are achieved and that the results of local analyses are comparable. Estimates of apparent survival rate, relative change in abundance, and proportions of newly marked and returning individuals can inform local management decisions while highlighting corresponding changes at other linked research stations. We have applied computer-assisted photo-identification and mark-recapture population modeling to whale sharks Rhincodon typus at Ningaloo Marine Park (NMP), Western Australia, to create a baseline trend for comparison with other regional aggregations of the species. We estimate several ecological parameters of interest, including an average apparent survival rate of 0.55 yr(-1) for sharks newly marked (new) and 0.83 yr(-1) for sharks captured in multiple seasons (philopatric). The average proportion of philopatric sharks is found to be 0.65 of the total population, and we derive an average population growth rate of 1.12 yr(-1) for them. Our analysis uncovered significant heterogeneity in capture and survival probabilities in this study population; our chosen model structures and data analysis account for these influences and demonstrate a good overall fit to the time-series data. The results show good correspondence between capture probability and an available measure of recapture effort, suggesting that unmodeled systematic effects contribute insignificantly to the model fits. We find no evidence of a decline in the whale shark population at NMP, and our results provide metrics of value to their future management. Overall, our study suggests an effective approach to analyzing and modeling mark-recapture data for a rare species using computer-assisted photo-identification and opportunistic data collection from ecotourism to ensure the quality and volume of data required for population analysis.
Three species of hammerhead sharks in the southwestern waters of Japan
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Concordance of genetic and fin photo identification in the great white shark, Carcharodon carcharias, off Mossel Bay, South Africa Range extension of the endangered great hammerhead shark Sphyrna mokarran in the Northwest Atlantic: preliminary data and significance for conservation
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IUCN Red List of Threatened Species
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