Entity

Time filter

Source Type

Gans, South Africa

O'Connell C.P.,University of Massachusetts Dartmouth | Andreotti S.,Stellenbosch University | Rutzen M.,Shark Diving Unlimited | MeYer M.,Branch Oceans and Coasts | He P.,University of Massachusetts Dartmouth
Ocean and Coastal Management | Year: 2014

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 zo≠ 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. © 2012 Elsevier Ltd. Source


O'Connell C.P.,University of Massachusetts Dartmouth | Andreotti S.,Stellenbosch University | Rutzen M.,Shark Diving Unlimited | Meyer M.,Branch Oceans and Coasts | And 2 more authors.
Journal of Experimental Marine Biology and Ecology | Year: 2014

The white shark (Carcharodon carcharias) is an apex predator and is a protected species that suffers from several sources of anthropogenic mortality, such as shark nets. Shark nets are devices used to minimize the interaction between beachgoers and potentially dangerous sharks; however, these nets have negatively impacted local and migratory shark populations, in addition to killing substantial quantities of other marine organisms. To address this issue, the present study developed and examined the effects of an alternative technology (the "Sharksafe" barrier) composed of two stimuli: (1) visual-artificial-kelp and (2) electrosensory-magnets, on C. carcharias behavior. Generalized linear mixed effect models were used to test hypotheses pertaining to the effects of treatment type, exposure quantity (i.e. habituation), conspecific density, and water visibility on shark behavior. Analyses based on forty-nine, one-hour trials illustrate that the swim patterns of all sixty-three individual C. carcharias was altered in the presence of the artificial kelp-the procedural control region, and the magnetic kelp-the magnetic region of the barrier (i.e. procedural control and magnetic regions reduced entrance frequency and increased avoidance and pass around frequency). Also, preliminary observations illustrated that the barrier had no observable impact on Cape fur seal (Arctocephalus pusillus pusillus) behavior. The C. carcharias-specific repellency associated with the Sharksafe barrier and the ability of the barrier to withstand harsh environmental conditions warrant future experiments to assess its exclusion capabilities on predatory sharks and possible application to replace shark nets. © 2014 Elsevier B.V. Source


Andreotti S.,Stellenbosch University | Rutzen M.,Shark Diving Unlimited | Wesche P.L.,University of Johannesburg | O'Connell C.P.,University of Massachusetts Dartmouth | And 3 more authors.
African Journal of Marine Science | Year: 2014

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. © 2014 Copyright © NISC (Pty) Ltd. Source


Andreotti S.,Stellenbosch University | Von Der Heyden S.,Stellenbosch University | Henriques R.,Stellenbosch University | Rutzen M.,Shark Diving Unlimited | And 3 more authors.
Journal of Biogeography | Year: 2016

Aim: To determine the genetic structure of the white shark population around the South African coastline and, by including data from animals sampled elsewhere in the world, to provide new insights into white shark evolution at the global scale. Methods: Mitochondrial and microsatellite analyses were performed on 302 free-ranging white sharks collected from five sites along the South African coastline. This was augmented with 58 GenBank sequences originating from five distinct global populations. Genetic diversity, local population sub-structuring analyses and global phylogeographical patterns were determined. Results: Four mtDNA haplotypes restricted to South Africa were recovered. One common haplotype was shared by 89% of all the individuals and was 13 bp different from the second most common haplotype shared by 10% of the remaining sharks. No local geographical sub-structuring was evident for either mtDNA or nuclear DNA. Both data sets show a remarkably low level of genetic diversity (mtDNA: h = 0.205, π = 0.0027; nDNA: Na = 7.6, Ho = 0.675). At the global scale, three distinct geographical clades were detected which could not be connected with 95% confidence in the haplotype network. Main conclusions: Results indicate that the observed South African mtDNA biogeographical pattern and diversity levels may be a consequence of a severe bottleneck or a recent colonization event from one or two sources. Globally, the population of white sharks can be differentiated into three mtDNA clades confined to (1) the Mediterranean and Indo-Pacific Oceans (Australia and California), (2) the North West Atlantic (Florida) and Indian Ocean (South Africa), and (3) a single divergent haplotype restricted to South Africa. The pattern is most likely the result of a combination of site philopatry, isolation by distance, infrequent long-distance dispersal, isolated founder events and the closure of the Isthmus of Panama. © 2015 John Wiley & Sons Ltd. Source


Andreotti S.,Stellenbosch University | Rutzen M.,Shark Diving Unlimited | Van Der Walt S.,Stellenbosch University | Von Der Heyden S.,Stellenbosch University | And 4 more authors.
Marine Ecology Progress Series | Year: 2016

The loss of apex marine predators has been reported to have a cascade of detrimental effects on marine ecosystems; however, the general lack of empirical data can severely limit our understanding of the ecological interactions among marine species. In this study we propose an integrated approach using mark-recapture and genetic techniques to assess population estimates of white sharks Carcharodon carcharias. Between 2009 and 2011, 4389 dorsal fin photographic identifications were collected in Gansbaai, South Africa, from 426 white sharks and used in markrecapture analyses. Saturation of new sightings occurred once 400 individuals were catalogued and the open population model POPAN suggested ranges between 353 and 522 individuals (95% confidence) and a point estimate of N = 438. Between 2010 and 2013, 302 biopsy samples were collected from 233 white sharks and used for a comparative genetic population estimate. Analyses of 14 microsatellite markers revealed a contemporary effective population size (CNe) of 333 individuals (95% CI = 247-487, pcrit = 0.02). These values were at least 52% less than those estimated in previous mark-recapture studies. Using this combination of techniques, we propose a Ne:N ratio of 0.76 for white sharks, which advances our ability to accurately make inferences on elasmobranch population numbers in general. Given the low population numbers of white sharks along the South African coastline, we predict a negative effect on the ecological stability of the marine environment in this region. © 2016 Inter-Research. Source

Discover hidden collaborations