Browsing by Author "Kock, Alison A"

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    Effects of an electric field on white sharks: in situ testing of an electric deterrent
    (Public Library of Science, 2013) Huveneers, Charlie; Rogers, Paul J; Semmens, Jayson M; Beckmann, Crystal; Kock, Alison A; Page, Brad; Goldsworthy, Simon D
    Elasmobranchs can detect minute electromagnetic fields, <1 nVcm -1 , using their ampullae of Lorenzini. Behavioural responses to electric fields have been investigated in various species, sometimes with the aim to develop shark deterrents to improve human safety. The present study tested the effects of the Shark Shield Freedom7™ electric deterrent on (1) the behaviour of 18 white sharks ( Carcharodon carcharias ) near a static bait, and (2) the rates of attacks on a towed seal decoy. In the first experiment, 116 trials using a static bait were performed at the Neptune Islands, South Australia. The proportion of baits taken during static bait trials was not affected by the electric field. The electric field, however, increased the time it took them to consume the bait, the number of interactions per approach, and decreased the proportion of interactions within two metres of the field source. The effect of the electric field was not uniform across all sharks. In the second experiment, 189 tows using a seal decoy were conducted near Seal Island, South Africa. No breaches and only two surface interactions were observed during the tows when the electric field was activated, compared with 16 breaches and 27 surface interactions without the electric field. The present study suggests that the behavioural response of white sharks and the level of risk reduction resulting from the electric field is contextually specific, and depends on the motivational state of sharks.
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    The influence of environmental variables on the presence of white sharks, Carcharodon carcharias at two popular Cape Town bathing beaches: A generalized additive mixed model
    (Public Library of Science, 2013) Weltz, Kay; Kock, Alison A; Winker, Henning; Attwood, Colin; Sikweyiya, Monwabisi
    Shark attacks on humans are high profile events which can significantly influence policies related to the coastal zone. A shark warning system in South Africa, Shark Spotters , recorded 378 white shark ( Carcharodon carcharias ) sightings at two popular beaches, Fish Hoek and Muizenberg, during 3690 six-hour long spotting shifts, during the months September to May 2006 to 2011. The probabilities of shark sightings were related to environmental variables using Binomial Generalized Additive Mixed Models (GAMMs). Sea surface temperature was significant, with the probability of shark sightings increasing rapidly as SST exceeded 14°C and approached a maximum at 18°C, whereafter it remains high. An 8 times (Muizenberg) and 5 times (Fish Hoek) greater likelihood of sighting a shark was predicted at 18°C than at 14°C. Lunar phase was also significant with a prediction of 1.5 times (Muizenberg) and 4 times (Fish Hoek) greater likelihood of a shark sighting at new moon than at full moon. At Fish Hoek, the probability of sighting a shark was 1.6 times higher during the afternoon shift compared to the morning shift, but no diel effect was found at Muizenberg. A significant increase in the number of shark sightings was identified over the last three years, highlighting the need for ongoing research into shark attack mitigation. These patterns will be incorporated into shark awareness and bather safety campaigns in Cape Town.
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    Predictors of white shark Carcharodon carcharias presence at two recreational beaches in a major metropole
    (2017) Loosen, Kristina; O'Riain, M Justin; Kock, Alison A; Winker, Henning
    The presence of white sharks Carcharodon carcharias in False Bay, South Africa follows a clear seasonal cycle. In the austral winter months, white sharks aggregate around Seal Island with female sharks in particular moving to the inshore areas in summer. What triggers these fine scale migrations at one of the largest white shark aggregation sites in the world remains a subject of debate. Previous research identified the environmental variables, water temperature and lunar phase, as significant influences on white shark presence inshore. In this study, I attempt to investigate the influence of prey fish availability on the presence of white sharks at two inshore areas of False Bay, namely Muizenberg and Fish Hoek. In addition, I explored the influence of sea surface temperature (SST), El Niño/Southern Oscillation (ENSO), dolphin presence, diatom patch presence and the occurrence of purse-seine fishing (treknetting) on shark presence. A total of 1209 shark sightings were recorded from 1 January 2006 - 31 December 2015 with 742 sightings at Muizenberg and 467 at Fish Hoek. I used randomization tests to determine whether the occurrence of shark sightings was randomly distributed across the environmental and biological variables and determined the dependencies between the predictor variables. I confirmed that shark sightings were clearly seasonal and sightings peaked between 17.49 - 18.57°C. Fish presence revealed a similar peak at 17.94°C. Randomization tests indicate that a spotter is 66% more likely to detect a shark when fish are present, which supports my prediction of a prey mediated cue. The occurrence of treknetting was also found to be non-randomly linked to white shark presence, increasing the probability to spot a shark to 38% instead of 20% at random. The presence of diatom patches was found to be negatively correlated with SST; with numbers increasing in colder waters. The presence of dolphins, as other predators in False Bay, didn't show a significant pattern with any of the analysed variables. The influence of ENSO on the presence of white sharks was also hypothesized and the data revealed that there was a significant influence of weak negative ENSO values on their inshore occurrence. In addition to providing a better understanding of the ecology of white sharks in False Bay, my results can also be used in shark safety and education programs to reduce the risk of shark human conflict.
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    Summer at the beach: spatio-temporal patterns of white shark occurrence along the inshore areas of False Bay, South Africa
    (BioMed Central, 2018-05-22) Kock, Alison A; Photopoulou, Theoni; Durbach, Ian; Mauff, Katya; Meÿer, Michael; Kotze, Deon; Griffiths, Charles L; O’Riain, M Justin
    Background Understanding white shark (Carcharodon carcharias) habitat use in coastal areas adjacent to large cities, is an important step when formulating potential solutions to the conservation conflict that exists between humans and large predatory sharks. In this study, we present the findings of a 2.5-year study of white shark occurrence and movement patterns adjacent to the City of Cape Town in False Bay, South Africa, with a focus on spring and summer months. Fifty-one white sharks were monitored annually at three offshore and twelve inshore sites by VR2 acoustic receivers, over 975 days from 1 May 2005 to 31 December 2007. Results Occurrence patterns at inshore sites during spring and summer were analysed using a generalized additive mixed model (GAMM) with a spatial term (longitude, latitude), time of day and year included as explanatory variables for site use. We found that sharks occurred more frequently at inshore sites along the northern and northwestern shores, compared to the rest of the bay, and they transitioned most frequently between four adjacent beach sites that encompass the most popular recreational water use areas in Cape Town. There was significant diel variation, with higher shark occurrence around midday, and a peak in shark occurrence in 2005, when human-shark interactions also peaked. However, we found no effect of shark size on occurrence patterns at inshore sites. Conclusions White sharks showed the highest levels of occurrence at specific inshore sites between Muizenberg and Strandfontein beach, and thus inclusion of these sites within False Bay’s marine protected area (MPA) network or recognition as Ecological or Biological Significant Areas (EBSAs) should be a future consideration. These insights into white shark habitat use at inshore sites in False Bay are important for successfully applying the principles of marine spatial planning (MSP) and for making science-based policy decisions. Furthermore, this information can be used to reduce potential shark-human conflict by incorporating it into future shark safety education campaigns.