Afraid of sharks? Check the moon

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shark spotter INDEPENDENT NEWSPAPERS Shark spotters assist scientists with research into great white sharks. Picture: Jason Boud

Cape Town - If you’re on your way to the beach and want to minimise your chances of an encounter with a great white shark while you’re there – and who doesn’t? – first check the water temperature and the phase of the moon.

According to new research, and based on several hundred observations by the Shark Spotters at Fish Hoek and Muizenberg, white sharks are much more active at new moon – including during the day over this period– and when the surface sea temperature is more than 14°C and heading towards 18°C or warmer.

The authors of the research suggest that these “significant” correlations are most probably because of an increase in the sharks’ natural prey at such times or because they create better hunting conditions for them – not because the sharks prefer warm water.

And they argue that a greater understanding of the behaviour of large, predatory sharks like great whites, bull sharks and tiger sharks can be used to increase the safety of sea users “in a proactive and environmentally friendly way”. The research paper “The influence of environmental variables on the presence of white sharks, Carcharodon carcharias, at two popular Cape Town bathing beaches” is published in the online, open-source research journal PLOS ONE , and the principal author is Kay Weltz of the Marine Research Institute at UCT’s zoology department, who conducted the study as part of her MSc research.

Co-authors are shark specialist Dr Alison Kock, who is the research manager of the Shark Spotters, UCT marine scientists Dr Henning Winker and Dr Colin Attwood, and Shark Spotters manager Monwabisi Sikweyiya.

The study involved analysing 378 white shark records from Muizenberg and Fish Hoek over a five-year period, with data being collected by the dedicated observers employed through the city’s Shark Spotter programme. Statistical models were used to predict the likelihood of spotting white sharks at these beaches under different environmental conditions.

“We tested a range of environmental variables, and found a significant relationship between the probability of a white shark sighting and water temperature and lunar phase,” Weltz explained.

These trends were consistent for both beaches, with the probability of shark sightings increasing rapidly as the water temperature exceeded 14°C and approached a maximum at 18°C, whereafter it remained high.

The models used by the researchers predicted an eight times greater likelihood of sighting a white shark at Muizenberg when the water was 18°C than when it was 14°C, while at Fish Hoek it was five times greater.

The lunar phase was also significant, with a prediction of a four times greater likelihood of a shark sighting at new moon than at full moon at Fish Hoek, and 1.5 times greater at Muizenberg.

The authors said the data had also confirmed a significant increase in the number of shark sightings over the past three years, but that it was not clear whether this increase reflected an increase in shark numbers, or a change in distribution or in shark activity patterns.

“The more we study this white shark population, the more we understand that their movement and behaviour is not random.

“There are patterns and rhythms to their behaviour – areas, times and conditions of higher or lower encounter probability – and we can use this information to reduce the risk of negative shark encounters,” Kock said. - Cape Argus

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