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

Abstract

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.

A synthesis of three decades of socio-ecological change in False Bay, South Africa: setting the scene for multidisciplinary research and management

“…Since 2003, shark sightings at Seal Island have exhibited annual peaks and troughs, with an overall decreasing trend in sighting rates, although this was not statistically significant (Hewitt, 2014). Conversely, shark sightings inshore, at Muizenberg and Fish Hoek, exhibited a significant increase over a similar period (Weltz et al, 2013). Fluctuations in sighting rates remain poorly understood, but are likely related to environmental factors and associated prey availability (Weltz et al, 2013;Hewitt et al, 2018).…”

“…Conversely, shark sightings inshore, at Muizenberg and Fish Hoek, exhibited a significant increase over a similar period (Weltz et al, 2013). Fluctuations in sighting rates remain poorly understood, but are likely related to environmental factors and associated prey availability (Weltz et al, 2013;Hewitt et al, 2018). Over the last two years (2016-2018), white shark sightings in False Bay have been the lowest on record (A.…”

“…The first documented shark attack in South Africa took place in False Bay over 100 years ago, and over the next 60 years shark bites were sporadic with an average of one attack every six years (Wallett, 1978;Cliff, 2006). By the 1960s, reporting had improved and between 1960 and 2015, 29 shark bites, five of them fatal, took place in False Bay bringing the average up to one attack every two years (Cliff, 2006;Weltz et al, 2013). To address a growing public concern (Nel and Peschak, 2006), the City of Cape Town and partners have employed shark spotters at eight of its beaches to provide an early warning system for recreational water users (Kock et al, 2012), as well as a shark exclusion net at Fish Hoek (Kock and O'Riain, 2015).…”

Patterns of long-term climate variability and predation rates by a marine apex predator, the white shark Carcharodon carcharias

“…Taken together with the results of Clark et al (1996), teleost distribution may be vacillating with wind speed and water temperature, with possible bottom-up impacts at the level of white shark predation patterns, or white sharks may be responding to similar environmental drivers as the fish monitored by Clark et al (1996). In another study, Weltz et al (2013) monitored visual sightings of white sharks at beaches in False Bay (Fish Hoek and Muizenberg), finding that probability of shark sighting increased as SST rose from 14 to 18°C. Although we did not directly measure sightings, frequency distribution of daily T w recorded for days when shark predation on seals were observed show that attacks peaked at 14− 14.5°C and that very few occurred above 18°C (T w > 18°C: 6.4% of days for (K+AP) h −1 > 0, 6.2% of days for K h −1 > 0, and 5.1% of all observation days).…”

The current status and management of South Africa's chondrichthyan fisheries

“…Smale et al 2012;Towner et al 2013;Weltz et al 2013). Fisheries research, by contrast, has been patchy and has been focused primarily on the biology and life history of the larger targeted and marketable species, or those caught in large quantities in research surveys (e.g.…”