Summer at the beach: spatio-temporal patterns of white shark occurrence along the inshore areas of False Bay, South Africa

Understanding the movement and habitat use of vulnerable marine top predators is essential for devising improved conservation and management strategies [1–3]. This is particularly important for large predatory shark species that aggregate in coastal areas threatened by diverse anthropogenic activities. Threats include intensive shore-based fishing, prey loss, pollution, culling to reduce shark bites, and the transformation or disturbance of natural habitats [4–6]. Movement and occurrence patterns of top predators can be included in marine spatial planning (MSP) e.g. to design marine protected area (MPA) boundaries or identify areas of conservation and biodiversity interest, such as Ecological or Biological Sensitive Areas (EBSAs) [7–9].

White sharks (Carcharodon carcharias) are apex predators that have small populations, spend considerable time near the coast and are vulnerable to human impacts due to being long-lived and having slow growth and low fecundity [10–12]. Consequently, since 2009, they have been listed as Vulnerable on the IUCN Red List of Threatened Species and protected in several countries [13]. Both juvenile and sub-adult white sharks use coastal areas extensively [14–21] with evidence of high fidelity at discrete beach sites in both Australia [17] and South Africa [16, 18].

In False Bay, South Africa, [20] reported both marked seasonal and sexual segregation of island versus inshore habitat use by white sharks. Both sexes aggregate around Seal Island over the austral autumn and winter, with females moving inshore in spring and summer [20]. Along the inshore regions of False Bay, potential teleost and elasmobranch prey resources are being heavily overfished [22–26] and white sharks are occasionally caught in three fisheries, namely recreational rock and surf fishing, beach purse-seine fishing and experimental fishing gear ([27], unpublished data). Thus, future MSP could benefit white sharks by conserving important prey resources, habitats and/or mitigating incidental catches [7–9].

The aims of this study are two-fold; first, to describe white shark movements between inshore and island sites year round, and second, to make inferences about the fine-scale spatial and temporal patterns of occurrence at a number of different inshore sites, during spring and summer when shark sightings peak inshore. We use passive acoustic telemetry data to test the null hypotheses that all sites along the inshore area of False Bay, South Africa are used equally, irrespective of shark size, and that there are no diel or annual differences in white shark occurrence at these inshore sites. Results from the study will improve our understanding of how a threatened apex predator uses an inshore area adjacent to a major metropole, and identify sites with higher occurrence that could be priority conservation areas. In addition, understanding where and when large predatory sharks overlap with recreational ocean users may assist relevant authorities in their goal to use non-lethal management to reduce the spatial overlap between people and sharks [28].

In a previous related study, [20] demonstrated a clear seasonal shift in white shark occurrence from Seal Island in autumn and winter, to the inshore region of False Bay in spring and summer. In this study, we expanded on these findings by describing the fine-scale spatial and temporal patterns of occurrence at inshore sites. Our results demonstrate clear spatial and temporal patterns of white shark occurrence, with the highest occurrence along the northern and northwestern shores of False Bay, most movement between four adjacent beach sites, most visits at midday, and in 2005. The significance of these findings and their potential application in MSP and shark safety campaigns are discussed below.

White sharks visited the northern and northwestern shore of False Bay significantly more often than sites on the eastern or western shores during spring and summer (Figs. 2 and 3). The high white shark occurrence at Strandfontein is noteworthy, with up to 12 tagged sharks (33% of tagged sharks detected inshore) recorded on a single day and any tagged shark detected at the site for up to 95 consecutive days, and up to 22 consecutive days for the same shark. There are very few seals seen at Strandfontein and Muizenberg beaches in spring and summer [30] and thus preference for this site and adjacent inshore sites, must be driven by factors other than seal presence, unlike for Seal Island. The marked preference of white sharks for distinct inshore sites along the northern and northwestern beaches of False Bay, supports the findings of [16, 18] who actively tracked white sharks in Mossel Bay, South Africa and reported prolonged visits to similar distinct beach habitats. The reasons for visits to these habitats, which are dominated by open beach habitat, remain unclear, with [16] suggesting it is used primarily for resting and socializing (based on the slow rate of movement recorded), in between more active foraging visits to a nearby seal colony. By contrast, both [17, 47] suggest that juvenile white sharks are using the inshore beach habitat primarily for foraging, with the former proposing an energy-conserving ‘sit and wait’ foraging strategy to encounter passing shoals of seasonally abundant fish that migrate along the inshore. White sharks can switch between a ‘sit-and-wait’ and ‘active searching’ (patrolling) movements when hunting [48], which could explain the high movement rates and large number of consecutive days present at sites between Fish Hoek and Strandfontein. Furthermore, studies have demonstrated a negative relationship between the level of habitat complexity and predator foraging success in aquatic environments [49, 50]. Rocky areas with dense kelp beds, like those along the eastern and western regions of False Bay, provide refugia for prey and impede the movement of large, fast swimming sharks which prefer to forage in uncluttered habitats [50], such as the open water around Seal Island [51], or in this case the sandy substrates typical of coastal beaches such as Strandfontein. Similar findings are available for terrestrial ambush predators, like lions, which track the seasonal distribution and abundance of their preferred prey, but on a fine-scale select habitat where prey is easier to catch, rather than areas where prey densities are highest [52].

White sharks may also be attracted to the warmer waters of the northern sites of False Bay [31, 32] with two studies showing a positive association between white shark sightings and warmer waters (≥18 °C) along the inshore [53, 54]. These studies propose that the association is likely due to an increase in prey availability, rather than thermal preference for warmer water at such a narrow temperature range (18 – 22 °C), as white sharks are often detected across a wider temperature range (13 – 25 °C) and can tolerate even colder temperatures [55–57]. In support of this is that white sharks are common at Seal Island, False Bay in winter when the average water temperature is 8 °C colder than the average summer water temperature [31]. However, fish and elasmobranch prey species have been confirmed to be more abundant along the inshore areas of False Bay during spring and summer, especially in the northern regions of False Bay when the water is warmer [32–34, 58]. Strandfontein in particular is a very well-known fishing location for various line-fish species e.g. kob (Argyrosomus spp.) and smooth hound sharks (Mustelus mustelus). Professional shark spotters [28] also record large schools of fish moving along the inshore in spring and summer months and the probability of detecting a white shark at both Fish Hoek and Muizenberg beaches has been shown to be significantly higher when prey fish are present [54]. Additionally, the warmer waters along the northern and northwestern shores may also facilitate a net positive energy budget for sharks, especially when prey is patchy, because they need to expend less energy in warmer waters [55, 59].

The findings that tagged white sharks spent considerable time at sites that are dominated by sandy beach habitat, high movement between Fish Hoek and Strandfontein, their generally high energy requirements [59], direct observations of foraging behavior close to beach habitat and the co-incidence of migratory prey fish along the inshore, suggest that white sharks are actively feeding in these areas dominated by open, beach habitat. However, it is clear that new methods, including cameras attached to animals (e.g. Crittercam [60]), stable isotope analysis and surveys of prey availability at different sites are needed to support or refute the hypothesis that beach habitat is preferred in False Bay during spring and summer because it facilitates foraging on migratory fish species. Future research should aim to collect in situ water temperature and associated information on prey availability at these sites.

We hypothesized that small white sharks (< 3 m), might use inshore habitats differently to larger (> 3 m) white sharks because they do not predate on Cape fur seals as frequently [35, 36]. Partridge Point (located on the western region of False Bay) is the only inshore site in False Bay where seals regularly haul-out [30] and when seals are observed along the inshore it is usually in small groups next to kelp forests along the western or eastern areas of False Bay and rare to see them along the northern shore (A. Kock, personal observation). However, we found that the number of visits to Partridge Point and movement to this site was very low for both size categories, suggesting that it is used to transit through, rather than hunt seals. Overall, we found that white sharks in both size categories preferred visiting and moving between sites on the northwestern and northern shore, with no size segregation observed in our study.

Understanding white shark spatial and temporal patterns at inshore sites in False Bay is important when applying the principles of MSP and decision-making regarding policy. The inshore region of False Bay is heavily impacted by fishing [22–25]. While the Cape fur seal population in False Bay seems to be stable [29], the same cannot be said for coastal fish populations [24, 26] and other shark populations in False Bay e.g. soupfin sharks (Galeorhinus galeus) [25]. It is possible that loss or changes in distribution of prey could impact the distribution, and spatial and temporal movements of white sharks in False Bay, in addition to inadvertently driving sharks to seek alternative prey sources. It has been demonstrated that marine reserves can benefit marine megafauna, and that megafauna can help establish target areas and boundaries for ecosystem reserves [7]. This study has confirmed that white sharks have very high levels of occurrence at Muizenberg and Strandfontein in particular and thus inclusion of these sites within False Bay’s MPA network or identification as EBSAs should be a future consideration. Such a move would increase monitoring and control of activities which may threaten white sharks or their prey resources (e.g. beach purse-seine fisheries, line-fisheries and shore angling) in False Bay. Sandy beach habitats and associated ecosystems are under-represented in the current MPA network, thus inclusion of these sites would increase the amount of sandy habitat conserved. It is also important to address potential threats to white sharks along the inshore. Perhaps highest on the list of known threats is the capture of white sharks by shore-based fisherman, either deliberately or as by-catch when fishing for other shark species. The banning of capture gear e.g. drones, large hooks, large baits and steel traces in areas with high white shark occurrence (e.g. Strandfontein) may reduce these risks, although the enforcement of such bans remains problematic. Although the data for this study was collected 10 years ago, current monitoring of the white shark population using telemetry and observational data indicates that the patterns identified here are consistent with present day observations ([28, 61] Shark Spotters, unpublished data). Resistance to extending the MPA to include these sites may come from both commercial and recreational users of these sections of False Bay and hence this proposal will have to be subject to a detailed risk assessment and extensive consultation with all of the relevant stakeholders.

The spring and summer spatial (inshore) and temporal (diel and annual) peaks in white shark occurrence have implications for both humans and sharks. We found a marked variation in the mean total number of annual visits, which peaked in 2005. This peak in shark sightings coincided with a spate of shark bites in False Bay with incidents reported in Fish Hoek and Muizenberg [53, 62]. White shark occurrence is highest in spring and summer which corresponds with the annual peak in human recreational activities and movement is highest between Fish Hoek and Strandfontein, which includes False Bay’s most popular swimming and surfing beaches [28]. Since 2000, 13 white shark bites, of which four (33%) were fatal, have been recorded on water users in False Bay, with Fish Hoek beach having the most bites (50%) and fatalities (75%) (Shark Spotters, unpublished data). Although rare, shark bites put tremendous pressure on management authorities like the Department of Environmental Affairs, Oceans and Coasts Branch and the City of Cape Town municipality, to implement lethal control programs e.g. drums lines or gill nets like those employed along the KwaZulu-Natal coast [63, 64]. Cape Town municipality has, however, opted to support a non-lethal policy at this stage [63] which aims to reduce the spatial overlap between sharks and recreational beach users through both a Shark Spotter programme [28], and an exclusion net at Fish Hoek beach (22 March 2013 Media Release, online at: www.capetown.gov.za) and through improved education and awareness of high risk sites and times. Our results indicate that current shark safety services are located at some of the sites with the highest shark occurrence (Fish Hoek to Muizenberg), but that the area between Muizenberg to Strandfontein is a high risk area which needs further attention. The information from our study should be incorporated into shark safety education campaigns, especially where shark spotters or exclusion nets are not available, so that individuals using these areas for recreational ocean activities better understand their personal risk when compared to using other areas around False Bay.

White sharks are capable of extreme migrations across ocean basins [10], yet exhibit fine-scale preferences for specific sites, for prolonged periods of time. In False Bay, white sharks moved between, and visited four specific inshore sites along the sandy beach, north and northwestern areas of the bay, significantly more than other inshore sites. This information can be used to interrogate the effectiveness of existing MPA boundaries, help design new MPA boundaries or identify EBSAs. White sharks do not benefit much from the current MPA network in False Bay, as high occurrence sites, such as Strandfontein and Seal Island are not included in the network. Our results suggest that they could benefit in two different ways if these sites were included in the future. Firstly, to conserve important prey resources and secondly, to reduce being caught incidentally by fisheries. An enhanced understanding of white shark spatial and temporal patterns can also be used in shark safety and awareness campaigns to reduce the spatial and temporal overlap of people and sharks in this region.