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Effects of environmental conditions on predator–prey interactions between white sharks (Carcharodon carcharias) and Cape fur seals (Arctocephalus pusillus pusillus) at Seal Island, South Africa
Abstract
Effects of environmental factors on frequency and success rate of 2,546 natural predatory attacks by white sharks, Carcharodon carcharias, on Cape fur seals, Arctocephalus pusillus pusillus, were studied over an 8-year period at Seal Island, South Africa. Attacks occurred primarily during winter months (June–August). Attack frequency increased significantly during northerly winds, during high tides, and within 400m of the island, but predatory success rate decreased with proximity to the island. Attacks occurred over a depth range of 5–31m, with significantly more occurring at depths of 26–30m. Attack frequency and success rate increased significantly at low light levels. These results are compared with published effects of environmental factors on white shark predation frequency at the Farallon Islands, California, and discussed in terms of the Predation Cycle. Suggestions for future work at this site are offered.
... Females tend to perform a higher number of behaviours when the sea is calm, while males behave more complexly when the sea is slightly rough. The water agitation around the bait gear may mislead the predator into reckoning a live prey or may represent a crypsis advantage for the attack [35]. In Gansbaai, females usually swim near coastal areas, while males spend more time off Dyer Island [25]. ...
... The time of day was strongly connected with light intensity in previous publications; therefore, this factor was split accordingly in this study. Different authors found an influence on the white shark behaviour, mostly associated with the ability of the shark and the prey to detect each other [8,35,[77][78][79]. Being a highly opportunistic predator, the white shark willingly scavenges on available carrion, garbage, and fish caught on lines [42]. ...
... Here, both females and males performed more complex ethograms under high tides, although the results were not statistically significant. Hammerschlag et al. [35] reported that a shark needs a critical minimum depth in order to approach prey undetected. The water depth from the sampling place was likely sufficient for the animals carrying out a higher number of interactions with the bait. ...
The white shark (Carcharodon carcharias) is a charismatic species and, consequently, one of the most studied and protected sharks. This species can be found in a wide range of temperatures and depths, showing site fidelity and migrating across the oceans. This offers a challenge to understanding the processes influencing their lifecycle and, more importantly, assessing anthropogenic disturbances to their populations. These predators’ behaviour has been linked to diverse abiotic factors. Here, an ethological approach was used to understand the influence of environmental variables on white shark behaviour. A different environmental impact was found between the activity of females and males toward the bait. Females performed a higher number of behaviours under daylight, lower sea surface temperatures, short wavelets, clear and cloudy skies, under La Niña events, elevated moonlight and high tides. Males behaved with more complexity at dawn, medium sea surface temperatures, large wavelets, few clouds, high tides, and elevated moonlight. The world’s aquatic habitats are experiencing significant physiochemical shifts due to human-induced climate change. Knowledge about how white sharks respond to environmental factors is essential to guide management and conservation actions.
... It is relatively simple to study the white shark (WS) Carcharodon carcharias (Linnaeus, 1758) predatory behavior, thanks to the ease with which it can be observed from the surface attacking and feeding on pinniped colonies close to rocky islands, where white sharks congregate [1]. Predator-prey interactions can be influenced by environmental factors and depend on the sensory capabilities of a predator and its prey [2]. Accordingly, these factors can drive the former's ability to detect the prey, the latter's ability to avoid the attack, behavioral patterns, and predator activity peaks within marine ecosystems [3]. ...
... Tide range is the most important environmental factor that influences prey choice. At Seal Island in South Africa, Hammerschlag et al. [2] stated that attack frequency to Cape fur seals increased significantly during high tides within 400 m of the island and over a depth range of 5-31 m where seals swim over shallow reefs. During high tide, the minimum depth from which sharks could approach seals was closer to shore, increasing the availability of prey and the possibility of predation. ...
... Higher average visibility and, consequently, low oktas values seemed to facilitate the attack on the seal-shaped decoy in Gansbaai. Our findings do not support the Seal Island results of Hammerschlag et al. [2] that observed a higher predatory success of WSs on Cape fur seals during low light levels and low underwater visibility. This inconsistency could be linked to two reasons: (i) stalking and ambush with low visibility are important for successful prey capture in many fishes, as well as sharks, including C. carcharias [12,[30][31][32]. ...
The influence of environmental factors on prey discrimination of bait-attracted white sharks was studied over a six-year period (2008–2013) at Dyer Island Nature Reserve (Gansbaai, South Africa). Across 240 bait-attracted feeding events observed in this period, both immature and mature white sharks were attracted by the seal-shaped decoy rather than the tuna bait, except for the years 2008 and 2011. Tide ranges, underwater visibility, water temperature, and sea conditions were, in decreasing order, the factors which drove white sharks to select the seal-shaped decoy. High tide lowered the minimum depth from which sharks could approach seals close to the shore, while extended visibility helped the sharks in making predatory choices towards the more energy-rich prey source, the odorless seal-shaped decoy. On the contrary, warmer water is associated with an increase in phytoplankton that reduces underwater visibility and increases the diversity of teleosts including tuna—a known prey of white sharks—driving the sharks to favor the tuna bait. Overall, sea conditions were almost always slightly rough, ensuring a good average underwater visibility. Recommendations for future research work at this site are presented.
... Seasonal variations and consequent changes in environmental parameters greatly influence C. carcharias abundance, segregation and activity Martin et al. 2005;Hammerschlag et al. 2006;Robbins 2007;Kock et al. 2013;Towner et al. 2013), and individuals of different sizes and life stages are likely to have a varied tolerance to abiotic factors (Hussey et al. 2012;Kock et al. 2013). For example, White et al. (2019) reported that juvenile C. carcharias abundance in Southern California Bight was highest when water temperatures were greater than 19°C. ...
... Several studies have also demonstrated that C. carcharias foraging behaviour and predation is influenced by variations in ocean temperature (Weltz et al. 2013;Skubel et al. 2018), water visibility Hammerschlag et al. 2006), lunar phase (Fallows et al. 2016), tidal height and swell Pyle et al. 1996). There is strong evidence to suggest that low ambient light and poor water visibility increase C. carcharias stealth resulting in successful predation upon pinnipeds at the surface (Strong et al. 1992;Pyle et al. 1996;Hammerschlag et al. 2006). ...
... Several studies have also demonstrated that C. carcharias foraging behaviour and predation is influenced by variations in ocean temperature (Weltz et al. 2013;Skubel et al. 2018), water visibility Hammerschlag et al. 2006), lunar phase (Fallows et al. 2016), tidal height and swell Pyle et al. 1996). There is strong evidence to suggest that low ambient light and poor water visibility increase C. carcharias stealth resulting in successful predation upon pinnipeds at the surface (Strong et al. 1992;Pyle et al. 1996;Hammerschlag et al. 2006). ...
The seasonal occurrence and temporal sexual segregation of great white sharks Carcharodon carcharias have been widely documented in various temperate and sub-tropical waters across the globe. Yet, there is limited understanding of the relationship between the life stages and habitat use of C. carcharias , particularly in the Southern Cape. In this study, we investigated the population dynamics of C. carcharias in Mossel Bay, South Africa, between 2009 and 2013, using skipper logbooks and citizen research data obtained by a cage-diving vessel. A total of 3064 sharks, ranging in life history stages from young-of-the-year to subadult, were sighted during 573 trips. Juveniles dominated the sightings throughout the study, and there was marked sexual segregation, with females dominating the total sightings of sharks. C. carcharias were most abundant during the cooler, winter season, with females differing in abundance seasonally and males maintaining a low abundance throughout the year but peaking in the winter. In addition, sea surface temperature was the best indicator of C. carcharias presence. Abundance was greatest when vertical water visibility exceeded 3 m, with cloud cover influencing overall abundance negatively. Likely reasoning for the aggregation of C. carcharias in Mossel Bay includes the favourable conditions and abundance of food. Juvenile sharks may also utilise this area as a training ground to learn from larger conspecifics. This research demonstrates that information on population size and structure of C. carcharias can be obtained effectively through a compilation of logbook and citizen science data to assess and identify potential critical habitats in the quest to develop appropriate management strategies. This research also shows value in commercial cage-diving operations deriving international data sets needed to assess global populations of C. carcharias .
... Aggregation sites are key areas for the survival of this threatened species, and are therefore relevant in terms of sustainable management, scientific research, and conservation (Huveneers et al. 2018, Rigby et al. 2019, White et al. 2019. Previous studies have shown that environmental factors can influence the white shark's distribution, reproduction, diving behaviour, and the probability of interaction with humans , Hammerschlag et al. 2006, Robbins 2007, Weltz et al. 2013, Hoyos-Padilla et al. 2016. However, the majority of such studies has been carried out in coastal environments, so there is scarce information about the ecology of white sharks in remote oceanic areas (Huveneers et al. 2018). ...
... In continental areas such as the Farallon Islands, California, environmental conditions such as high waves, low visibility, cloudy skies, new moon, and high tides have been related to an increase in the occurrence of white sharks, since these conditions are favourable for the predation of northern elephant seals Mirounga angustirostris . A similar effect has been observed in Seal Island, South Africa, by a significant increase of attacks on Cape fur seals Arctocephalus pusillus pusillus related to wind direction and intensity, low tides, and low sunlight (Hammerschlag et al. 2006). ...
... data). However, an increase in white sharks' occurrence could also be due to favourable environmental conditions or an increased prey availability in the area , Hammerschlag et al. 2006, Weltz et al. 2013. ...
Guadalupe Island, Mexico, is one of the most important white shark (Carcharodon carcharias) aggregation sites in the Eastern Pacific. In the waters surrounding Guadalupe Island, cage diving has been carried out since 2001 during August-November; however, there is scarce information regarding the factors associated with this seasonal aggregation. The purpose of this study was to describe the probability of occurrence of white sharks relative to spatial, temporal, and environmental factors in Guadalupe Island. Generalized additive models (GAMs) were used to describe the effect of sea surface temperature, water visibility, tide, moon phase, cloud cover, time of day, and location on white shark occurrence. GAMs were generated from a data set of 6266 sightings of white sharks, classified as immature males, mature males, immature females, and mature females. A sexual segregation related to month was observed, where females arrived after males during late September. GAMs evidenced a segregation of white sharks according to the analysed variables, which is consistent with previous observations in this locality. Environmental preferences for each white shark category are potentially influenced by feeding habits, sexual maturation, and reproduction. This study constitutes a baseline of the effect of the environment on the occurrence of white sharks in Guadalupe Island, which can be used in further studies regarding management and conservation in future climatic and anthropogenic scenarios. Its relevance is related to the understanding of its ecology in oceanic environments and the presence of this threatened species during the ecotourism season.
... Prey may also respond to predation risk by temporally shifting activity to a safer period, such as times when predators are less active or not present (Lima & Bednekoff 1999, Turner & Montgomery 2003, Brown & Kotler 2004). It has been suggested that Cape fur seals Arctocephalus pusillus pusillus employ all 3 of the above strategies to avoid predation from patrolling white sharks Carcharodon carcharias while traversing between their colonies and their offshore feeding grounds (Hammerschlag et al. 2006, Laroche et al. 2008, Johnson et al. 2009b. Cape fur seals typically colonise small, rocky, offshore islands as well as mainland sites along the southern African coastline (David et al. 1986). ...
... Cape fur seal pups first begin to enter the water during the late autumn and winter months (May to August; David et al. 1986). During these periods of heightened foraging behaviour of lactating females and naïve pups entering the water for the first time, white sharks are consistently patrolling inshore waters, particularly within a few hundred metres of Cape fur seal colonies (Martin et al. 2005, Hammerschlag et al. 2006, Laroche et al. 2008. To avoid predation during foraging trips, Cape fur seals have been reported to use predator avoidance strategies such as grouping (safety in numbers), and diving to avoid vulnerability at the surface (Laroche et al. 2008), as well as mobbing behaviour (Johnson et al. 2009b). ...
... To avoid predation during foraging trips, Cape fur seals have been reported to use predator avoidance strategies such as grouping (safety in numbers), and diving to avoid vulnerability at the surface (Laroche et al. 2008), as well as mobbing behaviour (Johnson et al. 2009b). It has also been suggested that Cape fur seals may limit their traversing periods to particular times of day when ambient light conditions might reduce their risk of being predated (Hammerschlag et al. 2006, Laroche et al. 2008. ...
White sharks (Carcharodon carcharias) are highly visual predators, leading to the hypothesis that the predation risk for foraging Cape fur seals (Arctocephalus pusillus pusillus) might differ with ambient light conditions. This study investigated the relationship between environmental fluctuations of ambient light and the traversing behaviour of Cape fur seals in and out of their colony at Mossel Bay, South Africa to better describe potential predator avoidance strategies. A total of 12,144 traversing events were observed over a four-year period and there was an overall trend for Cape fur seals to traverse less often but in relatively larger group sizes during periods when white sharks are suggested to be more active. Specifically, Cape fur seal activity was reduced during winter when white sharks are most actively hunting, and most traversing behaviour occurred at night when Cape fur seals were less likely to be detected by white sharks. However, among nocturnal observations Cape fur seal group sizes increased significantly with moonlight. Although nocturnal predations of Cape fur seals by white sharks have been observed before in Mossel Bay, this is the first study to indicate Cape fur seals might respond to the increased risk of improved white shark visual acuity during moonlit nights by seeking safety in numbers while foraging. Further investigations are needed to assess the effect of the lunar cycle on white shark nocturnal hunting behaviour, but observations presented here suggest that white sharks may pose a bigger threat to Cape fur seals under the light of a full moon.
... At continental aggregation sites, the ethology of C. carcharias has been used to describe hunting strategies Martin, 2003;Martin, Hammerschlag, Collier, & Fallows, 2005;Tricas, 1985), effects of environmental factors on abundance and hunting strategies (Hammerschlag, Martin, & Fallows, 2006;Pyle, Klimley, Anderson, & Henderson, 1996), and local movements of white sharks around pinniped colonies (Goldman & Anderson, 1999;Klimley et al., 2001). Regarding the surface behaviour of bait-attracted white sharks, Sperone et al. (2012) observed eight behavioural responses that showed a complex tactical situation with multiple responses for the acquisition of baits, such as predatory and social behaviours previously observed for South African white sharks (Martin et al., 2005;Sperone et al., 2010). ...
... Regarding tourist activity, the methods in this paper could be used by authorities during citizen science programmes, specifically for the obtention of pictures for photo identification of individuals. As has been observed in other studies, the implementation of these kinds of programme could provide useful information related to the interaction with other species (Hoyos-Padilla, Papastamatiou, O'Sullivan, & Lowe, 2013), demography by a mark-recapture technique (Towner, Wcisel, Reisinger, Edwards, & Jewell, 2013), and ecological data if environmental factors are considered (Becerril-García, 2015;Hammerschlag et al., 2006). ...
... The comparison of social interactions between different environments in oceanic and continental aggregation sites could be an opportunity to describe interactions during competition situations, along with the effect of the environmental and human factors on the frequency and types of behaviour. These comparisons could be useful for future conservation actions in these aggregation sites, as agonistic interactions can be detected and avoided; for example, if a higher frequency of encounters, aggressions, or accidents with the cages are observed during certain environmental conditions(Green & Giese, 2004;Hammerschlag et al., 2006;Johnson & Kock, 2006;Laroche et al., 2007). ...
1. Cage diving is the most important activity for the sustainable use of white sharks
(Carcharodon carcharias). However, information related to their behaviour during
ecotourism is scarce.
2. This study provides useful information for monitoring C. carcharias during cage‐
diving activities around Guadalupe Island, Mexico. Surface behaviour of 106 white
sharks was recorded for 87 days on‐board six cage‐diving boats in 2012, 2013,
and 2014.
3. Of the observed sharks, 63% were immature specimens (n = 67) and 37% were
considered mature (n = 39). Seventy‐one per cent were males (n = 75) and 29%
were females (n = 31).
4. Interactions were classified into one of the 11 behaviours: parading, close inspection, horizontal attack, vertical attack, bait catching, feeding, not feeding, buoy
catching, encounter, escape, and staying.
5. Parading, close inspections, and horizontal attacks were performed more often by
mature males, whereas immature females performed more vertical attacks, with no
differences between mature and immature males.
6. A total of 1,542 ethograms were registered. Each ethogram consisted on average
of 6.3 ± 5.6 behaviours with a significant transitional pattern from horizontal
attacks to parading and close inspections, and from vertical and horizontal attacks
to bait being caught.
7. A pattern related to feeding in a simple stimulus response reflex was observed. The
shark's length seems to play an important role in the efficiency of the attacks, presumably resulting from the experience of mature individuals. Intentional feeding
should be avoided to prevent negative effects related to ecotourism.
8. This study constitutes a baseline for future research on white shark behaviour. It
can be applied in other regions regardless of environmental conditions, quantity
and size of the boats, and types of bait. Using this standard method could improve
the monitoring, management, and conservation of this vulnerable species.
... Although Steiner et al. (2007) conducted their study in an estuary, C. limbatus in the Grand Strand could be displaying similar behaviour at piers. Furthermore, the influence of barometric pressure on C. limbatus presence has also been documented for juvenile C. limbatus (Heupel et al., 2003) along with other shark species (Hammerschlag et al., 2006;Udyawer et al., 2013). Juvenile C. limbatus appeared to leave their nursery after a large decrease in barometric pressure associated with a tropical storm event in Florida (Heupel et al., 2003). ...
... have investigated the effects of large declines in barometric pressure associated with storm events with sharks in estuarine systems and discovered that responses to severe events were species specific. Additionally,Hammerschlag et al. (2006) suggested that barometric pressure influenced white shark Carcharodon carcharias (L. 1758) feeding patterns at Seal Island, South Africa.Hammerschlag et al. (2006) attributed this change in feeding pattern to an increase in seal activity prior to a severe storm event. ...
... of large declines in barometric pressure associated with storm events with sharks in estuarine systems and discovered that responses to severe events were species specific. Additionally,Hammerschlag et al. (2006) suggested that barometric pressure influenced white shark Carcharodon carcharias (L. 1758) feeding patterns at Seal Island, South Africa.Hammerschlag et al. (2006) attributed this change in feeding pattern to an increase in seal activity prior to a severe storm event. Hurricane Matthew affected the study site in 2016; unfortunately, receivers were removed prior to landfall in order to prevent equipment loss. Despite the lack of data during tropical storm events, our study observed a 25% increase i ...
... During feeding forays, seals can do little to control their exposure to the risk of attack by white sharks, if the waters off of the island on which they raise their pups do not provide structural cover (Wcisel et al. 2015). White shark predation at some breeding colonies is intense, with attack rates as high as 1.97 attacks/ hour and success rates as high as 0.55 kills/attack (Hammerschlag et al. 2006). White sharks are not present at all colonies, so seals on other islands experience very little risk of white shark predation at their breeding colony (Bonfil et al. 2005;Dudley 2012). ...
... White sharks are not present at all colonies, so seals on other islands experience very little risk of white shark predation at their breeding colony (Bonfil et al. 2005;Dudley 2012). Risk also varies seasonally, because white sharks feed on seals only during the winter months (Hammerschlag et al. 2006;Jewell et al. 2013), and these patterns create strong spatial and temporal variation in predation risk between sites and seasons. At sites and times where predation risk is high, adult seals cannot leave the island to eat without passing through a band of high-risk, nearshore water where sharks concentrate their attacks. ...
... can do to control the acute risk of predation other than swimming through the zone of danger as quickly as possible (Hammerschlag et al. 2006;Wcisel et al. 2015). Because white sharks attack from below in a rapid rush from dark waters, attacks are unpredictable during a seal's passage through the high-risk area (Hammerschlag et al. 2006). ...
Inducible defences against predators evolve because they reduce the rate of direct predation, but this benefit is offset by the cost (if any) of defence. If antipredator responses carry costs, the effect of predators on their prey is partitioned into two components, direct killing and risk effects. There is considerable uncertainty about the strength of risk effects, the factors that affect their strength, and the mechanisms that underlie them. In some cases, antipredator responses are associated with a glucocorticoid stress response, and in other cases they are associated with trade‐offs between food and safety, but there is no general theory to explain this variation. Here, I develop the control of risk (COR) hypothesis, predicting that proactive responses to predictable and controllable aspects of risk will generally have food‐mediated costs, while reactive responses to unpredictable or uncontrollable aspects of predation risk will generally have stress‐mediated costs. The hypothesis is grounded in laboratory studies of neuroendocrine stressors and field studies of food‐safety trade‐offs. Strong tests of the COR hypothesis will require more studies of responses to natural variation in predation risk and the physiological consequences of these responses, but its explanatory power can be illustrated with existing case studies.
... 23.4− 26.7°C, Goldman 1997), recurrent predation patterns are likely due to behavioral optimization of predation for energy intake rather than for a need to maintain optimal body temperature (as opposed to teleost fishes (Pink et al. 2016)). False Bay, South Africa, is the location of a long-term monitoring program on the predation rates of white sharks on Cape fur seals Arctocephalus pusillus pusillus (Martin et al. 2005, Hammerschlag et al. 2006, 2017 as well as a site of ongoing climate and temperature monitoring (Dufois & Rouault 2012). Accordingly, the system provides an opportunity to investigate for possible relationships between climate-driven temperature variability and predation rates by a marine apex predator. ...
... Seals travel to and from the island via porpoising at the surface . Thus, white shark attacks on seals occur at the water surface, where they can be documented (Hammerschlag et al. 2006. Attacks are concentrated on the southern side of the island, close to shore (within 2 km). ...
... Attacks are concentrated on the southern side of the island, close to shore (within 2 km). Thus, by positioning at the south end of Seal Island where most predatory activity occurs, a single vessel can survey at least 270° uninterrupted to a distance of at least 3.5 km (Hammerschlag et al. 2006, Martin et al. 2009). Observations were made at Seal Island each month, by 1 boat, between 1999 and 2013. ...
Understanding potential responses of aquatic animals to climate variability is important, given the wide-ranging implications of current and future climatic change scenarios. Here, we used long-term data from natural predator?prey interactions between white sharks Carcharodon carcharias and Cape fur seals Arctocephalus pusillus pusillus in False Bay, South Africa, paired with environmental monitoring to examine potential relationships between temperature variability and shark predation rates on seals. Based on generalized linear modelling of a dataset of 941 shark attacks on seals collected over 15 years (1999?2013) during the austral winter (May?September) season, we found water temperature was included as a significant predictor of daily and monthly variability in predation rates. However, the signal of temporal variability over the season emerged as a more predominant predictor. Moreover, inter-annual variability in predation rate appeared linked to other environmental factors (wind, water visibility, and the occurrence of El Niño and La Niña events) rather than water temperature. These data suggest that water temperatures on an intra-annual scale might contribute to predation patterns in white sharks either directly or indirectly (e.g. due to associated changes in prey availability), but do not implicate water temperature as a primary driver in this scenario, or at an interannual scale. It is possible that (1) the metabolic demand of white sharks may be modulated against temperature variability by their partially endothermic nature, and (2) the predation patterns of white sharks on seals are the result of a complex interplay between ambient physical conditions and broader oceanographic, biological, and ecological factors.
... All images by C. Fallows. [Color figure can be viewed at wileyonlinelibrary.com] seals during winter months (Hammerschlag et al. 2006, Towner et al. 2016). However, among the different colonies targeted by sharks, predation risk to seals can vary significantly due to landscape features that either offer protection to seals traversing the gauntlet or leave them exposed to unpredictable risk of attack (Wcisel et al. 2015). ...
... 1B). Such landscape features are absent from False Bay (Fig. 1C), where predation rates on seals average 6.7 attacks per day during the cool season, with success rates as high as 0.55 kills per attack (Hammerschlag et al. 2006). Rarely do spatially discrete natural prey populations share the same general food supply, while being exposed to such distinct variation in predation risk as in the current study system. ...
... These "high shark abundance" seal colonies include False Bay, Mossel Bay, and Geyser Rock, where shark habitat use and predation has been relatively well studied (Martin et al. 2005, Johnson et al. 2009, Fallows et al. 2012, Kock et al. 2013, Towner et al. 2016, Wcisel et al. 2015. During warm months, white sharks shift the focus of their hunting away from seals at the colonies, presumably to feed on teleosts and elasmobranchs, resulting in lower shark occurrence and predation pressure to seals at these high shark abundance colonies during the summer (Hammerschlag et al. 2006, Kock et al. 2013, De Vos et al. 2015a). There are also "low shark abundance" seal colonies within the region that are not targeted by sharks at any time of year (Bonfil et al. 2005, Kock et al. 2013, De Vos et al. 2015a). ...
Predators can impact ecosystems through consumptive or risk effects on prey. Physiologically, risk effects can be mediated by energetic mechanisms or stress responses. The predation-stress hypothesis predicts that risk induces stress in prey, which can affect survival and reproduction. However, empirical support for this hypothesis is both mixed and limited, and the conditions that cause predation risk to induce stress responses in some cases, but not others, remain unclear. Unusually clear-cut variation in exposure of Cape fur seals (Arctocephalus pusillus pusillus) to predation risk from white sharks (Carcharodon carcharias) in the waters of Southwestern Africa provides an opportunity to test the predation-stress hypothesis in the wild. Here, we measured fecal glucocorticoid concentrations (fGCM) from Cape fur seals at six discrete islands colonies exposed to spatiotemporal variation in predation risk from white sharks over a period of three years. We found highly elevated fGCM concentrations in seals at colonies exposed to high levels of unpredictable and relatively uncontrollable risk of shark attack, but not at colonies where seals were either not exposed to shark predation or could proactively mitigate their risk through antipredatory behavior. Differences in measured fGCM levels were consistent with patterns of risk at the site and seasonal level, for both seal adults and juveniles. Seal fGCM levels were not correlated with colony population size, density, and geographic location. Investigation at a high risk site (False Bay) also revealed strong correlations between fGCM levels and temporal variation in shark attack rates, but not with shark relative abundance. Our results suggest that predation risk will induce a stress response when risk cannot be predicted and/or proactively mitigated by behavioral responses.
... 23.4− 26.7°C, Goldman 1997), recurrent predation patterns are likely due to behavioral optimization of predation for energy intake rather than for a need to maintain optimal body temperature (as opposed to teleost fishes (Pink et al. 2016)). False Bay, South Africa, is the location of a long-term monitoring program on the predation rates of white sharks on Cape fur seals Arctocephalus pusillus pusillus (Martin et al. 2005, Hammerschlag et al. 2006, 2017 as well as a site of ongoing climate and temperature monitoring (Dufois & Rouault 2012). Accordingly, the system provides an opportunity to investigate for possible relationships between climate-driven temperature variability and predation rates by a marine apex predator. ...
... Seals travel to and from the island via porpoising at the surface . Thus, white shark attacks on seals occur at the water surface, where they can be documented (Hammerschlag et al. 2006. Attacks are concentrated on the southern side of the island, close to shore (within 2 km). ...
... Attacks are concentrated on the southern side of the island, close to shore (within 2 km). Thus, by positioning at the south end of Seal Island where most predatory activity occurs, a single vessel can survey at least 270° uninterrupted to a distance of at least 3.5 km (Hammerschlag et al. 2006, Martin et al. 2009). Observations were made at Seal Island each month, by 1 boat, between 1999 and 2013. ...
Understanding potential responses of aquatic animals to climate variability is important given the wide-ranging implications of current and future climatic change scenarios. Here, we used long-term data from natural predator–prey interactions between white sharks Carcharodon carcharias and Cape fur seals Arctocephalus pusillus pusillus in False Bay, South Africa, paired with environmental monitoring to examine potential relationships between temperature variability and shark predation rates on seals. Based on generalized linear modelling of a dataset of 941 shark attacks on seals collected over 15 years (1999–2013) during the winter (May–September) season, we found water temperature was included as a significant predictor of daily and monthly variability in predation rates. However, the signal of temporal variability over the season emerged as a more predominant predictor. Moreover, inter-annual variability in predation rate appeared linked to other environmental factors (wind, water visibility, and the occurrence of El Niño and La Niña events) rather than water temperature. These data suggest that water temperatures on an intra-annual scale might contribute to predation patterns in white sharks either directly or indirectly (e.g. due to associated changes in prey availability), but do not implicate water temperature as a primary driver in this scenario or at an interannual scale. It is possible that (1) metabolic demand of white sharks may be modulated against temperature variability by their partially endothermic nature, and (2) predation patterns of white sharks on seals are the result of a complex interplay between ambient physical conditions, and broader oceanographic, biological, and ecological factors.
... Gansbaai is a white shark aggregation site recognized worldwide, and it is the only location in the world where cage-diving trips operate daily, weather permitting [27]. White sharks frequently prey on Cape fur seals (Arctocephalus pusillus pusillus) at rookeries off southern Africa and particularly during autumn and winter, when juvenile seals leave to forage offshore for the first time, making them vulnerable to predation by white sharks [28][29][30][31]. Gansbaai is a white shark hotspot because the white sharks predate on the resident seals, but the diet also includes fishes and other elasmobranchs. ...
... Specifically, the results of the social interactions exhibited by pairs of bait-attracted white sharks in the Dyer Island's Nature Reserve show both competitive modules of follow and follow/give way, and "mild" observation modules [17], such as parallel swim and swim by: the latter, in fact, as shown in Figure 5, was the most observed social behavior. As already noted in the study by Sperone et al. (2012) [40] in Gansbaai, these "mild" observation modules reflect a type of extremely calm approach and an initial hesitation, thus confirming the studies by Bromilow (2014) [86] and the observations made by Martin et al. (2005) [29], who argued that the white shark certainly has a more aggressive approach with natural prey rather than with passive ones, when the investigation period is of longer duration. ...
Knowledge about the social behavior of sharks is a growing research field, but not many observations are available on the social interactions between pairs of sharks in the presence of passive surface bait and mainly related to aggregations. Between 2009 and 2018, in Gansbaai, South Africa, 415 white sharks were sighted, and 525 surface-generated social interactions were identified, exhibited by 169 different white sharks. The mean sighting rate was 0.91 (range 0.18-1.53) white sharks per hour. Eight patterns of social interaction were exhibited: swim by, parallel swim, follow/give way, follow, give way, stand back, splash fights, and piggyback. Non-random interactions occurred when pairs of specimens approached the passive surface bait, confirming that the white sharks made a real choice, showing a dominance hierarchy during the ten years of data collection. Evidence of non-random social interactions in the surface behavior of bait-attracted white sharks Carcharodon carcharias in Gansbaai's transient population was the goal of this research.
... [11][12][13]). More than two decades of monitoring shark-seal interactions at Seal Island in False Bay have revealed unusually clear-cut spatial and temporal variation in predation risk to seals from white sharks [14][15][16][17][18]. ...
... Around the island's perimeter, seals often enter the water to cool down and/or play in large groups, which is referred to as rafting behaviour [24]. During high predation risk periods, rafting seals remain close to the island's edge [17], where depths are shallow, and thus seals are relatively safe from attack from below [14,25] and can also quickly exit the water if a white shark is detected. ...
Predators can impact prey via predation or risk effects, which can initiate trophic cascades. Given widespread population declines of apex predators, understanding and predicting the associated ecological consequences is a priority. When predation risk is relatively unpredictable or uncontrollable by prey, the loss of predators is hypothesized to release prey from stress; however, there are few tests of this hypothesis in the wild. A well-studied predator-prey system between white sharks (Carcharodon carcharias) and Cape fur seals (Arctocephalus pusillus pusillus) in False Bay, South Africa, has previously demonstrated elevated faecal glucocorticoid metabolite concentrations (fGCMs) in seals exposed to high levels of predation risk from white sharks. A recent decline and disappearance of white sharks from the system has coincided with a pronounced decrease in seal fGCM concentrations. Seals have concurrently been rafting further from shore and over deeper water, a behaviour that would have previously rendered them vulnerable to attack. These results show rapid physiological and behavioural responses by seals to release from predation stress. To our knowledge, this represents the first demonstration in the wild of physiological changes in prey from predator decline, and such responses are likely to increase given the scale and pace of apex predator declines globally.
... Prey abundance and distribution is an important factor influencing the movements of marine predators (Taylor et al. 2018a), with foraging success directly related to particular environmental conditions (Cherry and Barton 2017). For example, white sharks (Carcharodon carcharias) are seasonally present in False Bay, South Africa, to prey on Cape fur seals (Arctocephalus pusillus) and rely on low-light periods to increase attack success rate (Hammerschlag et al. 2006). The moon can affect predator occurrence in coastal waters indirectly through tide levels and behaviour of potential prey shoals and directly through visibility (Afonso et al. 2014;Wintner and Kerwath 2018;Niella et al. 2021a). ...
... Heavy rainfall may override regular behavioural patterns of estuarine teleosts, with fish using deeper areas and becoming more active at night following periods of intense rain (Payne et al. 2013). In addition, fine-scale diel variability (i.e. over the circadian cycle of 24 h) in movement is closely related to energy expenditure, facilitating biological processes such as foraging (Hammerschlag et al. 2006;Taylor et al. 2018a) and reproduction (Walsh et al. 2013;Gannon et al. 2015). ...
Greater Sydney is the largest coastal city in Australia and is where bull sharks (Carcharhinus leucas) are present every summer and autumn. A decade of acoustic telemetry data was used to identify drivers of space use for bull sharks and their potential prey, according to standardised 6-h intervals using dynamic Brownian bridge movement models. Influences of environmental, physical, and biological variables on the areas of space use, location, and predator-prey co-occurrence were investigated with generalised additive mixed models. Rainfall in the catchment affected space use for all animals (i.e. teleost species and both sexes of sharks), with varying temporal responses. Male sharks responded most promptly to high rainfall moving upstream in < 1 day, followed by teleosts (2 to 7 days), and female bull sharks after 4 days. Environmental luminosity affected male shark dispersal and space use, possibly indicating use of visual cues for foraging. Physical characteristics of habitat were important factors driving spatial overlaps between predator and prey in estuarine areas. In sandy embayments < 10-m deep, males and female bull sharks overlapped with different species, whereas males and silver trevally (Pseudocaranx georgianus) co-occurred in deep holes (> 30 m). Shark size influenced overlap between sexes, with smaller females less likely to co-occur with larger males (~ 50 cm). Variability in space use suggests spatial segregation by sex and size in bull sharks, with individuals targeting similar prey, yet either in different areas or at different times, ultimately enabling them to exploit different resources when in the same habitats.
... White shark (Carcharodon carcharias) predation on these colonies is well documented. [1][2][3] It therefore follows that there should be evidence of similar behaviour in the geological past. Diedrich 4 showed that the correlation between the presence of seal fossils and abundance of shark teeth in the Proto North Sea may indicate the earliest specialisation for seal hunting in the fossil record in the northern hemisphere. ...
... From the middle of the bone to the medial border below the broken edge of the shaft, there are six bites caused by the shark biting down but not penetrating the bone causing only surface damage (CF1a; Figure 3b). Two bites -one towards the lateral surface and the other towards the medial surfaceshow evidence of the shark biting down and leaving grooves with ridges and grooves inside (CF1b, Figure 3c, d [1][2][3] In the vicinity of Langebaanweg, 'E' Quarry, offshore islands and a protected lagoon that was open to the ocean were present 5 Ma. 11,18 The islands were surrounded by shallow water 23 , making this ideal for haul out and rookeries. ...
A study of the Mio-Pliocene marine palaeoenvironment on South Africa’s west coast revealed aspects of the biology and behaviour of fossil marine mammals. Close examination showed that seals from Langebaanweg suffered from pathologies and bore marks of marine carnivore activity. This study adds to our knowledge of shark feeding behaviour in the geological past and is one of a few studies of sharks feeding on seals in the fossil record. Two incomplete seal humeri with shark tooth marks are the first documented evidence from South Africa’s Mio-Pliocene of such behaviour. These injuries show no healing, which suggests that the animals were most likely scavenged.
Significance:
• Fossil rich deposits at Langebaanweg contribute to the knowledge of South African Mio-Pliocene fossils by placing them in a global context.
• This study is one of five globally that have documented marine carnivores feeding on seals.
• This is the first description of white sharks feeding on seals from South Africa’s geological past.
• This study shows this behaviour was in place on South Africa’s coast as early as 5 million years ago.
• The injuries show no signs of healing, suggesting the most parsimonious explanation is that white sharks were scavenging seal carcasses.
... This may in part explain why tagged sharks were more likely to occur at shallow depths during the night than twilight, although the low number of records corresponding to the abbreviated twilight period relative to the number of day and night records may also have influenced our results. Given the high energetic cost associated with predation attempts, white sharks also modify their behaviour in relation to site-specific environmental conditions that affect their predatory success (Strong 1996;Goldman and Anderson 1999;Hammerschlag et al. 2006;Martin and Hammerschlag 2012;Huveneers et al. 2015). Seals are an important, energy-rich component of the diets of subadult and adult white sharks (Hussey et al. 2012), but periods of nearshore patrolling are energetically costly (Semmens et al. 2013;Watanabe et al. 2019). ...
... Seals are an important, energy-rich component of the diets of subadult and adult white sharks (Hussey et al. 2012), but periods of nearshore patrolling are energetically costly (Semmens et al. 2013;Watanabe et al. 2019). At other aggregation sites, white sharks reduce pinniped predation attempts when the probability of a successful strike declines, seemingly to optimise energy expenditure (Martin et al. 2005;Hammerschlag et al. 2006;Skubel et al. 2018). White sharks patrolling the dynamic Cape Cod coastline for seals may move offshore when local conditions reduce the likelihood of a successful strike by influencing prey behaviour or detectability (e.g. by affecting water clarity, which fluctuates substantially along the coast of Cape Cod; Lyons et al. 2009) or by increasing the energetic cost of swimming near the surf zone (Watanabe et al. 2019). ...
ContextOver the past decade, the coastal waters off Cape Cod, Massachusetts, have emerged as the only known aggregation site for the white shark (Carcharodon carcharias) in the western North Atlantic. During periods of seasonal residency, white sharks patrol the shoreline in search of pinniped prey, bringing them in close proximity to popular beaches where people recreate. AimTo examine whether white sharks off Cape Cod are more likely to occupy shallow depths (and consequently more likely to overlap with recreational water users) under certain conditions. Methods
We deployed short-term, pop-up satellite archival transmitting (PSAT) tags and acoustic transmitters on 14 subadult and adult white sharks off the coast of Cape Cod during the summer and fall of 2017. PSAT tags provided fine-scale depth and temperature data, which were combined with high-resolution location data obtained from an acoustic telemetry array, to identify the depth and temperature preferences of white sharks when resident in the area. Key resultsSharks spent the majority (95%) of tracked time at depths of 0–31m and at temperatures from 8.9°C to 20.7°C. During resident periods along Cape Cod, individuals spent almost half (47%) of their time at depths of less than 4.5m, but made frequent excursions to mid-shelf depths, alternating between the surf zone and deeper offshore waters. Sharks were slightly more likely to occupy shallow depths at night during the new moon. The relationship between shark depth and lunar phase varied over the course of the day, suggesting the mechanism underlying lunar effects differs among diel periods. Conclusions
Although the overall risk posed to humans by white sharks is low, there is a high potential for overlap between white sharks and recreational water users off Cape Cod. The risk of interaction may be slightly higher during periods when local environmental conditions favour the species’ predatory stealth by influencing prey behaviour or detectability. ImplicationsThis study provides the first glimpse into the fine-scale vertical habitat use of white sharks off Cape Cod, which can be used to better understand the risk to recreational water users and to inform public safety practices.
... Although water temperature and season are likely to be correlated, temperatures were quite different at Ōtākou and Te Whaka ā Te Wera, so both explanatory variables were included. Environmental factors such as water flow, sea state and cloud cover may affect sensory reception or detection of prey (Ebert 1991;Hammerschlag et al. 2006;Robbins 2007). The type of prey available and the location of the habitat may also influence the occurrence of shark species (Knip et al. 2010). ...
... Alternatively, the increasingly poor environmental conditions associated with higher sea state may have affected the ability of sevengill sharks to detect the presence of food in disturbed water conditions. Behavioural research on great white sharks (Carcharodon carcharias) suggests that abiotic factors affecting chemoreception and vision will likely influence predator-prey dynamics (Hammerschlag et al. 2006). Therefore, if factors such as near-surface water flow and water clarity were influenced by an increased sea state, the ability of sevengill sharks to detect the chum and bait may have been compromised. ...
Despite being a large, relatively abundant predator, the distribution and seasonal occurrence of the broadnose sevengill shark, Notorynchus cepedianus, in New Zealand is poorly understood. During 71 sampling trips conducted from July 2013 to May 2015, sharks were attracted to coastal sampling sites in southern New Zealand at Ōtākou/Otago Harbour and Te Whaka ā Te Wera/Paterson Inlet, Stewart Island, using chum. Using a logistic regression model, water temperature was identified as a key predictor of encountering sevengill sharks. In addition, location, cloud cover and sea state were also identified as influential predictors. At Ōtākou, a clear seasonal pattern of sevengill shark sightings emerged. Sharks were detected on 86% of survey trips in summer, whilst no sharks were detected in winter or spring. At Te Whaka ā Te Wera, sharks were sighted throughout all seasons, but a decline in shark encounters occurred during winter. This study represents the first systematic data on seasonal habitat use by sevengill sharks in New Zealand.
... * M. Sclafani ms332@cornell.edu 1 migration modes (Jonzén et al. 2011) and behavior can subsequently shape community structure and species coexistence (Hilborn 1975;Holt 1984). Predator movements, and foraging behavior and ability (Hammerschlag et al. 2006;Grigaltchik et al. 2012), can also be mediated by diel, seasonal or episodic environmental factors, including temperature (Sanford 2002), light intensity (Einfalt et al. 2012), atmospheric phenomena (Cherry and Barton 2017), turbidity (Lunt and Smee 2014), tidal exposure (Zamon 2001) and current velocity (Robinson et al. 2011). In general, these processes are less well known for marine taxa, especially in the field. ...
... In particular, ocean acidification may alter an organism's ability to detect and respond to critical chemical cues that guide decision-making (e.g., predators, food, mates) [22][23][24]. For instance, Jiahuan et al. (2018) found that the black sea bream's olfactory transduction was substantially hampered and a significant reduction in the in vivo contents of both GABA and ACh in elevated CO 2 conditions, altering their behavioral response to certain olfactory cues [25]. ...
Ocean acidification may diminish the response of many marine organisms to chemical cues that can be used to sense nearby food and predators, potentially altering community dynamics. We used a Y-maze choice experiment to investigate the impact of ocean acidification on the ability of mud snails (Ilyanassa obsoleta) to sense food cues in seawater. Mud snails have a well-adapted chemosensory system and play an important role in estuarine ecosystem functioning. Our results showed substantially diminished foraging success for the mud snail under acidified conditions, as snails typically moved towards the food cue in controls (pH 8.1) and away from it in acidified treatments (pH 7.6). These results, coupled with previous work, clearly demonstrate the magnitude at which ocean acidification may impair foraging efficiency, potentially resulting in severe alterations in future ecosystem dynamics.
... most actively hunting and patrolling at dusk and dawn, and spending the day at rest, digesting food and conserving energy [7,29,47,51]. Hammerschlag et al (2006) reported that GWS predations on cape fur seals were most frequent during low light intensities and predation success rates dropped dramatically as light increased during the day. High octane hunting behaviours and high activity levels of the early morning would have made GWSs especially conspicuous to observers, leading to higher SSRs during early morning sampling trips. ...
Whilst previous studies have described the impact of various environmental conditions on behaviour and abundance of great white sharks (GWS), existing knowledge gaps must be addressed in order to turn the tide on their population declines. This study used data collected by a diving tour operator, to investigate how environmental and anthropogenic variables affected the rate of GWS sightings. Observation data were collected by trained crew and volunteers alongside tourists, and combined with externally sourced environmental data. Hurdle modelling identified that the probability of sighting at least one GWS fluctuated seasonally (peaking during winter), but was also correlated with minimum running air temperature, water visibility and the length of time the boat stayed at anchor. The rate GWSs were sighted also rose in winter, and was associated with maximum running air temperature, the arrival time, seal activity, and the wind direction and speed. These findings indicate that environmental conditions directly impacted upon the sighting frequency, but also influenced habitat selection on a fine spatial scale. This study emphasises that collaboration with ecotourism companies could represent a valuable, inexpensive alternative for scientific data collection, as long as powerful statistical methods are used, the influence of human activity is considered and results are interpreted with consideration of the data collection methodology.
... However, prey distribution impacts diet preferences among populations of great white sharks. South African great white sharks prey preferentially on fur seal colonies (Hammerschlag et al., 2006;Martin et al., 2005). However, as the Mediterranean Sea no longer presents pinniped colonies, nowadays Mediterranean great white sharks mainly predate on large bony fishes like tuna, with which it shares migratory behaviours (Kabasakal, 2016). ...
Sharks occupy all living environments of the marine realm as well as some freshwater systems. They display varied and flexible feeding behaviors, but understanding their diet remains challenging due to their elusive ecology and the invasiveness of stomach content analyses in regard of their threatened status. As a potential alternative, we discuss the variability in δ44/42Ca values recorded in tooth enamel of size‐graded individuals belonging to three species of large sharks with distinct diets (Isurus oxyrinchus, Hexanchus griseus and Carcharodon carcharias). The preliminary results highlight shifts in diet linked to ontogeny (I. oxyrinchus and H. griseus) and spatial distribution (C. carcharias) characterizing feeding behavior in these species at individual and population level. These outcomes agree with the results of traditional stomach analyses supporting that non‐traditional stable isotopes thus represent new perspectives for the study of modern and extinct shark ecology. In addition, for the first time, the Sr/Ca elemental ratios measured in H. griseus reflect sexual differences that could be interpreted in terms of spatial segregation or physiological heterogeneities. This article is protected by copyright. All rights reserved.
... For instance, foraging can occur within hourly or daily spatiotemporal refuges, whereas reproductive opportunities may manifest on monthly or seasonal scales. Prey may not be able to exploit spatiotemporal refuges if they are too narrow or infrequent (e.g., in multipredator systems [108,123]), such that strong non-consumptive effects may still occur. Overall, the costs associated with temporal responses to risk are not well explored under field conditions, which currently limits our ability to predict downstream consequences on individual fitness, population dynamics, and community structure (e.g., [10,99,105]). ...
The landscape of fear (LOF) concept posits that prey navigate spatial heterogeneity in perceived predation risk, balancing risk mitigation against other activities necessary for survival and reproduction. These proactive behavioral responses to risk can affect individual fitness, population dynamics, species interactions, and coexistence. Yet, antipredator responses in free-ranging prey often contradict expectations, raising questions about the generality and scalability of the LOF framework and suggesting that a purely spatial, static LOF conceptualization may be inadequate. Here, we outline a ‘dynamic’ LOF framework that explicitly incorporates time to account for predictable spatiotemporal variation in risk–resource trade-offs. This integrated approach suggests novel predictions about predator effects on prey behaviors to refine understanding of the role predators play in ecological communities.
... * M. Sclafani ms332@cornell.edu 1 migration modes (Jonzén et al. 2011) and behavior can subsequently shape community structure and species coexistence (Hilborn 1975;Holt 1984). Predator movements, and foraging behavior and ability (Hammerschlag et al. 2006;Grigaltchik et al. 2012), can also be mediated by diel, seasonal or episodic environmental factors, including temperature (Sanford 2002), light intensity (Einfalt et al. 2012), atmospheric phenomena (Cherry and Barton 2017), turbidity (Lunt and Smee 2014), tidal exposure (Zamon 2001) and current velocity (Robinson et al. 2011). In general, these processes are less well known for marine taxa, especially in the field. ...
... * M. Sclafani ms332@cornell.edu 1 migration modes (Jonzén et al. 2011) and behavior can subsequently shape community structure and species coexistence (Hilborn 1975;Holt 1984). Predator movements, and foraging behavior and ability (Hammerschlag et al. 2006;Grigaltchik et al. 2012), can also be mediated by diel, seasonal or episodic environmental factors, including temperature (Sanford 2002), light intensity (Einfalt et al. 2012), atmospheric phenomena (Cherry and Barton 2017), turbidity (Lunt and Smee 2014), tidal exposure (Zamon 2001) and current velocity (Robinson et al. 2011). In general, these processes are less well known for marine taxa, especially in the field. ...
Quantifying predator–prey interactions and gaining insights into predator behavior are crucial for optimizing restoration strategies. However, such knowledge is often lacking for marine invertebrates. We examined potential impacts of predation by channeled Busycotypus canaliculatus and knobbed whelks Busycon carica on natural and planted populations of bay scallops in the Peconic Bays, New York, through laboratory and field investigations. In lab experiments, mean predation rates exhibited by small channeled whelks were low: 0.06 and 0.005 scallops d⁻¹ for adult and juvenile scallops, respectively. Predation rates of small knobbed whelks on juvenile scallops were 22 × higher. Eighty-six percent (86%) of scallops consumed by channeled whelks had undamaged shells, while 73% eaten by knobbed whelks had notched ventral margins. In field plots where scallop densities were manipulated via removals/plantings, whelks consumed ~ 2% of ~ 19,100 planted juveniles, whereas crabs and presumably finfish consumed > 40% overall. Acoustic telemetry revealed that tagged channeled whelks moved shorter distances and spent more time in plots planted with scallops versus those without scallops. Whelks spent more time in low versus high-density plots, but consumed far more scallops in the latter. In trials without scallops, whelk movement rates were 5 × higher, presumably due to increased exploratory behavior. Overall, whelks were most active during crepuscular hours and during periods of increasing wind speeds. Our results, combined with population abundance data, suggest that whelks (especially B. carica) are drawn to planted bay scallop aggregations, but probably contribute to relatively low overall mortality in the context of restoration efforts.
... Modelling with a spatial resolving power of 2.5 cpd would also reflect a scenario where a shark detects an object from a greater distance (approx. 9 m) than that used here, which may apply to a white shark that first identifies pinniped prey from below, at distances greater than 10 m, depending on water clarity [51,52]. ...
Shark bites on humans are rare but are sufficiently frequent to generate substantial public concern, which typically leads to measures to reduce their frequency. Unfortunately, we understand little about why sharks bite humans. One theory for bites occurring at the surface, e.g. on surfers, is that of mistaken identity, whereby sharks mistake humans for their typical prey (pinnipeds in the case of white sharks). This study tests the mistaken identity theory by comparing video footage of pinnipeds, humans swimming and humans paddling surfboards, from the perspective of a white shark viewing these objects from below. Videos were processed to reflect how a shark's retina would detect the visual motion and shape cues. Motion cues of humans swimming, humans paddling surfboards and pinnipeds swimming did not differ significantly. The shape of paddled surfboards and human swimmers was also similar to that of pinnipeds with their flippers abducted. The difference in shape between pinnipeds with abducted versus adducted flippers was bigger than between pinnipeds with flippers abducted and surfboards or human swimmers. From the perspective of a white shark, therefore, neither visual motion nor shape cues allow an unequivocal visual distinction between pinnipeds and humans, supporting the mistaken identity theory behind some bites.
... The white shark (WS) Carcharodon carcharias [1] is an important top predator and the largest fish predator in existence, reaching about 6 m in length, combining many particular features including large size, regional endothermy (restricted to swimming muscles, viscera, and brain), and coarsely serrated dentition [2]. Predatory behaviour was recently described by Martin et al. [3], who provided ethograms with frequency and event sequence analyses of behavioural units for the Seal Island white shark population in South Africa, and by Hammerschlag et al. [4], who reported the effects of environmental factors on the frequency and success rate of predatory attacks. The white shark is an interesting species in the study of shark behaviour thanks to the relative ease with which it can be observed from the surface, especially near pinniped colonies on rocky islands where sharks congregate [5]. ...
Between the years 2008 and 2013, six annual research expeditions were carried out at Dyer Island (Gansbaai, South Africa) to study the surface behaviour of white sharks in the presence of two passive prey: tuna bait and a seal-shaped decoy. Sightings were performed from a commercial cage-diving boat over 247 h; 250 different white sharks, with a mean total length (TL) of 308 cm, were observed. Of these, 166 performed at least one or more interactions, for a total of 240 interactions with bait and the seal-shaped decoy. In Gansbaai, there is a population of transient white sharks consisting mainly of immature specimens throughout the year. Both mature and immature sharks preferred to prey on the seal-shaped decoy, probably due to the dietary shift that occurs in white sharks whose TL varies between 200 cm and 340 cm. As it is widely confirmed that white sharks change their diet from a predominantly piscivorous juvenile diet to a mature marine mammalian diet, it is possible that Gansbaai may be a hunting training area and that sharks show a discriminate food choice, a strategy that was adopted by the majority of specimens thanks to their ability to visualize energetically richer prey, after having been attracted by the odorous source represented by the tuna bait.
... In particular, heterogeneous landscapes with spatially disparate resources can result in shifts in predator migration modes (Jonzén, et al. 2011) and behavior can subsequently shape community structure and species coexistence (Hilborn 1975;Holt 1984). Predator movements and foraging behavior/ability (Grigaltchik et al. 2012; Hammerschlag et al. 2006) can also be mediated by diel, seasonal or episodic environmental factors, including temperature (Sanford 2002 (Robinson et al. 2011). In general, these processes are less well known for marine taxa, especially in the eld. ...
Quantifying predator-prey interactions and gaining insights into predator behavior are crucial for optimizing restoration strategies; yet, such knowledge is often lacking for marine invertebrates. We examined potential impacts of predation by channeled ( Busycotypus canaliculatus ) and knobbed whelks ( Busycon carica ) on natural and planted populations of bay scallops in the Peconic Bays, New York, through laboratory and field investigations. Mean rates of predation by small channeled whelks were low: 0.06 and 0.005 scallops d ⁻¹ for adult and juvenile scallops; respective lab predation rates of small knobbed whelks on juvenile scallops were 24x higher. Channeled whelks usually did not damage shells of scallop prey (86%), but knobbed whelks typically notched the ventral shell margin (73%). In field plots where scallop densities were manipulated via removals/plantings, whelks consumed ~2% of ~19,100 planted juveniles; crabs and presumably finfish consumed over 40% in all trials overall. Acoustic telemetry revealed that tagged channeled whelks moved shorter distances and spent more time in plots planted with scallops versus those without scallops. Unexpectedly, whelks spent more time in low vs. high density plots, but consumed far more scallops in the latter. In trials without scallops, whelk movement rates were 5x higher, presumably due to increased exploratory behavior. Overall, whelks were most active during crepuscular hours and during periods of increasing wind speeds. Our results, combined with population abundance data, suggest that whelks (especially B. carica ) are drawn to high density scallop plantings and may be important causes of bay scallop mortality in the context of restoration efforts.
... Photo identification must be very detailed, and the database must include data such as sex, length, parasites, and scars on the body to avoid potential mistakes (Towner et al. 2013a;Micarelli et al. 2015). White sharks frequently prey on Cape fur seals (Arctocephalus pusillus pusillus) at rookeries off southern Africa and particularly during autumn and winter, when juvenile seals leave to forage offshore for the first time, making them vulnerable to predation by white sharks (Martin et al. 2005;Hammerschlag et al. 2006;Kirkman et al. 2006;Towner et al. 2013b). False Bay, Gansbaai and Mossel Bay are white shark hotspots because the white sharks predate on the resident seals. ...
In Gansbaai (South Africa), at Dyer Island Nature Reserve, a large White shark population is present and can be observed due to the support of local ecotourism operators authorised to reach the field observation sites. Between 2009 and 2019, it was possible to create a database including information about each individual observed. In total, 423 white sharks were sighted during 462 direct observation hours from the boat, that included 220 hours from the diving "cage". The mean sighting rate was 0.91 (range 0.18-1.53) sharks per hour and sighting rates dramatically declined in the last three years of the study period. Ninety-nine unique Photo-Ids of the dorsal fin were collected and only five re-sightings occurred, which indicate a transient behaviour for the Gansbaai White shark population. The sex ratio showed that females were always prevalent over males throughout the duration of the observations: the ratios were 1:2.2:0.8 for males, females, and unsexed sharks, respectively, and showed the prevalence of immature female individuals (immature: 51 males, 201 females, and 40 unsexed; adults: 49 males, 14 females, and 1 unsexed; undefined maturity: 5 males, 19 females, and 43 unsexed sharks). The predominance of immatures only applies to the females; there were as many immature males (51) as mature (49). The total length for all the individuals was between 150 cm and 500 cm (mean 308 cm, n = 423) with few young-of-the-year and adults recorded, indicating that Gansbaai Area is not a nursery area nor an adult aggregation site, but a seasonal feeding ground. The interannual sighting trend showed a consistent long-term increasing peak (ca. 4-5 years) and this could confirm that, in Gansbaai, the White shark frequency is not affected by ecotourism but, since 2017, a consistent loss of sightings was also due to recorded transient killer whales' unusual fatal attacks.
... A consistent pattern was observed with most species being more frequently caught during dark periods, i.e. afterdusk and in low moonlight. Similarly, white sharks (Carcharodon carcharias) make the most use of nearshore areas in eastern Australia during new moon periods (Lee et al. 2018) and rely on low-light conditions to increase predation success in South Africa (Hammerschlag et al. 2006). Nevertheless, feeding chronology has been observed to vary among carcharhinid sharks in the North Atlantic Ocean, indicating different periods of feeding intensity among species (Driggers III et al. 2012). ...
Oceanic islands are productive ecosystems, and so have higher densities of many marine predators. We investigated the dynamics of elasmobranch and teleost predators in coastal waters off Réunion Island, Indian Ocean, using fisheries-independent data from a preventative shark fishing program from January 2014 to December 2019. We developed a moonlight index that calculates exact moonlight through incorporating lunar azimuth, elevation angle and island topography. We quantified spatial-temporal and environmental drivers of occurrence using zero-inflated mixed models and assessed species-specific catchability in the program. A consistent segregated pattern was observed with higher occurrence of all species at dusk and after-dusk associated with lower luminosity. Scalloped hammerhead sharks (Sphyrna lewini) and giant trevally (Caranx ignobilis) were found to patrol coastal waters earlier in the day than the other species. Tiger (Galeocerdo cuvier) and bull (Carcharhinus leucas) sharks showed high spatial segregation, potentially reducing competition. Teleost predators were found more frequently inside the coral reef environment of the Marine Protected Area but there was no clear pattern for sharks. Seasonality was observed for giant trevally, stingrays, bull sharks, and giant guitarfish (Rhynchobatus australiae), with higher presence during early winter periods related to turbidity, photosynthetically available radiation, and temperature. Inter-annual variation in catch rates suggested that juvenile tiger sharks might be replacing bull sharks in nearshore habitats, and the consequences for mitigation of shark bite hazard are discussed. Operational alternatives are proposed to enhance reducing the impacts of preventative shark fishing upon critically endangered species, improve their conservation and ensure local ecosystem balance.
... Nonostante gli studi, le motivazioni che causano questi comportamenti sono poco comprese a causa delle caratteristiche degli ambienti poco facili da raggiungere e delle relative difficoltà nell'osservarli (Hammerschlag et al., 2006). I grandi squali bianchi sono endotermi (Tricas e McCosker, 1984) e presentano temperature dello stomaco che superano quelle esterne anche di 14.3° C (Goldman, 1997). ...
... Hypotheses that white sharks aggregate around this marine park solely to predate upon pinnipeds may overestimate the significance of this group of long-nosed fur seals (Arctocephalus forsteri), understating the value of other pinniped-rich foraging grounds, which warrant investigation (DSEWPaC, 2013; objective 7 -identify and protect critical white shark habitat, with an emphasis on key foraging areas). Additionally, the dietary importance of pinnipeds may be overstated, driven by the relative ease of observing breaching predation attempts (Hammerschlag, Martin, & Fallows, 2006;Martin, Hammerschlag, Collier, & Fallows, 2005) and that most known white shark aggregations are in the vicinity of pinniped colonies, despite the abundance of cetaceans and teleosts in white shark gut content (Hussey et al., 2012). Understanding the relative importance of different prey items, in the context of key foraging grounds, requires further research extending outside cage-diving locations. ...
Shark and ray tourism is growing in popularity and often necessitates attractants like bait and chum to encourage close encounters. Such practices remain contentious amongst stakeholders as they may affect the species they target. We used lipid and fatty acid profiles to investigate the effects of South Australia's cage-diving industry on the diet and nutritional condition of white sharks Carcharodon carcharias (n = 75). We found no evidence of dietary shifts or reduced nutritional condition after a >3 week period of tourism-exposed residency at the Neptune Islands where the cage-diving industry operates. White sharks fed on a variety of prey groups, similar to other populations around Southern Australia that are not exposed to ecotourism provisioning. These findings indicate that current cage-diving operations in South Australia do not alter white shark diet and nutritional condition where prey resources are abundant.
... Klimley et al. (2001) described the hunting strategy of this species in California in terms of movements and behaviour. Martin et al. (2005) provided an ethogram, with frequency and event sequence analyses of behavioural units, for a shark population in South Africa, and Hammerschlag et al. (2006) reported the effects of environmental factors on frequency and success rate of predatory attacks. Social interaction among white sharks, however, has received little attention, but increasing evidence suggests that this species is socially complex (Martin 2003). ...
... Since 2005, considerable attention has been given to the predator-prey relationship at the island. Studies have documented white shark predation strategies, predation rates and factors influencing predation success (Martin et al., 2005;Hammerschlag et al., 2006;Laroche et al., 2008;Fallows et al., 2012) physiological stress response of seals to natural variation in predation rates (Hammerschlag et al., 2017) as well as anti-predator strategies employed by seals De Vos and O'Riain, 2010;Martin and Hammerschlag, 2012;De Vos et al., 2015a, 2015b. ...
Over the past three decades, marine resource management has shifted conceptually from top-down sectoral approaches towards the more systems-oriented multi-stakeholder frameworks of integrated coastal management and ecosystem-based conservation. However, the successful implementation of such frameworks is commonly hindered by a lack of cross-disciplinary knowledge transfer, especially between natural and social sciences. This review represents a holistic synthesis of three decades of change in the oceanography, biology and human dimension of False Bay, South Africa. The productivity of marine life in this bay and its close vicinity to the steadily growing metropolis of Cape Town have led to its socio-economic significance throughout history. Considerable research has highlighted shifts driven by climate change, human population growth, serial overfishing, and coastal development. Upwelling-inducing winds have increased in the region, leading to cooling and likely to nutrient enrichment of the bay. Subsequently the distributions of key components of the marine ecosystem have shifted eastward, including kelp, rock lobsters, seabirds, pelagic fish, and several alien invasive species. Increasing sea level and exposure to storm surges contribute to coastal erosion of the sandy shorelines in the bay, causing losses in coastal infrastructure and posing risk to coastal developments. Since the 1980s, the human population of Cape Town has doubled, and with it pollution has amplified. Overfishing has led to drastic declines in the catches of numerous commercially and recreationally targeted fish, and illegal fishing is widespread. The tourism value of the bay contributes substantially to the country’s economy, and whale watching, shark-cage diving and water sports have become important sources of revenue. Compliance with fisheries and environmental regulations would benefit from a systems-oriented approach whereby coastal systems are managed holistically, embracing both social and ecological goals. In this context, we synthesize knowledge and provide recommendations for multidisciplinary research and monitoring to achieve a better balance between developmental and environmental agendas.
... However, only three studies in our review directly measured elasmobranch predation rates during dark periods. Of the latter, Hammerschlag et al. (2006) found white shark success rate of seal capture was highest during crepuscular periods (dawn), and Lowe et al. (1994) found Pacific electric rays [Tetronarce californica (Ayres, 1855)] to be extremely active at night, more actively hunting and seeking prey (shown by decreased latency times in rays attacking prey at night vs the day). ...
... Off the South African coastline, studies demonstrate that C. carcharias often frequent regions that are associated with Cape fur seal (A. pusillus pusillus) foraging corridors (Hammerschlag et al., 2006;Martin et al., 2005;Wcisel et al., 2015). Therefore, the predictability of A. pusillus pusillus movements through the high refuge area (i.e. ...
Large-scale and small-scale natural barriers have the ability to mediate the ecological dynamics within a region. In some instances, these barriers greatly influence the presence of predators and prey on spatio-temporal scales. For this study, we aimed to assess how varying densities of sea bamboo (Ecklonia maxima), a kelp species found within South African waters, could influence the presence of C. carcharias. Using baited remote underwater video systems (BRUVS), thirty-one different C. carcharias were identified and a total of 135 h of video were collected in high, moderate, and low kelp densities to create an organismal sighting index. Generalized linear models illustrated that the best fit model included the main effect of kelp density, with a clear inverse relationship between kelp density and C. carcharias sightings. However, although zero sharks were sighted within the high kelp density regions, ten different C. carcharias were sighted within the moderate kelp density regions, illustrating that the broad notion that C. carcharias do not navigate through kelp ecosystems is false and requires more specificity (e.g. high density kelp areas may result in decrease C. carcharias presence). In contrast, kelp density had no significant influence on the presence of five other elasmobranch species detected by the BRUVS. This study demonstrated that highly dense kelp forests (i.e. ≥1 stalk of kelp per 1 m 2) serve as a local natural barrier for large C. carcharias; however, the need for a more inclusive analysis (e.g. routine inclusion of current speed and multi-location assessment) is warranted.
... flipper or fins; Hammerschlag et al. 2018). Relevant environmental attributes widely found to decrease predation risk include increased habitat complexity (Schmitz 1998), proximity to refuge (Atwood et al. 2018), and visibility (Hammerschlag et al. 2006) as well as decreased depth (Rypel et al. 2007). These environmental attributes can be measured in situ or via remotely sensed data (e.g. ...
Understanding the ecological impacts of sharks on prey populations has become a research priority given widespread shark population declines due to overfishing, combined with significant conservation efforts. Accordingly, many studies have conducted analyses of shark stomach contents and/or used biomarkers, such as stable isotope signatures, to assess dietary pat-terns in order to infer ecological roles. Here, I summarize how relying on stomach contents and/or stable isotope signatures to assess the potential for sharks to initiate trophic cascades can be misleading and may significantly underestimate the strength of shark top-down predation effects on prey. However, a study approach that measures attributes of the sharks (e.g. hunting mode), potential prey (e.g. escape speed) and the environment (e.g. habitat rugosity) can provide greater insights for quantifying the magnitude of top-down predation effects of sharks and the potential for their population declines or recoveries to trigger trophic cascades. To aid future investigations,I provide a set of predictions, based on ecological theory, which would specifically lead to increases in the magnitude of shark predation effects on prey populations. I also present key study approaches currently being employed by researchers to test such predictions.
... In addition, Rozkov et al. (Rozhkov et al. 2014) reported that when the full moon approaches, mood worsens and anxiety traits increase; the authors argued that the gravitational pull was behind this moon-related effect. Also, there is evidence to suggest that moonlight may affect animal physiology and behaviour (DeVries et al. 2004;Donati et al. 2001;Hammerschlag et al. 2006;Hazin et al. 2008;Penteriani et al. 2010;Takemura et al. 2004;Wolfe and Tan Summerlin 1989). Still, despite the relatively high number of contact injuries recorded (nearly 600) in the current study, no effect of the moon cycle was observed. ...
Previous literature suggests that human behaviour and physiology are somehow altered by the moon-cycle, with particular emphasis on poorer sleep quality and increased aggressive behaviour during full moon. The latter variables can negatively impact athletes’ recovery and increase the likelihood of injury resulting from collision with another athlete. Therefore, the current study aimed to investigate the association between the lunar cycle and injury risk in professional football players (soccer). We monitored injuries and player exposure in the premier professional league in Qatar during four consecutive seasons (2013–2014 through 2016–2017). Acute (sudden-onset traumatic) injuries (n = 1184; 587 from contact with another player and 597 without player contact) recorded during matches and training were classified according to the lunar cycle characteristics on the date of injury: (i) moon illumination, (ii) lunar distance from earth and (iii) tidal coefficient, acquired from the lunar calendar and tide tables. We used a Poisson regression model to examine the relationship between injury risk and lunar cycle characteristics. We did not detect any association between injury risk and moon illumination, earth-to-moon distance or tidal coefficient, not for all acute injuries, nor for contact and non-contact injuries when examined separately. The findings suggest that the full moon or new moon or the gravitational pull have no effect on football injuries. Thus, organisers need not consult moon or tide tables when planning future event schedules.
... Lunar phase may either affect predator behaviour and abundance directly through abiotic factors such as lunar illumination (Poisson et al. 2010) and tide (e.g. Hammerschlag et al. 2006;Afonso et al. 2014) or indirectly by affecting biotic factor such as distribution of prey (di Natale and Mangano 1995). ...
The influence of environmental variables on shark catch in the bather-protection program along the eastern coast of South African was investigated for 11 commonly caught species (Carcharhinus limbatus, C. obscurus, C. brachyurus, C. plumbeus, C. brevipinna, C. leucas, Carcharodon carcharias, Carcharias taurus, Sphyrna lewini, S. zygaena, Galeocerdo cuvier). Data for the period 1986–1994 were analysed using generalised additive models and generalised additive mixed models. The influence of temporal and spatial factors was respectively considered and removed within a standardisation procedure to investigate and predict the influence of lunar cycle, sea-surface temperature (SST) and water visibility on daily catch. The catches of C. taurus and C. brevipinna were significantly affected by all three factors. Catches of all other species were affected by at least one of the three factors, e.g. six were affected by lunar phase, six by SST and seven by water visibility. The results suggested that measurable, predictable relationships exist between environmental conditions and presence and, consequently, catch of shark species in this program. Understanding these relationships could be useful to mitigate against unwanted catch and to further reduce risk for bathers.
... Secondly, we observed daily visits by sharks to the turtle-nesting sites almost exclusively at night, when turtles would be most available and vulnerable (Fig 4E), as green sea turtles are nocturnal nesters [60]. Similarly, great white sharks (Carcharodon carcharias) have been found to target cape fur seals (Arctocephalus pusillus pusillus) at their island entry and exit points during times of low light [61,62]. ...
The potential effectiveness of marine protected areas (MPAs) as a conservation tool for large sharks has been questioned due to the limited spatial extent of most MPAs in contrast to the complex life history and high mobility of many sharks. Here we evaluated the movement dynamics of a highly migratory apex predatory shark (tiger shark Galeocerdo cuvier) at the Galapagos Marine Reserve (GMR). Using data from satellite tracking passive acoustic telemetry, and stereo baited remote underwater video, we estimated residency, activity spaces, site fidelity, distributional abundances and migration patterns from the GMR and in relation to nesting beaches of green sea turtles (Chelonia mydas), a seasonally abundant and predictable prey source for large tiger sharks. Tiger sharks exhibited a high degree of philopatry, with 93% of the total satellite-tracked time across all individuals occurring within the GMR. Large sharks (> 200 cm TL) concentrated their movements in front of the two most important green sea turtle-nesting beaches in the GMR, visiting them on a daily basis during nocturnal hours. In contrast, small sharks (< 200 cm TL) rarely visited turtle-nesting areas and displayed diurnal presence at a third location where only immature sharks were found. Small and some large individuals remained in the three study areas even outside of the turtle-nesting season. Only two sharks were satellite-tracked outside of the GMR, and following long-distance migrations, both individuals returned to turtle-nesting beaches at the subsequent turtle-nesting season. The spatial patterns of residency and site fidelity of tiger sharks suggest that the presence of a predictable source of prey and suitable habitats might reduce the spatial extent of this large shark that is highly migratory in other parts of its range. This highly philopatric behaviour enhances the potential effectiveness of the GMR for their protection.
... The white shark is globally listed as vulnerable (IUCN 2016), with protection provided under a variety of international treaties and national legislative instruments throughout its distribution. Accordingly, studies on the species are numerous and much is now known about its biology and ecology (Bruce and Bradford 2012;Cliff et al. 1989;Domeier 2012;Francis 1996), movements (Bonfil et al. 2010;Boustany et al. 2002;Domeier andNasby-Lucas 2007, 2013;Weng et al. 2007a), predatory behaviour (Hammerschlag et al. 2006;Martin et al. 2005) and population structure (Blower et al. 2012;Gubili et al. 2012;Oñate-González et al. 2015;Pardini et al. 2000). However, most studies have focussed on sub-adult and adult white sharks because of their predilection to aggregate at readily accessible sites, such as pinniped colonies (Bruce et al. 2006;Chapple et al. 2011;Klimley et al. 2001;Robbins 2007;Towner et al. 2013). ...
Estuarine environments are known to provide important feeding, breeding, resting and nursery areas for a range of shark species, including some which are considered dangerous to humans. Juvenile white sharks (<3 m) are known to frequent inshore environments, particularly ocean beaches, but their presence in and use of estuaries and coastal embayments is unclear. Given that estuarine environments are often surrounded by highly populated areas, understanding how white sharks use these environments will not only assist with their conservation management, but also inform public safety policies. The use of estuarine environments by acoustic-tagged white sharks was investigated from 2009 to 2015 at Port Stephens, New South Wales and Corner Inlet, Victoria, both of which adjoin known nursery areas for the species. Juvenile white sharks were detected within both estuaries, with 20 individuals recorded within the Port Stephens estuary, including four on one day. Only one tagged shark was detected within Corner Inlet; however, monitoring effort and local tagging in the area was more restricted. Detections in Port Stephens were predominantly from October to January and peaked in November. This study demonstrates that the footprint of known nursery areas for white sharks in eastern Australia should be expanded to include their adjacent estuarine environments. Consequently, there is clear potential for them to be exposed to a range of anthropogenic estuarine impacts, and that human interactions are more likely over warmer periods (summer), when human use of such waterways is more prevalent.
... However, only three studies in our review directly measured elasmobranch predation rates during dark periods. Of the latter, Hammerschlag et al. (2006) found white shark success rate of seal capture was highest during crepuscular periods (dawn), and Lowe et al. (1994) found Pacific electric rays [Tetronarce californica (Ayres, 1855)] to be extremely active at night, more actively hunting and seeking prey (shown by decreased latency times in rays attacking prey at night vs the day). ...
It is commonly assumed that elasmobranchs (sharks, skates, and rays) are most active during dark periods (dawn, dusk, night). However, this assertion has not been critically evaluated. It is also unclear whether dark periods are primarily utilized for the performance of important life-history events, such as mating. If this were the case, low-light periods would be of significance to elasmobranch conservation as some anthropogenic activities (night fishing, lighting) could disproportionately impact fitness of species that are more active in the dark. Here, we review and summarize previous studies on elasmobranch behavior during nocturnal and crepuscular periods focusing on patterns of movement, habitat use, foraging, and reproduction. A review of 166 studies provided mixed results for widely-assumed increased elasmobranch activity when dark. Frequency of foraging and horizontal movement (distance travelled, activity space) were reported as greater only during crepuscular periods in the majority (>50%) of reviewed studies (28 of 43 and 78 of 125 studies, respectively), a pattern not evident during night. No pervasive patterns emerged for increased habitat use or reproductive behaviors during dark. We did not find any particular habitat type consistently supporting increased activity during dark, nor did we find evidence that higher trophic level elasmobranchs were more active when dark. Thus, generalizations about increased elasmobranch activity during dark periods are currently not supported. While research on the behavior of elasmobranchs during dark periods has been increasing, many knowledge gaps remain and we present a set of research priorities to assist in the development of future investigations.
... Secondly, we observed daily visits by sharks to the turtle-nesting sites almost exclusively at night, when turtles would be most available and vulnerable (Fig 4E), as green sea turtles are nocturnal nesters [60]. Similarly, great white sharks (Carcharodon carcharias) have been found to target cape fur seals (Arctocephalus pusillus pusillus) at their island entry and exit points during times of low light [61,62]. ...
Sleep is a crucial part of the daily activity patterns of mammals. However, in marine species that spend months or entire lifetimes at sea, the location, timing, and duration of sleep may be constrained. To understand how marine mammals satisfy their daily sleep requirements while at sea, we monitored electroencephalographic activity in wild northern elephant seals (Mirounga angustirostris) diving in Monterey Bay, California. Brain-wave patterns showed that seals took short (less than 20 minutes) naps while diving (maximum depth 377 meters; 104 sleeping dives). Linking these patterns to accelerometry and the time-depth profiles of 334 free-ranging seals (514,406 sleeping dives) revealed a North Pacific sleepscape in which seals averaged only 2 hours of sleep per day for 7 months, rivaling the record for the least sleep among all mammals, which is currently held by the African elephant (about 2 hours per day).
Despite the low chance of a person being bitten by a shark, there are serious associated costs. Electronic deterrents are currently the only types of personal deterrent with empirical evidence of a substantial reduction in the probability of being bitten by a shark. We aimed to predict the number of people who could potentially avoid being bitten by sharks in Australia if they wear personal electronic deterrents. We used the Australian Shark Attack File from 1900 to 2020 to develop sinusoidal time-series models of per capita incidents, and then stochastically projected these to 2066. We predicted that up to 1063 people (range: 185–2118) could potentially avoid being bitten across Australia by 2066 if all people used the devices. Avoiding death and injury of people over the next half-century is of course highly desirable, especially when considering the additional costs associated with the loss of recreational, commercial and tourism revenue potentially in the tens to hundreds of millions of dollars following clusters of shark-bite events.
Like landscapes of fear, animals are hypothesized to strategically use lightscapes based on intrinsic motivations. However, longitudinal evidence of state-dependent risk aversion has been difficult to obtain in wild animals. Using high-resolution biologgers, we continuously measured body condition, time partitioning, three-dimensional movement, and risk exposure of 71 elephant seals throughout their 7-month foraging migrations ( N = 16,000 seal days). As body condition improved from 21 to 32% fat and daylength declined from 16 to 10 hours, seals rested progressively earlier with respect to sunrise, sacrificing valuable nocturnal foraging hours to rest in the safety of darkness. Seals in superior body condition prioritized safety over energy conservation by resting >100 meters deeper where it was 300× darker. Together, these results provide empirical evidence that marine mammals actively use the three-dimensional lightscape to optimize risk-reward trade-offs based on ecological and physiological factors.
The twilight zone contains the largest biomass of the world’s ocean. Identifying its role in the trophic supply and contaminant exposure of marine megafauna constitutes a critical challenge in the context of global change. The white shark (Carcharodon carcharias) is a threatened species with some of the highest concentrations of neurotoxin methylmercury (MeHg) among marine top predators. Large white sharks migrate seasonally from coastal habitats, where they primarily forage on pinnipeds, to oceanic offshore habitats. Tagging studies suggest that while offshore, white sharks may forage at depth on mesopelagic species, yet no biochemical evidence exists. Here, we used mercury isotopic composition to assess the dietary origin of MeHg contamination in white sharks from the Northeast Pacific Ocean. We estimated that a minimum of 72% of the MeHg accumulated by white sharks originates from the consumption of mesopelagic prey, while a maximum of 25% derives from pinnipeds. In addition to highlighting the potential of mercury isotopes to decipher the complex ecological cycle of marine predators, our study provides evidence that the twilight zone constitutes a crucial foraging habitat for these large predators, which had been suspected for over a decade. Climate change is predicted to expand the production of mesopelagic MeHg and modify the mesopelagic biomass globally. Considering the pivotal role of the twilight zone is therefore essential to better predict both MeHg exposure and trophic supply to white sharks, and effectively protect these key vulnerable predators.
The twilight zone contains the largest biomass of the
world’s ocean. Identifying its role in the trophic supply and
contaminant exposure of marine megafauna constitutes a critical
challenge in the context of global change. The white shark
(Carcharodon carcharias) is a threatened species with some of the
highest concentrations of neurotoxin methylmercury (MeHg)
among marine top predators. Large white sharks migrate seasonally
from coastal habitats, where they primarily forage on pinnipeds, to
oceanic offshore habitats. Tagging studies suggest that while
offshore, white sharks may forage at depth on mesopelagic species,
yet no biochemical evidence exists. Here, we used mercury isotopic
composition to assess the dietary origin of MeHg contamination in
white sharks from the Northeast Pacific Ocean. We estimated that a
minimum of 72% of the MeHg accumulated by white sharks originates from the consumption of mesopelagic prey, while a maximum
of 25% derives from pinnipeds. In addition to highlighting the potential of mercury isotopes to decipher the complex ecological cycle
of marine predators, our study provides evidence that the twilight zone constitutes a crucial foraging habitat for these large predators,
which had been suspected for over a decade. Climate change is predicted to expand the production of mesopelagic MeHg and
modify the mesopelagic biomass globally. Considering the pivotal role of the twilight zone is therefore essential to better predict
both MeHg exposure and trophic supply to white sharks, and effectively protect these key vulnerable predators.
Shark attacks are rare but traumatic events that generate social and economic costs and often lead to calls for enhanced attack mitigation strategies that are detrimental to sharks and other wildlife. Improved understanding of the influence of environmental conditions on shark attack risk may help to inform shark management strategies. Here, we developed predictive models for the risk of attack by white Carcharodon carcharias, tiger Galeocerdo cuvier, and bull/ whaler Carcharhinus spp. sharks in Australian waters based on location, sea surface temperature (SST), rainfall, and distance to river mouth. A generalised additive model analysis was performed using shark attack data and randomly generated pseudo-absence non-attack data. White shark attack risk was significantly higher in warmer SSTs, increased closer to a river mouth (<10 km), and peaked at a mean monthly rainfall of 100 mm. Whaler shark attack risk increased significantly within 1 km of a river mouth and peaked in the summer months. Tiger shark attack risk increased significantly with rainfall. We performed additional temporal and spatio-temporal analyses to test the hypothesis that SST anomaly (SSTanom) influences white shark attack risk, and found that attacks tend to occur at locations where there is a lower SSTanom (i.e. the water is relatively cooler) compared to surrounding areas. On the far north coast of eastern Australia — an attack hotspot — a strengthening of the East Australian Current may cause white sharks to move into cooler upwelling waters close to this stretch of the coast and increase the risk of an attack.
Dyer Island is thought to host one of the most abundant populations of white sharks on the planet, this is often credited to the large (55 – 60,000) Cape fur seal colony at Geyser Rock. Yet relatively little work has ever been produced from the area. This may be attributed to the harshness in its location as a study site, exposed to wind and swell from west to east which limits research periods. This study accounts for over 220 hrs of manual tracking at Dyer Island with a further 68 added from the inshore shallow areas of the bay. Sharks focused their movements and habitat use to reefs or channels that allowed access to Cape fur seals. Movement-Based Kernel Estimates (MKDE) were used to compute home range estimates for shark movements through and around the heterogeneous structures of Dyer Island and Geyser Rock. Inshore two core areas were revealed, one being the major reef system at Joubertsdam and the other at a kelp reef where the tracked shark had fed on a Cape fur seal. At Dyer Island one core area was identified in a narrow channel, ‘Shark Alley’, here a second tracked shark foraged for entire days within meters of rafting Cape fur seals.
Studies on the feeding behaviour of large predatory sharks are inherently problematic because of the size of the animals, the difficulty in observing and handling them and the risks to the investigator. Observations on the feeding behaviour of the sevengill shark Notorynchus cepedianus over a seven-year period revealed this species to display at least four distinct foraging strategies. A particular strategy, or a combination of two or more, was employed depending on the size and type of prey being subdued. The four primary factors that appear to influence prey selection were prey size, availability and density, and scavenging. The behaviour of a prey species influences the predator's foraging and hunting behaviour. The success of the sevengill shark as an apical predator can be attributed, in part, to the feeding strategies it employs to subdue prey.
The degree to which white sharks, Carcharodon carcharias, are social while hunting is unclear. Our aim was to describe the behavior and interactions among white sharks hunting seals
near a seal colony. We attached ultrasonic beacons to five adult white sharks, 4.5–5.2 m long, and recorded their movements
and behavior toward each other over a 15-day period in October 1997 at Año Nuevo Island, California. This site is home to
colonies of four species of seals and sea lions. Two additional sharks, females 5.5 and 4.7 m in length, were later tracked
intensively during periods of 12 and 3 days during October 1998 and November 1999, respectively. We recorded stomach temperature
(indicative of feeding on warm-bodied seals) and swimming depths from the 5.5-m female, swimming speed and depth from the
4.7-m female. We monitored the movements and behavior of these sharks using an array of sonobuoys moored near the island;
the receptive field measured 1 km2. Our principal findings were: (1) the sharks spent a mean time of 39.5% of each day patrolling within the receptive field;
(2) no shark ever moved far out of it; (3) the sharks spent an equal amount of time and activity in the receptive field at
all times of the day, daytime, twilight, and nighttime; (4) movements with respect to the island rookery were most often back
and forth parallel to the shoreline, (5) tracks of three sharks, tagged at the same time and place, overlapped more often
than those of the other two sharks; and (6) some sharks patrolled certain areas in the field preferentially, but there was
no conclusive evidence that they defended these areas as territories. Feeding appeared to be infrequent: only two likely feeding
bouts occurred during a cumulative 78-day/shark period that individuals were monitored at Año Nuevo Island. The behavior and
movements of the sharks were consistent with a hunting strategy, in which individuals search for prey independently but, at
the same time, remain close enough to each other to “sense” and exploit a kill by any one of them by joining in on the kill
to feed.
A review of past behavioral ultrasonic telemetry studies of sharks and rays is presented together with previously unpublished material on the behavior of the lemon shark, Negaprion brevirostris, around the Bimini Islands, Bahamas. The review, focusing on movement behaviors of 20 shark and three ray species, reveals that elasmobranchs exhibit a variety of temporal and spatial patterns in terms of rates-of-movement and vertical as well as horizontal migrations. The lack of an apparent pattern in a few species is probably attributable to the scarcity of tracking data. Movements are probably governed by several factors, some still not studied, but data show that food, water temperature, bottom type, and magnetic gradient play major roles in a shark's decision of where and when to swim. A few species exhibit differences in behavior between groups of sharks within the same geographical area. This interesting finding warrants further research to evaluate the causes of these apparent differences and whether these groups constitute different subpopulations of the same species. The lack of telemetry data on batoids and some orders of sharks must be addressed before we can gain a more comprehensive understanding of the behavior of elasmobranch fishes. Previously unpublished data from 47 smaller and 38 larger juvenile lemon sharks, collected over the decade 1988–1998, provide new results on movement patterns, habitat selection, activity rhythms, swimming speed, rate-of-movement, and homing behavior. From these results we conclude that the lemon shark is an active predator with a strong, apparently innate homing mechanism. This species shows ontogenetic differences in habitat selection and behavior, as well as differences in movements between groups of individuals within the same area. We suggest three hypotheses for future research on related topics that will help to understand the enigmatic behavior of sharks.
This paper presents information on the movements of white sharks, Carcharodon carcharias, at the South Farallon Islands (SFI), central California. Acoustic telemetry techniques provided preliminary data on the diurnal space utilization, movement patterns and swimming depths of four white sharks, ranging from approximately 3.7 to 4.9m in length. Sharks swam within about 10m of the bottom to depths of approximately 30m, but in deeper water they tended to stray more from the bottom. Activity spaces for time periods tracked ranged from 1.84 to 9.15km2. Indications are that an inverse relationship exists between length and activity space. During the time tracked, larger individuals swam within particular areas around the islands whereas smaller individuals did not restrict their movements in the same manner. Values of a site attachment index were inversely related to length for all sharks tracked. The site attachment indices, apparent inverse relationship between total length and activity space and observations on telemetered and other known individuals support a hypothesis that larger sharks possess site fidelity in their search for prey at SFI, within and between years. With the high frequency of predation by white sharks on juvenile northern elephant seals at SFI in the fall, the majority of the sharks' movements are probably related to their search for these pinniped prey. These data provide preliminary evidence that white sharks at SFI may search for prey by swimming in a particular area over a number of days or weeks, traversing the area in a manner which maximizes coverage, and swimming close to the bottom or at a distance far enough from the surface to remain cryptic from prey.
Understanding the foraging behavior and spatial distribution of top predators is crucial to gaining a complete understanding of communities. However, studies of top predators are often logistically difficult and it is important to develop appropriate methods for identifying factors influencing their spatial distribution. Sharks are top predators in many marine communities, yet no studies have quantified the habitat use of large predatory sharks or determined the factors that might influence shark spatial distributions. We used acoustic telemetry and animal-borne video cameras ("Crittercam") to test the hypothesis that tiger shark (Galeocerdo cuvier) habitat use is determined by the availability of their prey. We also used Crittercam to conduct the first investigation of foraging behavior of tiger sharks. To test for habitat preferences of sharks, the observed proportion of time in each habitat for each individual was compared to the predicted values for that individual based on correlated random walk and track randomization methods. Although there was individual variation in habitat use, tiger sharks preferred shallow seagrass habitats, where their prey is most abundant. Despite multiple encounters with potential prey, sharks rarely engaged in prolonged high-speed chases, and did not attack prey that were vigilant. We propose that the tiger sharks' foraging tactic is one of stealth, and sharks rely upon close approaches to prey in order to be successful. This study shows that using appropriate analysis techniques and a variety of field methods it is possible to elucidate the factors influencing habitat use and gain insights into the foraging behavior of elusive top predators.
Between 1997 and 2003, there were 2088 natural predations by white sharks (Carcharodon carcharias) on Cape fur seals (Arctocephalus pusillus pusillus) and 121 strikes on towed seal-shaped decoys were documented from observation vessels at Seal Island, South Africa. White sharks at Seal Island appear to selectively target lone, incoming young of the year Cape fur seals at or near the surface. Most attacks lasted < 1 min and consisted of a single breach, with predatory success rate decreasing rapidly with increasing duration and number of subsequent breaches. A white shark predatory ethogram,composed of four phases and 20 behavioural units, is presented, including four varieties of initial strike and 11 subsequent behaviour units not previously defined in the literature. Behaviour units scored from 210 predatory attacks revealed that, for both successful and unsuccessful attacks, Polaris Breach was the most commonly employed initial strike, while Surface Lunge was the most frequent second event, closely followed by Lateral Snap. Examination of video footage, still images, and tooth impressions in decoys indicated that white sharks at Seal Island bite prey obliquely using their anterolateral teeth via a sudden lateral snap of the jaws and not perpendicularly with their anterior teeth, as previously supposed. Analysis of white shark upper tooth morphology and spacing suggest the reversed intermediate teeth of white sharks occur at the strongest part of the jaw and produce the largest wound. White sharks predatory success at Seal Island is greatest (55%) within one hour of sunrise and decrease rapidly with increasing ambient light; the sharks cease active predation on seals of sunrise and decreases rapidly with increasing ambient light; the sharks cease active predation on seals when success rate drops to +/- 40%; this is the first evidence of cessation of foraging at unproductive times by any predatory fish. At Seal Island, white shark predatory success is significantly lower at locations where frequency of predation is highest, suggesting that white sharks may launch suboptimal strikes in areas of greatest intraspecific competition; this is the first evidence of social influence on predation in any elasmobranch. Idiosyncratic predatory behaviours and elevated success rates of known individual white sharks at Seal Island suggest some degree of trial-and-error learning. A hypothetical decision tree is proposed that models predatory behaviour of white sharks attacking Capr fur seals at the surface.
Carcharodon carcharias was studied at Dangerous Reef, South Australia. A single bit action is composed of a uniform sequence of jaw and head movements. Various approach behaviors to baits were documented. Small sharks (<3 m) feed primarily on fish prey, while larger sharks feed on marine mammals, especially pinnipeds. Telemetric studies of white shark thermal biology show that they are warm-bodied, c4-5oC above ambient water temperature. -from Sport Fishery Abstracts
Pacific angel sharks ambush demersal fishes by rapidly lunging from the sea floor. These sharks consistently attacked rubber and plastic prey models during in situ experiments. Videotaped strikes ranged from approximately 30-100 msec in duration and included extreme cranial elevations (as much as 90°) toward the models. Eleven of 23 sharks tested attacked a fish model from beneath a transparent partition, which was designed to block near-field mechanical cues. Head-on approaches by the model elicited a higher frequency of attacks and shorter attack latencies than caudal approaches (frequency, P = 0.04; latency, P = 0.03). Night attacks may have been facilitated by turbulence-generated bioluminescence. Resting sharks and vacated depressions were usually adjacent to reefs, either facing or aligned parallel to margins of nearby reefs, and oriented toward upslope directions. Four sharks that were identified based on natural pigment patterns returned to the near proximities (< 3 m) of previously occupied ambush sites. Based on our experiments and observations, we suggest that (1) prey movement sensed visually is the most important cue for eliciting daytime attacks, (2) the sharks probably possess an anterodorsally directed field of vision, and (3) ambush sites are selected based on substrata characteristics likely to be associated with prey availability.
Spatial and temporal records of 146 predatory attacks by white sharks (Carcharodon carcharias) on four species of pinnipeds, one bird, and one human at the South Farallon Islands, Central California, from late Aug. to early Dec. 1986-89 are presented. During each 3.5-mo period, attacks were (1) unevenly distributed in bouts separated by hiatuses in predation, (2) paired temporally within the same day, (3) at similar times and locations on consecutive days, and (4) all during daylight hours. Predation was observed most often within 450 m of shore, with a decrease in attack frequency with increasing depth. Within this high-risk zone, predation was concentrated near coastal departure and entry points of pinnipeds, and the predatory attack positions formed linear patterns leading away from the island. Consecutive predatory attacks were often near each other, yet at times alternated between localities on either side of the island.
As a group, sharks have survived for more than 300 million years and are admirably adapted to their environment. Their complex nervous and reproductive systems are of great scientific interest. Although a hazard in certain areas, they are also human benefactors, providing a small but useful supplement to normal sources of fish flesh, as well as a particularly fine quality leather.
Basic population parameters and behaviours of great white sharks, Carcharodon carcharias,
were studied during four expeditions to Spencer Gulf, South Australia. In all, 58 white sharks
were observed, ranging in length from about 2.2 to 5.5 m TL (X= 3.7 m TL ± 0.7 s.d.); of these
sharks, 32 were subsequently tagged. Many sharks were observed repeatedly, the most
frequently resighted individual being seen on 22 days over a 197-day period spanning two
expeditions. Sharks in the study area were segregated by sex but not by size. Females were
most abundant at 'inshore' islands, whereas males occurred mainly at 'offshore' islands.
Further, there were no marked differences in abundance between summer and winter.
Nine ultrasonic trackings, lasting a maximum of 27.5 h, revealed three general horizontal
movement patterns: downstream circling, island patrolling, and inter-island cruising. Average
rate of movement was 3.2 km h-1 (n = 145 time intervals of 15 min each). Sharks generally
remained near the surface or along the bottom (about 20 m), spending relatively little time in
midwater. Daytime swimming depths were significantly shallower than those at night.
Shark predation on Cape fur seals, Arctocephalus pusillus pusillus, from the Eastern Cape coast of South Africa was investigated. Observations were made on an opportunistic basis over a five year period from 1991 to 1996. Evidence of shark predation was obtained from: (1) incidental observations of shark attacks on live Cape fur seals, and (2) counts of shark bite wounds on the bodies of live and dead seals. In Eastern Cape waters, potential shark predators include the white, Carcharodon carcharias; broadnose sevengill, Notorynchus cepedianus; bluntnose sixgill, Hexanchus griseus; shortfin mako, Isurus oxyrinchus; bull, Carcharhinus leucas; tiger, Galeocerdo cuvier; and dusky, Carcharhinus obscurus. Only the white and broadnose sevengill shark have been observed actively feeding on Cape fur seals. Shark bitten seals (n = 33) were observed throughout the year, with coastal records peaking in the autumn/winter period. Adult animals were observed with shark bite wounds more often than young. Wounds were usually located on the lower body, suggesting that attacks were made from behind. Fresh shark bite wounds were found on a minimum of 3.4% of seals found stranded on local beaches, and 0.3% of seals observed at the Black Rocks colony during the breeding season. Although predatory encounters appear to be common, long term studies are required before we can fully assess the extent of shark predation along the Eastern Cape coast. These records are the only known examples of shark inflicted injuries on southern fur seals Arctocephalus spp. south of 10°S.
A new automated ultrasonic telemetry system for monitoring the swimming activity of adult rainbow trout, Salmo gairdneri, at liberty in the wild is described. The transmitter detects bioelectric potentials (i.e. electromyograms) associated with the contraction of the epaxial myomeres during swimming. Transmitter output is relayed to the signal processing system via submerged hydrophones. The incoming signals represent averaged electromyograms which have been shown in earlier studies to correlate well with swimming activity and concurrent oxygen consumption of rainbow trout in the laboratory.
Electromyogram records obtained from rainbow trout released into a small lake and monitored for up to 4 weeks, indicate a fairly regular pattern of elevated midday activity contrasting with periods of relative quiescence during the evening and morning. This midday peak in locomotory activity is the result of an increased feeding activity which is evidently a response to a concomitant increase in the activity of the fish's prey.
Acoustic telemetry was used to follow 22 blue sharks,Prionace glauca (Linnaeus), over the continental shelf and slope in the region between George's Bank and Cape Hatteras between 1979 and 1986. The sharks frequently made vertical excursions between the surface and depths of several hundred meters. The oscillations, which were repeated every few hours, were largest in the daytime and were smaller in amplitude and confined to depths near the thermocline at night. This behavior was prominent in trials from August through March, but was not seen from June through July. Diving is discussed in terms of a hunting tactic and behavioral thermoregulation. Most of the sharks moved in a southeasterly direction from the release point and many maintained a constant course day and night for several days. The sharks may orient to the earth's magnetic field, or to the ocean's electric fields, allowing them to swim on a constant heading in the absence of celestial cues. These possibilities are discussed in the appendix.
Despite the recent upsurge of interest in shark research, the current status of knowledge of the behavioural repertoire of most species is alarmingly incomplete. Clearly, from the steadily decreasing numbers of sharks caught by commercial and sport fishermen, sharks are highly vulnerable to human exploitation. Although education is making inroads, there is still steady opposition to the enforcement of catch limits and management strategies for most species. Accurate life history and behavioural information is required to enforce management policies. Wetherbee et al. (1990) cited a case in which commercial fishermen accused the spiny dogfish of stripping the commerical and recreational fisheries of their herring and salmon catch. A detailed study of the spiny dogfish diet disproved their claims.
Sharks are clearly not mindless eating machines, as they have been labelled in the past. They are intelligent and have complex patterns of movement, space utilization, and social organization. Using a combination of remote and direct observational techniques, the scientific community is beginning to have a more complete understanding of these important apex predators in coral reef and oceanic ecosystems. More importantly, researchers who are interested in pursuing the fascinating field of shark behaviour still have a wide choice of direction.
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