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Article
Passive Prey Discrimination in Surface Predatory Behaviour of
Bait-Attracted White Sharks from Gansbaai, South Africa
Primo Micarelli 1, *, Federico Chieppa 1, Antonio Pacifico 1,2 , Enrico Rabboni 1and Francesca Romana Reinero 1
Citation: Micarelli, P.; Chieppa, F.;
Pacifico, A.; Rabboni, E.; Reinero, F.R.
Passive Prey Discrimination in
Surface Predatory Behaviour of
Bait-Attracted White Sharks from
Gansbaai, South Africa. Animals 2021,
11, 2583. https://doi.org/10.3390/
ani11092583
Academic Editors: Martina
Francesca Marongiu and
Tyrone Lucon-Xiccato
Received: 21 July 2021
Accepted: 1 September 2021
Published: 3 September 2021
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Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1
Sharks Studies Center—Scientific Institute, 58024 Massa Marittima, Italy; chieppafederico@gmail.com (F.C.);
antonio.pacifico86@gmail.com (A.P.); ENRICO.RABBONI@gmail.com (E.R.);
ricerca@centrostudisquali.org (F.R.R.)
2Department of Political Science and CEFOP-LUISS, LUISS Guido Carli University, 00197 Rome, Italy
*Correspondence: direzione@centrostudisquali.org; Tel.: +39-3896732796 or +39-0566919529
Simple Summary:
White sharks, in surface passive prey predatory behaviour, are initially attracted
by the olfactory trace determined by the bait and then implement their predatory choices to energeti-
cal richer prey, especially thanks to their visual ability, which plays an important role in adults and
immatures with dietary shifts in their feeding patterns. Gansbaai represents a hunting training area
for white sharks who are changing their diet.
Abstract:
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.
Keywords: white shark; behaviour; Gansbaai; prey choice; shark vision; shark olfaction
1. Introduction
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
]. White sharks
have the largest olfactory bulb among sharks, and thanks to their perceptions, they are able
to trace wounded prey, whale carcasses, seal colonies or sea lions even at great distances [
3
].
During hunting, sharks refer mainly to odorous stimuli from prey [
5
], but many sharks are
also thought to rely on their visual system for prey detection, predator avoidance, navigation,
and communication [
6
]. For many sharks, vision plays a vital role in their ecology, particularly
Animals 2021,11, 2583. https://doi.org/10.3390/ani11092583 https://www.mdpi.com/journal/animals
Animals 2021,11, 2583 2 of 9
in detecting and identifying prey [
7
–
9
]. It is widely accepted that white sharks undergo
an ontogenetic shift in prey preference [
2
,
10
–
13
]; both stomach-content and stable-isotope
analyses indicate that an ontogenetic shift is expressed by a change in the trophic level,
passing from a predominantly piscivorous diet when immature to a marine mammalian diet
when adult [
10
,
12
–
15
]. The estimated total length (TL) at which they undergo this dietary
shift varies between 200 cm and 340 cm [
2
,
10
–
13
,
16
,
17
]. White sharks’ teeth also reflect their
ontogenetic diet change: as adults, the teeth of their upper jaw have a single large cusp, which
is triangular and has serrated edges, while those of the lower jaw are tighter, smaller, narrower,
and slightly sharper. The lower teeth penetrate and hold the prey, the upper ones cut the flesh:
this enables both the predation of large prey such as pinnipeds, and the detachment of large
pieces of meat from carcasses [
18
]. Martin et al. [
3
] observed that Sea Island white sharks
appear to select the age class of Cape fur seal Arctocephalus pusillus pusillus [
19
], a group
with a defined size and a clear direction of movement, as well as a choice of hunting during
times and in locations that maximize their probability for predatory success. Regarding
the ontogenetic shift, French et al. [
20
] suggested that gender and individual specialization
are also key drivers in white sharks’ ecological variation, and that they remain important
throughout ontogeny; in fact, individuals may learn a variety of different movement tactics
for encountering and catching prey, and they may develop a preference for a particular tactic
based on their experiences [
21
,
22
]. The individual surface behaviour of white sharks in the
presence of a bait, in the same manner as their predatory [
3
,
23
,
24
] and social [
25
] behaviour,
is not a simple stimulus response reflex, but rather a complex tactical situation with plastic
responses [
24
]. Only a few studies have described the mechanisms that underlie patterns
of prey selection in white sharks
[9,26–28]
. The present study carried out along the South
African coasts in Gansbaai was aimed at providing a contribution to confirm whether, by
simulating the natural condition of scavenging, white sharks, in the presence of artificial
passive prey, such as a seal-shaped decoy and tuna bait, implemented a real food choice
based on vision. In the attempt to gather new information on the topic, this study on the
Gansbaai white shark transient population aimed to research the following: (i) the main
surface predatory behaviours of white sharks when using both baits and seal-shaped decoys;
(ii) the existence of possible interlinkages between predatory behaviour and other observed
endogenous factors, such as maturity; and (iii) white sharks’ tendency to exhibit a real food
choice, based on vision, rather than indiscriminate attacks on the two target passive preys,
which are the bait and seal-shaped decoy.
2. Materials and Methods
Observations and data collection were recorded in the Dyer Island Natural Reserve,
located 7.5 km south-east of Gansbaai, South Africa (34
◦
41
0
S; 19
◦
24
0
E). The reserve
includes Dyer Island and Geyser Rock (Figure 1): the first is a low-profile island ca. 1.5 km
long and 0.5 km wide, and it is characterised by the presence of different seabird colonies;
the second is ca. 0.5 km long and 180 m wide, and it hosts a colony of Cape fur seals,
Arctocephalus pusillus pusillus [19].
In Gansbaai (South Africa), at Dyer Island Nature Reserve, a large white shark popu-
lation is present and can be observed thanks to the support of local ecotourism operators
authorised to reach the field observation sites, with a prevalence of immature individuals [
29
].
The greatest proportion of sub-adults and potentially mature sharks also occurs in Seal Island,
False Bay, South Africa [
30
,
31
]. The 2008–2013 study periods occurred in the autumn season
between March and May and required a total of 247 h. Seal-shaped decoys and floating baits
of tuna pieces, of similar size, were the tested passive target preys; in line with the research
protocol adopted by Sperone et al. [
24
], the two baits were chosen first on the basis of the
odourless seal shape, and second, the production of odour. The decoy’s size was as close as
possible to that of a juvenile Cape fur seal (A. pusillus pusillus): 70 cm long and 32 cm wide,
with a diameter of about 60 cm (Figure 2a,b).
Animals 2021,11, 2583 3 of 9
Animals 2021, 11, x 3 of 9
Figure 1. Dyer Island and Geyser Rock Nature Reserve.
In Gansbaai (South Africa), at Dyer Island Nature Reserve, a large white shark pop-
ulation is present and can be observed thanks to the support of local ecotourism operators
authorised to reach the field observation sites, with a prevalence of immature individuals
[29]. The greatest proportion of sub-adults and potentially mature sharks also occurs in
Seal Island, False Bay, South Africa [30,31]. The 2008-2013 study periods occurred in the
autumn season between March and May and required a total of 247 h. Seal-shaped decoys
and floating baits of tuna pieces, of similar size, were the tested passive target preys; in
line with the research protocol adopted by Sperone et al. [24], the two baits were chosen
first on the basis of the odourless seal shape, and second, the production of odour. The
decoy’s size was as close as possible to that of a juvenile Cape fur seal (A. pusillus pusillus):
70 cm long and 32 cm wide, with a diameter of about 60 cm (Figure 2a,b).
(a) (b) (c)
Figure 2. (a) Decoys, (b) interaction shark versus seal-shaped decoy, and (c) tuna bait.
The ethological observations on the sharks were made from the 13 m “Barracuda”
boat owned by “Shark diving unlimited”, which was anchored at 100m-150 m off Dyer
Island. The boat was equipped with an upper deck from which it was possible to observe
and photograph the sighted sharks. The observations were also carried out in an anti-
shark cage, which was fixed, for the duration of the observations, to the side of the boat.
Throughout this study, sharks were identified by the same research team (Sharks Studies
Center—Centro Studi Squali—Istituto Scientifico, Massa Marittima, Italy), and the identi-
fication was based on the recognition of different anatomical features such as the dorsal
fin, the caudal fin, and the presence of scars and ectoparasites and their arrangement
[29,32,33]. It was also possible that one specimen was observed at various times through-
out the day and that all exhibited behaviours were recorded. All sharks’ total lengths were
estimated from the boat, referring to structures of a size known as the length of the cage.
Inside the cage, operators were equipped with a mask, boots, a semi-dry suit and weights,
remaining on the surface and maintaining a vertical position. When white sharks ap-
proached, the operators descended in a free dive to the bottom of the cage, in order not to
disturb the sharks with air bubbles and to be able to observe, up close, the behaviour of
the animals, the sex of the specimen and other body features. The cage was, therefore, an
excellent and useful tool for identifying the sex of each individual with precision and for
Figure 1. Dyer Island and Geyser Rock Nature Reserve.
Animals 2021, 11, x 3 of 9
Figure 1. Dyer Island and Geyser Rock Nature Reserve.
In Gansbaai (South Africa), at Dyer Island Nature Reserve, a large white shark pop-
ulation is present and can be observed thanks to the support of local ecotourism operators
authorised to reach the field observation sites, with a prevalence of immature individuals
[29]. The greatest proportion of sub-adults and potentially mature sharks also occurs in
Seal Island, False Bay, South Africa [30,31]. The 2008-2013 study periods occurred in the
autumn season between March and May and required a total of 247 h. Seal-shaped decoys
and floating baits of tuna pieces, of similar size, were the tested passive target preys; in
line with the research protocol adopted by Sperone et al. [24], the two baits were chosen
first on the basis of the odourless seal shape, and second, the production of odour. The
decoy’s size was as close as possible to that of a juvenile Cape fur seal (A. pusillus pusillus):
70 cm long and 32 cm wide, with a diameter of about 60 cm (Figure 2a,b).
(a) (b) (c)
Figure 2. (a) Decoys, (b) interaction shark versus seal-shaped decoy, and (c) tuna bait.
The ethological observations on the sharks were made from the 13 m “Barracuda”
boat owned by “Shark diving unlimited”, which was anchored at 100m-150 m off Dyer
Island. The boat was equipped with an upper deck from which it was possible to observe
and photograph the sighted sharks. The observations were also carried out in an anti-
shark cage, which was fixed, for the duration of the observations, to the side of the boat.
Throughout this study, sharks were identified by the same research team (Sharks Studies
Center—Centro Studi Squali—Istituto Scientifico, Massa Marittima, Italy), and the identi-
fication was based on the recognition of different anatomical features such as the dorsal
fin, the caudal fin, and the presence of scars and ectoparasites and their arrangement
[29,32,33]. It was also possible that one specimen was observed at various times through-
out the day and that all exhibited behaviours were recorded. All sharks’ total lengths were
estimated from the boat, referring to structures of a size known as the length of the cage.
Inside the cage, operators were equipped with a mask, boots, a semi-dry suit and weights,
remaining on the surface and maintaining a vertical position. When white sharks ap-
proached, the operators descended in a free dive to the bottom of the cage, in order not to
disturb the sharks with air bubbles and to be able to observe, up close, the behaviour of
the animals, the sex of the specimen and other body features. The cage was, therefore, an
excellent and useful tool for identifying the sex of each individual with precision and for
Figure 2. (a) Decoys, (b) interaction shark versus seal-shaped decoy, and (c) tuna bait.
The ethological observations on the sharks were made from the 13 m “Barracuda” boat
owned by “Shark diving unlimited”, which was anchored at 100–150 m off Dyer Island.
The boat was equipped with an upper deck from which it was possible to observe and
photograph the sighted sharks. The observations were also carried out in an anti-shark cage,
which was fixed, for the duration of the observations, to the side of the boat. Throughout
this study, sharks were identified by the same research team (Sharks Studies Center—
Centro Studi Squali—Istituto Scientifico, Massa Marittima, Italy), and the identification
was based on the recognition of different anatomical features such as the dorsal fin, the
caudal fin, and the presence of scars and ectoparasites and their arrangement [
29
,
32
,
33
].
It was also possible that one specimen was observed at various times throughout the day
and that all exhibited behaviours were recorded. All sharks’ total lengths were estimated
from the boat, referring to structures of a size known as the length of the cage. Inside the
cage, operators were equipped with a mask, boots, a semi-dry suit and weights, remaining
on the surface and maintaining a vertical position. When white sharks approached, the
operators descended in a free dive to the bottom of the cage, in order not to disturb the
sharks with air bubbles and to be able to observe, up close, the behaviour of the animals,
the sex of the specimen and other body features. The cage was, therefore, an excellent and
useful tool for identifying the sex of each individual with precision and for supporting the
identification made on board. The sex of each shark was determined by the surface and by
cage-diving observations, and also with underwater video recordings of the pelvic fin area:
the males were recorded if claspers were seen and the females if the lack of claspers was
verified and their pelvic fin area was filmed [
29
]. All other specimens were categorised as
being of unknown sex. In our study, we estimated the white shark’s size at sexual maturity
according to Hewitt et al. [
31
] and Micarelli et al. [
29
]: a mature male if the TL was
≥350 cm
and a mature female if the TL was
≥
450 cm. To attract the sharks, olfactory stimulants
(chum) were used, following the methods described in Laroche et al. [
34
], Ferreira and
Ferreira [
35
], and Sperone et al. [
25
]. The chumming was composed of sea water, cod liver
Animals 2021,11, 2583 4 of 9
oil (Gadus sp.), tuna blood and small pieces of fish [
25
,
36
]. Shark predatory behaviour was
induced using two types of surface passive prey placed into the water: the bait, consisting
of tuna pieces tied to a floating buoy positioned at the stern of the boat, and a seal-shaped
decoy, positioned at the bow at a distance from the tuna bait of at least 10 m. A constant
distance between the two-surface passive prey was maintained to isolate the seal-shaped
decoy from the odorous and bloody trail coming from the tuna bait. In order to analyse
the surface behaviour of white sharks in the presence of passive preys, tuna baits and an
odourless seal-shaped decoy, we performed a Chi-squared independence test to check
whether the types of prey (bait and seal-shaped decoy) were associated, and whether the
choice of prey was not causal. To investigate the presence of independency between two
causal variables, we used Pearson’s Chi-squared test, where H0 tests the null hypothesis of
independency between the variables, and H1 the alternative hypothesis of dependency.
To strengthen the results regarding the presence of causality between behaviour and prey,
we also used Cochran’s Q test. This is a non-parametric statistical test used to verify
whether k treatments (or number of studies) have identical effects. Generally, the test
statistic refers to two-way randomized block designs, where the response variable takes
only two possible outcomes coded as 0 or 1 denoting failure or success, respectively. It is
often used to assess whether different observers of the same phenomenon have consistent
results (interobserver variability). Cochran’s Q test is as follows: the null hypothesis (H
0
),
where there is no difference in the effectiveness of treatments (the choice is causal) and the
alternative hypothesis (H
1
), where there is a difference in the effectiveness of treatments
(the choice is not causal).
3. Results
3.1. Descriptive Analysis
Overall, 250 white sharks were sighted, with 166 having at least one or more inter-
actions for a total of 240 interactions: 41 adults, 183 immatures and 16 unsexed. The
variables denoting maturity and prey were transformed into dichotomous variables in
order to be quantitatively evaluated (Table 1). In the statistical analysis, the re-interaction
(more precisely, we observed 64 reinteractions) was counted as additional information
investigating the frequency among white sharks’ individual characteristics. During the
discriminant analysis, when studying the independency and heterogeneity among units,
potential re-interactions were counted as unique observations. The same occurred in the
empirical analysis accounting for non-linear methods, to investigate (potential) interactions
among the observed variables. Multiple interactions were dealt with by shrinking the
dataset with respect to every re-sighting. More precisely, the sample size ‘n’—used in
the analysis—refers to the ‘individuals’ (counted only one time for any observation) and
was weighted by the factors (e.g., behaviour and maturity). In this way, any (potential)
bias—affecting the robustness of estimates—was sufficiently avoided in the statistical
analyses. For the shrinking, a discriminant analysis involved in constructing hypotheses
for a significance test on an individual was used before the estimation procedure.
Table 1. Description of variables. Own computations.
Variable Label
Maturity (=1) adult, (=0) immature
Prey (=1) bait, (=0) seal-shaped decoy
3.2. Statistical Analysis
In the analysis of the white sharks (individual) and the two types of prey (bait and
seal-shaped decoy) (Figure 3), the preference of white sharks for the seal-shaped decoy
rather than the bait was proven through appropriate test statistics. More precisely, we ran
a Chi-squared test (Table 2) and Cochran’s test (Table 3) rejecting any causality concerning
Animals 2021,11, 2583 5 of 9
the shark’s choice under the null hypothesis. In this way, the descriptive statistics were
confirmed, and it was proven that the preference for the seal-shaped decoy was not causal.
Animals 2021, 11, x 6 of 9
Figure 3. Bait and seal-shaped decoy shark choice, between 2013 and 2008.
4. Discussion
Carcharodon carcharias is a “top predator” that has a wide distribution in temperate
and tropical areas [5,18] with a higher concentration in eight spots around the world [18],
including South Africa. Studies aimed at investigating the various methods of white
sharks’ surface artificial prey approach or their food choices are scarce: between 1989 and
1992, Anderson et al. [32] examined the predatory behaviour of the Californian Farallon
Islands sharks towards decoys with different shapes, with an approximate size of prey
represented by pinnipeds and also including the reproduction of a sea lion shape, for a
total of 159 h of observation. In that study, it was observed that vision played a major role
in the approach and predation activities: the sharks were attracted, not by smell, electric
fields or vibrations coming from the baits, but only by the presence of odourless prey on
the surface. Strong [9] carried out an experiment comparing two floating shapes, one with
the shape of a seal and the other with a square shape: during the first experiences with
these preys, the sighted sharks showed a significant preference for the seal shape. He also
stated that in his study there was little doubt that the sharks initially located the bait via
olfaction, and vision was clearly used to orient their actual approaches, but its relative
importance to most shark species remains, however, poorly understood. The present
study carried out along the South African coasts in Gansbaai was aimed at providing a
contribution to confirm whether, by simulating the natural condition of scavenging, white
sharks, in the presence of artificial passive prey, such as a seal-shaped decoy and tuna
bait, implemented a real food choice. Micarelli et al. [29] stated that, in Gansbaai, the white
shark population was always made up of a prevalence of immature individuals, and that
the mean TL recorded was 308 cm. It is important to remember that the estimated length
at which white sharks undergo dietary shifts varies between 200 cm and 340 cm in TL
[2,10–13,16,17]; between 2008 and 2013, only six specimens showed a total length <200cm,
of 250 specimens. Tricas and McCosker [14] showed a clear prevalence of an ichthyopha-
gous diet in immature specimens, while adult sharks preferred marine mammals at Dan-
gerous Reef, South Australia. Moreover, in our study, considering the tests performed to
distinguish the adult from the immature individuals, it emerged that the adult sharks
seemed to prefer the seal-shaped decoy, and the frequency of attacks on tuna bait by the
adult sharks was not significantly greater than that of the immature ones. However, since
the Gansbaai population of white sharks showed a mean TL of 308 cm, it is possible that
the majority of Gansbaai white sharks’ transient population had already undergone the
Figure 3. Bait and seal-shaped decoy shark choice, between 2013 and 2008.
Table 2.
Pearson’s Chi-squared test of independence. The first column denotes the sub-categories,
and the second column displays the corresponding statistical results in terms of p-values. The
significant codes are as follows: * significance at 10%, ** significance at 5%, and *** significance at 1%.
Chi-Squared Test of Independence on Prey
Full Sample 0.00 ***
Adult 0.00 ***
Immature 0.00 ***
Table 3. Cochran’s Q test (asymptotic p-value).
Test Statistic 175.98
Chi-squared distribution (critical value) 3.840
Df 1
p-value (one-tailed) 0.027
Significance level 0.050
All the test statistics are significant (at 1%) and consistent (p-value < 0.001).
3.3. Frequency Distribution
In order to investigate potential relationships between white sharks and types of
prey, the previous preliminary analysis was expanded to account for related frequency
distributions. More precisely, the relationship was maturity–prey, highlighting possi-
ble interactions between each shark’s life stage and hunting patterns. The relationship
maturity–prey highlighted that adult and even immature sharks preferred the seal-shaped
decoy, confirming previous results. We performed a Chi-squared test with each frequency
distribution accounted for. The main results, summarised in Table 2, highlight that the null
hypothesis of independency can be rejected according to the whole sample. More precisely,
the relationship between the types of prey (bait and seal-shaped decoy) is associated with a
p-value close to zero (=0.00). This finding shows that their choice is not causal. In Cochran’s
test, the treatments denote how the choice of different prey affects the sharks’ behaviour,
as shown in Table 3. Let k
−
1 = 1 be the degrees of freedom, with k denoting the types of
Animals 2021,11, 2583 6 of 9
prey; as the computed p-value is lower than the significance level (
α
= 5%), one should
reject the null and then accept the alternative hypothesis.
4. Discussion
Carcharodon carcharias is a “top predator” that has a wide distribution in temperate
and tropical areas [
5
,
18
] with a higher concentration in eight spots around the world [
18
],
including South Africa. Studies aimed at investigating the various methods of white sharks’
surface artificial prey approach or their food choices are scarce: between 1989 and 1992,
Anderson et al. [
32
] examined the predatory behaviour of the Californian Farallon Islands
sharks towards decoys with different shapes, with an approximate size of prey represented
by pinnipeds and also including the reproduction of a sea lion shape, for a total of 159 h of
observation. In that study, it was observed that vision played a major role in the approach
and predation activities: the sharks were attracted, not by smell, electric fields or vibrations
coming from the baits, but only by the presence of odourless prey on the surface. Strong [
9
]
carried out an experiment comparing two floating shapes, one with the shape of a seal and
the other with a square shape: during the first experiences with these preys, the sighted
sharks showed a significant preference for the seal shape. He also stated that in his study
there was little doubt that the sharks initially located the bait via olfaction, and vision was
clearly used to orient their actual approaches, but its relative importance to most shark
species remains, however, poorly understood. The present study carried out along the
South African coasts in Gansbaai was aimed at providing a contribution to confirm whether,
by simulating the natural condition of scavenging, white sharks, in the presence of artificial
passive prey, such as a seal-shaped decoy and tuna bait, implemented a real food choice.
Micarelli et al. [
29
] stated that, in Gansbaai, the white shark population was always made
up of a prevalence of immature individuals, and that the mean TL recorded was 308 cm. It
is important to remember that the estimated length at which white sharks undergo dietary
shifts varies between 200 cm and 340 cm in TL [
2
,
10
–
13
,
16
,
17
]; between 2008 and 2013, only
six specimens showed a total length <200 cm, of 250 specimens. Tricas and McCosker [
14
]
showed a clear prevalence of an ichthyophagous diet in immature specimens, while adult
sharks preferred marine mammals at Dangerous Reef, South Australia. Moreover, in
our study, considering the tests performed to distinguish the adult from the immature
individuals, it emerged that the adult sharks seemed to prefer the seal-shaped decoy, and
the frequency of attacks on tuna bait by the adult sharks was not significantly greater than
that of the immature ones. However, since the Gansbaai population of white sharks showed
a mean TL of 308 cm, it is possible that the majority of Gansbaai white sharks’ transient
population had already undergone the dietary shift, and this would explain why immature
specimens also showed a similar interest towards the seal-shaped decoy (Figure 4).
Regarding our question (which are the white sharks’ main surface predatory be-
haviours when using both baits and seal-shaped decoys?), it is possible to conclude from
our studies that, at least as far as the behaviour of adult specimens is concerned, the same
predatory trend observed near the Californian coasts emerged, but also included most
immatures with dietary shift. White sharks must be selective when there is an abundance
of food; therefore, according to the Optimal Foraging Theory [
37
], they prefer the most
caloric sources over low-energy ones [
38
]. In response to the question “Do white sharks
tend to exhibit a real food choice or indiscriminate attacks on the two passive prey (bait
and seal shaped decoy)?”, it is possible to conclude that there was a real food choice
rather than an indiscriminate attack, and that this strategy was adopted by the majority
of specimens, helped by the ability to visualize the energetically richer preys, also with
respect to the odorous source represented by the tuna bait. In this regard, it was possible to
observe, in this study, that, in the contemporary presence of surface prey, characterized
by different stimulating conditions, the white sharks preferred the odourless seal-shaped
decoy rather than the tuna bait, which produces a strong odour stimulus. In only two of
the six observation years, 2011 and 2008, we recorded the prevalence of the bait choice
instead of the seal-shaped decoy. We suppose that this different behaviour, as proposed
Animals 2021,11, 2583 7 of 9
by Sperone et al. [
39
], could be linked to environmental factors, such as cloud cover and
water visibility. This allows us to confirm that vision, in the Gansbaai transient population
of white sharks, as suggested by Anderson [
32
] and Strong [
9
], plays an important role
during the investigation and choice of prey, helping adults and immatures, which are in
progress with dietary shift, to optimize the result of their predatory activity.
Animals 2021, 11, x 7 of 9
dietary shift, and this would explain why immature specimens also showed a similar in-
terest towards the seal-shaped decoy (Figure 4).
Figure 4. Total length range (meters) of white sharks, between 2008 and 2013.
Regarding our question (which are the white sharks’ main surface predatory behav-
iours when using both baits and seal-shaped decoys?), it is possible to conclude from our
studies that, at least as far as the behaviour of adult specimens is concerned, the same
predatory trend observed near the Californian coasts emerged, but also included most
immatures with dietary shift. White sharks must be selective when there is an abundance
of food; therefore, according to the Optimal Foraging Theory [37], they prefer the most
caloric sources over low-energy ones [38]. In response to the question “Do white sharks
tend to exhibit a real food choice or indiscriminate attacks on the two passive prey (bait
and seal shaped decoy)?”, it is possible to conclude that there was a real food choice rather
than an indiscriminate attack, and that this strategy was adopted by the majority of spec-
imens, helped by the ability to visualize the energetically richer preys, also with respect
to the odorous source represented by the tuna bait. In this regard, it was possible to ob-
serve, in this study, that, in the contemporary presence of surface prey, characterized by
different stimulating conditions, the white sharks preferred the odourless seal-shaped de-
coy rather than the tuna bait, which produces a strong odour stimulus. In only two of the
six observation years, 2011 and 2008, we recorded the prevalence of the bait choice instead
of the seal-shaped decoy. We suppose that this different behaviour, as proposed by Sper-
one et al. [39], could be linked to environmental factors, such as cloud cover and water
visibility. This allows us to confirm that vision, in the Gansbaai transient population of
white sharks, as suggested by Anderson [32] and Strong [9], plays an important role dur-
ing the investigation and choice of prey, helping adults and immatures, which are in pro-
gress with dietary shift, to optimize the result of their predatory activity.
5. Conclusions
It is possible to confirm, based on the data collected in the expeditions between 2008
and 2013, that the Gansbaai white shark transient population shows food preferences that
are similar to those already observed in adult specimens in similar studies carried out
along the Australian and Californian coasts [14,32]. The presence of a white shark popu-
lation showing a mean TL of 308 cm, and only six specimens out of 250 showing a TL
<200cm, implies that for the majority of the Gansbaai white shark population, the change
in diet has already occurred or is in progress and, therefore, the interest in energetically
0
10
20
30
40
50
60
2.0-2.5 2.6-3.0 3.1-3.5 3.6-4.0 4.1-4.5 4.6-5.0
TL Range 2008-2013
Figure 4. Total length range (meters) of white sharks, between 2008 and 2013.
5. Conclusions
It is possible to confirm, based on the data collected in the expeditions between 2008
and 2013, that the Gansbaai white shark transient population shows food preferences that
are similar to those already observed in adult specimens in similar studies carried out along
the Australian and Californian coasts [
14
,
32
]. The presence of a white shark population
showing a mean TL of 308 cm, and only six specimens out of 250 showing a TL <200 cm,
implies that for the majority of the Gansbaai white shark population, the change in diet has
already occurred or is in progress and, therefore, the interest in energetically richer prey
can be important for the overall population. Gansbaai, therefore, could represent a hunting
training area for white sharks who are changing or have already changed their diet. It can
be assumed that white sharks are initially attracted by the olfactory trace [
5
] determined by
the bait and then implement their predatory choices to energetical richer prey, especially
thanks to their visual ability, which plays an important role in adults and immatures with
dietary shifts in their feeding patterns. The behaviour shown in the presence of the two
preys examined in this work is, therefore, linked to the different dietary needs of white
sharks in different stages of development. These needs are linked both to the modification
of the dental system and to the different energy necessary in the immature and adult stages.
As demonstrated by Laroche et al. [
34
] and Sperone et al. [
24
], the results presented in this
article are based on the assumption that white sharks do not respond to the presence of the
boat, but focus their attention mainly on the floating object, because the presence of a boat
has very little influence on the white sharks’ behaviour.
Author Contributions:
Conceptualization, P.M.; methodology, P.M.; software, A.P.; validation, P.M.,
F.R.R., F.C. and A.P.; formal analysis, A.P., P.M. and F.R.R.; investigation, P.M., F.R.R., E.R. and F.C.;
writing—original draft preparation, P.M.; writing—review and editing, P.M., F.R.R., F.C. and A.P.;
funding acquisition, P.M. All authors have read and agreed to the published version of the manuscript.
Funding:
This research was funded by the Sharks Studies Center—Scientific Institute, University of
Calabria and University of Siena.
Animals 2021,11, 2583 8 of 9
Institutional Review Board Statement:
The study and experimentation protocols were reviewed
and approved in accordance 166 with the Directive 2010/63/EU.
Data Availability Statement:
https://www.researchgate.net/project/Great-White-Shark-Carcharodon-
carcharias-Behaviour-Ecology-and-cotoxicology/update/61312b952897145fbd6df39c.
Acknowledgments:
This research was made possible by the financial support of the Sharks Studies
Center—Scientific Institute, University of Calabria and University of Siena. We are grateful to the
CSS team members that carried out 11 expeditions for their indirect financial support of this research,
and thanks are also due to Shark Diving Unlimited for the logistical assistance and all other field
assistance with data collection. All research protocols were approved by the University of Calabria
and Siena.
Conflicts of Interest: The authors declare no conflict of interest.
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