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Abstract and Figures

A sudden increase in the rates of shark attacks on humans at Reunion Island has been blamed by some on the implementation of a marine protected area (MPA) along the island's west coast, where attacks, primarily by bull sharks Carcharhinus leucas, were concentrated. We used passive acoustic telemetry to investigate the spatial distribution of bull sharks (N = 36) by quantifying their residency and their frequentation of the MPA and compared it to outside of the MPA. Over the study duration of 17 mo, 18 sharks were detected in the acoustic receiver array, most of which were detected more frequently outside the MPA (N = 148; mean ± SD = 41.5 ± 56.4 visits mo⁻¹ and 17.6 ± 30.5 h mo⁻¹) than inside the MPA (N = 218; 21.4 ± 28.1 visits mo⁻¹ and 7.2 ± 15.2 h mo⁻¹). However, we found individual variation in the sharks' use of the MPA. Thirteen sharks spent more time outside the MPA than inside, while 5 sharks (all females) spent significantly more time inside the MPA. These results suggest that the spatial distribution of bull sharks is not primarily centered in the MPA along the west coast of Reunion Island, although we identified specific locations where bull shark encounter probabilities are relatively high during particular times of the year. Such higher-risk areas could be targeted as part of the risk management strategy for changes in human uses in order to reduce the risks of negative shark-human interactions observed during the past decade.
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Residency and spatial distribution of bull sharks (Carcharhinus leucas) in and
around Reunion Island MPA
Marc Soria1*¶, Michael R. Heithaus2,Antonin Blaison, Estelle Crochelet, Fabien Forget5, Pascale
1 MARBEC, University Montpellier, CNRS, Ifremer, IRD, Sète, France
2Center for Coastal Oceans Research, Marine Sciences Program, Florida International University, North
Miami, FL 33181
3 MARBEC, University Montpellier, CNRS, Ifremer, IRD, Sète, France
4ARBRE - Agence de Recherche pour la Biodiversité à la Réunion, 18 rue des Seychelles, Lotissement
Horizon, 97436 Saint-Leu, La Réunion.
5MARBEC, University Montpellier, CNRS, Ifremer, IRD, Sète, France
6UMR 9220 ENTROPIE, IRD, La Réunion, France
* Corresponding author
E-mail: , +33767173453 (MS)
A sudden increase in the shark attacks rates on humans in Reunion Island has been blamed by some on
the implementation of a Marine Protected Area (MPA) along the Island’s West Coast where attacks,
primarily by bull sharks (Carcharhinus leucas), were concentrated. We used passive acoustic telemetry
to investigate the spatial distribution of bull sharks (N = 36) by quantifying their residency and their
frequentation of the MPA and compared it to outside of the MPA. Over the study duration of 17 months,
18 sharks were detected in the acoustic receiver array, most of which were detected more frequently
outside the MPA (N = 148; mean ± SD = 41.5 ± 56.4 visits month-1and 17.6 ± 30.5 hrs month-1
respectively) than inside the MPA (N = 218; mean = 21.4± 28.1 visits month-1 and 7.2 ± 15.2 hrs month-
1). There was, however, individual variation in sharks’ use of the MPA. Thirteen sharks spent more time
outside the MPA than inside the MPA while five sharks (all females) spent significantly more time
inside the MPA. These results suggest that the spatial distribution of bull sharks does not appear to be
primarily centered in the MPA along the west coast of Reunion island. There are, however, specific
locations where bull shark encounter probabilities are relatively high during particular times of the year.
Such higher risk areas could be targeted as part of the risk management strategy for changes in human
uses in order to reduce the risks of negative shark-human interactions observed during the past decade.
Keywords: bull shark, marine protected area, human-shark conflict, acoustic telemetry
1. Introduction
Despite of the substantial global shark populations decline (e.g. Ferretti et al. 2010), with approximately
25% of shark and ray populations threatened by extinction (Dulvy et al. 2014), some areas around the
world have seen shark populations stabilize or even begin to increase (e.g. Carlson et al. 2012). In some
areas this has raised concerns about the potential for negative shark-human interactions including
increased rates of depredation of fisheries (Gilman et al. 2008, MacNeil et al. 2009) or even shark bites
on humans (Taglioni & Guiltat 2015). Despite the low probability of shark bites on ocean users at a
global scale (, the rate of shark-human interactions
offshore of Reunion Island in the Southwest Indian Ocean has increased over the past decade
(Lagabrielle et al. 2018). Off Reunion Island, the incidence of shark bites rose from 1.2 bites yearly
between 1980 and 2010 to an average of 3.7 bites yearly between 2011 and 2015 (Taglioni & Guiltat
2015). Following these authors, the majority of incidents involved bull sharks (Carcharhinus leucas)
and people practicing board sports (e.g. surfing). When correcting for the total number of surfing hours,
the increase in incidents represents a 23-fold increase from 2005-2016 (Lagabrielle et al. 2018). Shark-
human incidents peak in winter and appear to have shifted from being distributed randomly around the
island before 2010 to the island’s west coast, the hub of Reunion’s coastal water activities (Lemahieu et
al. 2017).
Increasing shark-human incident rates, concentrated on one part of the island, has exacerbated conflicts
among ocean-users and has led to great interest in understanding potential causes for these patterns.
Previous analyses have suggested that seasonal patterns of shark abundance and the total numbers of
ocean users may drive some of observed patterns of shark-human incidents (Ferretti et al. 2015,
Chapman & McPhee 2016). Another factor that could potentially contribute to the increased incident
rates could be the rapid tourism development of the west coast over the past 20 years. Such
developments may have increased freshwater runoff to the coast, expanding habitat for juvenile bull
sharks. It is also possible that increased fishing pressure on reef sharks have reduced their populations
but there are not convincing data on this fishing activity. Tourism development has resulted in
eutrophication of the reef waters and over-exploitation of resources leading to declines in living coral
cover and fish biomass (McClanahan et al. 2007, Hughes et al. 2010, Naim et al. 2013) as well as
erosion of coral beaches (Mahabot 2016). Recognizing this, a Marine Protected Area (MPA) was
established in 2007 to restore and protect reef zones.
The creation of the MPA has been controversial with some ocean users. They consider that reduced
human uses combined with increased fish biomass in the MPA have attracted sharks towards the coastal
waters where ocean activities, especially surfing, occur. To address these concerns associated with
increasing shark-human incidents (Yemane et al. 2009), the CHARC (Connaissance de l’HAbitat des
Requins Côtiers de la Réunion) program was launched in October 2011 (FEDER convention of 28 June
2012; French State convention Bop 113 n°2012/03 and Region Reunion convention ref. POLENV
n°20120257). The aim of this broader project was to use acoustic telemetry to investigate spatiotemporal
patterns in the occurrence and residence times of bull and tiger (Galeocerdo cuvier) sharks that had been
implicated in incidents (Blaison 2017). In this study, we investigate the degree of residency and spatial
distribution of bull sharks in and out of Reunion Island’s MPA.
2.1. Study species
Bulls sharks are large carcharhinids that frequent warm coastal waters worldwide. Bull sharks in the
Indian Ocean are larger than those of the Atlantic Ocean, and can reach lengths exceeding 400 cm total
length (McCord & Lamberth 2009). Bull sharks are viviparous, with neonates generally occupying
coastal rivers, mangroves and estuaries for the first several years of life before moving to coastal waters
(Cruz-Martínez et al. 2004, Simpfendorfer et al. 2005). Large bull sharks are capable of taking large-
bodied prey and have broad diets that include cephalopods, crustaceans, teleosts, elasmobranchs and
marine mammals (Daly et al. 2013, Trystram et al. 2016). Larger individuals can restrict their
movements to particular areas along coasts (Yeiser et al. 2008, Carlson et al. 2010), however some
individuals are also known to undertake long distance movements or seasonal migrations depending on
location (Daly et al. 2014, Lea et al. 2015, Espinoza et al. 2016).
2.2. Study site
Reunion Island is a relatively young volcanic island (3 Ma) in the southern hemisphere (21°07’S /
55°32’E), located 700 km east of Madagascar in the southwest Indian Ocean (Fig. 1). The island is 2512
km2 and has 217 km of coastline. Like most volcanic islands, Reunion is characterized by its lack of
insular plateau (except in the north at Saint-Paul, and in the south at Saint-Pierre). Beyond this plateau,
the underwater slopes are very steep (ca. 10-20%) to a depth of 2,000 m (Piton & Taquet 1992). The
coastal ecosystems of Reunion include sandy and rocky bottoms as well as coral reefs. Fringing reefs
stretch over 25 km along the west and south-west coast, from Saint-Gilles to Saint-Pierre (Montaggioni
& Faure 1980). They form a natural coral barrier that bounds the reef flats and back-reef depressions,
and lie no further than 500 m from the beach. In February 2007, a 35 km2 marine protected area (MPA)
was established that extends ca. 36 km from Cap La Houssaye (Saint-Paul) to La Pointe aux Oiseaux
(Etang-Salé). Much of the existing reef habitat is included in the MPA (Letourneur et al. 2004, Fig. 1).
Our study was focused primarily along the western coast (leeward coast), from Saint-Paul’s Bay to
Saint-Pierre, and was centered around the MPA (Fig. 1).
Fig. 1 The study site along the west coast of Reunion Island. The MPA is colored differently according
to the different levels of protections ( low level, medium level, sanctuary). Shark release locations
are labeled with stars () and the position of the receivers with closed circles ().Receivers with “I” in
the receiver ID are considered inside the MPA and those with an “O” are outside the MPA.
2.3. Field methods
Occurrence and residence times of bull shark were assessed using passive acoustic telemetry (Heupel et
al., 2018). Sharks were captured along the west coast of Reunion Island between September 2012 and
March 2013, using horizontal drifting long-lines that were 0.2 to 1 km long and equipped with 20 to 200
baited 16/0 circle hooks (Blaison 2017). Most fishing occurred at dusk or overnight and soak times were
fixed at 3 hrs to minimize mortality. The CPUE (catch per unit effort), expressed as the number of
sharks per 100 hooks per hour, averaged 0.35 ± 1.07, 0-6.25, N = 115 sharks (Blaison et al. 2015). The
fishing effort was higher on the north-west coast (70% of the fishing effort was done in Saint-Paul’s Bay
and offshore from the harbor of Saint-Gilles and 30% in the south offshore from the harbor of Saint-
Pierre). CPUE were not significantly different between the different sites (N = 115; Kruskal-Wallis test,
H = 11,8, P = 0,07; i.e. Blaison et al. 2015).
Once captured, sharks were brought alongside a tagging vessel, they were measured and the sex was
recorded. Sharks were then inverted and, once they entered a state of tonic immobility (Henningsen
1994), transmitters (Vemco V16TP-4H, transmission interval 40-80 s, power output 158 db, estimated
battery life 845 days, N = 13, or V16-5H, transmission interval 40-80 s, power output 162 db, estimated
battery life of 482 days, N = 5) were implanted through a mid-ventral incision. Two independent
absorbable sutures were made to close the wound. All the fieldwork and protocols of handling and
tagging the sharks were approved by the Ethics Committee (n° 114) of the CYROI (Cyclotron Réunion
Océan Indien) in Reunion Island.
The array of acoustic receivers consisted of 36 Vemco VR2W receivers, deployed on average ca. 2 km
apart and 700 m from shore at depths of 10-60 m (Fig. 1). Detection ranges are known to vary with
environmental characteristics such as depth, sea conditions, surrounding noise and the presence of
thermoclines (Mathies et al. 2014). Therefore, 13 range tests were conducted in the study site. The range
to 50% detection probability was on average of 190 ± 80 m (N = 6) for the receivers placed less than 400
m from the shore (22% of the acoustic network and evenly distributed inside and outside the MPA) and
in average of 390 ± 90 m (N =7) for the receivers further offshore (78% of the acoustic network). We
can therefore presume that the detection ranges are comparable throughout the network, inside and
outside the MPA.
2.4. Data analysis
Data analysis was restricted to the period between the 1st of January 2013 and the 25th of May 2014.
During this period, all receivers were operational;14 receivers were deployed along roughly 27 km of
coastline outside of the MPA, and 22 receivers were deployed along the 36 km of coastline of the MPA
(Fig. 1). During the analysis period, 36 sharks were detected in the receiver array. We calculated the
proportion of time within the study zone by dividing the proportion of time a shark was within the
monitoring array (defined as having been detected on any receiver during a 60 min window) divided by
the total number of monitored hours (the time between tagging and either the end of the study period or
the estimated tag lifetime). The proportion of detection time inside MPA was calculated by dividing the
number of hours inside the MPA by the number of hours in the study zone.
To determine whether sharks used the MPA differently than areas outside of the MPA, we calculated the
number of visits each shark made to individual receivers as well as the duration of these visits. The
duration of a visit to a receiver was defined as the time between first being detected and when it was last
detected on that receiver before an absence (maximum blanking period; MBP) of more than one hour
(Ohta & Kakuma 2005, Capello et al. 2015). This period of absence from a receiver overestimates the
amount of time spent within the detection range of the receiver. Moreover, sharks that return within this
interval of one hour would have remained within the MPA or outside the MPA during this relatively
short absence. To ensure that results were not biased by the selection of a 60 min MBP, we analyzed the
effect of the MBP duration on the presence estimated inside and outside the MPA. This effect was tested
for MBPs of 1, 3 and 6 hrs. The number of times sharks returned to specific receivers (i.e. the number of
distinct visits) decreased by 50% (2326 to 1161) for a 3 hrs MBP and by 68% (2326 to 727) for a MBP
of 6 hr. The duration of presence per visit increased by 72% when increasing the MBP from 1 hr to 3 hrs
(0.38 ± 0.75 hr and 0.65 ± 1.50 hr, respectively) and more than 100% when increasing MBP from 1 hr to
6 hrs (0.91 ± 2.22 hr). While MBP affected the number and duration of visits, there was no difference in
the increase between receivers inside and outside the MPA [Kruskal-Wallis test: H (2, N = 441) = 0.045,
P = 0.977; Siegel post hoc test: Z1hr,3hr = 0.148, Z1hr.6hr = 0.205, Z3hr,6hr = 0.0567]. Therefore, MBP does
not affect the nature of our results. Results presented, use a MBP of 1 h.
The home-range of bull sharks inside the study zone was estimated by using the adehabitatHR R
package v0.4.15 (Calenge 2006). The home range for each shark was computed in order to visualize
their activity area within the receiver array and to determine the importance of the MPA in the spatial
distribution of sharks. If the MPA is attractive to sharks it can be expected that the latter plays an
important role in their spatial distribution. The function kernelUD estimates the utilization distribution
(UD) of each shark by considering that the animals use of space can be described by a bivariate
probability density function. The 90% home range was calculated from the UD estimates. The UD gives
the probability density to relocate the sharks at any place according to the coordinates (x, y) of the 36
receivers deployed in the study zone. The kernel estimation of the UD at a given point coordinate is
obtained by:
²exp di²
where h is a smoothing parameter, n is the number of relocations, and di is the distance of the
ith observation from the x, y coordinate. We determined kernel bandwidth h, by numerical optimization
using the optimal h (hopt) for a standard multivariate normal distribution (Horne & Garton 2006).
Since the number of receivers were not the same inside and outside the MPA, the duration of presence
and the number of visits per month, per shark and per receiver were weighted by the densities of
receivers in each area [with a factor of 1 inside the MPA, 1.32 outside the MPA in the north (4 receivers
along 9.8 km of coastline), and 1.05 outside the MPA in the south (10 receivers along 17.6 km of
coastline)]. The sums and means of the duration of presence and of the number of visits inside or outside
MPA were then calculated per month, for each shark or for each receiver. Only adult shark detected
during more than 2 months were included in the analyses (with the exception of two sharks that were
detected during four months but with less than 2 detections per month and were excluded of the
Presence times were Log and Box-Cox transformed and visits were Box-Cox transformed. These
transformations were appropriate (Chi-square test X12 = 12.8, P = 0.38 and X8 = 11.03, P = 0.20
respectively). We also did not detect differences in variances between data inside and outside the reserve
(Levene tests F (1,318) = 0.67, P = 0.41, and F (1,318) = 3.10, P = 0.08 respectively). Therefore, our
data met the assumptions of parametric tests.
We used mixed ANOVA models to determine first, whether the release sites (i.e. whether sharks were
captured inside or outside the MPA) could impact on the proportion of detection time and number of
visits inside the MPA and secondly whether the monthly presence of sharks varied between areas (i.e.
inside or outside the MPA). Area, month and interactions were fixed-effect factors, and we included the
individual identity of sharks as a random factor.
To group the sharks relative to their MPA use, we conducted a hierarchical cluster analysis (HCA) on
the monthly proportions of time spent in the MPA, using Ward's minimum variance method and
Manhattan distance which has the advantage of both having triangular inequality and offering better data
contrast than Euclidean distance. Tukey's HSD was used to correct for multiple comparisons.
Seasonality of presence by group was described following the marine seasons in Reunion island
(Conand et al. 2008). All statistical tests were carried out in Statistica 12.0 (StatSoft Inc.) or using
specific R packages. Results are presented as mean ± SD (min-max, N = x), unless otherwise indicated.
Of the 36 tagged sharks, 24 were caught and tagged inside the MPA and 12 outside the MPA. Only 18
were detected frequently enough to be included in analyses (i.e. during more than 2 months). Of the 18
sharks that were not included in the analysis, eight sharks were never detected and ten were detected too
rarely and sporadically (i.e. during only few months).
The final database contains 10804 visits from 103170 detections. The 18 individuals monitored on the
west coast of Reunion island included six males and 12 females, with two individuals detected in all 17
months, four detected during 14-16 months, seven detected during 9-12 months, and five sharks during
Table 1 Presence time and number of visits at receivers inside and outside the MPA between Jan 2013 - May 2014. Monthly mean presence time was
estimated from the sum of time of presence by month. Receiver codes with an I were located inside the MPA and those with an O were outside.
Inside MPA
Outside MPA
to the
coast (m)
Monthly mean
presence time
± SD (hour)
of visits
Mean visits ±
SD (by
to the
coast (m)
time (hour)
Monthly mean
presence time ±
SD (hour)
of visits
Mean visits
± SD (by
I1 1600 9 53.7 6 ± 7.6 122 13.6 ± 10.6 O1 400 7 15.3 2.2 ± 1.4 101 14.4 ± 8.3
1 ± 0.7
4.3 ± 2.4
36.5 ± 31.6
56.2 ± 38.5
0.2 ± 0.1
1.2 ± 0.4
22.8 ± 22.6
39.8 ± 28.9
I4 1050 6 95.6 15.9 ± 13.1 278 46.3 ± 34.6 O4 950 12 190.7 15.9 ± 15.2 434 36.2 ± 21.5
17.8 ± 24.5
33.4 ± 37.8
11.7 ± 13.3
54.4 ± 51.8
I6 1400 12 465.9 38.8 ± 43.3 698 58.2 ± 57.2 O6 400 12 517.9 43.2 ± 42.7 1373 114.5 ± 93
I7 850 12 22.1 1.8 ± 2.3 107 8.9 ± 8.3 O7 290 11 757.9 68.9 ± 61.5 1373 124.9 ± 95.8
1.6 ± 1.9
7.3 ± 4.1
5.3 ± 5.5
27.4 ± 23.8
I9 810 12 34.3 2.9 ± 2.9 69 5.8 ± 4.1 O9 460 12 32.8 2.7 ± 4.2 144 12 ± 11.3
I10 430 10 42.9 4.3 ± 3.8 149 14.9 ± 10.2 O10 630 12 128.3 10.7 ± 14.4 363 30.2 ± 28.3
4 ± 3.7
22.9 ± 17.3
3.7 ± 3.5
7.3 ± 5.5
I12 540 9 42.7 4.7 ± 10.7 34 3.8 ± 2.9 O12 730 5 5.7 1.1 ± 1 43 8.7 ± 5.1
I13 540 11 10.7 1 ± 0.9 59 5.4 ± 3.2 O13 520 10 10.2 1 ± 0.7 34 3.4 ± 1.4
8.7 ± 8.6
31.7 ± 17.1
4.9 ± 3.6
15.4 ± 9.1
I15 910 12 7.4 0.6 ± 0.4 44 3.7 ± 1.8
I16 700 12 106.8 8.9 ± 9.3 521 43.4 ± 33.8
3.5 ± 4
13.9 ± 9.8
I18 300 12 85.9 7.2 ± 6 568 47.3 ± 39
5 ± 5.5
10.9 ± 9.9
4 ± 3.4
17.6 ± 13.1
I21 850 11 87.3 7.9 ± 12.1 302 27.5 ± 35.1
5.8 ± 3.3
25.9 ± 13.3
7.2 ± 15.2
21.4 ± 28.1
17.6 ± 30.5
41.5 ± 56.4
Table 2 Summary statistics of the 18 bull sharks from January 2013 to May 2014. The asterisks denote the sharks whose presences inside and outside the MPA
are significantly different (Tukey’s HSD test). ♀: female; ♂: male. I code is for receivers inside the MPA and O code for those outside.
General informations Inside MPA Outside MPA
receiver to
time (day)
time (day)
of time
of detection
time inside
Mean presence
time ± SD (in
h month-1)
Mean visits
± SD (by
of visits
Mean presence
time ± SD (in
h month-1)
Mean visits
± SD (by
of visits
1 ♀ *
41.5 ± 37.8
66.4 ± 58.5
0.4 ± 0.4
4.2 ± 4.4
42.6 ± 58.7
88 ± 89.4
4.6 ± 3.2
18.9 ± 14.2
24.5 ± 23.6
67 ± 54
2.9 ± 1.3
11.2 ± 6.4
4 ♀ *
12.2 ± 7.4
47.1 ± 28.8
4.2 ± 3.4
18.5 ± 11.6
5 ♀ *
5.7 ± 5.7
22 ± 18.4
1.4 ± 1.2
5.4 ± 4.2
16.1 ± 12.3
56.9 ± 40
23.3 ± 16.8
46.6 ± 22.7
6.6 ± 5
43 ± 34
6.4 ± 3.9
19.4 ± 10.2
8.4 ± 3.9
31 ± 6
16.9 ± 5.9
65.1 ± 10.3
4.3 ± 3.3
21.9 ± 15
10 ± 9.1
36.9 ± 17.2
13.2 ± 14.8
24.2 ± 23.5
29 ± 36.8
51.4 ± 47.2
1.7 ± 2
10 ± 10.7
1.8 ± 1.5
7.8 ± 5.6
12 ♀ *
7.3 ± 9.8
21.2 ± 21.7
19.3 ± 21.6
38.9 ± 28.5
13 ♂ *
4.8 ± 5.3
22.2 ± 20.9
16 ± 14.4
38 ± 25.8
14 ♀ *
10 ± 12.6
36 ± 41.6
25.4 ± 26.5
61.4 ± 42
15 ♀ *
4.1 ± 8.3
10.2 ± 18.5
16.5 ± 14.1
36 ± 25.4
3.9 ± 2.5
29 ± 29.7
20.1 ± 26.8
59.2 ± 81.5
17 ♀ *
2.7 ± 2.7
14.7 ± 14.6
10.6 ± 8.6
19 ± 10.6
18 ♂ *
0.8 ± 0.8
6.1 ± 4
19.2 ± 24.7
46.7 ± 48.4
10.8 ± 17.3
32 ± 35.6
15 ± 19.5
35.3 ± 33.2
3-6 months. These sharks spent between 2.3% - 29.3% of their time in range of at least one receiver
(mean = 11.5% ± 7.0, N = 18).
Overall, sharks made more visits and spent longer times within the range of receivers found outside the
MPA than inside (Table 1).
Fig. 2 90% Kernel home-range densities of sharks that were detected in the array of acoustic receivers.
Green boxes indicate the latitudinal extent of the MPA. Shark release locations are denoted by stars
().Shark codes are indicated in each box. Inset indicates (shaded rectangle) the localization of the
study zone relative to Reunion Island. Red shark codes denotes sharks that spent more time within the
MPA (Group 1, see Table 1 and Fig. 3).
Based on Kernel home range estimates, 12 of 18 sharks that entered the array moved over extensive
areas of the coast and ranges extended over the entire study zone (Fig. 2). Two sharks had a restricted
occupancy, one (N°1) in the north part of the MPA, the other north of the MPA (N° 10). Two other
individuals (N° 17 and 18) occupied two areas outside the MPA area (in North and South of the study
zone) but did not enter coastal waters of the MPA (Fig. 1, 2 and Table 2).
Sharks released within the MPA spent approximately equal amounts of time (in hour month-1) around
receivers inside and outside the MPA [13.08 ± 21.07 h (0.05-109.95), N = 81 and 13.28 ± 20.91 h (0.04-
103.90), N = 87 respectively; ANOVA F (1,166) = 0.02, P = 0.89], while those that were tagged outside
the MPA spent considerably more time at receivers outside the reserve than inside [16.62 ± 17.93 h
(0.10-111.32), N = 87 and 8.00 ± 10.27 h (0.05-42.27), N = 65 respectively; ANOVA F (1,150) = 19.05,
P = 0.001]. Sharks in the latter category also were detected for longer overall proportions of time as
The amount of time sharks spent at receivers varied significantly with the interaction : area x shark
identity and the main effects of area, month, and shark identity (Table 3). In general, sharks spent more
time outside the MPA with small but significant variation across months. Individual sharks varied in the
duration of time they spent in the study zone and inside versus outside the reserve (Table 3).
Table 3 Results of the ANOVA mixed model for effects of MPA, month and shark on the presence time
of tagged bull sharks in the study zone.
Factors Effect df
df error MS
error F p-value
Area x Month
Area x Shark
< 0.001
Month x Shark
Area x Month x Shark
Fig. 3 Hierarchical clustering dendrogram of sharks (cf. shark codes in Table 1) based on the proportion
of time spent in the MPA.
The hierarchical cluster analysis revealed two groups. The Group 1 (Fig. 3, Table 2) was composed of
five individuals (N° 1, 2, 3, 4, 5), all females, which spent more time (>70% of the total presence time)
in the MPA and visited it more often (Table 4).
Table 4 Total and monthly mean presence time and number of visits at receivers inside and outside the
MPA for two behavior types resolved by hierarchical cluster analysis. P values are based on ANOVA.
Group MPA N Total presence
time (h)
Monthly mean
presence time
± SD (h)
number of
Mean visits ±
SD (by
22.0 ± 28.9
51.8 ± 47.6
2.6 ± 2.7
11.4 ± 10.4
7.6 ± 10
26.2 ± 29
17.6 ± 20.5
40.4 ± 34.2
However, the difference between areas was significant for only three sharks (N°1, 4 and 5; Table 2).
Except for the shark N°1, these sharks ranged widely (see home-ranges Fig. 2). Four sharks of five (1, 2,
3 and 5, Table 2) in Group 1 spent more time at the three receivers offshore of Saint-Gilles harbor (I4, I5
and I6, Fig. 1) than at the others receivers, inside the MPA [9.7 ± 14.2 h month-1 receiver-1 (0.05-55.34),
N= 55 and 0.7 ± 1.2 h month-1 receiver-1 (0.03-7.84), N= 257 respectively; ANOVA, F (1,310) = 94.34,
P= 0.001]. In addition, these sharks were observed mainly between April and June when water
temperatures are dropping (Fig. 4).
Fig. 4 Proportion of time individual sharks were detected across seasons (from Jan-Mar : Summer, from
Apr-Jun: Cooling, from Jul-Sep: Winter and from Oct-Dec: Warming).
Group 2 (Fig. 3 and Table 2) was composed of thirteen individuals (from N°6 to N°18) which spent
more time outside the MPA than inside and made more visits to receivers outside the MPA (Table 4).
Post hoc analyses indicated that presence times were significantly different inside and outside the MPA
for six of thirteen individuals (Table 2). In general these sharks were widely dispersed in the study zone
with the exception of two individuals (N° 10 and 18) that were detected mostly outside the MPA (Fig.
2). Outside the MPA, they occupied mostly receivers on either side of the MPA: O2, O3 and O4 in
Saint-Paul’s Bay and O5, O6, O7 in Saint-Louis’s Bay (see Fig. 1) than the other receivers outside the
MPA (4.6 ± 14.2 h month-1 receiver-1 (0.03-82.28), N = 477 and 1.0 ± 1.9 h month-1 receiver-1 (0.03-
18.26), N = 327 respectively; ANOVA, F (1,802) = 153.04, P = 0.001). Except for two sharks,
exclusively observed in summer (N° 8 and 16), these sharks were present throughout the year and did
not display marked seasonality (Fig. 4).
Given the high rate of bull shark-human incidents and increasing public concern, it is important to
understand whether MPA affect the spatial distribution of bull sharks. In the absence of movement data
prior the implementation of the MPA and in the absence of knowledge on the habitat quality for bull
sharks inside and outside the MPA, it is impossible to pinpoint whether the MPA implementation
modified the movements and residency of bull shark along the west coast of Reunion island.
Nonetheless, we found that 50% of the tagged sharks (18 individuals) were never detected in the coastal
network and most of the 18 sharks that remained within the acoustic array spent more time outside than
inside the MPA. Generally, their home-ranges appear to extend along the coast of the study area. Finally,
only 5 (21%) of the 24 individuals tagged inside the MPA regularly frequented this area. This suggests
that tagging location did not appear to have an effect on the sharks’ movements and that bull sharks were
not using the MPA more heavily than surrounding areas.
Reunion Island’s MPA was created in 2007 in order to restore coral reef biodiversity and augment fish
stocks by managing the human activities taking place within it. For coastal sharks, such as the bull shark,
this could provide them with an opportunity to find feeding sites if prey resources recovered (Garla et al.
2006, Knip et al. 2012a). While MPAs in other areas have been shown to result in rebounds in shark
numbers fairly quickly (Knip et al. 2012b, Edgar et al. 2014), this pattern seems not verified for bull
sharks off Reunion. Indeed, our tracking data do not suggest that bull sharks preferentially select areas
within the MPA. This difference could be due to the continued presence of extractive fishing in the
Reunion MPA and relatively low biomass of potential prey within the MPA. Indeed, from 2008 to 2014,
fish biomass in the reserve increased from ca 400 to 500 kg ha-1, but only in the full sanctuary zones of
the MPA (Bigot et al. 2016) that represent ca 5% of the reserve’s area (see Fig. 1). This small increase in
biomass is unlikely to be sufficient to drive shifts in shark numbers. Moreover, biomass on Reunion’s
reefs is generally low (~500 kg ha-1) compared to biomass levels observed on other Indian Ocean coral
reefs (McClanahan et al. 2011, Chabanet et al. 2016) and may represent only a modest attraction for
large-bodied predators like bull sharks. Shark use of the MPA was spatially heterogeneous. The one
location where shark activity was concentrated inside the MPA was offshore of Saint-Gilles, where
professional and recreational fishing are authorized. In the perspective of implementing an efficient
strategy of warning and prevention in Reunion island, this further suggests to focus more on the habitat
use, movements and site fidelity of sharks than on the impact of MPA which is unlikely a cause of
increased incidents.
Sharks often exhibit inter-individual variation in behaviors [e.g. (Heithaus et al. 2002, Matich et al.
2011)]. Here, we found that individuals varied considerably in their use of the MPA and temporal
patterns of occurrence. Broadly, sharks could be grouped into those that were present virtually year-
round and used waters outside or at the boundaries of the MPA more than those inside and another
smaller group of female sharks that occurred primarily Apr-Jun, when waters temperatures are dropping,
and used waters off Saint-Gilles more often than other individuals.
Variation in the abundance and behavior of bull sharks has been attributed to several factors including
temperature (Carlson et al. 2010, Brunnschweiler et al. 2010, Matich & Heithaus 2012, Drymon et al.
2014), dissolved oxygen levels (Heithaus et al. 2009), salinity (Simpfendorfer et al. 2005, Curtis 2008)
and water turbidity (Cliff & Dudley 1991, Taylor 2007, Froeschke et al. 2010).
At Reunion Island several factors could be responsible for the increase in shark-human interactions
including benthic substrate, sea temperature and period of day (Lagabrielle et al. 2018), turbidity and
swell height (Taglioni et al., 2018). Multiple rivers and ravines provide freshwater inputs to the coastal
waters. During rainfall events, turbid outflow waters rich in organic matter (Piper & Normark 2009)
would not only reduce visibility in coastal waters but also reduce water salinity, conditions that might be
attractive to bull sharks (Werry et al. 2018). Over the recent decades, the fast expansion of the urban
zones on the west coast of the island might have increased the soil sealing and by consequence the rate
and quantity of stormwater runoff flowing to the sea (Shuster et al. 2014). Consistent with this
hypothesis is the finding that the highest occurrence of sharks outside the MPA was in the two bays
located at the mouths of the two largest rivers of the west coast (Fig. 1), on both sides of the MPA
(receivers O2, O3 and O4 in Saint-Paul’s bay and O5, O6, O7 in Saint-Louis’s Bay).
Overexploitation of stocks of coastal and deep-sea demersal fish in Reunion observed since the early
2000’s (Le Manach et al. 2015) may have reduced the availability of potential prey for bull sharks,
inducing them to forage over wider areas and nearer to the coast irrespective of the presence of the
MPA. Anthropogenic changes to the environment could also influence shark behavior and habitat use
(Wong & Candolin 2015, Hays et al. 2016). For example, the presence of the harbor of Saint-Gilles,
where fish carcasses are discarded regularly (Loiseau, pers comm) would offer feeding opportunities and
could attract sharks (Hazin et al. 2008, Papastamatiou et al. 2011). Consistent with this hypothesis is the
finding that the three receivers in the MPA with the greatest presence were offshore of Saint-Gilles
harbor (I4, I5 and I6, see Fig. 1). Another hypothesis is that the preferred use of the Saint-Gilles site by
four adult females during April to June is linked to reproduction. Indeed, this period overlaps with the
apparent mating period of bull sharks in the intertropical zone [March -June; (Stevens & McLoughlin
1991, Espinoza et al. 2016)]. Recently, Pirog et al. 2019 reported that the mating period in Reunion
island should occur during the cold season (June-September). The hypothesis stated above suggests that
a pre-spawning shark aggregation could occur near Saint-Gilles harbor before the mating period. In
addition to external factors like turbidity, swell height and human activity (Taglioni et al. 2018),
localized movements might influence the occurrence of attacks. For example, the site of Saint-Gilles,
where four of the five sharks observed in the MPA were mainly present, is one of the most popular surf
spot of Reunion island. Therefore, a high level of shark-human interactions could be expected on this
site. Specific analysis on fine-scale movements along the coast related to biotic and abiotic factors could
help to test this hypothesis.
While passive acoustic telemetry is an important tool in studies of elasmobranch habitat use, it has
limitations (e.g.Kessel et al. 2014, Heupel et al. 2018). The first challenge is detection area within the
network of receivers. For example, sharks can spend a considerable portion of their time outside of a
monitoring array as was the case for most of the sharks tagged in this study. In our study at Reunion
Island, the receivers only covered ca. 40% of the coastal zone of the study area and did not extend far
into offshore waters where sharks likely spend considerable time. It also did not extend along the coast
further away from the MPA. However, the array was optimized to determine whether individuals were
spending extended periods of time nearshore where shark-human interactions might occur both inside
and outside the MPA.
With a highly mobile species like bull sharks, and relatively large detection ranges, movements along
the coastline have a high probability of being detected by receivers. Importantly, this design is unlikely
to bias results towards greater use of waters inside or outside the reserve. The weighting factors used to
take into account the difference of the densities of receivers inside and outside the MPA should not have
influenced the results. A second challenge is the number of tagged sharks that could be analyzed in our
study. Of the 36 sharks tagged, only 18 were detected enough times to warrant inclusion in analyses.
This sample, however, is large enough to gain insights into general patterns of visitation and residence
times for individuals using coastal waters and provides evidence that sharks are not using MPA waters
more often than those outside the MPA. Finally, spatial or temporal variations in detection ranges of the
receivers might confound data analysis. While the potential impact of such variation is difficult to fully
quantify, the design of the array and range testing suggest that patterns were unlikely to have been
driven by variation in detection ranges of receivers. Indeed, receivers were deployed in acoustically
similar environments inside and outside the MPA and only eight receivers of 36 (22%) were situated
near the coastline and the coral reefs where background noise could reduce detection ranges.
Nevertheless, these nearshore’s receivers were evenly distributed inside and outside the MPA. Lastly,
receiver recoveries were performed every 4-5 months to reduce the potential effects of biofouling
(Heupel et al. 2008).
In summary, our results suggest that although some sharks may use specific areas inside the MPA during
limited time periods, they do not seem to use more the habitat in the MPA than around the MPA. Indeed,
sharks generally were not detected in coastal waters after release or spent more time in waters outside
the MPA than inside the MPA. Concerning the influence of release positions on residence time in or out
of the MPA, we are currently studying the relationship between fine scale individual movements and
potential social interactions amongst sharks. Further studies that employ additional field and analytical
methods, increase sample sizes, extend the temporal period of observation and integrate data on
environmental and biotic factors will provide further insights into the factors driving bull shark habitat
use along the coast of Reunion Island. Together with biological and ecological studies, social science
studies on the perception by the different ocean users of the wildlife as both carrier of damages and
fascination (Dickman 2010) are also necessary to develop policies that could reduce shark-human
We are grateful to all members of the institutions and associations involved in the shark program (IRD,
CRPMEM, University of Reunion Island, Globice, Kélonia, ARVAM, Squal’Idées, RNMR, Ifremer), as
well as all volunteers who assisted with shark fishing and tagging and made our work possible. Finally,
we thank anonymous reviewers and the guest editor for their comments and advice, which helped to
improve this paper. This study received financial support from the European Union (convention FEDER
ref. 2012-dossier Presage n° 33021), the French government (BOP 113 n°2012/03) and the Regional
Council of Reunion Island (POLENV n°20120257). Research grants provided to the second author by
the Regional Council of Reunion Island. The funders had no role in study design, data collection and
analysis, decision to publish, or preparation of the manuscript.
Bigot L, Bruggemann H, Cadet C, Cauvin B, Chabanet P, Durville P, Guillaume M, Mulochau T, Penin
L, Tessier E, Urbina I (2016) Point I du suivi de « l’effet réserve » sur les communautés
ichtyologiques et benthiques récifales - Secteurs de St Gilles/ La Saline et de Saint-Leu - Etat des
lieux à 7 ans après la création de la réserve naturelle nationale marine de La Réunion. Rapport
final. ECOMAR, IRD, MNHN. 59 pp, La Réunion.
Blaison A (2017) Écologie comportementale des requins bouledogue (Carcharhinus leucas) sur les côtes
de La Réunion : application à un modèle de gestion du « risque requin ». Ph. D. Thesis,
Université de la Réunion, 2017. Fr. <NNT: 2017LARE0009>, La Réunion
Blaison A, Jaquemet S, Guyomard D, Vangrevelynghe G, Gazzo T, Cliff G, Cotel P, Soria M (2015)
Seasonal variability of bull and tiger shark presence on the west coast of Reunion Island, western
Indian Ocean. Afr J Mar Sci 37:199–208.
Brunnschweiler JM, Queiroz N, Sims DW (2010) Oceans apart? Short-term movements and behaviour
of adult bull sharks Carcharhinus leucas in Atlantic and Pacific Oceans determined from pop-off
satellite archival tagging. J Fish Biol 77:1343–1358.
Calenge C (2006) The package “adehabitat” for the R software: A tool for the analysis of space and
habitat use by animals. Ecol Modell 197:516–519.
Capello M, Robert M, Soria M, Potin G, Itano D, Holland K, Deneubourg J-L, Dagorn L (2015) A
methodological framework to estimate the site fidelity of tagged animals using passive acoustic
telemetry. PLoS One 10:e0134002.
Carlson JK, Hale LF, Morgan A, Burgess G (2012) Relative abundance and size of coastal sharks
derived from commercial shark longline catch and effort data. J Fish Biol 80:1749–1764.
Carlson JK, Ribera MM, Conrath CL, Heupel MR, Burgess GH (2010) Habitat use and movement
patterns of bull sharks Carcharhinus leucas determined using pop-up satellite archival tags. J
Fish Biol:661–675.
Chabanet P, Bigot L, Nicet J-B, Durville P, Massé L, Mulochau T, Russo C, Tessier E, Obura D (2016)
Coral reef monitoring in the Iles Eparses, Mozambique Channel (2011–2013). Acta Oecol
Chapman BK, McPhee D (2016) Global shark attack hotspots: identifying underlying factors behind
increased unprovoked shark bite incidence. Ocean Coast Manag 133:72–84.
Cliff G, Dudley SFJ (1991) Sharks caught in the protective gill nets off Natal, South Africa. 4. The bull
shark Carcharhinus leucas Valenciennes. S Afr J Mar Sci 10:253–270.
Conand F, Marsac F, Tessier E, Conand C (2008) A ten-year period of daily sea surface temperature at a
coastal station in Reunion Island, Indian Ocean (July 1993 April 2004): patterns of variability
and biological responses. West Ind Ocean J Mar Sci 6.
Cruz-Martínez A, Chiappa-Carrara X, Arenas-Fuentes V (2004) Age and growth of the bull shark,
Carcharhinus leucas, from southern Gulf of Mexico. J Northw Atl Fish Sci 35:367–374.
Curtis TH (2008) Distribution, movements, and habitat use of bull sharks (Carcharhinus leucas, Müller
and Henle 1839) in the Indian River Lagoon system, Florida. Doctoral dissertation, University of
Florida Gainesville. 130 pp.
Daly R, Froneman PW, Smale MJ (2013) Comparative feeding ecology of bull sharks (Carcharhinus
leucas) in the coastal waters of the southwest Indian Ocean inferred from stable isotope analysis.
PLoS One 8:e78229.
Daly R, Smale MJ, Cowley PD, Froneman PW (2014) Residency patterns and migration dynamics of
adult bull sharks (Carcharhinus leucas) on the east coast of Southern Africa. PLoS One
Dickman AJ (2010) Complexities of conflict: the importance of considering social factors for effectively
resolving human–wildlife conflict. Anim Conserv 13:458–466.
Drymon JM, Ajemian MJ, Powers SP (2014) Distribution and dynamic habitat use of young bull sharks
Carcharhinus leucas in a highly stratified northern Gulf of Mexico estuary. PLoS One 9:e97124.
Dulvy NK, Fowler SL, Musick JA, Cavanagh RD, Kyne PM, Harrison LR, Carlson JK, Davidson LN,
Fordham SV, Francis MP, Pollock CM, Simpfendorfer CA, Burgess GH, Carpenter KE,
Compagno LJ, Ebert DA, Gibson C, Heupel MR, Livingstone SR, Sanciangco JC, Stevens JD,
Valenti S, White WT (2014) Extinction risk and conservation of the world’s sharks and rays.
eLife 3.
Edgar GJ, Stuart-Smith RD, Willis TJ, Kininmonth S, Baker SC, Banks S, Barrett NS, Becerro MA,
Bernard ATF, Berkhout J, Buxton CD, Campbell SJ, Cooper AT, Davey M, Edgar SC, Försterra
G, Galván DE, Irigoyen AJ, Kushner DJ, Moura R, Parnell PE, Shears NT, Soler G, Strain EMA,
Thomson RJ (2014) Global conservation outcomes depend on marine protected areas with five
key features. Nature 506:216–220.
Espinoza M, Heupel MR, Tobin AJ, Simpfendorfer CA (2016) Evidence of partial migration in a large
coastal predator: opportunistic foraging and reproduction as key drivers? PLoS One
Ferretti F, Jorgensen S, Chapple TK, De Leo G, Micheli F (2015) Reconciling predator conservation
with public safety. Front Ecol Environ 13:412–417.
Ferretti F, Worm B, Britten GL, Heithaus MR, Lotze HK (2010) Patterns and ecosystem consequences
of shark declines in the ocean. Ecol Lett 13:1055–1071.
Froeschke J, Stunz GW, Wildhaber ML (2010) Environmental influences on the occurrence of coastal
sharks in estuarine waters. Mar Ecol Prog Ser 407:279–292.
Garla RC, Chapman DD, Wetherbee BM, Shivji M (2006) Movement patterns of young Caribbean reef
sharks, Carcharhinus perezi, at Fernando de Noronha Archipelago, Brazil: the potential of
marine protected areas for conservation of a nursery ground. Mar Biol 149:189.
Gilman E, Clarke S, Brothers N, Alfaro-Shigueto J, Mandelman J, Mangel J, Petersen S, Piovano S,
Thomson N, Dalzell P, Donoso M, Goren M, Werner T (2008) Shark interactions in pelagic
longline fisheries. Mar Policy 32:1–18.
Hays GC, Ferreira LC, Sequeira AMM, Meekan MG, Duarte CM, Bailey H, Bailleul F, Bowen WD,
Caley MJ, Costa DP, Eguíluz VM, Fossette S, Friedlaender AS, Gales N, Gleiss AC, Gunn J,
Harcourt R, Hazen EL, Heithaus MR, Heupel M, Holland K, Horning M, Jonsen I, Kooyman
GL, Lowe CG, Madsen PT, Marsh H, Phillips RA, Righton D, Ropert-Coudert Y, Sato K,
Shaffer SA, Simpfendorfer CA, Sims DW, Skomal G, Takahashi A, Trathan PN, Wikelski M,
Womble JN, Thums M (2016) Key questions in marine megafauna movement ecology. Trends
Ecol Evol 31:463–475.
Hazin FHV, Burgess GH, Carvalho FC (2008) A Shark attack outbreak off Recife, Pernambuco, Brazil:
1992–2006. Bull Mar Sci 82:199–212.
Heithaus MR, Dill L, Marshall G, Buhleier B (2002) Habitat use and foraging behavior of tiger sharks
(Galeocerdo cuvier) in a seagrass ecosystem. Mar Biol 140:237–248.
Heithaus MR, Delius BK, Wirsing AJ, Dunphy-Daly MM (2009) Physical factors influencing the
distribution of a top predator in a subtropical oligotrophic estuary. Limnol Oceanogr 54:472–482.
Henningsen AD (1994) Tonic immobility in 12 elasmobranchs: use as an aid in captive husbandry. Zoo
Biol 13:325–332.
Heupel MR, Kessel ST, Matley JK, Simpfendorfer CA (2018) Acoustic telemetry. In: Shark Research:
emerging technologies and applications for the field and laboratory, CRC Press, Taylor &
Francis Group. Carrier J. C., Heithaus M. R., Simpfendorfer C. A., Boca Raton, FL, USA, p
Heupel MR, Reiss KL, Yeiser BG, Simpfendorfer CA (2008) Effects of biofouling on performance of
moored data logging acoustic receivers. Limnol Oceanogr Methods 6:327–335.
Horne JS, Garton EO (2006) Likelihood cross-validation versus least squares cross-validation for
choosing the smoothing parameter in Kernel home-range analysis. J Wildl Manage 70:641–648.
Hughes TP, Graham NAJ, Jackson JBC, Mumby PJ, Steneck RS (2010) Rising to the challenge of
sustaining coral reef resilience. Trends Ecol Evol 25:633–642.
Kessel ST, Cooke SJ, Heupel MR, Hussey NE, Simpfendorfer CA, Vagle S, Fisk AT (2014) A review of
detection range testing in aquatic passive acoustic telemetry studies. Rev Fish Biol Fish 24:199–
Knip DM, Heupel MR, Simpfendorfer CA (2012a) To roam or to home: site fidelity in a tropical coastal
shark. Mar Biol 159:1647–1657.
Knip DM, Heupel MR, Simpfendorfer CA (2012b) Evaluating marine protected areas for the
conservation of tropical coastal sharks. Biological Conservation 148:200–209.
Lagabrielle E, Allibert A, Kiszka JJ, Loiseau N, Kilfoil JP, Lemahieu A (2018) Environmental and
anthropogenic factors affecting the increasing occurrence of shark-human interactions around a
fast-developing Indian Ocean island. Sci Rep 8.
Le Manach F, Bach P, Boistol L, Robinson J, Pauly D (2015) Artisanal fisheries in the world’s second
largest tuna fishing ground - Reconstruction of the Seychelles’ marine fisheries catch, 1950–
2010. In: Fisheries catch reconstructions in the Western Indian Ocean, 1950–2010, University of
British Columbia. Fisheries Centre Research Reports, Le Manach F, Pauly D, Vancouver,
Canada, p 99–110
Lea JSE, Humphries NE, Clarke CR, Sims DW (2015) To Madagascar and back: long-distance, return
migration across open ocean by a pregnant female bull shark Carcharhinus leucas. J Fish Biol
Lemahieu A, Blaison A, Crochelet E, Bertrand G, Pennober G, Soria M (2017) Human-shark
interactions: The case study of Reunion island in the south-west Indian ocean. Ocean Coast
Manag 136:73–82.
Letourneur Y, Chabanet P, DurviLLe P, Taquet M, Teissier E, Parmentier M, Quero J-C, Pothin K
(2004) An updated checklist of the marine fish fauna of Reunion Island, South-Western Indian
Ocean. Cybium 28:199–216.
MacNeil MA, Carlson JK, Beerkircher LR (2009) Shark depredation rates in pelagic longline fisheries: a
case study from the Northwest Atlantic. ICES J Mar Sci 66:708–719.
Mahabot M-M (2016) Suivi morphodynamique des plages récifales de La Réunion en contexte
d’observatoire. Ph. D. Thesis, Université de La Réunion. 264 pp. https://hal-ens.archives-, La Réunion
Mathies N, Ogburn M, McFall G, Fangman S (2014) Environmental interference factors affecting
detection range in acoustic telemetry studies using fixed receiver arrays. Mar Ecol Prog Ser
Matich P, Heithaus M (2012) Effects of an extreme temperature event on the behavior and age structure
of an estuarine top predator, Carcharhinus leucas. Mar Ecol Prog Ser 447:165–178.
Matich P, Heithaus MR, Layman CA (2011) Contrasting patterns of individual specialization and
trophic coupling in two marine apex predators: specialization in top marine predators. J Anim
Ecol 80:294–305.
McClanahan T, Ateweberhan M, Graham N, Wilson S, Sebastián C, Guillaume M, Bruggemann J
(2007) Western Indian Ocean coral communities: bleaching responses and susceptibility to
extinction. Mar Ecol Prog Ser 337:1–13.
McClanahan TR, Graham NAJ, MacNeil MA, Muthiga NA, Cinner JE, Bruggemann JH, Wilson SK
(2011) Critical thresholds and tangible targets for ecosystem-based management of coral reef
fisheries. Proc Natl Acad Sci USA 108:17230–17233.
McCord ME, Lamberth SJ (2009) Catching and tracking the world’s largest Zambezi (bull) shark
Carcharhinus leucas in the Breede estuary, South Africa: the first 43 hours. Afr J Mar Sci
Montaggioni LF, Faure G (1980) Récifs coralliens des Mascareignes (Ocean Indien). Coll. Trav. Centre
Univ., O. Indien, Université de La Réunion. 151 pp. ISSN 0337-100X.
Naim O, Tourrand O, Faure G, Bigot L, Cauvin B, Semple S, Montaggioni LF (2013) Fringing reefs of
Reunion Island and eutrophication effects - Part 3: long-term monitoring of living coral. Atoll
Res Bull 596:1–35.
Ohta I, Kakuma S (2005) Periodic behavior and residence time of yellowfin and bigeye tuna associated
with fish aggregating devices around Okinawa Islands, as identified with automated listening
stations. Mar Biol 146:581–594.
Papastamatiou YP, Cartamil DP, Lowe CG, Meyer CG, Wetherbee BM, Holland KN (2011) Scales of
orientation, directed walks and movement path structure in sharks. J Anim Ecol 80:864–874.
Piper DJW, Normark WR (2009) Processes that initiate turbidity currents and their influence on
turbidites: a marine geology perspective. J Sediment Res 79:347–362.
Pirog A, Magalon H, Poirout T, Jaquemet S (2019) Reproductive biology, multiple paternity and
polyandry of the bull shark Carcharhinus leucas. Journal of Fish Biology 0. doi:
Piton B, Taquet M (1992) Océanographie physique des parages de l’Ile de la Réunion (Océan Indien).
Rapport Scientifique Orstom Le Port. 40 pp,
Shuster WD, Dadio S, Drohan P, Losco R, Shaffer J (2014) Residential demolition and its impact on
vacant lot hydrology: implications for the management of stormwater and sewer system
overflows. Landsc Urban Plan 125:48–56.
Simpfendorfer CA, Freitas GG, Wiley TR, Heupel MR (2005) Distribution and habitat partitioning of
immature bull sharks (Carcharhinus leucas) in a Southwest Florida estuary. Estuaries 28:78–85.
Stevens JD, McLoughlin KJ (1991) Distribution, size and sex composition, reproductive biology and
diet of sharks from Northern Australia. Mar Freshw Res 42:151–199.
Taglioni F, Guiltat S (2015) Le risque d’attaques de requins à La Réunion. Éléments d’analyse des
attaques et contextualisation d’une gestion contestée. EchoGéo.
Taglioni F, Guiltat S, Teurlai M, Delsaut M, Payet D (2018) A spatial and environmental analysis of
shark attacks on Reunion Island (1980–2017). Mar Policy.
Taylor S (2007) Population structure and resource partitioning among carcharhiniform sharks in
Moreton Bay, Southeast Queensland Australia. Ph. D. Thesis, University of Queensland. 247 pp.
Trystram C, Rogers KM, Soria M, Jaquemet S (2016) Feeding patterns of two sympatric shark predators
in coastal ecosystems of an oceanic island. Can J Fish Aquat Sci 74:216–227.
Werry JM, Sumpton W, Otway NM, Lee SY, Haig JA, Mayer DG (2018) Rainfall and sea surface
temperature: key drivers for occurrence of bull shark, Carcharhinus leucas, in beach areas. Glob
Ecol Conserv 15:e00430.
Wong BBM, Candolin U (2015) Behavioral responses to changing environments. Behav Ecol 26:665–
Yeiser BG, Heupel MR, Simpfendorfer CA (2008) Occurrence, home range and movement patterns of
juvenile bull (Carcharhinus leucas) and lemon (Negaprion brevirostris) sharks within a Florida
estuary. Mar Freshwater Res 59:489–501.
Yemane D, Shin Y-J, Field JG (2009) Exploring the effect of Marine Protected Areas on the dynamics
of fish communities in the southern Benguela: an individual-based modelling approach. ICES J
Mar Sci 66:378–387.
... Two species were involved in these incidents, the tiger shark Galeocerdo cuvier and the bull shark Carcharhinus leucas, the latter being responsible for the majority of incidents (Taglioni et al., 2019). Human-shark incidents seem to peak in austral winter and are mostly concentrated on the island's west coast where most coastal water activities occur (Lemahieu et al., 2017;Soria et al., 2019); prior to 2010, such incidents were randomly distributed around the island. ...
... High levels of individual variability in behaviour and space use were observed. While young sharks appeared to be present all year round, the detection of larger individuals increased during autumn and winter (Blaison et al., 2015;Soria et al., 2019), which is probably a result of increased mating activity. Soria et al. (2019) suggested that a pre-mating aggregation could occur in the north near the Saint-Gilles harbour from April. ...
... While young sharks appeared to be present all year round, the detection of larger individuals increased during autumn and winter (Blaison et al., 2015;Soria et al., 2019), which is probably a result of increased mating activity. Soria et al. (2019) suggested that a pre-mating aggregation could occur in the north near the Saint-Gilles harbour from April. For example, 'shark17' shifted between four behavioural clusters, which corresponded to a higher closeness in movement patterns in the north at the end of 2012 and in the south at the end of 2013 ( Figure S2). ...
• Knowledge about spatial and temporal variability in the distribution and abundance of predators is necessary to adapt measures to mitigate human–wildlife interactions. • Acoustic telemetry and network analyses were used to investigate the spatial ecology of bull sharks, the species responsible for most shark bites in Reunion Island, one of the world's shark bite hotspots. • The west coast of the island was not used uniformly by every individual, with size predicting the movements of sharks along the coast. • Node-based metrics – closeness, node strength, and cumulated continuous residency times – derived from up to 181 monthly movement networks from 20 individuals, revealed that smaller sharks (<250 cm total length) primarily used the south-west coast while larger individuals spent most of their time in the northern region with regular visits to multiple areas along the coast. • This study provides essential knowledge on bull shark behaviour and central areas used at different periods of the year, which correlates well with the dynamics of observed shark bites. Our approach provides a non-invasive alternative to help predicting and anticipating human–shark conflicts and avoid shark culling programmes detrimental to the conservation of large predators such as sharks.
... Following a spate of shark attacks in Reunion Island in the Western Indian Ocean ( Fig. 1; Werbrouck et al., 2014), a tagging program was introduced to investigate the movements and behaviour of bull and tiger sharks (Soria et al., 2019). After the fifth fatal attack in July 2013, which took place in Saint-Paul Bay (SPB) on the north-west coast ( Fig. 1), growing pressure from beach user associations and the local economic sector forced the Reunion authorities (French government, together with the regional and local city councils) to initiate an experimental shark control program, using baited lines. ...
... In addition to the receivers installed on the four SDL moorings, a network of ten other receivers, termed the CHARC network (Soria et al., 2019), was used to assess the movements of tagged sharks outside SPB from November 2012 to October 2014 (Fig. 1). As some receivers were lost during the study period, the deployment duration of each receiver varied (Fig. 3A). ...
... The former may be indicative of longer-term patterns of presence/absence along the west coast of Reunion Island, possibly related to the biennial female reproductive cycle (Pirog et al., 2019). In addition, bull sharks have been shown to travel along the entire coast of Reunion over different times of the year (Soria et al., 2019) therefore the probability of visiting SPB increases with time. The results in which fewer sharks were detected during the SDL deployment period, compared to the period before deployment, seem counterintuitive. ...
Following a series of shark attacks, local authorities in Reunion Island developed an experimental shark control programme using innovative fishing gear, namely Shark Management Alert in Real Time (SMART) drumlines (SDL). From January to November 2014, four SDL were deployed 24 h per day, four days per week to target bull (Carcharhinus leucas) and tiger (Galeocerdo cuvier) sharks and to test the fishing efficacy of the SDL. Presence and residence time of 19 acoustically tagged bull and 19 tiger sharks, which had been tagged up to two years before the SDL deployment, were modelled against different SDL configurations, which included bait type and presence of bait or catches on the hooks, as well as environmental parameters before, during and after SLD deployment. There was insufficient acoustic data from the tiger sharks for any analyses. Bull sharks spent less time in nearshore waters when drumlines were deployed, and their presence was influenced by sea surface temperatures (SST), rainfall and time of day. There was no difference in the number of bull sharks detected in the SDL deployment area compared with surrounding sites. As SDL catch rates were only poorly correlated with presence of tagged sharks, their efficacy in catching sharks present in the area could not be accurately determined. Overall, the results show that SDL, deployed without chum and baited with small, whole, low-fat fish cannot be considered as "shark magnets", which could attract dangerous bull sharks inshore where they would pose a threat to the safety of surfers and other sea users. The detection of tagged bull sharks moving into and out of the SDL fishing area indicates that these fishing devices do not provide an impenetrable barrier to the passage of this potentially dangerous species.
... Since 2007 a great part of the fringing reefs on the western coast of Réunion Island has been protected by an MPA. Our results did not point to any greater use of the MPA by any of the sharks or the other elasmobranch species, corroborating previous telemetry research that indicated bull sharks actually spent more time outside the MPA than within it (Soria et al. 2019). Conversely, both barracuda and giant trevally made greater use of the MPA, even though they share dietary habits with bull and tiger sharks and would therefore be expected to exhibit similar distribution patterns. ...
... Along the south-west part of the island, the continental shelf is narrow with deep canyons (Fig. 1a). Bull sharks show site fidelity to this region (Soria et al. 2019) and here we identified significant overlap in distribution with giant guitarfish in these shallower waters. In contrast, tiger sharks occurred mostly over deeper isobaths corroborating previous studies in this region and elsewhere (Afonso et al. 2014;Blaison et al. 2015). ...
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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.
... Available distribution maps are also fragmentary regarding the presence of C. leucas in the southwestern Indian Ocean, and they do not display the recent state of knowledge. For example, nowadays, the presence of C. leu cas at Réunion Island is a well-known fact (trystram et al. 2016;martin & Jaquemet 2019;PiroG et al. 2019aPiroG et al. , 2019bsoria et al. 2019Guyomard et al. 2020;le croizier et al. 2020;chynel et al. 2021;hoarau et al. 2021;mariani et al. 2021;mourier et al. 2021;niella et al. 2021b), but the island was not included in any of the past distribution maps for the species. The same applies to the presence of C. leucas in the Seychelles. ...
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The bull shark (Carcharhinus leucas Valenciennes, 1839) is a large, primarily coastally distributed shark famous for its ability to penetrate far into freshwater bodies in tropical, subtropical, and warm-temperate climates. It is a cosmopolitan species with a geographical range that includes the coastlines of all major ocean basins (Atlantic Ocean, Indian Ocean, Pacific Ocean). As a consequence, freshwater occurrences of C. leucas are possible everywhere inside its geographic range. Carcharhinus leucas is a fully euryhaline, amphidromous species and possibly the widest-ranging of all freshwater tolerating elasmobranchs. This species is found not only in river systems with sea access that are not interrupted by human impediments but in hypersaline lakes as well. Rivers and estuaries are believed to be important nursery grounds for C. leucas, as suggested by observations of pregnant females in estuaries and neonates with umbilical scars in rivers and river mouths. Due to the physical capability of this species to enter riverine systems, the documentation of its occurrence in fresh and brackish water is essential for future conservation plans, fishery inspections, and scientific studies that focus on the link between low salinity habitats, shark nurseries, and feeding areas. The author’s review of the available literature on C. leucas revealed the absence of a comprehensive overview of fresh and brackish water localities (rivers and associated lakes, estuaries) with C. leucas records. The purpose of this literature review is to provide a global list of rivers, river systems, lakes, estuaries, and lagoons with records and reports of this species, including a link to the used references as a base for regional, national, and international conservation strategies. Therefore, the objective of this work is to present lists of fresh and brackish water habitats with records of C. leucas as the result of an extensive literature review and analysis of databases. This survey also took into account estuaries and lagoons, regarding their function as important nursery grounds for C. leucas. The analysis of references included is not only from the scientific literature, but also includes semi-scientific references and the common press if reliable. The result of 415 global fresh and brackish water localities with evidence of C. leucas highlights the importance of these habitats for the reproduction of this species. Moreover, gaps in available distribution maps are critically discussed as well as interpretations and conclusions made regarding possible reasons for the distribution range of C. leucas, which can be interpreted as the result of geographic circumstances, but also as a result of the current state of knowledge about the distribution of this species. The results of the examination of available references were used to build a reliable and updated distribution map for C. leucas, which is also presented here.
... 1). Each time an acoustic tag enters within the detection radius (± 400 m) of receiver [13], its ID and time stamp is recorded. This dataset was used to assert the presence of the sharks in the coastal water of the island throughout the study duration. ...
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Two bull sharks ( Carcharhinus leucas ) were tagged in coastal waters off Reunion Island in the tropical Indian Ocean and where tracked for 174 and 139 days using both popup satellite archival tags (pSAT) and acoustic tags. Both sharks spent a majority of their time inshore (58.1% and 89.9% in the male and the female respectively). The female performed short excursions. The male alternated residence time along the coast with wide ranging movements and performed one extensive open-ocean excursion near a seamount situated at more than 200 km from the island. The differences in the residency and home range of both sharks probably reflect different patterns of foraging and mating behaviors in the male and the female. These results underline the importance of developing risk-mitigation management taking into account the movements of sharks, and of double tagging in telemetry studies that attempt to measure the degree of fidelity of a species.
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The rapid expansion of human activities threatens ocean-wide biodiversity. Numerous marine animal populations have declined, yet it remains unclear whether these trends are symptomatic of a chronic accumulation of global marine extinction risk. We present the first systematic analysis of threat for a globally distributed lineage of 1,041 chondrichthyan fishes-sharks, rays, and chimaeras. We estimate that one-quarter are threatened according to IUCN Red List criteria due to overfishing (targeted and incidental). Large-bodied, shallow-water species are at greatest risk and five out of the seven most threatened families are rays. Overall chondrichthyan extinction risk is substantially higher than for most other vertebrates, and only one-third of species are considered safe. Population depletion has occurred throughout the world's ice-free waters, but is particularly prevalent in the Indo-Pacific Biodiversity Triangle and Mediterranean Sea. Improved management of fisheries and trade is urgently needed to avoid extinctions and promote population recovery.
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This paper analyses data related to the 57 shark attacks that were recorded on Reunion from 1980 to 2017, against the backdrop of an Indian Ocean island that is particularly vulnerable to shark attacks. To address this issue of vulnerability, the discussion focuses on the respective weight of environmental, contextual and individual variables. The most pertinent parameters to explain the occurrence of attacks on Reunion are as follows: time of day, month and turbidity. Two specific features of Reunion Island can be added to those: first, the high mortality rate of the attacks (46% vs a world average of 11%), and secondly, the average increase in the number of attacks between 2011 and 2017, despite the average drop in the number of ocean users. To understand and explain this rise, three variables are identified: water turbidity, swell height and victim activity. In addition, the multiple correspondence analysis, despite the limited number of attacks, provides correlations between some variables: on the one hand, attack outcome, turbidity, swell height, and, as regards attacks before or after 2011, board sports and swell height. Keywords: Reunion Island; Human-shark interaction; Hazard; Vulnerability; Coastal water sports; Spatial & environmental analysis
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Climate and weather-based drivers of shark movement are poorly known, yet vital for determining measures for effective conservation of shark populations and the management of shark-human interactions at different time scales. The bull shark, Carcharhinus leucas, an IUCN ‘Near-threatened’ species, is globally distributed in subtropical to tropical regions and is implicated in many attacks on humans because of its euryhaline habitat-use. However, drivers determining rapid transitions of this species among habitats along the freshwater-estuarine-marine continuum are yet to be fully understood. To identify triggers for movement by this species into beach areas we used conditional (binomial and gamma) generalised linear modelling (CGLM) of historical bull shark catches from the Queensland Shark Control Program (QSCP) collected from 1996 to 2007 across 1783 km of the Queensland coastline, Australia. We then compared catches before and after key weather events (such as floods) between 2006 and 2014 and used passive long-term acoustic telemetry to monitor movements of bull sharks into beach areas to test the model predictions. The CGLM showed that bull shark catch (occurrence) in beach areas is driven by rainfall and further influenced by sea surface temperature. Our model suggests that ≥100 mm total rainfall in the catchment associated with each beach area is significantly correlated with increased bull shark catch 1–8 days after the rainfall, a relationship also confirmed by the movements of acoustic tagged sharks between estuarine and beach areas. These trends provide the first predictive relationship for identifying increased risk of bull shark-human interactions in beach areas. They also suggest that the activity patterns of bull sharks are correlated with rainfall and this makes them particularly susceptible to localised, short-term changes in weather and long-term climate change.
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Understanding the environmental drivers of interactions between predators and humans is critical for public safety and management purposes. In the marine environment, this issue is exemplified by shark-human interactions. The annual shark bite incidence rate (SBIR) in La Réunion (Indian Ocean) is among the highest in the world (up to 1 event per 24,000 hours of surfing) and has experienced a 23-fold increase over the 2005-2016 period. Since 1988, 86% of shark bite events on ocean-users involved surfers off the leeward coast, where 96% of surfing activities took place. We modeled the SBIR as a function of environmental variables, including benthic substrate, sea temperature and period of day. The SBIR peaked in winter, during the afternoon and dramatically increased on coral substrate since the mid-2000s. Seasonal patterns of increasing SBIR followed similar fluctuations of large coastal shark occurrences (particularly the bull shark Carcharhinus leucas), consistent with the hypothesis that higher shark presence may result in an increasing likelihood of shark bite events. Potential contributing factors and adaptation of ocean-users to the increasing shark bite hazard are discussed. This interdisciplinary research contributes to a better understanding of shark-human interactions. The modeling method is relevant for wildlife hazard management in general.
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An uncommon series of shark attacks, mostly involving surfers, occurred on the West coast of Reunion Island between 2011 and 2013, causing eight deaths. Following these events, which resulted in social, economic and political upheaval, and referred to as the "shark crisis", a scientific program with the aim of understanding shark behavior and ecology in Reunion Island was launched in 2012. It integrated spatial and temporal monitoring protocol of coastal uses allowing for the study of shark attack repercussions on the dynamics of 15 types of uses. In this paper, we bring shark and users observations together in order to assess human-shark interactions. Firstly, we assess the impacts that shark attacks have triggered in terms of users spatiotemporal distribution between 2011 and 2013. Secondly, we explore human-shark interactions in 2013 using cross-mapping techniques. Results show that three areas (Saint-Gilles, Trois-Bassins, Etang-Salé) have high levels of potential interaction and should be of high interest for the local authorities and stakeholders for further mitigation policies. Although further studies are needed to better understand the link between shark presence and shark attack, this study provides a first insight into human-shark interactions in Reunion Island.
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It is a golden age for animal movement studies and so an opportune time to assess priorities for future work. We assembled 40 experts to identify key questions in this field, focussing on marine megafauna, which include a broad range of birds, mammals, reptiles, and fish. Research on these taxa has both underpinned many of the recent technical developments and led to fundamental discoveries in the field. We show that the questions have broad applicability to other taxa, including terrestrial animals, flying insects, and swimming invertebrates, and, as such, this exercise provides a useful roadmap for targeted deployments and data syntheses that should advance the field of movement ecology.
To improve understanding of bull shark Carcharhinus leucas reproductive biology, we analysed reproductive traits from 118 bull sharks caught along Reunion Island coasts (Western Indian Ocean), including 16 gravid females. Specific microsatellite loci were used to investigate the frequency of multiple paternity. Males and females reached maturity at c. 234 cm and 257 cm total length (LT), respectively, and litter sizes ranged from 5 to 14 embryos. Analysis of the 16 litters collected in various months of the year indicated that parturition occurs between October and December, with a size at birth c. 60–80 cm LT and that the gestation period is probably c. 12 months. Assuming a 1 year resting period and a period of sperm storage (4–5 months) between mating (in June–September) and fertilisation, the reproductive cycle of bull sharks at Reunion Island would be biennial. At least 56.25% of the litters investigated were polyandrous, sired by 2–5 males. Several males that each sired several litters conceived during the same or distinct mating seasons were detected, suggesting both a seasonal aggregation of sharks to mate and some male fidelity to mating site. Altogether, these findings provide valuable information for both shark risk management and conservation of the species in the Western Indian Ocean. This article is protected by copyright. All rights reserved.
Ces travaux visent à inscrire le monitoring des plages récifales de l’île de La Réunion dans la « Stratégie Nationale de Gestion Intégrée des Zones Côtières » (2012). Elle oriente les recherches vers une démarche labellisée sur le long terme qui se concrétise par la mise en place de protocoles de mesures normalisés déployés sur des sites ateliers. Le site de l’Ermitage devient, en 2012, le premier site atelier en zone tropicale et de type bioclastique labellisé à l’échelle nationale (AllENVI, puis INSU en 2014). La dynamique des plages d’arrière-récif demeure à ce jour peu étudiée à travers le monde. Les plages bioclastiques de La Réunion sont le produit de récifs coralliens décrits comme dégradés par les biologistes depuis les années 80. Elles sont confrontées à une très forte anthropisation. Des formes marquées d’érosion se lisent dans ces paysages littoraux. La révision des protocoles de suivi de la topographie des plages tout en exploitant les suivis historiques, vise à illustrer la pluralité des dynamiques en contexte d’arrière-récif. La diversité des processus et des échelles spatio-temporelles impliqués dans le fonctionnement hydro-sédimentaire des plages récifales nécessite la mise en œuvre de méthodes d’observation in situ adaptées, comparables et reproductibles. Dans cette étude nous exploitons surtout les suivis topographiques des plages à l’échelle évènementielle, saisonnière et pluriannuelle. Par l’analyse morphologique et volumétrique des séries de profils topographiques, la variabilité morphosédimentaire en zone intertidale et supratidale est décrite. La significativité de la mesure de la mobilité du trait de côte est également questionnée.
Stomach contents and stable carbon and nitrogen isotope analyses (δ¹³C and δ¹⁵N) were used to investigate the trophic ecology of two apex predators, tiger sharks (Galeocerdo cuvier) and bull sharks (Carcharhinus leucas), from Reunion Island to describe their dietary habits at both the population and individual levels. In this oceanic island, the tiger and bull sharks were more piscivorous and teutophagous than noted in previous research from other localities. The δ¹³C values suggested that bull sharks depended on more neritic organic matter sources than tiger sharks, confirming a coastal habitat preference for bull sharks. Moreover, the total length of the bull shark influenced δ¹³C values, with smaller individuals being more coastal than larger individuals. All indicators suggest that there is a higher degree of similarity between individual tiger sharks compared with the more heterogeneous bull shark population, which is composed of individuals who specialize on different prey. These results suggest that the two species have different functions in these coastal habitats, and thus, they must be considered independently in terms of conservation and management.