Large-scale movements and site delity of bull sharks (Carcharhinus leucas) at Reunion Island (Indian ocean)

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 reect 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 delity of a species.


Abstract
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 re ect 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 delity of a species.

Background
Bull sharks occur in warm inter-tropical zones, mostly on continental shelves of Africa, America and Australia [1,2]. They are reported as being mostly philopatric with some seasonal migrations along the coast [3]. It is one of the rare euryhaline species of sharks allowing females to give birth in estuaries and/or mangroves. Juveniles stay in such ecosystems several years before reaching the oceanic coastal zones [4]. According to IUCN, bull shark is a near threatened species. It is a fragile top predator because of slow growth. Like other apical predators, he is essential for the proper functioning of coastal tropical and subtropical ecosystems [5]. Globally, shark populations around the world, including bull sharks, have been under intense shing pressure throughout their range [6] resulting in a substantial decline in populations.
At the same time, bull sharks have been implicated in attacks on humans, particularly in Reunion island.
Since 2011, the mean number of shark bites per year increased markedly in Reunion Island, from 1.1 to 3 for the periods 1980-2010 and 2011-2019 respectively [7]. Between 2011 and 2019, 27 attacks (including 11 fatal shark bites) occurred in Reunion Island, representing on average 10% of fatal attacks worldwide during the same period. This is considerable when compared to the population of Reunion Island (863000 habitants in 2016). To this date, little is known about the large scale movement behavior of bull sharks, especially around small oceanic islands with narrow insular shelf habitat [8,9]. There is therefore a critical need to improve our understanding of habitat use and especially site delity and movements of this species to mitigate the negative consequences for humans and sharks [10][11][12]. This study aims to determine extent of the movements of bull sharks when they leave the coastal waters of Reunion's island.

Methods
Two bull sharks were equipped with two different types of electronic tags: pop-up archival transmitting tags (MiniPAT-247A PSAT tags, Wildlife Computers, http://wildlifecomputers.com) and coded acoustic transmitters (V16TP-4x; delay 40-80 s, power output 158 dB, battery life of 845 days, Vemco). The array of acoustic receivers consisted of 44 receivers (Vemco VR2W) located around Reunion Island deployed on average ca. 2 km apart and 700 m from shore at depths of 10-60 m (Fig. 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. Tags were programmed to detach after 192 days. The Wildlife Computers software WC-DAP Global Position Estimator Data (V 2.00.0027) was used to process the raw light data [14] to generated two location estimates per day: one at 12h00 and one at 24h00 (UTC).
We used the particle ltering modelling approach described in Tremblay et al. 2009 to estimate likely locations every 8 hours. Constraints such as sea surface temperature and maximum diving depth (DD) were not used to re ne position estimates. This was due to that lack of horizontal thermal gradient and as DD of the sharks was relatively shallow and did not re ect the ocean sea-oor in the area. The

Results
Details on tagging and tracking data are summarized in Table 1. Table 1 Characteristics of tagging and tracking data of the male and the female bull shark Consequently, we only used the data collect prior to this date for the analysis. The pop-up location was at 36 km south of the tagging place and 2 km offshore (21°19'S, 55°23'E).
The proportions of time spent in the coastal waters were 58.1% and 89.9% in the male and the female, respectively. Figure 2 indicates the tracks as a succession of location probability elds. It appears that female had limited large-scale movements and remained in the coastal waters southwest of Reunion Island while performing short offshore excursions.  (Table 2), following by 6 other short excursions between forty and ninety kilometers to the coast. The female performed only three excursions during few days near the shore ( Fig. 3 and Table 2).

Discussion
Results showed that both bull sharks were regularly found inshore, suggesting possible delity to Reunion Island. This insular delity is similar to that described in previous studies on bull sharks [8,16].
The coastal delity of the female was also observed in New-Caledonia [16] and could be related to mating activities of bull sharks during cooling season. Mating, likely occurs during this period as it was suggested by Pirog et al. 2019 on bull sharks at Reunion island or in Tiger sharks in Hawaii as suggested by [12].
The offshore excursions of the male did not resemble large scale migrations as previously recorded for this species [18] or in other shark species [19,20], however, it demonstrate the ability of bull sharks to leave the coastal waters and occasionally foray into the open ocean. Intensive coastal movements punctuated by repeated offshore excursions over several hundreds of kilometers is common in sharks.
For example, this behavior was observed in great white sharks [21], where several individuals simultaneously occurred in an offshore area and it was hypothesized that these potential meeting points, or "cafés", were motivated by feeding or mating. The offshore excursion of the male bull shark was oriented toward a ridge situated > 200 km from the island. Such features are known to have increased productivity via water enrichment associated with localized upwelling [22] and could potentially offshore feeding area.
Using a double-tagging approach, we were able to improve considerably the track quality by adding highly precise locations [23]. We have recorded data over several months both in a male and a female, with sizes representative of individuals typically found in Reunion waters. The consistency of the data with others works on the inshore-offshore patterns of horizontal movements suggests that our results are relevant to infer the coastal delity pattern of bull sharks around Reunion island. However, the causality of the repetitive offshore excursions of remain unknown for bull sharks and should be further examined in future studies.
Declarations Figure 1 Positions of the 44 acoustic receivers around Reunion Island (red dots) and tagging location (pink stars) of both the male (M) and the female (F) bull sharks.  Timeline displaying distance from the coast estimations (solid line) and acoustic detections of the individuals (red triangles) of bull sharks in the coastal water. Pink line indicates excursions over 20 km and minimum 2 days duration.