# Life stage-specific habitat use and trophic ecology of the Critically Endangered Great hammerhead (Sphyrna mokarran)

Tatiana Boube, Charlie Huveneers, Yannis P. Papastamatiou, Siméon

Wong-Sang, Fabrice Charleux, Jean-Claude Gaertner

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Tatiana Boube, Charlie Huveneers, Yannis P. Papastamatiou, Siméon Wong-Sang, Fabrice Charleux, et al.. Life stage-specific habitat use and trophic ecology of the Critically Endangered Great hammerhead (Sphyrna mokarran). Reviews in Fish Biology and Fisheries, 2025, 10.1007/s11160-025-10001-8 hal-05312680

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# Life stage-specific habitat use and trophic ecology of the Critically Endangered Great hammerhead (*Sphyrna mokarran*)

Tatiana Boube<fnref n="1" />, Charlie Huveneers<fnref n="2" />, Yannis P. Papastamatiou<fnref n="3" />, Siméon Wong-Sang<fnref n="4" />, Fabrice Charleux<fnref n="5" />, Jean-Claude Gaertner<fnref n="5" />

**Keywords:** ontogeny, mating, papping, nursery, space use, diet

## ABSTRACT

The great hammerhead (*Sphyrna mokarran*) is listed as Critically Endangered by the IUCN Red List and is one of the most iconic shark species worldwide. Despite its emblematic status, there are major knowledge gaps regarding its habitat use and ecology – particularly during early life stages. In recent years, however, considerable advances have been made in our understanding of great hammerhead biology – especially in previously data-deficient regions such as the northern Indian Ocean and South Pacific Islands. Here, we review and reassess the literature on the habitat use and dietary ecology of *S. mokarran*, with specific consideration of these factors across the species life stages. This review provides new key insights into the species' habitat requirements, which are essential to assess its resilience to climate change. Emerging findings suggests that *S. mokarran* may have more limited dispersal capabilities than previously assumed, exhibiting restricted migratory behavior and a stronger reliance on coastal or insular habitats. Moreover, recent research has begun to describe previously undocumented critical habitats, including mating, papping, and nursery areas—vital for population recovery. Regional differences in habitat use and diet are becoming evident, highlighting the importance of region-specific assessments of vulnerability and population-level conservation strategies. In this context, some regions—such as the South Pacific—may play a particularly important role in the species' survival and thus merit increased research and conservation efforts.

<footnote n="1">¹Univ Polynésie française, Ifremer, IRD, Institut Louis-Malardé, SECOPOL, F-98702 Faa'a, Tahiti, Polynésie française, France</footnote>

<footnote n="2">²Flinders University, Sturt Rd, Bedford Park, South Australia 5042, Australia</footnote>

<footnote n="3">³Florida International University, 11200 SW 8th St, Miami, FL 33199, USA</footnote>

<footnote n="4">⁴Avatoru, Rangiroa, Polynésie française, France</footnote>

<footnote n="5">⁵IRD, Ifremer, Institut Louis-Malardé, Univ Polynésie française, SECOPOL, F-98702 Faa'a, Tahiti, Polynésie française, France</footnote>
<figure><img src="image_2.png" /><figcaption>**Fig. 1 – Graphical Abstract:** Summary of critical habitats and primary dietary preferences across life stages, based on current available research. Dominant prey items are represented by an orange icon, while life stages are indicated by the size of the great hammerhead icon (Adobe Photoshop 26.7.0; Fabrice Charleux).</figcaption></figure>
INTRODUCTION

The great hammerhead shark, *Sphyrna mokarran* (Rüppell, 1837), stands out as one of the ocean's most recognizable and cryptic apex predators. As the largest member of the *Sphyrnidae* family, commonly reaching lengths of up to four-five meters (Springer, 1963), *S. mokarran* is known for its solitary nature and relatively low abundance compared to other hammerhead species (Forselledo et al., 2022; Harry et al., 2011; Gilbert, 1967; Compagno, 1984; Taniuchi, 1974). Its distribution spans tropical to sub-tropical regions, primarily occupying coastal areas (Cadenat and Blache, 1981; Compagno, 1984; Gallagher and Klimley, 2018), though it occasionally ventures into pelagic waters (Fourmanoir, 1961; Graham et al., 2016; Hammerschlag et al., 2011; Raoult et al., 2019). Its k-selected life-history traits have been well studied between the North-West (NW) Atlantic / Gulf of Mexico (GOM) and Australian regions, with late sexual maturity (8 – 20 years of age) (Cortés et al., 2015; Harry et al., 2011; Piercy et al., 2010), low fecundity (litter size ranging 6 - 42 pups) and long gestation periods (10 – 11 months), following a biennial reproductive cycle for females while males are capable of reproducing annually (Cadenat and Blache, 1981; Cliff, 1995; Compagno, 1984; Cortés et al., 2015; Harry et al., 2011; Stevens and Lyle, 1989). Despite the species having one of the longest lifespans among sharks (up to 44 years) (Piercy et al., 2010; Passerotti et al., 2010), these life-history characteristics contribute to *S. mokarran* having a relatively low biological productivity (r = 0.07) compared to other costal-pelagic elasmobranchs (Branstetter, 1990; Cortés et al., 2015). Combined with the well documented physiological vulnerability of the species to capture (Butcher et al., 2015; Gallagher et al., 2017, 2014a; Gulak et al., 2015; Jerome et al., 2018; Morgan and Burgess, 2007; Prohaska et al., 2025; Shiffman and Hammerschlag, 2014), *S. mokarran* has been globally listed as Critically Endangered on the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species (Rigby et al., 2019). Additionally, it has been included in Appendix II of the Convention on Migratory Species (CMS, 2019) and the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES, 2019).

In recent years, the capacity of *S. mokarran* to adapt to the projected environmental changes driven by climate change, and to sustain stable populations, has attracted increasing scientific attention (Black, 2024; Cerutti-Pereyra et al., 2024; Chin et al., 2010; de Souza Junior and Teixeira Lessa, 2025; Santos et al., 2024). Addressing this issue requires a solid understanding of the species' habitat preferences across different life stages and geographic regions, yet existing knowledge remains fragmented and incomplete. The growing body of research on *S.*
*mokarran*, combined with its Critically Endangered status, underscores the urgent need to synthesize available information to inform effective conservation and management strategies, particularly as regional differences in environmental stressors may have variable impacts on the species. To date, two genetically different subpopulations of *S. mokarran* have been identified, in the NW Atlantic and the Australian region (Naylor et al., 2012; Testerman, 2014), suggesting potential local adaptations. These subpopulations display notable differences in life-history traits, such as growth rates k ~ 0.08 in Australian waters (Harry et al., 2011) to ~0.13 in the NW Atlantic/GOM (Cortés et al., 2015; Piercy et al., 2010). Juvenile growth during the first year of life also shows substantial geographic variability. For instance, annual growth averages ~25 cm.y⁻¹ in eastern Australia (n = 100; size at birth ~71 cm) (Harry et al., 2011), ~ 40 cm.y⁻¹ in the NW Atlantic (n = 216; size at birth 66 - 71 cm) (Macdonald et al., 2021; Piercy et al., 2010), and reaches up to 83.3 cm.y⁻¹ in the Arabian Gulf (n = 69; mean size at birth = 71.2 cm Total Length [TL]) (Hsu et al., 2021). These marked differences reinforce the importance of adopting a population-level perspective when assessing the species' ecology and vulnerability, as local environmental conditions likely shape growth dynamics and other life-history parameters.

The most recent comprehensive review by Gallagher and Klimley (2018) highlighted a major gap in data outside the NW Atlantic and GOM. However, since then, a notable number of studies (n = 22) have emerged from regions where the species had previously been understudied or unrecorded, including the Caribbean Sea, the South-West (SW) Atlantic, the Arabian Gulf, and throughout the Pacific. Among these regions, the South Pacific is of particular interest, following the recent description of a natural aggregation site in French Polynesia (Boube et al., 2023), a region known for its exceptionally high shark and ray abundance and diversity (MacNeil et al., 2020). To our knowledge, this is the only documented natural aggregation site of *S. mokarran* in the world beyond the extensively studied, provisioning-induced gatherings observed in the Florida/Bahamas region (Guttridge et al., 2017; Heim et al., 2021). This review aims to consolidate and synthesize current knowledge of *S. mokarran* habitat use, diet, and ecological requirements (e.g., temperature, salinity) throughout its life stages and distribution. It also re-evaluates the species' vulnerability in light of recent findings, with a particular focus on the South Pacific, a region that may be instrumental in supporting the species long-term survival. By examining the ecological role of this region, the review highlights its potential significance in shaping effective conservation efforts for this highly threatened and elusive species.
METHOD

Our review followed the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) process (Page et al., 2021). We identified peer-reviewed articles, reports, and dissertation of doctoral theses with an approval committee, between 1919¹ and June 2025 using the search engines *Google Scholar* and *Web of Science* and the search terms “*Sphyrna mokarran*”, “*mokarran*”, “Great hammerhead” within titles, abstracts, and key words. The reference lists of peer-reviewed articles focusing on the species were also examined. Despite this broad search spectrum (in terms of keywords, time period, and type of paper) we identified 250 articles. From these, we selected publications that provided information on diet, movements, life history characteristics, critical habitats (e.g., foraging, nursery, and pupping areas) and vulnerability to climate change. Publications that focused solely on catch records were excluded. This resulted in 143 publications being retained **(Online Resource 1)**. For each publication, we extracted information on sample size, total length, sex, ontogenetic stage, habitat characteristics, and study location when available. The geographic distribution of studies is uneven across ocean basins, with the majority originating from the NW Atlantic / GOM (n = 58). This is followed, in descending order, by studies from Australian region (n = 21), the Pacific Islands (n = 13), the Western Indian Ocean (n = 9), the SW Atlantic (n = 7), the Caribbean Sea (n=6), the Eastern Pacific (n=5) and the Eastern Atlantic (n = 3) **(Fig. 2; Online Resource 1)**. Additionally, 24 studies had a global scope or were not specific to any particular region.

Understanding habitat preferences of *S. mokarran* across its life stages is essential, as each stage
may have distinct physiological or resource requirements (Grubbs, 2010). Accordingly,
whenever possible, we present the findings from our literature review across three life stages:
neonate/young-of-the-year (YOY), juveniles (defined as individuals from the end of their first
year until reaching sexual maturity), and adults. The size threshold distinguishing juveniles
from adults is based on region- and sex-specific L50 estimates, as follows: 225 cm (males) and
210 cm (females) stretched total length (TL) in North Australia (Stevens and Lyle, 1989); 228
cm stretched TL (sexes combined) in East Australia (Harry et al., 2011); 293 (males) and 337
cm TL (females) in the SW Indian Ocean (Cliff, 1995; Forselledo et al., 2022); 237 (males) and
284 cm TL (females) in the NW Atlantic (Piercy et al., 2010); and 257 (males) and 259 cm TL
(females) in the GOM, based on equation from Piercy et al. (2010) (Moncrief-Cox, 2021). The

<footnote n="1">¹ For studies before Gilbert (1967), species identification was based on photographic evidence and detailed descriptions provided in the articles, as *S. mokarran* was often confused with *Sphyrna tudes* or *Sphyrna zygaena* prior to Gilbert (1967).</footnote>
size at maturity in the Arabian Gulf is considered to be similar to that of the SW Indian and NW
Atlantic Oceans (Hsu et al., 2021). The distribution of studies on *S. mokarran* varied
considerably across life stages (Fig. 2): 20 studies examined one or two early life stages
(neonate /YOY and Juvenile); 23 studies examined two life stages between neonates/YOY or
juveniles and adults, 20 studies included data on all three life stages; 44 focused on adults and
sub-adults, while 35 studies did not specify any life stages, some being general reviews or
addressing topic at the species level.

<figure><img src="image_3.png" /><figcaption>**Fig. 2:** World map showing research effort on *S. mokarran* across its life stages. Each point marks a reported location (tagging or observation), with multiple locations per life stage possible within a single study. Point colours correspond to life stages, as detailed in the legend. Clusters of points (dotted circles) indicate multiple studies, with “n” specifying the number of studies in each cluster. Zoom panels A and B respectively encompass 18 and 49 studies. The dorsal-fin icon denotes studies providing only broad or imprecise geographic information. (Adobe Photoshop 26.7.0; Fabrice Charleux).</figcaption></figure>

**1. Habitat use**

**a. Emerging evidence of mating and pupping habitats**

The reproductive habitats of *S. mokarran*, where mating and pupping occur, remain largely
unknown due to the lack of direct observations. Recently, Guttridge et al., (2025) documented
a potential pre- or post-copulatory interaction in the Bahamas: two unidentified individuals
were observed swimming slowly on their sides in close contact for about three minutes at ~12
m in a reef habitat during the northern hemisphere summer. Pre-mating behaviour may have
also been observed in the South Pacific at Rangiroa atoll during the austral winter, where four
males were observed following a mature-sized female along the outer reef slope at 15 - 20 m
(pers. obs. C. Mastnak; **Online Resource 2**). As with many large-bodied sharks, knowledge of
the environmental conditions required for mating is limited, and current evidence remains
indirect.

Pupping in *S. mokarran* was initially theorized to occur offshore in deep waters, due to the rarity
of neonate sightings and catches in coastal areas (Castro, 1993; Harry et al., 2011; Hueter and
Tyminski, 2007; McCandless et al., 2002; Stevens and Lyle, 1989), and continues to be alluded
to in recent studies (Barker et al., 2017; Casselberry et al., 2025; Hlavin and Macdonald, 2025).
However, an increasing number of studies suggest that *S. mokarran* gives birth in shallow bays,
mangroves, and inshore reefs. In the GOM and along the U.S. coast, Gallagher et al. (2014a)
propose that pregnant great hammerheads enter shallow tropical coastal bays to give birth. In
southern Belize, a mangrove habitat has been identified as a potential pupping and nursery
ground (IUCN, 2007), while pupping has been reportedly observed in a bay area of Little
Cayman (Gore et al., 2024). Guttridge et al. (2017) proposed that coastal waters off South
Carolina and Georgia could serve as key pupping sites during the northern hemisphere summer,
with confirmed pregnant females migrating from the Bahamas to the South Carolina coastline
during that time (Guttridge et al., 2022). These females exhibited a strong preference for surface
waters, spending over 90% of their time at depths shallower than 30 meters (Guttridge et al.,
2022), consistent with findings by Schlaff et al. (2014), which suggests that female sharks seek
warmer, shallow waters for reproductive benefits. Drawing on repeated seasonal migrations of
female *S. mokarran* between the Bahamas and specific regions of the southeast U.S. coastlines
during the pupping and mating season, along with the consistent capture of YOY individuals in
those same areas, Guttridge et al. (2025) suggest that the species exhibits pupping and/or mating
philopatry to those sites. In the South Pacific, a pupping event was observed in July 2006 within
the Rangiroa lagoon in a shallow sandy area (< 1 m) (pers. obs., S. Wong-Sang; species
identified by its large first dorsal, habitat shown in **Online Resource 3**). The event occurred
mid-morning in an area influenced by a rising tidal current flowing over the atoll reef ring.
Blacktip reef sharks (*Carcharhinus melanopterus*) and lemon sharks (*Negaprion brevirostris*)
were seemingly attracted to the pupping event and attempted to prey on the newborns. The
observer witnessed the birth of seven pups, each emerging with its cephalofoil folded inward.
To our knowledge, this observation provides the first detailed documentation of parturition in
*S. mokarran*, offering valuable insights into potential pupping habitats, in line with sporadic reports from the West Atlantic. It highlights the significance of coastal and shallow environments - such as mangroves, estuaries, and lagoon waters – as potential parturition habitats and challenges the long-held belief that pupping occurs offshore. Pupping in this species has been documented in late spring to early summer in both hemispheres - for example, June–July in the NW Atlantic and November–January in northern and eastern Australia (Clark and Von Schmidt, 1965; Harry et al., 2011; Stevens and Lyle, 1989). In contrast, this observation points to geographical variation in the timing of pupping, indicating that pupping may occurs during the austral winter in the South Pacific.

b. Early-life stages rely on coastal nursery habitats

*Sphyrna mokarran* was initially believed not to rely on nursery areas (Harry et al., 2011; Stevens and Lyle, 1989) due to a lack of reliable data, which was likely hindered by the potential for misidentification with *Sphyrna lewini* (Barker et al., 2017; Cadenat and Blache, 1981; Compagno, 1984; Fourmanoir, 1961; Gilbert, 1967; Forselledo et al., 2022). However, Macdonald et al. (2021) has since demonstrated the existence of such critical habitat for the species based on Heupel et al. (2007) criteria. Sightings and captures of neonates, YOY, and juveniles *S. mokarran* across various regions and ocean basins are also increasingly being reported, including in Florida (Carlson et al., 2021; Hlavin and Macdonald, 2025; Macdonald et al., 2021; McCandless et al., 2002; Tinari and Hammerschlag, 2021), Texas (Hueter and Tyminski, 2007), South Carolina (Barker et al., 2017), Puerto Rico (Espinoza et al., 2024), Belize (Pikitch et al., 2005), Brasil (Andrade et al., 2016; Pinheiro et al., 2016), West Africa (Clavareau et al., 2020), South Africa (Cliff, 1995), the Red Sea (Spaet and Berumen, 2015), the Arabian Gulf (Hsu et al., 2022, 2021; Lin et al., 2024), Australia (Chin et al., 2017; Harry et al., 2011; Lubitz et al., 2023; Simpfendorfer et al., 2014; Yates et al., 2015; Kinney, 2011), Fiji (Vierus et al., 2018), and Guatemalan Pacific (Hérnandez et al., 2024). Neonates, YOY and juveniles *S. mokarran* show a strong dependence on coastal, shallow and turbid habitats, including mangrove shorelines (Hueter and Tyminski, 2007; IUCN, 2007), and estuaries or bays (Barker et al., 2017; Carlson et al., 2021; Hueter and Tyminski, 2007; IUCN, 2007; Macdonald et al., 2021; Yates et al., 2015), with possible tidal influence (Harry et al., 2011; Vierus et al., 2018), but also beach areas (Branstetter, 1990; Sadowsky, 1965) and inshore barrier reefs (Pikitch et al., 2005). Substrate descriptions indicate a preference for sandy or muddy bottoms (Carlson et al., 2021), often interspersed with seagrass beds (Barker et al., 2017; Hueter and Tyminski, 2007; Simpfendorfer et al., 2014; Vierus et al., 2018; Yates et al., 2015) and coral
patches (Macdonald et al., 2021). Different early-life stages of *S. mokarran* may display varying environmental preferences both across stages and regions. For temperature, neonates and YOY are predominantly found in waters exceeding 29°C in the western Pacific (Vierus et al., 2018), whereas sites in the GOM exhibit a wider thermal range (23.9 - 31.5°C) (Carlson et al., 2021; Tinari and Hammerschlag, 2021) (Table 1; Fig.3). For juveniles, temperature data are only available in the GOM, where they appear to tolerate a broader temperature range than neonates/YOY (20.4 - 32.9°C) (Carlson et al., 2021; Drymon and Wells, 2017; Hueter and Tyminski, 2007; McCandless et al., 2002) (Table 2; Fig.3). Similarly, in terms of salinity, neonates and YOY in the West Pacific were recorded in higher salinity ranges (27.2 - 44.6 ppt) than in the GOM (20.8 - 31.0 ppt) (Carlson et al., 2021; Hueter and Tyminski, 2007; McCandless et al., 2002; Vierus et al., 2018) (Table 1). In the GOM, juveniles were recorded at wider ranges of salinity than neonate/YOY (15.8 - 37.6 ppt) (Carlson et al., 2021; Drymon and Wells, 2017; Hueter and Tyminski, 2007; McCandless et al., 2002) (Table 1 and 2). Juveniles can also tolerate a broader range of dissolved oxygen (DO) levels compared to YOY (3.4 - 7.8 mg/L vs. 5.3 - 7.6 mg/L) (Carlson et al., 2021; Hueter and Tyminski, 2007) (Table 1 and 2). This suggests that neonates and YOY may be more vulnerable to fluctuations in environmental conditions, which warrants further investigation, particularly in the Indo-Pacific region, where data remains limited. Additionally, no habitat preferences have been reported between males and females YOY and juveniles, supporting Stevens and Lyle's (1989) findings that sexual segregation in *S. mokarran*'s early life stages is less pronounced compared to other hammerhead species, such as *S. lewini*. However, in some coastal fisheries of Brazil, juvenile males are reportedly dominating the catches (Pinheiro et al., 2023).

<table><caption>**Table 1:** Environmental parameters recorded at sites where neonate/YOY *S. mokarran* have been observed. Values are presented per site and per study, with N indicating the number of individuals recorded at each site.</caption><thead><tr><th></th><th>N</th><th>Temperature (°C)</th><th>Salinity (ppt/PSU)</th><th>Dissolved oxygen (mg/L)</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="12">GULF OF MEXICO</td><td>8</td><td>23.9 - 31.1</td><td>21.9 - 33.0</td><td>5.30 - 6.11</td><td rowspan="3">McCandless et al., 2002</td></tr><tr><td>3</td><td>30.0 - 30.1</td><td>32.2 - 34.2</td><td>6.25 - 6.25</td></tr><tr><td>5</td><td>30.0 - 30.0</td><td>34.5 - 34.9</td><td>-- - --</td></tr><tr><td>18</td><td>23.9 - 31.5</td><td>20.8 - 33.9</td><td>5.3 - 7.6</td><td rowspan="3">Hueter and Tyminski, 2007</td></tr><tr><td>3</td><td>30.0 - 30.1</td><td>32.2 - 34.2</td><td>6.3 - 6.3</td></tr><tr><td>5</td><td>30.0 - 30.0</td><td>34.5 - 34.9</td><td>-- - --</td></tr><tr><td>3</td><td>26.4 (25.2 - 28.3)</td><td>29.4 (27.6 - 31.0)</td><td>5.76 (5.57 - 6.05)</td><td>Carlson et al., 2021</td></tr></tbody></table>
<table><tr><td>NW ATLANTIC</td><td>2</td><td>28 - 35 (Castro, 1993)</td><td>Barker et al., 2017</td></tr><tr><td>ARABIAN GULF</td><td>43<sup>a</sup></td><td>often &gt; 35°C (Sheppard et al., 1992)</td><td>Year-round average: 42 (Vaughan et al., 2019)</td><td>Hsu et al., 2021</td></tr><tr><td>RED SEA</td><td>83<sup>a</sup></td><td>24 - 35 °C depending on season (Ngugi et al., 2012)</td><td>up to 40 (Ngugi et al., 2012)</td><td>Spaet and Berumen, 2015</td></tr><tr><td>SW PACIFIC</td><td>11</td><td>29.9 - 31.3<br>30.4 - 31.9<br>29.4 - 31.6<br>31.2 - 31.9</td><td>34.2 - 34.9<br>27.2 - 43.4<br>33.1 - 44.6<br>34.1 - 35.2</td><td>Vierus et al., 2018</td></tr></table>

<sup>a</sup> No distinction between neonate/YOY and juveniles in the study

<table><caption>**Table 2:** Environmental parameters recorded at sites where juveniles *S. mokarran* have been observed. Values are presented per site and per study, with N indicating the number of individuals recorded at each site.</caption><thead><tr><th></th><th>N</th><th>Temperature (°C)</th><th>Salinity (ppt/PSU)</th><th>Dissolved oxygen (mg/L)</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="10">GULF OF MEXICO</td><td>20</td><td>23.0 - 31.0</td><td>15.8 - 32.0</td><td>5.37 - 6.15</td><td rowspan="4">McCandless et al., 2002</td></tr><tr><td>14</td><td>20.9 - 30.0</td><td>27.1 - 34.8</td><td>6.09 - 6.09</td></tr><tr><td>13</td><td>25.0 - 32.9</td><td>27.5 - 35.9</td><td>4.22 - 6.76</td></tr><tr><td>1</td><td>31.0</td><td>28.1</td><td>--</td></tr><tr><td>33</td><td>23.0 - 31.5</td><td>15.8 - 32.7</td><td>5.4 - 7.2</td><td rowspan="5">Hueter and Tyminski, 2007</td></tr><tr><td>21</td><td>20.4 - 31.2</td><td>24.4 - 37.6</td><td>5.4 - 7.4</td></tr><tr><td>28</td><td>25.0 - 32.9</td><td>27.5 - 35.9</td><td>4.2 - 7.2</td></tr><tr><td>5</td><td>25.1 - 31.0</td><td>24.9 - 28.1</td><td>5.4 - 7.8</td></tr><tr><td>24</td><td>22.0 - 30.9</td><td>26.3 - 35.3</td><td>3.4 - 6.6</td></tr><tr><td>2</td><td>F: 24 - 31</td><td></td><td></td><td rowspan="2">Drymon and Wells, 2017</td></tr><tr><td>1</td><td>M: 22 - 25</td><td></td><td></td></tr><tr><td rowspan="2">ARABIAN GULF</td><td>2</td><td>26.1 (24.6-27.5)</td><td>31.75 (31.7 - 31.8)</td><td>6.23 (6.20-6.25)</td><td rowspan="2">Carlson et al., 2021</td></tr><tr><td>3</td><td>28.0 (24.9-29.6)</td><td>34.9 (34.0 - 36.6)</td><td>6.17 (6.12-6.28)</td></tr><tr><td>RED SEA</td><td>83<sup>a</sup></td><td>often &gt; 35°C (Sheppard et al., 1992)</td><td>Year-round average: 42 (Vaughan et al., 2019)</td><td></td><td>Hsu et al., 2021</td></tr><tr><td></td><td></td><td>24 - 35°C depending on season (Ngugi et al., 2012)</td><td>up to 40 (Ngugi et al., 2012)</td><td></td><td>Spaet and Berumen, 2015</td></tr></tbody></table>

<sup>a</sup> No distinction between neonate/YOY and juveniles in the study
<figure><img src="image_4.png" /><figcaption>**Fig. 3:** Thermal ranges across the species' ontogeny and distribution, based on data from Table 1, 2, and 3. Rectangles show minimum and maximum values per sites sampled (blue = Neonate/YOY; orange = Juvenile; green = Adult). Number of studies per region and life stage is indicated by "n =", as individual studies may include data from multiple sites. Mean temperature value is represented by a horizontal line when available (RStudio 2024.09.0)</figcaption></figure>

In all recorded observations of neonate and YOY *S. mokarran*, the surveyed sites also served as nursery grounds for other shark species, including other hammerhead species (e.g., *S. lewini*, *Sphyrna tiburo*, *Sphyrna zygaena*, *Sphyrna gilberti*, and *Sphyrna media*) (Barker et al., 2017; Carlson et al., 2021; Espinoza et al., 2024; Hérnandez et al., 2024; Hueter and Tyminski, 2007; Macdonald et al., 2021; Pikitch et al., 2005; Vierus et al., 2018). The overlap between early life stages of different shark species, referred to as communal nurseries areas by Simpfendorfer and Milward (1993), is hypothesized to reduce both inter- and intraspecific predation risks, as adults sharks tend to remain separate from their pups. However, regional overlap between adults and YOY *S. mokarran* has also been reported (Chin et al., 2017; Clavareau et al., 2020; Cliff, 1995; Harry et al., 2011; Hlavin and Macdonald, 2025; Macdonald et al., 2021; Yates et al., 2015), but with no evidence of cannibalism - except for a single, and old study involving a highly mature *S. mokarran* (Coles, 1919). The downside of these shared nurseries is the potential interspecific competition for resources (Simpfendorfer and Milward, 1993), which may explain the high dietary specialization observed in YOY *S. mokarran*, primarily feeding on *Platycephalidae* (Cliff, 1995; Hsu et al., 2022), which is likely less targeted by other shark species. In the nursery area of Biscayne Bay, Miami, Hlavin and Macdonald, (2025) reported a high degree of isotopic niche overlap among early ontogenetic stages of *S. mokarran*,
potentially reflecting intraspecific competition for resources. More broadly, the co-occurrence
of YOY and juvenile *S. mokarran* with early life stages of other shark species across various
regions suggests shared ecological needs and predator avoidance strategies, potentially leading
to increased resource limitation.

Further investigation is needed on postnatal horizontal movements and space use of S. mokarran, especially whether neonates remain in the parturition area (Barker et al., 2017). In the Arabian Gulf, stable isotope analysis showed that both neonates and juveniles rely heavily on shallow coastal waters as primary feeding grounds, suggesting restricted space use with horizontal movements occurring primarily near shorelines (Hsu et al., 2022, 2021). YOY and juveniles are often reported co-occurring in the same areas (Carlson et al., 2021; Espinoza et al., 2024; Harry et al., 2011; Hlavin and Macdonald, 2025; Hsu et al., 2022, 2021; Hueter and Tyminski, 2007; Lin et al., 2024; Macdonald et al., 2021; McCandless et al., 2002; Yates et al., 2015), which suggests prolonged residency to nursery grounds throughout early ontogenetic stages or site fidelity with extent of movements increasing as individuals mature. Emerging data are beginning to provide insights into the horizontal movements of juveniles. Clavareau et al., (2020) reported seasonal purse-seine capture of juvenile *S. mokarran* off the coasts of Gabon and Mauritania in West Africa. Additionally, using stable isotope analyses in the NW Atlantic, Hlavin and Macdonald (2025) reported a seasonal shift in juvenile *S. mokarran* from bay to coastal resources. This suggests a seasonal movement along coastlines as the sharks mature, potentially as a strategy to reduce competition with other ontogenetic stages. In the Gulf of Mexico, Drymon and Wells (2017) tracked three large juveniles (187 - 250 cm TL) over a three-week period, documenting movement distances ranging from 22 to 387 km. These findings indicate considerable variability in movement, with greater dispersal and seasonal movements during the juvenile stage compared to the YOY and neonates. Information on vertical space use during the early stages is limited, but neonates and juveniles are typically found in shallow waters (<15 m) (Carlson et al., 2021; Cliff, 1995; Drymon and Wells, 2017; Hueter and Tyminski, 2007; Vierus et al., 2018; Yates et al., 2015) and have been shown to spend over 90% of their time at depths shallower than 6 m (Drymon and Wells, 2017). One study on juveniles in the GOM reported vertical migrations with mean depths ranging from 37.2 to 50.8 m, and a maximum depth of 98 m (Drymon and Wells, 2017). Similar to horizontal movements, vertical space use appears to increase as individuals mature.
c. Reevaluating adult habitat uses in *Sphyrna mokarran: limited migrations and habitat ubiquity*

*Sphyrna mokarran* is considered to be a migratory, wide - ranging species (Gallagher and Klimley, 2018; Rigby et al., 2019), capable of migrations exceeding 1,000 km (Guttridge et al., 2022, 2017; Hammerschlag et al., 2011; Kohler and Turner, 2001; Queiroz et al., 2016), which were suggested to be driven by aggregations and movements of preys (Guttridge et al., 2017; Hammerschlag et al., 2011; Lowerre-Barbieri et al., 2021). However, increasing evidence suggests that most populations are more resident to core habitats than initially reported (Chin et al., 2017; Gore et al., 2024; Graham et al., 2016; Guttridge et al., 2025; Lubitz et al., 2023), with intraspecific variation in residency and site fidelity patterns (Boube et al., 2023; Griffin et al., 2022; Guttridge et al., 2017). These varying philopatric movements appear consistent between sexes (Casselberry et al., 2025; Guttridge et al., 2025) and are likely associated with the year-round or seasonally predictable availability of preys in those areas (Calich et al., 2021; Casselberry et al., 2022; Griffin et al., 2022; Guttridge et al., 2025; Lowerre-Barbieri et al., 2021; Lubitz et al., 2023). Recent studies in the NW Atlantic have demonstrated that provisioned great hammerheads modify their spatial behavior in response to food availability, potentially leading to a reduction in their overall space use (Heim et al., 2024, 2021). The species exhibits partial migration in the NW Atlantic/GOM, with a portion of the population residing year-round in the Florida Keys, showing seasonal shifts between inshore channels and outer reef habitats (Casselberry et al., 2025). Others undertake seasonal return-migrations along established corridors, in defined areas of the NW Atlantic or GOM coastlines (Calich et al., 2021; Casselberry et al., 2025; Guttridge et al., 2022; Lowerre-Barbieri et al., 2021). In North and East Australia, Lubitz et al. (2023) demonstrated that adult *S. mokarran* exhibit restricted spatial ranges, likely driven by the year-round availability of batoid prey populations. However, Raoult et al. (2020), suggest that individuals recorded in the Southeast may range further north along the Australian coastline, based on stable isotope analysis. Body size may also influence the extent of movement, as smaller adults (TL < 3 m) are less likely to engage in long-distance migrations (Heupel et al., 2020). Prey density and availability are therefore key factors influencing the movements, migration, and philopatric behaviour of *S. mokarran*.

Genetic structuring exhibits regional variability, with certain populations displaying relatively high inbreeding rates and apparent genetic homogeneity (Brunjes et al., 2024; Stanhope et al., 2023). For instance, in Northern and Eastern Australia, as well as Papua New Guinea, movement across the continental shelf facilitates gene flow, contributing to lower genetic differentiation (Brunjes et al., 2024; Chin et al., 2017; Heupel et al., 2020). In contrast, other
populations exhibit greater genetic structuring, such as those in Australia, Indonesia, and the Red Sea, where geographic barriers, including the Timor Trench and Wallace's Line, likely limit connectivity (Brunjes et al., 2024). These findings suggest that large-scale dispersal is more restricted than initially assumed, although this might be biased by a small number of individuals maintaining gene flow between regions (Gonzalez et al., 2017). Overall, although the movement behavior of the species appears complex, recent evidence challenges the traditional view of *S. mokarran* as a highly migratory species. Instead, findings suggest that its horizontal movements are more restricted than previously assumed and shaped by both regional and intraspecific variability.

Vertical space use in adult *S. mokarran* has been comparatively less studied than its horizontal movements. A significant number of studies have documented the species' frequent use of shallow coastal habitats, with depths rarely exceeding 50 m (Baum and Blanchard, 2010; Chin et al., 2010; Cliff, 1995; Compagno, 1984; Fourmanoir, 1961; Griffin et al., 2022; Guttridge et al., 2022; Heithaus et al., 2007; Henderson et al., 2021; Lubitz et al., 2023; Miller et al., 2014; Raoult et al., 2019; Roemer et al., 2016; Sadowsky, 1965, 1971; Stevens and Lyle, 1989). Adults have not been recorded at depths beyond 300 m (Rigby et al., 2019), in contrast to *S. lewini*, which is known to reach bathypelagic depths exceeding 1,000 m (Anderson et al., 2022). Satellite telemetry data from Guttridge et al. (2022) further confirm the species' strong association with very shallow waters (less than 30 meters), and reveal intraspecific variation in vertical movement patterns, regardless of sex. Diel vertical behavior was evident in 90% of the tracked sharks (n = 11), with most individuals occupying deeper waters (~50 meters) during daylight hours and moving closer to the surface at night (Guttridge et al., 2022).

Sexual segregation is well documented in *S. lewini* (Gallagher and Klimley, 2018; Klimley, 1987), but has never been defined for *S. mokarran* (Gallagher and Klimley, 2018), or is considered less pronounced (Stevens and Lyle, 1989). This may be attributed to the solitary nature of *S. mokarran*, in contrast to *S. lewini*, which frequently forms large social groups, making patterns of sexual segregation more readily observable. However, previous studies have highlighted habitat partitioning between male and females, associated with temperature preferences. Males are more commonly found in cooler, offshore waters (Cliff, 1995; Drymon and Wells, 2017 [based on limited sample]; Hammerschlag et al., 2011; Macbeth et al., 2009), while females occupy warmer, inshore areas, likely benefiting feeding and parturition (Gallagher et al., 2014b; Guttridge et al., 2022; Harry et al., 2011; IUCN, 2007; Tinari and Hammerschlag, 2021). Sexual segregation in *S. mokarran* is often observed on a seasonal basis.
For example, in the South Pacific, a population exhibited a seasonal sex-biased use of a pass, with females predominating in summer (Southern Hemisphere) and males in winter (Boube et al., 2023). In the South-West (SW) Indian Ocean, Cliff (1995) reported sexual segregation only during winter months, with males being predominantly captured. In the NW Atlantic, *S. mokarran* has been observed at a provisioning site, where the population was female-dominated during winter (Guttridge et al., 2017; Heim et al., 2021), although males were recorded at elevated temperature in the summer (Guttridge et al., 2025). While not consistently observed across all studies and regions, these results suggest that adult males and females may exhibit different space-use patterns, potentially influenced by temperature and seasonal variations (Spencer, 2024).

A review of environmental parameters highlights *S. mokarran* broad ecological tolerance and ubiquitous presence across diverse habitats. The temperature preferences of adults vary among geographic region (**Table 3; Fig.3**) with the species seemingly tolerating cooler temperatures in the SW Indian Ocean (mean: 23.1 °; range: 18.5 - 26.1°C) (Cliff, 1995) compared to the NW Atlantic and GOM, where the year-round average is closer to 25.0 - 27.0°C, and ranges between 13.5° to 34.0°C depending on seasons (Calich et al., 2018; Grace and Henwood, 1997; Gulak et al., 2015; Guttridge et al., 2025, 2022; Heithaus et al., 2007; Lear et al., 2021; Tinari and Hammerschlag, 2021). In the South Pacific, the species inhabits a higher temperature range of 26.6 – 28.9°C, based on measurements taken exclusively during the summer months (Boube et al., 2023). Several studies report an affinity for temperature discontinuities and edge habitats (Chin et al., 2010; Gore et al., 2024; Guttridge et al., 2025; Heithaus et al., 2007; Queiroz et al., 2016), areas often associated with prey aggregations (Cliff, 1995; Compagno, 1984; Guttridge et al., 2022; Heupel et al., 2020). Further research is needed to clarify whether this inter-regional variability in thermal tolerance reflects broad global tolerance or region-specific adaptation. Regarding salinity and DO preferences, available data are insufficient to compare differences between genders and regions (**Table 3**). Records in the NW Atlantic and GOM indicate that adults are captured across a wide range of DO levels (1.98 - 12.78 mg/L) (Grace and Henwood, 1997; Tinari and Hammerschlag, 2021). They have also been observed hunting in areas with extremely low DO levels (Roemer et al., 2016), likely tolerating such conditions temporarily during foraging activities (Gulak et al., 2015). Their small mouth gape necessitates increased ventilation rates to meet oxygen demands of burst swimming strategies, making prolonged occupancy in deoxygenated environments energetically costly (Gallagher et al., 2014a). Measure in salinity associated with the species occurrences typically fall within a relatively
narrow and standard oceanic range of 33 - 36 ppt (Table 3) (Grace and Henwood, 1997; Parker and Bailey, 1979), although there is one report of the species being observed in a river system in the SW Indian Ocean (Cliff, 1995). Additionally, the species is frequently observed in highly productive waters, likely associated with oceanic primary productivity (Guttridge et al., 2025, 2017; Lubitz et al., 2023; Queiroz et al., 2016) (Heupel et al., 2020), and occurs in both turbid (Cliff, 1995; Griffin et al., 2022; Lubitz et al., 2023) and clear-water environments (Boube et al., 2023; Fourmanoir, 1961; Guttridge et al., 2017).

Adults forage in a diverse array of coastal habitats, including inshore, warm, shallow (< 6m) sandy bottoms (Doan and Kajiura, 2020), coral banks (Fourmanoir, 1961; Strong et al., 1990), mangroves shorelines (Roemer et al., 2016), as well as deep and shallow channels influenced by currents and tides (Boube et al., 2023; Chapman and Gruber, 2002; Fourmanoir, 1961; Heithaus et al., 2007; Mourier et al., 2013; Roemer et al., 2016). These dynamic environments shape the species' spatial use (Casselberry et al., 2022) by providing both feeding opportunities and conditions that potentially support recovery from the energetic costs of predation. Specifically, they reduce the energy required for swimming to maintain adequate oxygenation (Roemer et al., 2016). Diel influences on foraging behavior have recently been examined in the NW Atlantic, revealing that *S. mokarran* forages over broader spatial scales and swims at higher speeds during daylight hours to maintain consistent prey encounter rates (Spencer et al., 2025b). However, peak activity, characterized by burst swimming, occurs at nighttime (Lear et al., 2021; Spencer et al., 2025b). The lunar cycle can also influence predator-prey dynamics, with *S. mokarran* synchronizing its horizontal movements with prey aggregations occurring during specific moon phases (Boube et al., 2023; Griffin et al., 2022). Together, these findings underscore the species' physiological adaptability in exploiting a variety of dynamic coastal environments, but also highlight its reliance on specific temporal and environmental cues to optimize foraging efficiency. A significant knowledge gap remains concerning the species' preferences for DO levels, primary productivity, and salinity ranges, as well as the extent to which these factors may influence *S. mokarran* movements and habitat use.

<table><caption>**Table 3:** Environmental parameters recorded at sites where adults *S. mokarran* have been observed. Values are presented per site and per study, with N indicating the number of individuals recorded at each site.</caption><thead><tr><td>N</td><td>Temperature (°C)</td><td>Salinity (ppt/PSU)</td><td>Dissolved oxygen (mg/L)</td><td>Reference</td></tr></thead></table>
<table><tbody><tr><td rowspan="10">NW ATLANTIC</td><td>238</td><td>27.0 ± 2.8 (20 - 32)</td><td>34.26 ± 3.69 (17.00 - 41.00)</td><td>1.98 - 12.78</td><td>Tinari and Hammerschlag, 2021</td></tr><tr><td>16</td><td>Wet season:<br/>28.4 ± 0.8 (27.0 - 29.6)</td><td></td><td></td><td>Calich et al., 2018</td></tr><tr><td>14</td><td>Dry season:<br/>22.8 ± 2.2 (18.5 - 26.2)</td><td></td><td></td><td>Guttridge et al., 2022</td></tr><tr><td>11</td><td>Mean daily:<br/>25.4 ± 1.1 (22.9 - 27.0)</td><td></td><td></td><td>Guttridge et al., 2022</td></tr><tr><td>32</td><td>24 - 28</td><td></td><td></td><td>Guttridge et al., 2022</td></tr><tr><td>35</td><td>20 - 24</td><td></td><td></td><td>Guttridge et al., 2022</td></tr><tr><td>1</td><td>16 - 20</td><td></td><td></td><td>Guttridge et al., 2022</td></tr><tr><td>42</td><td>Higher probability ~ 18°C,<br/>Probability decrease > 26°C</td><td></td><td></td><td>Guttridge et al., 2022</td></tr><tr><td>1</td><td>21.9 ± 0.4 (17.0 - 27.9)</td><td></td><td></td><td>Guttridge et al., 2022</td></tr><tr><td>15</td><td>13.5 - 28.5</td><td>34.3 - 36.3</td><td>3.2 - 6.7</td><td>Grace and Henwood (1997)</td></tr><tr><td>7</td><td>Wet season: 28 - 30<br/>Dry season: 25 - 26</td><td></td><td></td><td>Guttridge et al., 2025</td></tr><tr><td rowspan="2">GULF OF MEXICO</td><td>NA</td><td></td><td>33 - 34</td><td></td><td>Parker and Bailey (1979)</td></tr><tr><td>15</td><td>Dry season: 23.5 - 26.3</td><td></td><td></td><td>Lear et al., 2021</td></tr><tr><td>SW INDIAN</td><td>158</td><td>23.1 (18.5 - 26.1)</td><td></td><td></td><td>Cliff, 1995</td></tr><tr><td>SW PACIFIC</td><td>50</td><td>27.7 (26.6 - 28.9)</td><td></td><td></td><td>Boube et al., 2023</td></tr></tbody></table>

d. Aggregative behaviour

Although *S. mokarran* is primarily considered a solitary species (Rigby et al., 2019; CMS, 2019), occasional aggregations have been reported but are much less frequent and understood compared to aggregations of *S. lewini* and *S. zygaena*. In West Africa, Cadenat and Blache (1981) reported group movements (number unspecified) of *S. mokarran* occurring between October and January. Similarly, Brainard et al. (2005) documented a school of 15 great hammerheads in deep waters along the edge of a terrace at Jarvis Island, South Pacific. In French Polynesia, groups of up to nine adults have been recorded in a restricted area of a pass during the austral summer, a transition zone between the open ocean and the lagoon (Boube et
al., 2023). It is not unusual to observe two or more individuals swimming in close proximity and following one another in this pass, suggesting that it could be used by *S. mokarran* to aggregate temporarily (McInturf et al., 2023) and where intraspecies interactions might occur. However, the extent and nature of these intraspecific interactions are currently unknown. Large brain to body mass has been documented for *S. mokarran*, which is a trait often associated with complex social behavior and intraspecific interactions, among other cognitive abilities (Santos et al., 2024; Yopak, 2012). These aggregations in *S. mokarran* might be underestimated due to the species' low abundance, the rarity of observations in the wild, and the likely sporadic nature of such events.

## 2. Trophic ecology

### a. YOY and juvenile diet

The diet of juveniles *S. mokarran* in Australia and SW Indian Ocean, was initially described as being primarily composed of demersal elasmobranch (i.e., guitarfish – *Rhinobatidae*, stingrays – *Dasyatidae*, and small *Carcharhinidae*), along with demersal or benthic teleost (e.g., sea catfish – *Ariidae*, and flatheads – *Platycephalidae*) (Cliff, 1995; Stevens and Lyle, 1989). Stomach content analysis of *S. mokarran* caught in the Arabian Gulf shows that YOY primarily prey on benthic teleost, particularly *Platycephalidae*, during their first two years, with females transitioning to elasmobranchs (e.g., *Myliobatidae*) and males to demersal species (e.g., *Epinephelus sp.*) in the third year (Hsu et al., 2022). However, stable isotope analysis in the same region indicates that neither sex nor ontogenetic stage influences the diet of *S. mokarran* (Lin et al., 2024). *Platycephalidae* species typically inhabit sandy substrates, supporting the idea that YOY and early juveniles use these habitats for foraging. The elevated and constant δ¹³C values in neonates and juveniles further confirm their strong reliance on shallow coastal areas as feeding grounds (Lin et al., 2024). Stable isotope analysis of two immature individuals from the Australian east coast showed contrasting ontogenetic shift in diet, with a 225 cm TL *S.mokarran* exhibiting a recent ontogenetic shift from a teleost-based diet to one dominated by benthic elasmobranchs, while a 210 cm TL individual predominantly fed on teleost (Lubitz et al., 2023). In contrast, in Biscayne Bay, Miami, stable isotope analysis revealed that YOY primarily relied on teleost and batoid from inside the bay, before transitioning to a coastal teleost-dominated diet, with consumption of batoid decreasing (Hlavin and Macdonald, 2025). Recent studies provide new insights into ontogenetic dietary shifts during the early life stages, revealing that individuals primarily consume benthic fish. As *S. mokarran* approach maturity,
their diet gradually shifts toward a greater reliance on elasmobranchs and demersal fish,
although notable geographic variations in dietary composition have been observed.

b. Adults foraging ecology: apex predator with individual dietary preference

As an apex predator (Cortés, 1999), *S. mokarran* may play an important role shaping the trophic dynamics of coastal ecosystems (Dedman et al., 2024; Raoult et al., 2019; Roff et al., 2016). Various prey handling and attack strategies have been documented for *S. mokarran*, underscoring its active hunting behavior (Chapman and Gruber, 2002; Doan and Kajiura, 2020; O'Connell, 2018; Roemer et al., 2016; Strong et al., 1990), agility (Kajiura et al., 2003) and sensory advantages (Kajiura, 2001; Kajiura et al., 2005). The species is characterized by a complex brain structure with a high level of foliation, a feature believed to support maneuverable and complex predation strategies (Yopak, 2012).

The diet of adults *S. mokarran*, particularly their reliance on small coastal elasmobranchs, has been well documented (Chapman and Gruber, 2002; Clark and Von Schmidt, 1965; Cliff, 1995; Mourier et al., 2013; Roemer et al., 2016; Strong et al., 1990). This dietary preference has earned the species a reputation as the 'shark that eats sharks' - as well as rays - a notion further supported by recent studies from the South and West Pacific, NW Atlantic, and the Caribbean Sea (Boube et al., 2023; De Bruyn et al., 2021; Doan and Kajiura, 2020; Gore et al., 2024; Lubitz et al., 2023; Moustaka and Strydom, 2020; Raoult et al., 2020, 2019). In the NW Atlantic, *S. mokarran* has also been reported feeding on large teleost such as permits, barracudas, and tarpon (Casselberry et al., 2022; Griffin et al., 2022; Guttridge et al., 2025; Lowerre-Barbieri et al., 2021). The benefits of *S. mokarran*'s hunting strategy and dietary choices were recently investigated in the NW Atlantic, showing that a shark-based diet enables individuals to go extended periods without feeding - up to two months depending on the season (Spencer et al., 2025a). Their annual metabolic energy requirements correspond to an average monthly intake of one blacktip shark, 2.5 stingrays, or 10 reef fish, indicating that a diet dominated by sharks reduces the time and effort spent foraging. For pregnant females with an average litter size (~15 pups), energy needs increase by approximately 15-17%, depending on the type of prey consumed, and by 32-34% for those carrying large litters (~33 pups) (Spencer et al., 2025a).

Population-level variability in diet, with distinct foraging behaviors observed across geographically separated populations, likely reflecting regional ecological conditions and prey availability rather than distinct regional preferences (Guttridge et al., 2025; Lubitz et al., 2023; Raoult et al., 2019). Intraspecific dietary specialization has also been documented, with some individuals showing a stronger reliance on barracudas, stingrays, or small sharks, irrespective
of sex (Guttridge et al., 2025). Additionally, no differences in δ¹³C and δ¹⁵N between adult
males and females have been reported from the NW Atlantic, GOM, Caribbean Sea or Australia,
suggesting minimal variation in diet and trophic ecology between sexes at the adult stage
(Guttridge et al., 2025; Lubitz et al., 2023; Sanchez, 2017). As adults continue to age, both their
trophic level and resource use increase; however, this shift does not appear to be primarily
driven by body size (Raoult et al., 2020, 2019).

3. Vulnerability of S. mokarran in the anthropocene and the role of Pacific Islands in its conservation

a. Vulnerability of S. mokarran to climate change and human-induced stressors

The increasing body of research on *S. mokarran* since Gallagher and Klimley (2018) provides an opportunity to re-evaluate the species' future vulnerability in the context of climate change and the growing impacts of human activities. The species' functional specialization, which optimizes its predatory efficiency, might have compromised its capacity to physiologically respond to capture-induced stress (Gallagher et al., 2014a). Great hammerheads are exposed to capture as bycatch or as a target in commercial (Baum et al., 2003; Braccini and Waltrick, 2019; Clavareau et al., 2020; Dudley, 2002; Dudley and Simpfendorfer, 2006; Gallagher et al., 2014b; Gulak et al., 2015; Morgan and Carlson, 2010; Sodré et al., 2024), artisanal (Jabado et al., 2015; Oliveira Junior et al., 2022; Diop and Dossa 2011) and recreational fisheries (Gallagher et al., 2017; Shiffman et al., 2014; Shiffman and Hammerschlag, 2014) worldwide. Its high value in the shark fin trade (Abercrombie et al., 2005; Clarke et al., 2006; Fields et al., 2018), compounded by illegal trade activities (Cardeñosa et al., 2018), further exacerbates population declines. The challenges in properly distinguishing hammerhead species have also hindered accurate population assessments (Bezerra et al., 2016; Celona and De Maddalena, 2005). In addition, the species k-selected life-history traits - characterized by slow growth, late maturity, and low reproductive rates (Cliff, 1995; Harry et al., 2011; Piercy et al., 2010; Stevens and Lyle, 1989) - further limit its capacity to recover from overexploitation, contributing to a marked decline in its populations across all oceans (Baum and Blanchard, 2010; Butcher et al., 2015; Dudley and Simpfendorfer, 2006; Ferretti et al., 2008; Morgan and Carlson, 2010; Myers et al., 2007; Pérez-Jiménez, 2014; Sodré et al., 2024). Genetic studies on *S. mokarran* genome also indicate historical population decline over the last 250,000 years, with a high level of inbreeding (Stanhope et al., 2023).

Recent studies have examined the projected responses of *S. mokarran* populations to climate
change (Black, 2024; Cerutti-Pereyra et al., 2024; Chin et al., 2010; de Souza Junior and
Teixeira Lessa, 2025; Santos et al., 2024), with some indicating a relatively lower level of concern than previously thought. The species exhibits moderate habitat specificity (Chin et al., 2010) and can be considered ubiquitous, occurring across a wide variety of coastal environments, including bays, estuaries, beaches, coral reefs, and lagoons (Chin et al., 2010; Gallagher and Klimley, 2018; Heithaus et al., 2007; Lubitz et al., 2023; Miller et al., 2014; Raoult et al., 2019, Forselledo et al., 2022). This ecological flexibility could potentially help *S. mokarran* adapt to and mitigate the impacts of habitat degradations. Additionally, its relatively broad diet (i.e., teleost, sharks and rays) may reduce its vulnerability to shifts in resource distribution (Chin et al., 2010). The species also demonstrates tolerance to a wide range of environmental conditions (i.e., DO levels, temperature) during its adult stage, suggesting potential adaptability to projected changes (Spencer, 2024). Bathymetric changes due to rising sea levels, along with shifts in sea surface temperature, salinity, and chlorophyll-a concentration are the most significant factors influencing *S. mokarran* distribution according to Santos et al (2024), possibly even enhancing habitat suitability of the species in a moderate scenario of climate change, while reducing it for *S. lewini* and *S. zygaena*. Recent studies have also demonstrated the resilience of *S. mokarran* in heavily impacted ecosystems. For instance, while the Arabian Gulf is under extreme environmental stress and anthropogenic pressures (Jabado et al., 2015), it also serves as a feeding ground for juvenile and neonate *S. mokarran* (Hsu et al., 2022; Lin et al., 2024). Similarly, *S. mokarran* is considered an "urban adapter" in the urbanized waters of Miami (Hammerschlag et al., 2022), exhibiting high residency with no detectable avoidance of boat activity (Rider et al., 2021). Biscayne Bay, a degraded and exploited area, also serves as a nursery ground for the species (Macdonald et al., 2021), further underscoring *S. mokarran*'s high adaptability and resilience in the face of habitat degradation, possibly reducing concerns for the species in the context of climate change (de Souza Junior and Teixeira Lessa, 2025).

While a comprehensive assessment of *S. mokarran*'s tolerance to climate change and anthropogenic pressures is beyond the scope of this review, several key factors must be considered when evaluating its vulnerability. The species' strong dependence on shallow coastal habitats for pupping, nursery, and foraging throughout its life cycle highlights its potential susceptibility to projected coastal environmental changes - including rising temperatures, ocean acidification, deoxygenation, and salinity fluctuations - as well as continued overexploitation (Cerutti-Pereyra et al., 2024; Mycoo et al., 2022). Notably, environmental parameters' ranges vary markedly by region, sex, and ontogenetic stage, with neonates and YOY displaying
narrower thermal, salinity, and DO ranges compared to older life stages, and males tending to occur in cooler waters than females. These differences suggest that climate change impacts on *S. mokarran* will likely vary across its range (Bindoff et al., 2019; Schlaff et al., 2014). The species' limited dispersal capacity (Brunjes et al., 2024) and naturally low abundance may further constrain its ability to shift distributions in response to habitat degradation. Moreover, with nearly 60% of coastal sharks and rays threatened by habitat loss, pollution, and overfishing (Sherman et al., 2023), decrease in prey availability are likely to challenge *S. mokarran*'s foraging efficiency depending on regions. Overall, while some studies forecast habitat expansion or suggest low climate vulnerability, such projections may overlook the species' distinct environmental needs, which vary across life stages, sexes, and regions.

**b. The Pacific Islands: a vast region with significant potential for *S. mokarran* conservation**

The occurrence of *S. mokarran* in the Pacific Islands (i.e., South and Central Pacific) was initially questioned in early studies (Gilbert, 1967), despite the species being well-documented in both the western (Gilbert, 1967; Stevens and Lyle, 1989) and eastern Pacific, where large individuals had been reported (Gudger, 1947; Tovar-Ávila and Gallegos-Camacho, 2014). However, over the past two decades, sightings and captures have confirmed its presence across a wide range of the Pacific Islands, including Fiji (Vierus et al., 2018), Kiribati (Mundy et al., 2010; Zgliczynski et al., 2013), the Marshall Islands (Bromhead et al., 2012), New Caledonia, Tuvalu, Tonga, the Solomon Islands, Nauru (Lack and Meere, 2009), Hawaii (Randall, 2007), and French Polynesia (Mourier et al., 2013; Siu et al., 2017). Despite these records, research on *S. mokarran* in the Pacific Islands remains scarce, with only one dedicated study documenting a substantial philopatric population in two atolls of the Tuamotu Archipelago (Boube et al., 2023). The lack of fishery data and limited information on the species' movements, life history, and genetic traits, is largely attributed to the technical and logistical challenges of data collection in this remote Pacific region (Clarke et al., 2015). This highlights both the need for, and the potential impact of, targeted scientific efforts in the Pacific. The importance of such research is further emphasized by the fact that several Pacific Island territories have been established as shark sanctuaries, including the Republic of Palau, the Marshall Islands, French Polynesia, the Cook Islands, New Caledonia, and the Federated States of Micronesia—together covering more than 80% of the world's shark sanctuary area (13,742,401 km²) (Ward-Paige, 2017; Ward-Paige and Worm, 2017). Many of these territories have also implemented offshore fishing regulations to mitigate overexploitation (Gillett, 2011). For instance, in French Polynesia, industrial offshore fishing is restricted to a domestic fleet of 80 longliners operating
within a huge Exclusive Economic Zone (EEZ) of nearly 5 million km² (Belli, 2024). This approach supports prey population stability for *S. mokarran* (e.g., elasmobranchs and teleosts), offering a potential conservation advantage relative to other regions. Although coastal artisanal fisheries remain less regulated, the combination of shark sanctuary status and relatively well-managed offshore fishing activity across several Pacific nations offers a promising framework for the long-term conservation of *S. mokarran*, its prey species, and associated habitats. This is particularly significant in light of recent evidence suggesting that *S. mokarran* populations may be less dispersive than previously assumed. If individuals are indeed capable of completing their entire life cycle within the protected waters of EEZs, these sanctuary areas could play a critical role in supporting local population stability and resilience. Strengthening protection within these zones—especially in key nursery, aggregation, and foraging habitats - could thus be instrumental in preventing further population declines and fostering regional recovery of this endangered species. Additionally, these oceanic islands and atolls may function as key biodiversity 'stepping stones' (Galbraith et al., 2024; Treml et al., 2008) ensuring population connectivity and facilitating gene flow across the vast expanse of the Pacific.

The geographic isolation of the Pacific Islands, relative to continental coastlines, provides another unique advantage in the context of climate change. Unlike other shark sanctuaries, such as the Bahamas, the South - Central Pacific region is less influenced by land-sea interactions and associated anthropogenic pressures, potentially reducing cumulative stressors on marine species. The region encompasses a wide range of geomorphological features (Nunn et al., 2016) which give rise to diverse habitat types, depth profiles, thermal regimes, and productivity gradients (Farabaugh et al., 2024). Among these, open atoll ecosystems appear particularly favorable, supporting high shark abundance and diversity, including *S. mokarran* (Farabaugh et al., 2024). Open atolls comprise a variety of habitats within a small spatial area, (i.e., fringing reefs, seagrass beds, sandy or muddy bottoms, and brackish zones resembling mangroves) (Bacchet et al., 2017). These heterogeneous environments can support the different ontogenetic stages and functional habitats identified for *S. mokarran*. Located in tropical to subtropical regions, open atolls maintain year-round suitable temperatures (> 26°C) (Chisholm et al., 2001; Farabaugh et al., 2025) and salinity conditions (Vollbrecht et al., 2021; Michel et al., 1971) for both adults and early life stages. Open atolls are also characterized by highly productive lagoons, where currents and tides facilitate nutrient circulation, creating productivity hotspots. These zones act as oases within the oligotrophic expanse of the open ocean, a phenomenon known as the "atoll mass effect" (Steibl et al., 2024), which boosts primary productivity in the
lagoon and enhances nutrient export through strong currents in the passes (Vollbrecht et al., 2021). Such productive environments, support high concentrations of sharks and rays, offering abundant feeding opportunities for *S. mokarran* year-round. Importantly, these productive systems function independently of continental coastal processes and are much less affected by terrestrial runoff and pollution, thereby offering relatively undisturbed ecological conditions. In this context, these sanctuaries could become refuges of global interest for *S. mokarran*. In contrast, populations of the species have already vanished from certain parts of the East Pacific, likely due to decades of intense fishing pressure (Pérez-Jiménez, 2014). This highlights the need for comprehensive research efforts across this understudied region. Improved understanding of population structure, ecological requirements, and spatial dynamics is essential to support effective conservation of *S. mokarran* in this potentially pivotal region.

**Acknowledgements**

This work was made possible thanks to the financial support of Blancpain and Banque de Tahiti, as well as Andromède Océanologie. We also thank Céline Mastnak for her contribution and the Direction de l'Environnement (DIREN) of French Polynesia (Arrêté n°004116). This is contribution #2061 from the Institute of Environment at Florida International University.

**Author Contributions**

T.B, C.H., Y.P.P. and J-C.G. designed the manuscript; T.B conducted the literature review and wrote the first draft; C.H., Y.P.P. and J-C.G. provided inputs, critical revisions and editing. S.W-S provided substantial data for the manuscript; F.B. prepared figures 1 and 2; All authors reviewed and approved the final version.

**Statements and declarations**

**Funding**

This review was financially supported by “Blancpain” and “Banque de Tahiti”.

**Competing interests**

The authors have no competing interests to declare that are relevant to the content of this article.

**Ethics approval**

Not applicable, this review does not include human or animal research.
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