Transoceanic dispersal and connectivity of a white shark (Carcharodon carcharias) between southern Africa and Southeast Asia

Introduction

Population connectivity, the degree of exchange of individuals among spatially separated populations, is essential for functioning marine ecosystems (Cowen et al. 2007). It influences dispersal and migration patterns, the formation of genetic population structure, source–sink population dynamics, and responses to climate change and other anthropogenic pressures (Kool et al. 2013; Beger et al. 2022). Furthermore, understanding marine connectivity and migration patterns is important to identify threats across a species’ range and opportunities for effective management (Queiroz et al. 2019; Sequeira et al. 2019).

Satellite-linked tags have transformed our understanding of animal movements, habitat use, and behaviors by providing unprecedented insights into their migrations across vast oceanic regions (Bonfil et al. 2005; Boyle et al. 2009; Orlov et al. 2020; Ramos et al. 2023). This knowledge has important applications for conservation and management, including identifying key foraging areas, migration corridors, and estimating overlap with fisheries to assess potential interactions and risks (Sequeira et al. 2019; Queiroz et al. 2019).

White sharks are well known for their transoceanic migrations (Bonfil et al. 2005; Blower et al. 2012; Duffy et al. 2012; Domeier and Nasby-Lucas 2013; Spaet et al. 2020). These border-crossing movements, along with their high vulnerability to overfishing, has supported their listing under CITES Appendix II and the Convention on Migratory Species (Rigby et al. 2022). However, they also show fidelity to coastal aggregation sites throughout their global range (Bonfil et al. 2005; Jorgensen et al. 2010; Domeier and Nasby-Lucas 2013; Skomal et al. 2017; Franks et al. 2021; Kock et al. 2022). Furthermore, in South Africa, many historically abundant coastal white shark aggregations have recently shown declines in sightings. The lack of a clear understanding of the population status, distribution in the region and drivers of movement continues to hinder effective management of the species (Bowlby et al. 2022, 2023, 2024; Kock et al. 2022; Gennari et al. 2024).

Our understanding of South African white shark distribution relies on telemetry, genetics, and catch-record data (Pardini et al. 2001; Bonfil et al. 2005; Andreotti et al. 2016; Kock et al. 2022). These sources indicate that the southern African coastline is an important area for juvenile and subadult sharks (<360 cm TL) of both sexes. Sharks in the south-western Indian Ocean pelagic realm are predominantly subadults and adults, and often females (Kock et al. 2022). Catches of large, mature individuals in Kenya, Tanzania, and Madagascar suggest potential shifts in habitat preference on reaching maturity. However, there is limited direct evidence linking these individuals to South Africa (Cliff et al. 2000; Zuffa et al. 2002), even though satellite tracking shows movement between cold-temperate South Africa and tropical Mozambique and Madagascar (Kock et al. 2022). Direct evidence of longitudinal dispersal from South Africa is currently documented by the movements of one individual from Dyer Island, Gansbaai to Ningaloo on the western coast of Australia, and supported by genetic data from six individuals with South African-like mtDNA haplotypes collected in eastern Australia and Tasmania (Pardini et al. 2001; Bonfil et al. 2005; Blower et al. 2012; Andreotti et al. 2016).

Pardini et al. (2001) found that maternally inherited mitochondrial genomes (mtDNA) were distinct between South African and Australasian white shark populations, but microsatellites in the nuclear genome were not significantly distinct. Bonfil et al. (2005) proposed that the genetic results from Pardini et al. (2001), which suggested that dispersal was limited to males, could be explained by philopatric female sharks from South Africa mating in Australia, but giving birth in South Africa. Further evidence of separate genetic clades between continents suggests that despite long-distance movements, genetic differences among shark populations in different regions remain distinct (Gubili et al. 2011; Blower et al. 2012; O’Leary et al. 2015; Andreotti et al. 2016). More recently, high-resolution genome-wide markers have shown that white sharks from South Africa, Australia and New Zealand form a distinct genetic clade from North Atlantic/Mediterranean, and North Pacific individuals (Wagner et al. 2024). These findings have highlighted the need for further research into the frequency of movement and connectivity of South African white sharks beyond their known ranges.

Through unexpected events, here we document the first recorded physical link and connectivity of a white shark between southern Africa and Southeast Asia. This discovery, facilitated by an incidental capture and a non-transmitting satellite tag, has highlighted collaboration among fishers, scientists and conservationists. It underscores the need for international cooperation and raises further questions about white shark dispersal and connectivity, especially considering changing ocean environments and shark distributions in South Africa.

Methods

On 9 May 2012, a SPOT-258 satellite transmitter (Wildlife Computers Inc., Redmond, WA, USA) was deployed on the first dorsal fin (Fig. 1a) of a 390 cm total-length (TL) female white shark. On the basis of her size, the female white shark was classified as a subadult (Malcolm et al. 2001; Márquez-Farías et al. 2024). Tagging took place aboard the M/V Ocearch and was part of a larger study investigating the movement of the species in southern Africa (Kock et al. 2022). Precise measurement and tagging of this individual occurred at Dyer Island, a productive Cape fur seal colony (Arctocephalus pusillus pusillus) situated in the temperate Atlantic Ocean, 8 km offshore of Gansbaai, South Africa (34°40′S, 019°25′E, Fig. 2, blue triangle). Data collection was conducted under the South African Department Forestry, Fisheries and the Environment: Oceans and Coasts permitting authority (Permit #RES2012/OCEARCH/umbrella-project) and adhered to the legal requirements of South Africa.

Fig. 1.

(a) Left-side dorsal photograph from the M/V Ocearch platform during tagging in May 2012 in Gansbaai, South Africa. (b) Left-side dorsal photograph from a shark cage diving charter in September 2012 in Gansbaai, South Africa (SharkWatchSA). (c) Left-side dorsal photograph taken at the landing site in Indonesia in 2016.

Fig. 2.

Top panel: SPOT satellite tag transmissions (black circles) for the white shark (Carcharodon carcharias) tagged at Gansbaai, South Africa, in May 2012. Dashed line in grey indicates the shortest straight-line distance between the last transmission and the capture location. Overviews corresponding to lower panels are indicated by lines and colour. Lower-left, blue border, shows inshore detections near the tagging site (tagging site indicated in blue triangle). Lower-right orange border depicts the region where the shark was captured in Indonesia in November 2016 (orange circle). Pink diamond indicates location of a previously documented catch (Fahmi and Dharmadi 2014). Purple square is an undocumented sighting of a large white shark filmed by recreational divers in Nusa Penida, Bali, and reported by popular media in 2019.

Results

Discussion

We have presented here the first documented physical link and connectivity between southern Africa and Southeast Asia, exhibiting the longest recorded one-way point-to-point dispersal of more than 10,000 km for a white shark. The tagging site in Gansbaai, South Africa, is renowned as a prime location for winter feeding on Cape fur seals from May to September, and provides a critical temperate, kelp-associated habitat for white sharks (Jewell et al. 2014; Towner et al. 2016). In contrast, the final capture location in Indonesia (10°20′S, 120°27′E; Fig. 2, orange circle) is dominated by reef building coral and tropical sea temperatures (Wilson et al. 2011). This striking transition highlights the species’ capacity for long-range dispersal between vastly different ecological niches. The movement between southern Africa and Southeast Asia surpasses the 9,000 km straight-line distance recorded for another subadult female white shark from Gansbaai, South Africa, to the west coast of Australia (Bonfil et al. 2005). This report marks only the second recorded transoceanic dispersal event for the species from South Africa, highlighting that despite extensive research, much remains to be learned about white sharks’ dispersal patterns in the southern hemisphere.

Transoceanic migrations and large-scale movements between cooler temperate and tropical waters have been well documented for white sharks across various regions (Boustany et al. 2002; Bonfil et al. 2005, 2010; Bruce et al. 2006; Domeier and Nasby-Lucas 2007, 2008; Weng et al. 2007; Guttridge et al. 2024). Whereas previous studies have reported movements between the temperate waters of South Africa and tropical regions such as Mozambique (Kock et al. 2022), the migration observed in this study spans a much greater distance, crossing a large ocean basin and providing new insights into the spatial dynamics of white shark movements.

In addition to the surprising connectivity between southern Africa and Southeast Asia, there are few documented occurrences of white sharks in Southeast Asia (Duffy 2016). The species is rarely reported in Indonesia, with the first confirmed record coming from Dompu, West Nusa Tenggara, where an estimated 6 m adult male white shark was harpooned by fishers onboard a demersal longliner (Fahmi and Dharmadi 2014; Fig. 2, pink diamond). However, given this discovery and the misidentification of the tagged shark from this study as a longfin mako, white shark presence in Indonesia may be more common than previously believed. Isurus oxyrinchus and I. paucus are documented in longline fisheries in Indonesia and can be mistaken for C. carcharias. In fact, misidentifications of mako species with C. carcharias are believed to occur in Indonesian fisheries statistics as exhibited in the present record (Oktaviyani et al. 2023). Shark catch recording programs conducted in several locations across Indonesia use an identification guidebook from White et al. (2006) that does not include an identification key for C. carcharias. This raises concerns about the trade of threatened species and underscores the need for improved species identification training, reporting, and when possible, access to DNA-based species identification/confirmation in markets.

Long-distance and transoceanic migrations expose white sharks to heightened mortality risks, such as capture in longline fisheries (Queiroz et al. 2019). Furthermore, shark fishing is widespread across Indonesian territorial waters, with diverse fish landing sites exhibiting varying facilities, characteristics, and conditions (Dewi et al. 2021). This record and the previous record from Indonesia provide evidence that some trade in white shark products is occurring, perhaps being complicated by species misidentification. The lack of sufficient information on catch data, potential yields, species diversity, biology, exploitation levels, and institutional socio-economic resources of sharks and rays in Indonesia hampers the development of a robust framework for sustainable fisheries management (Dharmadi et al. 2015; Dewi et al. 2021). However, contemporary trade in this species is not unique to Southeast Asia (Shivji et al. 2006). Nevertheless, as a signatory to CITES, Indonesia is obligated to regulate the utilization and trade of species listed in the CITES Appendices (Nurbani et al. 2021). Standardization and training in species identification, data recording and traceability is ongoing in the country and may be improved with updated species keys that reflect the discovery reported here (Oktaviyani et al. 2023). Simple educational resources and training has proven effective at improving the accuracy and reliability of species identification and reporting in similar fisheries (Macbeth et al. 2018).

The presence of South African sharks in the far eastern regions of the Indian Ocean aligns with our current understanding of genetic differentiation between populations of the species. Philopatric evidence, indicated by heterogeneity in mtDNA and uniformity in nuclear DNA based on microsatellite data, suggests that even though transoceanic movements may occasionally result in rare breeding or parturition, they are not expected to significantly contribute to gene flow (Pardini et al. 2001; Bonfil et al. 2005; Gubili et al. 2011; Blower et al. 2012; Andreotti et al. 2016; Wagner et al. 2024). The record offers some insight into white shark growth rates, although it is important to note that the reported length estimate may have a margin of error owing to measurement challenges during landing, because the shark was cut up and re-assembled for estimation. Over its 4.5 years at liberty, the shark grew an estimated 83 cm (18.4 cm per year), a growth rate faster than previously reported for subadults of this species (Christiansen et al. 2016). The discovery of such a widely displaced individual highlights the importance of gaining a clearer understanding of the distribution patterns of subadult and adult sharks in southern Africa and the mechanisms driving their dispersal. Given the increased pressure on this population, and a lack of data on white shark reproduction and fecundity, there is a critical need to satellite tag more large white sharks, particularly females. However, accessing adult sharks of both sexes has proven challenging, particularly after the displacement of white sharks from previous aggregation sites in the Western Cape, South Africa (Towner et al. 2022). Efforts might benefit from a focus on searching for white sharks off the southern coasts of Mozambique and Madagascar, and possibly Indonesia, where migratory females appear to be active. Furthermore, an overdue comparison of photo identification catalogues across countries would also complement telemetry and genetic datasets.

The South African white shark population size, trends, and the impact of killer whales (Orcinus orca), fisheries, and shark nets are currently the focus of extensive research and scientific debate (see Bowlby et al. 2022, 2023, 2024; Kock et al. 2022; Gennari et al. 2024). Recent studies have documented novel interactions between white sharks and killer whales (Towner et al. 2023, 2024), the displacement of white sharks by killer whales (Towner et al. 2022, 2023), and the vulnerability of white shark populations to human-induced mortality (Bowlby et al. 2022; Kock et al. 2022). These findings emphasize the need to fully understand the movements and connectivity between remote geographic areas and how frequently these occur. Increased redistribution may be expected if continued predatory displacement persists as an additional coastal threat to white sharks in South Africa.

The series of events documented here has highlighted the crucial role of the fishing industry and community engagement in scientific discovery, emphasizing the importance of multinational communication and collaboration. Simultaneously, the chronology of events also illustrates the importance of confirming information provided by fishers, questioning sources and following up on reports. Without the trusted working relationships built among conservation agencies, local fishers and the government, it is likely that this discovery would have remained unknown. Project Hiu has successfully recovered four tags through their relationships with the local fishing community, with two others highlighting the movements of tiger sharks from Australia to Indonesia. Establishing connections and building respect between conservationists and shark fishers forms a strong foundation for scientific advancement in the area. In a low socio-economic community that stands to benefit greatly from the indiscriminate capture of sharks, understanding how to navigate financial incentives to support scientific discovery and provide an alternative to fishing sharks is an ongoing challenge. The discovery made here may encourage continued communication and transparency regarding discovered tags and inspire fishers to have pride in their involvement with the conservation and research of sharks. The opportunity to also use this discovery to provide further training and materials for identifying sharks in these fisheries should be prioritised.

Our discovery has shown the previously unknown area of occurrence for white sharks from South Africa, expanding our understanding of their extensive range and connectivity among distant regions. It has also advanced our scientific understanding of white shark biology and behavior, emphasizing the importance of ongoing international research, collaboration and conservation efforts to protect these apex predators and their habitats. Most importantly, it has highlighted areas of focus for future improvements in understanding white shark ecology and conservation.