Unusual encounters during BCSS’s long-term spatial mapping monitoring reveal the extraordinary ecology—and mounting vulnerability—of whale sharks in Mozambique and the broader Western Indian Ocean.
In September and again in November 2025, researchers from the Bazaruto Center for Scientific Studies (BCSS) documented two rare encounters with Rhincodon typus—the planet’s largest fish and one of its most endangered.
The sharks were sighted in the St. Sebastian area in the greater Bazaruto Seascape, outside the official Bazaruto Archipelago National Park (BANP) but within the surrounding sanctuary waters. Both individuals were encountered in shallow water between 5–10 meters, exhibiting calm, slow-travel behaviour and—at least initially—open-mouth feeding, a classic indicator of productive surface waters.
Image by BCSS/Orlando Miranda & Salvador Colvee: A rare encounter with a Whale shark (Rhincodon typus) swimming through shallow waters in the Bazaruto seascape.
Why These Sightings Matter
For many field programs, whale sharks are a familiar sight. But for BCSS, these encounters were extraordinary. Despite over eight years of continuous boat-based spatial mapping time-series surveys, covering thousands of kilometers of transects across seasons and ecosystems, these two sharks are the only whale sharks BCSS has ever recorded in the greater Bazaruto seascape.
And this rarity matters. Not only do whale sharks serve as indicators of ocean health, but their long-term absence—and sudden appearance—within a seascape as dynamic as Bazaruto may reflect shifting ecological conditions, regional productivity changes, or broader oceanographic signals still unfolding across the Western Indian Ocean (WIO).
The sightings also underscore the vital importance of consistent fieldwork. BCSS’s multi-year spatial mapping program and Scientific Training Program—which brings early-career researchers into real-time conservation science—made these two encounters possible, documented, and scientifically meaningful.
Image by BCSS/Orlando Miranda & Salvador Colvee: A BCSS researcher filming the 2025 whale shark (Rhincodon typus) encounter.
WIO Whale Shark Populations
The Western Indian Ocean is home to some of the world’s most distinctive whale shark populations. From Djibouti and the Seychelles in the north, to Mafia Island (Tanzania), Nosy Be (Madagascar), and southern Mozambique’s famous Praia do Tofo, each region hosts aggregation sites shaped by unique environmental drivers.
According to the scientific literature (Rohner et al. 2014, 2015, 2018; Cagua et al. 2013, 2015; Prebble et al. 2018), whale sharks in the WIO are mostly juvenile-dominated populations, highly dependent on coastal productivity hotspots and showing surprisingly limited connectivity between regional aggregation sites.
This means a whale shark frequently observed at Mozambique’s Praia do Tofo is unlikely to be the same animal seen at Mafia Island (Tanzania), Nosy Be (Madagascar), the Seychelles, or the Maldives. Their ecological boundaries are far more local than once assumed. Understanding these distinctions is essential for conservation—and for interpreting what the 2025 Bazaruto sightings might mean.
Southern Mozambique, particularly the Inhambane Province around Praia do Tofo, is one of the world’s best-documented whale shark sites.
Long-term datasets show:
- Unusual year-round presence, rather than seasonal aggregation
- High densities (up to 29 sharks per 100 km)
- A population dominated by juvenile sharks aged 10–20 years
- Strong coastal fidelity, even though individuals are capable of traveling thousands of kilometres
- Foraging behaviour tied to complex upwelling systems that create continuous plankton productivity
What makes Mozambique unique is the consistent productivity of its shelf waters. Three concurrent upwelling mechanisms—shelf-edge upwelling, divergent upwelling, and Delagoa Bight vortex-driven upwelling—produce rich feeding patches year-round. This contrasts sharply with other WIO regions where productivity is seasonal, driving sharks to migrate.
Image by BCSS: Aerial view of BCSS Seascape and Bazaruto Island.
Bazaruto: A Very Different Seascape
But the Bazaruto Seascape is different. Despite being located only ~400 km north of Tofo, BCSS’s Bazaruto time-series shows:
- Zero whale shark sightings for eight years, until the two encounters in 2025
- Sparse surface plankton blooms, especially sergestid shrimps associated with feeding events in Tofo
- Distinct hydrology: fewer strong upwelling events, more stable temperatures, and more lagoonal habitats
- Lower prevalence of the patchy, high-biomass prey layers whale sharks seek.
This suggests that while southern Mozambique is a whale shark hotspot, the Bazaruto Seascape lies outside the species’ preferred feeding corridors, reflecting the localised nature of whale shark ecology.
Interestingly, Tanzania’s Mafia Island offers a fascinating contrast. Surface sightings there show a strong seasonal pattern (peaking October–February), but acoustic telemetry reveals sharks are actually present year-round, diving deeper and moving offshore during “off-season” months. This “cryptic residency” highlights a critical lesson: a lack of surface sightings does not necessarily equate to absence.
However, the Bazaruto sharks encountered in 2025 were not deep or cryptic—they were at the surface, slow-swimming, exhibiting typical shallow-water behaviour. These individuals were feeding and navigating through productive water layers, suggesting localised prey or environmental conditions at the time of sighting.
Comparisons With Madagascar and the Seychelles
In Nosy Be (Madagascar), whale sharks appear annually between September and December, forming a growing tourism hotspot.
In the Seychelles, aggregations are highly seasonal (June–October) and strongly tied to monsoon-driven productivity pulses.
Both regions display higher male bias (up to 82.5% male populations) than Mozambique, where male proportion is closer to 39%. These patterns reinforce a central point: whale shark ecology in the WIO is patchy, seasonally influenced, and site-specific.
Image by BCSS/Orlando Miranda & Salvador Colvee: A close-up of the whale shark (Rhincodo typus) encountered by BCSS divers in 2025.
Interpreting the 2025 Whale Shark Encounters
BCSS has conducted one of the longest continuous boat-based spatial mapping expeditions in the WIO. Operating multiple transects year-round, the team collects:
- Bathymetric profiles
- Water quality and temperature data
- Plankton samples
- Megafauna presence/absence data
- Photo-ID and behavioural observations
This dataset is invaluable precisely because of its consistency. For eight years, whale shark encounters were conspicuously absent. Then, in 2025—two individuals in two months. In ecology, such events are not noise—they are signals, and potential explanations include:
1. Regional productivity pulses
A localised upwelling or nutrient injection may have created a temporary prey hotspot. Such
events are rare but not impossible in the Bazaruto area.
2. Shifts in whale shark movement corridors.
Changes in prey distribution, temperature, or regional currents may have temporarily
redirected whale sharks through the archipelago.
3. Broader climate-linked ocean changes.
The WIO is warming faster than the global ocean average. Changes to boundary currents or eddy formation could alter shark pathways.
4. Statistical coincidence.
Eight years of zeros does not guarantee absence; it merely reflects long-term rarity. The
2025 encounters may simply represent a rare but natural occurrence.
What These Rare Sightings Reveal About Bazaruto’s Megafauna
Regardless of reason, documenting these encounters is essential for understanding the long-term ecology of Bazaruto’s megafauna. The first whale shark, photographed on a calm morning in September 2025, was estimated at around 7 meters, a size consistent with juvenile aggregations in Mozambique. Swimming in ~12 meters of water, the shark exhibited slow travel behaviour, no visible injuries or fishing gear scars.
The second encounter occurred in November 2025, just 2 km from the first location but in ~14 meters of water.
Both sharks appeared to be males based on the presence of paired claspers on the inside margin of the pelvic fins. In total, the two encounters represent the first documented whale sharks in the Bazaruto Seascape.
Understanding the Encounters
Drawing on the provided scientific sources, we can contextualise these encounters. The WIO whale shark population shows:
- Strong juvenile dominance.
- Limited connectivity between aggregation sites.
- High local residency, despite capacity for long-distance dispersal.
- Male-biased populations, except Mozambique which has more balanced ratios.
The two sharks encountered by BCSS closely matched the juvenile profile of the regional
population, and the next steps for BCSS researchers will be to run the photo-ID pictures across the regionally available datasets – stay tuned for potential matches!
Image by BCSS/Orlando Miranda & Salvador Colvee: A close-up of the whale shark (Rhincodo typus) encountered by BCSS divers in 2025.
So, why are they so rare in Bazaruto?
Despite hosting manta rays, dolphins, dugongs, sea turtles, reef sharks, and large pelagic fish, Bazaruto is a poor whale shark site. Possible explanations may include lack of strong upwelling. Without the complex upwelling mechanisms of Tofo, Bazaruto does not consistently produce the prey biomass whale sharks target. Hydrodynamics may be another explanation. The archipelago’s lagoonal structure and shallow sandbanks create different productivity regimes—more stable but less intense. Prey availability may also contribute to this situation.
Whale sharks are classified as Endangered on the IUCN Red List. The WIO population faces multiple threats, including increasing gill net use in Mozambican waters, tourism pressure in Madagascar and Tanzania, vessel strikes, climate-driven shifts in prey distribution, offshore industrial development. The limited regional connectivity shown by scientific studies means that each aggregation needs localised protection. What happens in Mozambique stays in Mozambique. A decline in one region is unlikely to be “buffered” by influx from other populations.
For Bazaruto, the sightings highlight two needs:
- Continued long-term monitoring: BCSS’s spatial mapping is one of the only consistent datasets for the BANP megafauna community.
- Integration of regional datasets: Connecting BCSS observations with broader WIO whale shark networks (e.g., through photo-identification platforms) may reveal whether the 2025 individuals have been seen in southern Mozambique or elsewhere.
The ability to document and analyse rare megafauna events like these depends on consistent, methodologically rigorous field research—the kind BCSS has been conducting since 2017. This is also why the BCSS Scientific Training Program plays an increasingly important role in developing future conservation scientists.
Participants in the program
- Join BCSS researchers on spatial mapping expeditions.
- Learn photo-identification techniques used for megafauna research.
- Acquire practical skills in plankton sampling, oceanography, and species behaviour recording.
- Participate in data analysis that directly feeds into publications and management decisions.
The 2025 whale shark encounters were documented by a team that included early-career researchers and Scientific Training Program participants—giving them a once-in-a-lifetime experience while contributing to real conservation science.
For aspiring marine scientists, opportunities like this are rare. For whale sharks in Bazaruto, they are rarer still.
A Seascape Full of Questions and Possibilities
Whether these encounters represent a shift in whale shark ecology or a fleeting anomaly remains unknown. But every documented encounter in a data-poor region like Bazaruto—especially after eight years of absence—carries scientific significance.
As BCSS continues its mission to study and protect the Bazaruto Seascape, such encounters remind us of the ocean’s unpredictability, beauty, and need for informed stewardship.
And for the early-career scientists joining these expeditions through the BCSS Scientific Training Program, they are reminders of why field science still matters—and why being present, persistent, and observant is the heart of discovery.
References
Andrzejaczek, S., Meeuwig, J., Rowat, D., Pierce, S., Davies, T., Fisher, R., & Meekan, M. (2016). The ecological connectivity of whale shark aggregations in the Indian Ocean: A photo-identification approach. Royal Society Open Science.
Brooks, K., Rowat, D., Pierce, S., Jouannet, D., & Vély, M. (2010). Seeing spots: Photo-identification as a regional tool for whale shark identification. Western Indian Ocean Journal of Marine Science.
Cagua, E. F., Cochran, J., Rohner, C., Prebble, C. E. M., Sinclair-Taylor, T., Pierce, S. J., & Berumen, M. (2015). Acoustic telemetry reveals cryptic residency of whale sharks. Biology Letters.
Cagua, E. F., Cochran, J., Rohner, C., Igulu, M., Rubens, J., Pierce, S., & Berumen, M. (2013). Demographics and feeding ecology of whale sharks at Mafia Island, Tanzania. Western Indian Ocean Journal of Marine Science.
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Rohner, C., Armstrong, A. J., Pierce, S., Prebble, C. E. M., Cagua, E. F., Cochran, J., Berumen, M., & Richardson, A. (2015). Whale sharks target dense prey patches of sergestid shrimp off Tanzania. Journal of Plankton Research.
Rohner, C., Weeks, S. J., Richardson, A., Pierce, S., Magno-Canto, M. M., Feldman, G., Cliff, G., & Roberts, M. (2014). Oceanographic influences on a global whale shark hotspot in southern Mozambique. PeerJ Preprints.
Rohner, C., Richardson, A., Jaine, F. R., Bennett, M., Weeks, S. J., Cliff, G., Robinson, D., Reeve-Arnold, K., & Pierce, S. (2018). Satellite tagging highlights the importance of productive Mozambican coastal waters to the ecology and conservation of whale sharks. PeerJ.
More information:
For questions about this article, please contact:
Ekaterina Kalashnikova, Bazaruto Archipelago – Ocean Observatory Bazaruto Center for Scientific Studies ekaterina.kalashnikova@bcssmz.org
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