Decoding bull shark behaviour in Seychelles’ coastal waters |15 December 2025

  Removing an acoustic receiver prior to downloading the data and replacing the batteries

Along the northeast coast of Mahé, the Marine Conservation Society Seychelles (MCSS), in collaboration with the Seychelles Fisheries Authority, the Seychelles Ports Authority, and the Oceanographic Research Institute, has embarked on a multi-year study to better understand one of the ocean’s most misunderstood predators: the bull shark (Carcharhinus leucas).

Bull sharks are listed as ‘Near Threatened’ by the International Union for Conservation of Nature (IUCN), with populations declining due to overfishing and habitat loss. This research, funded by the SeyCCAT Blue Grants Fund, is not just about sharks, it is about the health of the entire marine ecosystem. By studying how juvenile bull sharks use Seychelles’ shallow-water habitats, MCSS is hoping to uncover insights that will inform conservation strategies, safeguard biodiversity, and strengthen the resilience of coastal ecosystems.

Bull sharks are remarkable for their adaptability. Unlike most sharks, they can thrive in both saltwater and freshwater environments, having been recorded thousands of kilometres up rivers and even in inland lakes. They play a vital role in maintaining the balance and resilience of coastal ecosystems. As apex predators, they regulate populations of fish and other marine animals, preventing any single species from becoming overly dominant and disrupting the food web. They are ecosystem engineers, and their presence helps sustain biodiversity by targeting sick, weak, or overabundant individuals, which strengthens prey populations genetically and ecologically.

Mahé’s northeast coast represents a critical pupping ground for bull sharks, offering sheltered shallow-water habitats where juveniles can grow and develop. These areas provide a relatively safe environment, with abundant food resources and reduced predation pressure compared to offshore waters. By supporting the early life stages of bull sharks, the northeast coast plays a vital role in sustaining local and regional populations and ensuring the long-term viability of the species in Seychelles’ waters and beyond.

In November 2025, MCSS completed its second season of field research focused on the habitat preferences and spatio-temporal movements of bull sharks along Mahé’s northeast coastline. Following the successful tagging of 13 juvenile and sub-adult bull sharks in 2024, an additional 5 sharks were fitted with acoustic transmitters during the recent fieldwork. These small devices, surgically implanted into each shark, emit unique sound pulses at regular intervals. Ten receivers, deployed along the coastline, act as listening stations. Whenever a tagged shark swims within range, its presence is recorded, creating a digital footprint of its movements.

The November fieldwork also allowed MCSS to retrieve and service the receivers, downloading a year’s worth of data that reveals where these young sharks have been, how they use coastal habitats, and how their behaviour changes over time. While the northeast coast of Mahé has long been an important nursery area for bull sharks, these same habitats are also among the most modified by human activity. Land reclamation for the Port of Victoria, man-made islands, and coastal development all exert pressure on the ecosystem. With batteries replaced, the receivers will continue collecting data into 2026, adding to a growing dataset that promises to answer questions that remain central to understanding how these sharks shape Seychelles’ coastal ecosystems and help design conservation measures that balance ecological needs with human livelihoods.

As well as answering questions in relation to local habitat use, regional collaboration is an important aim of the project. Acoustic telemetry networks allow scientists across the Indian Ocean to share data, tracking sharks as they move beyond Seychelles’ waters. This cooperative approach is vital for migratory species that do not recognise political boundaries. Moreover, the study contributes to global knowledge of shark ecology and Seychelles’ research adds valuable data to international efforts aimed at safeguarding the species.

Finally, the project is not only about data collection, but it also aims to foster awareness of the bull shark’s ecological importance. Sharks often suffer from negative public perception, and it is hoped that research like this can help to reframe them as guardians of marine health rather than threats.

Contributed