Shark sightings by scuba divers reveal the movements of marine predators throughout the year, according to a study published February 26, 2025 in the open-access journal PLOS One by George Balchin, William Hughes and colleagues at the University of Sussex, U.K., and Aquaplanet Dive Center, South Africa.
Many sharks move through different habitats as they follow food or search for mates. Since they are major predators, they change the shape of the ecosystems they visit. Examining these movements is key to understanding the health of ocean habitats as well as the impacts of human activity, but it is notoriously difficult to get the long-term data needed. In this study, Balchin and colleagues compiled citizen science data on shark sightings from professional scuba divers working in diving tourism. The dataset included over 5,300 sightings of six shark species, including hammerhead sharks, tiger sharks and bull sharks, between 2013 and 2019 in the Protea Banks reef of South Africa.
The data revealed that all six species came and went with the seasons, though they followed their own schedules, with hammerhead sightings more frequent in the spring and tiger sharks more abundant in autumn, for example. The team also identified changes from year to year. During the studied period, for instance, hammerhead shark, blacktip shark and ragged-tooth shark sightings increased; tiger shark sightings were stable, and bull shark and dusky shark sightings decreased. In some cases, these changes are likely to reflect broad-scale changes in the behavior or population of these species – for example, population growth in those shark species which now have protected status.
This information regarding which sharks are present at which times is very valuable for planning fishing guidelines or conservation efforts throughout the year. Furthermore, this study demonstrates the potential of diving tourism as a source for citizen science data. The authors suggest that future studies might expand their reach to new areas, as well as accounting for potential biases in this study's data by involving more varied habitats or types of dives.
The authors add: "Our study provides a really good example of how shark diving operators and scientists, working together, can provide the important long-term information on shark population dynamics that's needed for shark conservation."
In your coverage, please use this URL to provide access to the freely available article in PLOS One: https://plos.io/4hY4zWO
Citation: Balchin GP, Schuller A, di Stefano I, Robertson M, Pollard K, Hughes WOH (2025) Seasonality, long-term trends and co-occurrence of sharks in a top predator assemblage. PLoS ONE 20(2): e0318011. https://doi.org/10.1371/journal.pone.0318011
Author countries: U.K., South Africa
Funding: The study was funded by the Fisheries Society of the British Isles https://fsbi.org.uk (GPB). The funders did not play any roles in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.
