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Basking sharks (Cetorhinus maximus) are the second-largest fish in the world, and they’re largely solitary, following highly individual migration routes. Yet off the coast of Ireland, they sometimes gather at the surface in groups of up to 1,000. Researchers are not sure why as to why that is. Food appears to be an obvious explanation, but observations suggest it isn’t the whole story. These gatherings also put the sharks at risk of boat strikes, which makes understanding them a conservation priority.
Previous models have captured long-range migration trends but struggled to reproduce behaviour at finer scales. Chlorophyll a, the usual stand-in for the sharks’ zooplankton prey, has also matched sightings data inconsistently.
A new study in Individual-based Ecology takes a different approach. Dr. Chelsea Gray of George Mason University and the Irish Basking Shark Group, with colleagues, built the first individual-based model of basking sharks. It simulates zooplankton directly, rather than relying on a proxy, and follows sharks around the Inishowen Peninsula in Co. Donegal from 1982 to 2018. The team tested four behavioural versions against three decades of public sightings: sharks seeking food only, sharks seeking other sharks only, sharks seeking both, and random movement as a control.
So what did 36 simulated years reveal, and what could it mean for protecting these animals? We reached out to Dr. Chelsea Gray to ask about her motivation, her biggest surprises from the simulations, and the insights the model could offer for conservation.
Pensoft (P): What motivated you to conduct research on basking sharks?
Chelsea Gray (CG): I am a member of the Irish Basking Shark Group and my previous research was focused on public perceptions of basking sharks as well as interest in basking shark tourism in Ireland. While completing that research, basking sharks were downgraded by the IUCN from ‘vulnerable’ to ‘endangered’. I was also interested in individual-based modelling.
I recognized that basking sharks, with their distinctive migratory and social behavior, were an ideal subject for an individual-based model (IBM), and that this kind of research could directly support conservation.
Ireland and other nations are currently developing or revising their ocean conservation strategies, so this is the right moment to build models and simulations that can inform policy. I have also carried out qualitative research on how to communicate IBM results to policymakers, so that this work can be put to practical use.
P: The model simulated sharks and their zooplankton prey over 36 years. What was the most surprising thing that happened when you ran the simulations, and did the random-movement sharks ever get close to behaving like the real ones?
CG: What surprised me most was how well the model replicated the boom-and-bust cycle of shark aggregations seen in the real world.
This is a complex model, and it took considerable trial and error to get it running and to find settings that produced realistic results. When I finally compared the time series, particularly those of the Food and Food/Social submodels under Test A, the match with real-world sightings data was striking.
The version in which sharks sought both food and each other matched the sightings data most closely, suggesting aggregations emerge from a combination of environmental and social drivers.
The random-movement sharks never behaved like real sharks, under any settings. Their aggregations never exceeded two or three individuals, and they formed noticeably fewer aggregations overall. Testing made it clear that the emergent behaviour in the Food and Food/Social submodels resulted from the sharks’ decision-making. This was reassuring, because it shows the model’s results could not have occurred by chance.
P: These gatherings happen at the surface, where boat strikes are a risk. Could this research help keep sharks safe? For example, could the model one day help predict when and where aggregations will form so that boat traffic can be managed?
CG: The model is still at an early stage. As our ability to measure the in-situ preyscape and to remotely sense phytoplankton and zooplankton improve, future iterations of the model will allow for more precise understanding of where the sharks are aggregating. In its current form, it can provide some insight into when (based on zooplankton availability) an aggregation might occur, but not where. It does, however, lay the groundwork for further development. It also serves as a proof of concept, showing that individual-based modelling is a valuable approach to simulating and understanding shark behaviour, even though it demands substantial effort up front.
The model also allows us to develop and test future scenarios, which is essential when considering climate change and marine protected areas. Currently, we don’t know much about the impact of climate change on shark migration, however we do have some evidence that their main prey (calanoid copepods) are shifting northwards.
Climate change could therefore impact the timing and location of future shark aggregations, and we will need to account for this when developing voluntary or statutory guidelines to reduce human threats such as ship strikes. Because the model is adaptable, it is particularly well suited to testing multiple scenarios of this kind.
P: Why did you choose Individual-based Ecology to publish this research?
Individual-Based Ecology (IBE) fills a very important niche when it comes to environmental research. When previously looking to publish this article, it was difficult to find a journal that would be interested in a proof-of-concept model, especially one that doesn’t quite have policy application. IBMs also remain underused in environmental research, which makes it hard to find editors equipped to assess both the model design and ecological merits of a study.
The model is freely available in the paper’s supplementary material, and Dr. Gray welcomes questions and ideas from researchers interested in developing or experimenting with individual-based models of basking sharks.
Read the full paper:
Gray C, Rothman DS, Peters-Burton E, Smith C, Parsons ECM, Garretson A (2026) An individual-based model of basking sharks in Ireland. Individual-based Ecology 2: e176474. https://doi.org/10.3897/ibe.2.176474