Why do basking sharks gather off Donegal? A new study points to two influences: food availability and social attraction. Its simulations suggest food affects how often gatherings form, while attraction to other sharks shapes how large they become. The work draws on 2,136 Irish sightings recorded between 1982 and 2018.
Researchers tested four explanations for shark movements around the Inishowen Peninsula against historical observations. The findings help separate the role of feeding from social behaviour. They also lay the groundwork for models that could help identify where sharks are likely to gather.
Why Basking Sharks Gather Off Donegal
The closest match to the sightings came from simulated sharks seeking both food and other sharks. Food alone wasn’t enough. Models based only on food, social attraction or random movement rarely produced groups larger than two animals.
The study, An individual-based model of basking sharks in ireland, published in Individual-based Ecology, found that the combined food-and-social model produced lower error and more closely matched observed gathering patterns than the other three approaches.
Here’s the useful distinction: how often gatherings form and how big they get are separate questions. Food availability influenced how many aggregations formed; social attraction helped explain their size. Both were needed to reproduce the observed patterns most realistically.
The paper’s authors are Chelsea Gray, Dale S. Rothman, Erin Peters-Burton, Cynthia Smith, E.C.M. Parsons and Alexis Garretson. Gray’s affiliations include George Mason University and the Irish Basking Shark Group in Donegal. The team also has affiliations with the University of Exeter, Tufts University and The Jackson Laboratory.
Decades of Sightings Provided the Test
The Irish Basking Shark Group and Irish Whale and Dolphin Group documented the sightings used to assess the models. Of the 2,136 sightings across Ireland, 969 involved two or more sharks—about 45% of the total.
Around the Inishowen Peninsula, the records contained 458 sightings. Again, approximately 45% involved aggregations. The simulations focused on shark movements around this north Donegal peninsula, while the historical dataset covered Ireland as a whole.
Those records gave researchers a way to test whether simple movement rules could reproduce the group patterns seen at sea. There’s an important limit, though: the sightings didn’t directly reveal what motivated individual sharks to join a gathering.
How the Shark Simulations Worked
An individual-based model gives each simulated animal its own movement rules. Group patterns then develop as the animals interact with their surroundings and one another. In other words, the model starts with individual sharks rather than a ready-made group.
The researchers tested four versions:
- Sharks seeking food only.
- Sharks seeking other sharks only.
- Sharks seeking both food and other sharks.
- Sharks moving randomly, as a control.
The model represented patches of zooplankton directly. Previous approaches had used chlorophyll a, associated with phytoplankton, as an indirect indicator of prey availability. Those approaches produced inconsistent results when compared with shark sightings.
Each simulated day, sharks assessed whether their patch contained enough zooplankton to support the animals present. Where they moved next depended on the behavioural rules being tested.
Roughly 2,000 exploratory runs helped establish settings for sensing distance, swimming speed, prey availability and the proportion of patches containing food. The researchers assessed model performance using mean error, mean absolute error and root mean square error. Final testing involved 50 trials of a 200-shark model for each of the two strongest parameter sets.
What the Findings Establish—and What Remains Uncertain
The results support a combined environmental and social explanation for basking shark gatherings. Still, this is model-based evidence. A closer match to historical sightings supports the behavioural rules being tested; it doesn’t establish the motivation behind every gathering.
Social attraction in the model meant sharks seeking other sharks. That result alone can’t tell us whether a particular group formed for courtship or another social purpose. Company, perhaps—but the model doesn’t tell us why.
Separate observations have documented dozens of sharks feeding, travelling and breaching around Mullaghmore Head and surrounding waters in Donegal Bay. Those observations show group behaviour elsewhere along the coast. They are geographically distinct from the Inishowen simulation area, not observations from the same location.
The researchers identify better environmental and behavioural data as the next opportunity to strengthen the model. Improved zooplankton information could eventually help predict large aggregations and identify areas of greater or lower collision risk for boats and fishing vessels. A useful possibility, but that predictive use remains a future application.
