New research using decades of Irish basking shark sightings suggests that food availability determines how often sharks gather, while social attraction influences the size of their groups.
Simulated research on basking shark behaviour off Donegal has found that both food and company help explain why the animals gather in groups.
The new study, published in the journal Individual-based Ecology, tested four competing behavioural rules against one of the longest-running citizen science datasets in shark research.
As Bioengineer reports, between 1982 and 2018, the Irish Basking Shark Group and the Irish Whale and Dolphin Group documented 2,136 sightings across Ireland, of which 969 — 45% — involved two or more sharks.
Around Donegal's Inishowen Peninsula alone, 458 sightings were reported, again with 45% involving aggregations.
Malin Head has long been a focus of basking shark research, including satellite-tracking work that followed a female shark from Donegal to tropical waters.
The area includes Malin Head and Lough Swilly, identified by the IUCN as an Important Shark and Ray Area, and is known for breaching sharks, a behaviour hypothesised to be linked to courtship. Previous research off Ireland's west coast has also examined reproductive behaviour involving groups of sharks.
Modelling Shark Behaviour
Scientists used individual-based models (IBMs), which work from the bottom up.
“Instead of assuming an average shark moving through an average sea, an IBM populates a spatially explicit environment with simulated individuals that each sense, decide, and move on their own — and whose interactions with their surroundings generate population-level patterns,” Bioengineer explains.
The approach had not previously been applied to basking sharks, despite the species being particularly suited to it. Although largely solitary, basking sharks are known to form poorly understood aggregations ranging from two individuals to groups of up to 1,000, with feeding, courtship or both proposed as explanations.
The scale of these gatherings was illustrated by reports of 125 basking sharks off the Clare coast.
“Each simulated day, every shark faced a decision: whether its current patch held enough zooplankton to support the sharks already there, and whether to seek food, other sharks, both, or — in the control case — nothing at all,” the report says.
Getting the parameters right took roughly 2,000 exploratory model runs. The team calibrated settings for how far sharks could sense, how fast they swam, how much zooplankton a patch needed to contain, and what proportion of patches held prey.
The researchers then validated the best configurations against the sightings data using mean error, mean absolute error and root mean square error.
The final confirmatory runs involved 50 trials of the full 200-shark model for each of the two best parameter sets.
Food Determines Frequency, Company Influences Size
The scientists say the results suggest a division of roles between feeding and social behaviour.
Food availability drives how many aggregations form, while social attraction shapes how large they become.
Bioengineer says the implications extend beyond shark behaviour.
Surface aggregations coincide with seasonal boat traffic, putting sharks at risk of collisions. In 2022, Ireland granted basking sharks protected wild animal status under Section 23 of the Wildlife Act, prohibiting interference with their feeding and reproductive behaviour.
Implications for Conservation
Effective conservation requires knowing where and when sharks are likely to gather, yet directly observing these elusive animals is difficult, and tagging is prohibitively expensive.
The findings also have implications for marine conservation planning off north Donegal, where basking shark concentrations have previously been highlighted.
The researchers suggest that individual-based modelling offers a flexible framework that can incorporate improved environmental and behavioural data as they become available.
With better zooplankton data, such models could eventually help predict large aggregations or provide estimates of higher- and lower-risk areas for boaters and fishers.
The full study is available in Individual-based Ecology.
