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First Individual-Based Model of Basking Sharks

Chelsea Gray

First Individual-Based Model of Basking Sharks Replicates Complicated Social and Environmental Drivers of Behavior 

A first for basking shark science: individual-based modelling proves a viable way to simulate how these sharks behave. 

A new study published in Individual-Based Ecology presents the first individual-based model (IBM) of basking shark behavior. The model simulates shark movement around the Inishowen Peninsula in County Donegal, Ireland, from 1982 to 2018. It shows that basking shark aggregations emerge from a combination of environmental and social drivers, not food availability or chance alone. 

Basking sharks (Cetorhinus maximus) are the second-largest fish in the world. They are largely solitary and follow highly individual migration routes. Yet they also form surface aggregations ranging from pairs to groups of up to 1,000 animals, and the reasons remain poorly understood. These gatherings put sharks at risk of boat strikes, which makes understanding them a conservation priority. 

Previous models captured long-range migration trends but could not reproduce behaviour at finer scales. Satellite chlorophyll a, the usual stand-in for the sharks' zooplankton prey, has also matched sightings data inconsistently. This study takes a different approach: it simulates zooplankton directly to capture fine-scale prey patchiness. 

The researchers tested four behavioral sub-models against three decades of sightings data from the Irish Basking Shark Group and the Irish Whale and Dolphin Group: 

  1. Sharks seeking food only 
  2. Sharks seeking other sharks only 
  3. Sharks seeking both food and other sharks 
  4. Random movement, as a control 

The combined food-and-social sub-model produced the most realistic results, with lower error and closer correspondence to observed sightings than the alternatives. The food-only, social-only and random sub-models rarely generated aggregations larger than two sharks. 

"This paper has been a long time in the making, and I'm proud to finally share it," said lead author Chelsea Gray, Researcher with the Irish Basking Shark Group "Most importantly, it shows that an IBM can realistically replicate basking shark behaviour. This first iteration lays the groundwork, and I see enormous potential for new developments, collaborations and experimental modelling." 

The model offers a flexible framework that can incorporate improved environmental and behavioral data as they become available. It is freely available in the paper's supplementary material. Gray invites researchers interested in developing or experimenting with IBMs for basking sharks to get in touch with questions or ideas. 

The paper is available here: https://doi.org/10.3897/ibe.2.176474

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