In a recent study, researchers investigated how sardines that find themselves on a predator's radar collectively employ collective evasive manoeuvres and how their predators respond to outsmart them.

Using computational modelling and aerial video footage, the team from the Cluster of Excellence “Science of Intelligence” (SCIol), the Humboldt-Universität zu Berlin, the Leibniz Institute of Freshwater Ecology and Inland Fisheries, and Cambridge University focussed on the predator-prey behaviour of striped marlins and sardine shoals in the open ocean. 

Their findings were published in the Communications Biology journal. 

The “fountain effect” 

When striped marlin attack a sardine shoal by swimming towards it, the sardines split into two subgroups, creating an arched trajectory that goes around the predator and rejoins as one at its tail. Called the “fountain effect,” this is a collective response that emerges when predators try to break up prey groups and isolate individuals. 

This evasive strategy helps the sardines outmaneuver the faster but less maneuverable marlin. By regrouping after the attack, they increase their chance of survival through the benefits of belonging to a larger group. 

This strategy results in a tradeoff between maximising the distance of individual prey from the predator and minimising the time they need to return to the bigger group after an attack.

“Using agent-based computer simulations, we discovered that there is an optimal ‘prey-fleeing angle’ of 30 degrees, which not only produces a fountain-like pattern at the collective level but also maximises individual survival chances independent of the attack direction,” said lead author and researcher Palina Bartashevich, from Institute for Theoretical Biology at Humboldt-Universität zu Berlin. 

From the side or from behind?

Based on the empirical data collected under natural conditions, the marlin generally attack sardine shoals either at the side and from behind. Side attacks usually gave rise to the fountain effect; if the sardines were attacked from behind, they would cohesively evade the attack as a whole in one single direction.

Using their computer model, Bartashevich said that they could predict that “if the prey in the group uses the mentioned optimal fleeing angle of 30°, predators are more effective in attacking from the side of the school."

“That’s because attacking from the sides represents the best compromise between getting close to the prey and increasing their splitting time, making it an efficient strategy for a predator balancing both objectives at the same time,” she added.