Dance of survival: How sardines outsmart striped marlins in the open ocean

In the vast, featureless expanse of the open ocean, sardines rely on their collective behavior to survive against predators such as the striped marlin. 

A recent study, published in the journal Communications Biology, has delved into the predator-prey interactions between sardine shoals (Sardinops sagax caerulea) and striped marlins (Kajikia audax).

The research uncovers how sardines use self-organized maneuvers to evade predators – and how predators exploit these strategies.

The study was conducted by a team of scientists from the Cluster of Excellence “Science of Intelligence” (SCIoI), Humboldt-Universität zu Berlin, the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), and the University of Cambridge.

The team utilized computational modeling and aerial video footage to analyze these behaviors.

Sardine survival strategy: The “fountain effect” 

One of the most striking maneuvers observed during attacks is the “fountain effect.” This collective response emerges when a sardine shoal splits into two subgroups, creating an arched trajectory around the predator before rejoining behind it. 

This pattern is an effective defensive mechanism that helps sardines outmaneuver faster, less agile predators.

The fountain effect not only allows prey to evade attacks but also enables the separated subgroups to reunite, preserving the benefits of group cohesion. 

According to the experts, the fountain effect helps slower-moving prey to outmaneuver the fast but less maneuverable predator. This also allows the separated subgroups to rejoin after the attack – retaining the benefits of belonging to a larger group – and to be fully prepared for the next attack.

Sardines maximizing their survival chances

Despite its advantages, the fountain effect comes with trade-offs. Sardines face a balance between distancing themselves from the predator and minimizing the time it takes to regroup. 

The researchers’ simulations revealed that the optimal “prey-fleeing angle” is 30 degrees, which maximizes survival chances but increases the time required for the shoal to recover its cohesion.

Lead researcher Palina Bartashevich is an expert in collective behavior at Humboldt-Universität zu Berlin. 

“Using agent-based computer simulations, we discovered that there is an optimal ‘prey-fleeing angle’ of 30°, which not only produces a fountain-like pattern at the collective level but also maximizes individual survival chances independent of the attack direction,” explained Bartashevich.  

“But we also showed that this comes at a cost: the group takes longer to recover after an attack.”

Marlins work around the sardines’ strategy

Striped marlins capitalize on this delay by attacking from the sides, a strategy that disrupts the group’s cohesion and isolates individual prey. 

The study confirmed that side attacks were the most effective for marlins, as they increase the time it takes for the group to reassemble.

The researchers observed distinct differences in sardine behavior based on the direction of the marlins’ attacks. Side attacks often triggered the fountain effect, causing the shoal to split and regroup. 

Conversely, back attacks led to more cohesive evasive movements, with the entire group fleeing in a single direction rather than splitting.

“With our predator-prey computer model, we were able to 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,” Bartashevich explained. 

“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.”

Constant battle between predator and prey

The study highlights the dynamic and adaptive nature of predator-prey interactions. Sardines use collective behaviors to enhance their chances of survival, while marlins refine their hunting techniques to counter these defenses. This ongoing evolutionary arms race underscores the complexity of life in the ocean’s open waters.

By combining computational modeling with real-world observations, the researchers were able to provide new insights into the strategies used by both predators and prey. 

“Our simulations, as explained above, provide a plausible explanation as to why the prevalence of both side and back attack directions and the observed prey response can be the case, suggesting that there is an ongoing arm wrestle between predators and prey in the strategies they use to improve their own success at the expense of the other,” Bartashevich concluded.

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