In the vast, open ocean with no place to hide, sardines group together for protection. When they cannot rely on speed to escape, the sardines' best chance is to outmaneuver the predator altogether. However, predators also seek ways to find weak spots in the group's defence. In a study published in Communications Biology, researchers from the Cluster of Excellence "Science of Intelligence" (SCIoI), the Humboldt-Universität zu Berlin, the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), and Cambridge University uncover the connection between how prey's collective escape patterns emerge and why predators adopt specific strategies to attack them. Using a combination of computational modeling and observations from the aerial video footage, an interdisciplinary research team investigated the predator-prey behaviour of striped marlins (Kajikia audax) and sardine shoals (Sardinops sagax caerulea) in the open ocean. Their findings reveal that individual prey in groups follows simple decision-making rules, which lead to complex, collective self-organized manoeuvers – and that this response is something predators can capitalize on.
The "fountain effect" helps sardines escape the marlin…
In the striped marlin predator-prey system, there is a certain stage of the hunt in which the fish school under attack splits into two subgroups, creating an arched trajectory that goes around the attacking predator and rejoins the group at its tail. This is the so-called "fountain effect", a collective anti-predator response that emerges when predators try to break up the prey groups and isolate individuals. Getting isolated from the group, a single sardine becomes an easy target for a predator. Simply put: staying in the group gives you a better chance to survive. Not only does the "fountain effect" allow slower-moving prey to outmaneuver the fast but less maneuverable predator, but it also allows the separated subgroups to rejoin after the attack, retaining the benefits of belonging to a larger group, and be fully prepared for the next attack.
…but the marlin can exploit it too
However, the researchers have found that this fascinating phenomenon is subject to trade-offs. The individual escape rule provides 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°, which not only produces a fountain-like pattern at the collective level but also maximizes individual survival chances independent of the attack direction," explained researcher Dr Palina Bartashevich, lead author of the study. "But we also showed that this comes at a cost: the group takes longer to recover after an attack." This suggests that predators can also potentially exploit the "fountain effect", despite it being a defensive mechanism against them.
Predators often attempt to fragment prey schools, as this reduces group size, which can increase capture success. The empirical data collected under natural conditions showed that the marlins generally attack the sardine shoals from the side and from behind, with side attacks most frequently leading to the fountain effect. 'Non-fountain' evasive manoeuvres, where the entire school evaded the attack cohesively in one direction rather than splitting up and re-joining, occurred predominantly when groups were attacked from the back.
"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," said Bartashevich. "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."
It's a constant arm wrestle between predator and prey
The researchers were also able to predict that prey are more effective in escaping when attacked from behind, verifying modelling predictions with empirical observations from aerial footage. "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," explained Palina Bartashevich.
