Herring perform stronger collective evasive reactions when previously exposed to killer whales calls

Schooling in fish is understood as a strategy reducing the risk of predation. Yet, it remains unsolved whether schooling fishes can change the structural properties of their collective in order to minimize risk and whether such adjustments promote efficient group-level responsiveness. We conducted a simulated-predator encounter experiment in a sea-cage on a large wild-caught Atlantic herring school (~60000 individuals). First, we tested whether herring schooling dynamics changed in response to vocalizations of killer whales (feeding calls), a main predator of herring in the wild. We also investigated if herring collective evasive reactions during simulated attacks varied after pre-exposure to killer whale vocalizations. Collective escape reactions (collective diving) were stronger when herring were previously exposed to killer whale vocalizations. However, herring did not modify their schooling dynamics (e.g., school density, school vertical distribution in the water column, fish swimming speed, or correlation strength between individuals, that measures how aligned the fish are as a function of distance) in response to the killer whale feeding calls alone. Overall, our results demonstrate that structural and dynamic changes at the school-level are not necessarily required for the execution of strong collective escape maneuvers, but risk awareness influences collective responsiveness and information transfer among schooling fish.