A feast for the eyes—and for dolphins. Here’s the wondrous way bait balls form.

With the feeding frenzy in full swing, the dark blue sea water is a flurry of movement. Dozens of dolphins dart up from below while seabirds dive underwater, all trying to snatch a snack from a large, dark shape that keeps shifting. Upon closer look, this morphing buffet isn’t a singular creature—it’s a whole school of fish.

This is what happens when predators such as dolphins, sharks, tuna, and seabirds descend on a bait ball—a group of fish trying to protect themselves from a hungry onslaught of predators by gathering in a dense cluster.

It’s rare, but “baleen whales, which also feed on small schooling fish, may join the party too,” says Laura González García, scientific and technical coordinator at Futurismo Azores Adventures.

Bait balls are all about safety in numbers. Being packed together makes the fish look like a much bigger animal. “This formation exposes fewer fish to dangers at the periphery of the ball,” says González García, who is also a researcher at Institute of Marine Sciences – OKEANOS at the University of the Azores. “Chances of survival for a single fish drastically increase inside the group.”

Their lateral line—sense organs that detect movement, vibrations, and pressure changes—enables the school to move in perfect unison. “Synchronized movements confuse predators,” she says, and make it harder to pick out one individual from the group.

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But this self-preservation strategy doesn’t always work. Intelligent dolphins use clever strategies to turn the bait balls to their advantage.

Working as a team, “several dolphins will encircle the ball to keep it confined,” González García says, “while other dolphins swim through or on the sides of the ball and take their chance to hunt.” They use whistles to coordinate and switch roles regularly so everyone gets enough to eat—each of them swallowing one fish at a time, head first so it goes down easier.

In many locations, the pod can pin the fish against the seabed or reef, but in deep waters—like the Portuguese archipelago of the Azores—the dolphins hem the fish in from the sides and underneath. Here, “confining fish against the bottom is not even an option,” notes González García.

Bait balls are an effective foraging tactic, giving fish fewer chances to escape. If dolphins hunt one fish at a time instead and the intended victim gets away, they needlessly waste a lot of energy.

The sight of a bustling bait ball has “a feeling of organized chaos, where everyone knows their role and accomplishes the mission with millimetric precision,” says González García. “It’s nature at its best.”