Bait ball
A tight sphere that sardines or anchovies form when attacked from all sides by predators.
When small schooling fish are attacked by dolphins, sharks, tuna, seabirds or whales at once, they press together into a dense, swirling ball, often near the surface. Each fish tries to get to the middle, away from the edges. The ball protects some fish, but it also concentrates them, and predators take turns driving through it. Bait balls are a famous sight of the Sardine Run off South Africa's east coast between about May and July, and in the Sea of Cortez and off Mexico's Pacific coast.