A large school of sardines in the Monterey Bay Aquarium’s “Open Sea Exhibit”
(taken from a Yahoo! News story today).

Sardines school to avoid predators, said curator Paul Clarkson. “It’s a safety-in-numbers gambit,” he told OurAmazingPlanet. “This makes it harder for a predator to target any single individual.”

This classic schooling behavior is seen in a wide variety of fish. But exactly how so many individuals can move as one remains mysterious. It is thought to involve ultra-quick collective processing of visual cues in the environment, likely aided by the fish’s lateral line organs, which sense water currents and pressure.

The presence of predators like mahi-mahi,  bluefin tuna and hammerhead sharks in the same exhibit keep the fish from getting complacent, Clarkson said. (Sardines will school even without predators present, just not in tight formations like this one.) Generally, however, there isn’t much predation going on, since the predators are kept well-fed, he said.

Sardines were heavily fished in the first half of the 20th century, according to the aquarium, and overfishing led their populations to collapse. The fish has recovered somewhat since the 1980s, though, and there is now a modest sardine fishery off the California coast.