Usually the oceans' biggest fish—which reach lengths of up to 40 feet (12 meters)—stick to themselves as they cruise the world's tropical waters looking for plankton and other small prey. (See picture: "Smallest Whale Shark Discovered—On a Leash.")
But aerial and surface surveys spotted at least 420 of the sharks rubbing fins as they gorged on eggs freshly spawned by little tunny fish, a relative of the mackerel.
"To see a group of that many all in one place was phenomenal—to the point where you couldn't navigate a boat through that without having concern for the fish. That's impressive," said study co-author Mike Maslanka, head of the Nutrition Science Department at the Smithsonian Conservation Biology Institute in Virginia.
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"It doesn't go understated that they're not small animals," added Maslanka, whose study was published in April in the journal PLoS ONE. "You don't realize how big they are until you're swimming beside them or you pull a boat up next to them."