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A woman on a boating trip in the Florida Keys died Thursday after a 75-pound spotted eagle ray jumped out of the water and struck her in the head. Injury by leaping fish isn’t as uncommon as you might think. Why do fish jump?
To eat—or to avoid getting eaten. Fish that have been chased to the surface of the water might hurtle into the air to confuse a pursuer, which either won’t notice where its prey went or will be unable to predict where it will land. Schools of mullets, for instance, will often pop above the water’s surface as they’re fleeing barracudas or groupers. Stingrays, large as they can be, are the favorite food of bull sharks and hammerhead sharks, though it’s not clear whether the airborne ray in Florida was evading a predator. Getting spooked by something large and loud like a motorboat—like the Asian carp in this video—might also evoke the same response.
Different fish have different jumping techniques. Tropical arowanas, among the best leapers around, coil into an S-shape before straightening out quickly and thrusting their bodies into the air. The arowanas jump in order to catch insects and small birds or mammals above the water. Great white sharks also exit the water in pursuit of a meal; they swim toward prey with such speed that they sometimes launch into the air with a seal clasped in their jaws.
Taking a leap out of the water isn’t particularly hard for fish. Most of them are capable of jumping, and usually do so by swimming quickly toward the surface of the water. (Bottom dwellers like flounder prefer to hide than to outrun predators.) Some fish are extra adept. Salmon, for instance, hurl themselves up waterfalls when it’s time to spawn. Flying fish push out of the water with their tail fins, then spread open their pectoral fins, which resemble wings; they can glide for hundreds of feet.
Going above the surface provides a big advantage: Fish move a lot faster through air than through water. Think of how much more effort it takes for us to swim than to walk; this is partly because water is more than 800 times denser than air and so creates more friction when we move. * Because fish evolved strong muscles to move through the dense medium they live in, the force that propels them through water will send them skyrocketing through the air. (Of course, most fish can’t control their motion once in midair.)
Some scientists believe jumping may serve a purpose in courtship and dominance for toothed whales, a group that includes dolphins, killer whales, and belugas. Males are thought to caper around to attract females or to push each other as a way of staking out a position in the social hierarchy. Jumping and crashing back into the water may also be akin to scratching for some fish. Whales, sharks, and large fish like tuna are often bothered by remoras—foot-long fish with suckers—and parasites like copepods that attach to their skin. Falling back into the water might dislodge these creatures; researchers have also calculated that a dolphin’s spin in the air generates enough centripetal force to throw off the suckers.
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Explainer thanks Ray Davis of the Georgia Aquarium, Philip Motta of the University of South Florida, George Parsons of Shedd Aquarium, and Greg Sass of NOAA’s National Marine Fisheries Service.
Correction, March 26, 2008: The original story misstated the relative density of water to air; water is more than 800 times, not 100 times, denser than air. (Return to the corrected sentence.)
