Published: September 2008
In the Whirl
Adorned for the hunt, with fin raised and changeable colors flashing, a sailfish in the Gulf of Mexico circles a ball of sardines, preparing to strike.
By Jennifer S. Holland
National Geographic staff

The hunt is on. Fifty miles northeast of Isla Mujeres in the Gulf of Mexico, sailfish prowl through blue waters.

Frigatebirds hang like arrows above the sea, dipping down now and then to grab a meal. Following their lead, Anthony Mendillo, sportfishing guide and expert sailfish chaser, steers the Keen M toward the flocks. Sure enough, below the birds a school of sardines hundreds strong moves as one, flashing in the sun with each turn. Dozens of long shadows orbit the ball of frantic fish: the hunters.

Sailfish and sardines are migratory and widely distributed, with populations in multiple oceans. But from January into June, Istiophorus platypterus and Sardinella aurita meet fish to fish in this stretch of sea. For predator and prey the continental shelf here makes ideal habitat. Plankton-rich shallows, nourished by rivers draining the mainland and ocean currents pushing between Cuba and the Yucatán, promise ample food.

The hunt seems almost mammalian. Sailfish—which often travel in loose groups—clearly join forces. Males and females alike circle the prey, pushing the school into tighter formation, and taking a few bites in turn. Each forward rush is punctuated by a startling flare of the dorsal fin, which more than doubles the hunter's profile.

Color bursts may serve not only to unsettle prey but also to warn other sailfish to stay back, helping avoid collisions. "Given their pointy noses and swimming speed, this would be important," Fritsches says. Indeed, sailfish bills—elongated upper jaws that the hunters whip left and right to batter prey, and likely wield against sharks, marlins, and other enemies—are dagger sharp. Yet despite their rapid-fire strikes, reports of sailfish skewering one another are hard to find. The fish take turns—and it appears no one loses an eye or goes hungry in the frenzy.

The sardines, too, work in concert. Detecting each other's proximity and movement, they shift in synchrony, each fish both leader and follower. The fish mass slides like a drop of mercury, mesmerizing, with a shimmer that may help to confuse predators.

But no hypnotic dance can fully protect the sardines, which will hide in a squirming mass under any bit of flotsam—even a snorkeler. The sailfish simply wait within striking distance for their prey to be exposed. Soon the hunt is back on, predators again corralling, swatting, swallowing. After a rush to mop up the leftovers, the deadly game is over, and the sailfish retreat. In their wake, drifts of glinting sardine scales fall slowly into the blue.

An iridescent flash along the body, often in silvery blue stripes, adds to the effect. Darkly pigmented cells called melanophores are "like venetian blinds," says neurobiologist Kerstin Fritsches of the University of Queensland, in Australia. Ordinarily the animal appears dull, but "during stress or excitement, the cells contract their pigment to expose gorgeous metallic colors in the skin below."