Sailfish hunt in packs and work together to drive bait fish up from the depths to shallow sunlit water where it's easier to see them.
When the ball is dense enough, they will drive into it with the long bills and startle the bait fish by opening their big dorsal fins (the "sail" of the sailfish) with a sudden whoosh, and at the same time they flash irridescent colors along their bodies.
They use these scare techniques to separate out smaller, more manageable groups of fish for easier eating. You can see these spectacular fish driving and cutting a baitball in this video, as photographer Paul Nicklen explains what's going on.
Photo by Paul Nicklen, National Geographic
From the Upcoming ueue, submitted by Marilyn Terrell.