Sailfish & Baitballs
The sailfish (Istiophorus platypterus) is generally considered to be the fastest fish in the sea, with an estimated top speed exceeding 60mph (100 kmph). They are members of the billfish family, along with swordfish and marlins. Sailfish can grow to more than 10 feet in length and weigh over 200 lbs / 90kg.
Sailfish have been observed to hunt cooperatively. A group of sailfish may surround a school of baitfish (e.g. sardines), herding them into a tight, rounded grouping called a "baitball". The baitball is forced toward the surface, where the smaller fish are trapped and can be picked off one at a time. When circling a baitball, sailfish intermittently raise their dorsal fin, while flashing bright colors, perhaps to frighten the sardines into a tighter grouping.