This amazing underwater footage shows an aggregation of Sailfish, Istiophorus platypterus, feeding on a school of baitfish.

The scientific name Istiophorus platypterus literally means 'flat-winged sail bearer', in reference to the huge dorsal fin.  The movie clearly shows the dorsal fin being raised and lowered during manoeuvring and while herding the baitfish.

 

Reference:
Pepperell, J. 2010. Fishes of the Open Ocean. A natural history & illustrated guide. University of NSW Press. Pp. 266.