I think the dolphins are taking advantage of the normal tendency of the fish to school, seeing as they do it when approached by tuna, not able to blow bubbles or do other things. The balling of the school of fish is a general respone to any threat, not just dolphins. As a behavior it must have survival value, if it has persisted so long in fish. Maybe the predator becomes confused and can't single out any one fish to grab, somewhat like zebras on the plains of Africa. While it is true that fish ball up in the face of dolphins, the dolphins are not the sole cause of the behavior.