In Hemingway in Cuba, the authors suggest that this passage is based on Hemingway's own battle, in the 1930s, to land a huge tuna, only to have it devoured by sharks.
He was sure the tuna was world-record size. . . . Ernest heard Dos [Passos] shout his congratulations. . . . Ernest grinned and then looked back to his prize. But his joy and triumph dissipated as he saw one, three, five shark fins surface and begin moving in on the tuna. Like wolves they cut off any escape and hit with teeth bared. In the thrashing of blood and foam, the sharks ripped off twenty-five to thirty pounds of meat with each bite.
Over the melee, a new sound arose. 'Rat, tat, tat, tat--rat, tat, tat, tat!' Bill Leeds stood on the bow of his yacht, firing down at the streaking fins below, a Thompson submachine gun in hand. . . .
But there were too many of them, even with the Thompson. The sharks were feeding on their dead brothers as well as on the great tuna. Within minutes, the spotlight showed an ocean of red.
The tuna was finally brought alongside Cook's boat; all that remained was the head, backbone, and tail. The sharks continued to hit what meat was left in the water. Angry, Ernest grabbed his rifle and fired three rounds into the feeding pack. . . .
Using block and tackle, the two Bimini mates hauled what was left abord. The tuna's head would tip the scale at 249 pounds. . . . (pages 49-50).