Hemingway describes the shovelnose shark attacking the fish, “he came like a pig to the trough.” what makes this simile effective?

After Santiago slaughters the immense marlin in The Old Man and the Sea, by Ernest Hemingway, he lashes it to the side of his watercraft. The fish is eighteen feet long, longer than the watercraft, and Santiago knows he won't make great time going home in light of the additional weight and clumsiness of the fish. He doesn't get much of anywhere before the principal sharks strike the fish.