Maybe you’ve seen that picture taken by the Sea Shepperds in the Faroe Islands where they’re currently killing dolphins as a traditionnal hobby.
Appart from the disgusting aspect of that image, isn’t it amazing how nature created such perfectly shaped and foiled fins ? NACA didn’t invent anything…
“The sail is normally kept folded down and to the side when swimming, but it may be raised when the sailfish feels threatened or excited, making the fish appear much larger than it actually is. This tactic has also been observed during feeding, when a group of sailfish use their sails to “herd” a school of fish or squid.”