Small and skittish fish are hard to photograph. Breathing underwater is surprisingly loud noisy. I learned this listening to the sound track of the movies from my camera. And I am a big thing in a black dive suit. Indeed this must be intimidating to a small fish. The spot on the dorsal fin is to distract and mislead the eye into thinking the fish is headed in a different direction or larger than it is. Sometimes you sneak up and get the shot. Or else we would not be looking at this image. It helps to not blow bubbles as you sneak.