When a fish is scared it will swim away faster than usual and with a trail of bubbles behind it - this will last around 2-3 seconds and then it will slow down and resume normal swimming. To scare a fish/make it flee take a larger fish and swim it into the fish you wish to scare and it will shoot away. If this is a camera mission you may need to try this more than once as they can swim offscreen faster than you can snap a picture!