To catch a Shark do the following. When it is raining in November go to were the ocean meets the river from 6pm onwards keep running out of the acre and back in, scaring away all the little fish, until you see a huge shadow (about the size of the Coelacanth's). Cast out your line try to catch it. If you dont catch keep on trying. It is worth 20,000 bells if sold at Tom Nooks.